2 TMS K⁺ and water channel (conducts K⁺ (K_D = 8 mM); blocked by Na⁺ (190 mM) (Renart et al., 2006) and tetrabutylammonium (Iwamoto et al., 2006)). Ion permeation occurs by ion-ion contacts in single file fashion through the selectivity filter (Köpfer et al. 2014). A narrow pore lined with four arrays of carbonyl groups is responsible for ion selectivity, whereas a conformational change of the four inner transmembrane helices (TMS2) is involved in gating (Baker et al. 2007). Two gates have been identified; one is located at the inner bundle crossing and is activated by H⁺ while the second gate is in the selectivity filter (Rauh et al. 2017). The C-terminal domain mediates pH modulation (Hirano et al., 2011; Pau et al., 2007). KcsA exhibits a global twisting motion upon gating (Shimizu et al., 2008).  Activity is influenced by the phase of the lipid bilayer (Seeger et al. 2010), and occupancy of nonannular lipid binding sites increases the stability of the tetrameric complex (Triano et al. 2010).  The open conformation of KcsA can disturb the bilayer integrity and catalyze the flipping of phospholipids (Nakao et al. 2014).  This protein is identical to the KcsA orthologue (P0A333) in Streptomyces coelicolor.  The stability of the pre domain in KcsA is stabilized by GCN4 (Yuchi et al. 2008).  The potential role of pore hydration in channel gating has been evaluated (Blasic et al. 2015).  Having multiple K⁺ ions bound simultaneously is required for selective K⁺ conduction, and a reduction in the number of bound K⁺ ions destroys the multi-ion selectivity mechanism utilized by K⁺ channels (Medovoy et al. 2016).  The channel accomodates K⁺ and H₂O molecules alternately in a K⁺-H₂O-K⁺-H₂O  series through the channel (Kratochvil et al. 2016). Insertion of KcsA is spontaneous and directional as the cytosolic part of the protein does not translocate across the membrane barrier. Charged residues, not hydrophobic residues, are crucial for insertion of the unfolded protein into the membrane via electrostatic interactions between membrane and protein.  A two-step mechanism was proposed. An initial electrostatic attraction between membrane and protein represents the first step prior to insertion of hydrophobic residues into the hydrocarbon core of the membrane (Altrichter et al. 2016). Bend, splay, and twist distinguish KcsA gate opening, filter opening, and filter-gate coupling, respectively (Mitchell and Leibler 2017). Details of the water permeability have been presented. Water flow through KcsA is halved by 200 mM K⁺ in the aqueous solution, which indicates an effective K⁺ dissociation constant in that range for a singly occupied channel. (Hoomann et al. 2013). A parameterized MARTINI program can be used to predict the hinging motions of the protein (Li et al. 2019).

Skc1 (KcsA) of Streptomyces lividans

Voltage-sensitive Na⁺ channel, Na_V1.7 (Cox et al., 2006). The human orthologue, SCN3A when mutated causes cryptogenic pediatric partial epilepsy (Holland et al., 2008). Batrachotoxin (BTX) is a steroidal alkaloid neurotoxin that activates NaV channels through interacting with transmembrane domain-I-segment 6 (IS6) of these channels. Ginsenoside inhibits BTX binding (Lee et al. 2008). VGSCs are heterotrimeric complexes consisting of a single pore-forming alpha subunit joined by two beta subunits, a noncovalently linked beta1 or beta3 and a covalently linked beta2 or beta4 subunit (Hull and Isom 2017).  The binding mode and functional components of the analgesic-antitumour peptide from Buthus martensii Karsch to human voltage-gated sodium channel 1.7 have been characterized (Zhao et al. 2019).

Voltage-sensitive Na⁺ channel of Rattus norvegicus

The insect (cockroach) Na⁺ channel. Batrachotoxin, pyrethroids, and BTG 502 share overlapping binding sites (Du et al., 2011).  Insecticides tagetting Na⁺ channels include indoxacarb and metaflumizone (Casida and Durkin 2013). They preferably bind to and trap sodium channels in the slow-inactivated non-conducting state, a mode of action similar to that of local anesthetics (Jiang et al. 2015).  Asp802 is involved in gating and action, but not binding, of pyrethroid insecticides (Du et al. 2010).

Na⁺ channel of Blattella germanica (O01307)

Sodium channel of 2215 aas, VmNa.  An L925V mutation in the channel domain renders the honey bee mites resistant to pyrethroids such as tau- fluvalinate and flumethrin (González-Cabrera et al. 2013).

VmNa of Varroa destructor

Type 2 Na⁺ channel, SCN2A or NaV1.2, of 2,005 aas and 24 TMSs.  Mutations give rise to epileptic encephalophathy, Ohtahara syndrome (Nakamura et al. 2013). They may also give rise to autism (ASD) (Tavassoli et al. 2014). This protein is orthologous to the rat Na⁺ channel, TC# 1.A.1.10.1 and very similar to the type 1 Na⁺ channel (1.A.1.10.7). Na_V1.2 has a single pore-forming alpha-subunit and two transmembrane beta-subunits. Expressed primarily in the brain, Na_V1.2 is critical for initiation and propagation of action potentials. Milliseconds after the pore opens, sodium influx is terminated by inactivation processes mediated by regulatory proteins including calmodulin (CaM). Both calcium-free (apo) CaM and calcium-saturated CaM bind tightly to an IQ motif in the C-terminal tail of the alpha-subunit. Thermodynamic studies and solution structure (2KXW) of a C-domain fragment of apo 13C,15N- CaM (CaMC) bound to an unlabeled peptide with the sequence of the rat NaV1.2 IQ motif showed that apo CaMC (a) was necessary and sufficient for binding, and (b) bound more favorably than calcium-saturated CaMC. CaMN  apparently does not influence apo CaM binding to NaV1.2IQp (Mahling et al. 2017).

SCN2A of Homo sapiens

Voltage-sensitive Na⁺ channel of 2821 aas and 24 TMSs (Cohen et al. 2009). Pyrethroid, an insecticide, binds to insect Na⁺ channels at two sites called pyrethroid,receptors, PyR1 (initial) and PyR2, located in the domain interfaces II/III and I/II, respectively, and binding residues have been identified (Du et al. 2015).

Na⁺ channel of Drosophila melanogaster

The voltage-gated Ca²⁺ channel (VDCC; CAV2), α-subunit of 2027 aas and 24 TMSs in four domains, each with six transmembrane segments and EEEE loci in the ion-selective filter, typical of VDCCs in vertebrates. CAV2 primarily localizes in the distal part of flagella and is transported toward the flagellar tip via intraflagellar transport (IFT) although CAV2 accumulates near the flagellar base when IFT is blocked. Thus, Ca²⁺ influx into Chlamydomonas flagella is mediated by the VDCC, CAV2, whose distribution is biased to the distal region of the flagellum, and this is required for flagellar waveform conversion (Fujiu et al. 2009).

CAV2 of Chlamydomonas reinhardtii (Chlamydomonas smithii)

The sodium channel of 1989 aas and 24 TMSs.  80% identical to the characterized channel of the crayfish (Astacus leptodactylus (Turkish narrow-clawed crayfish) (Pontastacus leptodactylus)) in which functional regions responsible for the selectivity filter, inactivation gate, voltage sensor, and phosphorylation have been identified (Coskun and Purali 2016).

Na⁺ channel of Cancer borealis (Jonah crab)

The voltage-gated sodium channel of 2147 aas and 24 TMSs.  Several mutations in the structural gene give rise to pyrethroid resistance (kdr) (Saavedra-Rodriguez et al. 2007).

Na⁺ channel of Aedes aegypti (Yellowfever mosquito) (Culex aegypti), the most prevalent vector of dengue and yellow fever viruses.

Voltage-gated Na⁺ Channel protein of 2,139 aas and 24 TMSs.  Mediates voltage-dependent sodium ion permeability of excitable membranes.  3-d modeling revealed spacial clustering of evolutionarily conserved acidic residues at extracellular sites (Vinekar and Sowdhamini 2016).

PARA sodium channel of Anopheles gambiae (African malaria mosquito)

Sodium channel protein, α-subunit, FPC1, of 2050 aas and 24 TMSs.  The 3-d structure has been solved by cryoEM to 3.8 Å resolution (Shen et al. 2017). One residue at the corresponding selectivity filter (SF) locus in each repeat, Asp/Glu/Lys/Ala (DEKA), determines Na⁺ selectivity. The S1 to S4 segments in each repeat form a voltage-sensing domain (VSD), wherein S4 carries repetitively occurring positive residues essential for voltage sensing. There are seven extracellular glycosylation sites (Shen et al. 2017).

FPC1 of Periplaneta americana (American cockroach) (Blatta americana)

Sodium channel Nav1.4-beta complex of 1820 and 209 aas, respectively.  Voltage-gated sodium (Nav) channels initiate and propagate action potentials. Yan et al. 2017 presented the cryo-EM structure of EeNav1.4, the Nav channel from electric eel, in complex with the beta1 subunit at 4.0 Å resolution. The immunoglobulin domain of beta1 docks onto the extracellular L5I and L6IV loops of EeNav1.4 via extensive polar interactions, and the single transmembrane helix interacts with the third voltage-sensing domain (VSDIII). The VSDs exhibit ""up"" conformations, while the intracellular gate of the pore domain is kept open by a digitonin-like molecule. Structural comparison with closed NavPaS shows that the outward transfer of gating charges is coupled to the iris-like pore domain dilation through intricate force transmissions involving multiple channel segments. The IFM fast inactivation motif on the III-IV linker is plugged into the corner enclosed by the outer S4-S5 and inner S6 segments in repeats III and IV, suggesting a potential allosteric blocking mechanism for fast inactivation (Yan et al. 2017). The PDB# for the complex is 5XSY, and that for the two subunits are 5XSY_A and 5XSY_B. Domain 4 TMS 6 of Nav1.4 plays a key role in channel gating regulation, and is targeted by the neurotoxin, veratridine (VTD) (Niitsu et al. 2018).

Nav1.4-beta subunits of Electrophorus electricus (Electric eel) (Gymnotus electricus)

Voltage-gated sodium channel of 1836 aas and 24 TMSs, PaFPC1. The 3-d structure has been determined (Shen et al. 2017).  It mediates the voltage-dependent sodium ion permeability in excitable membranes.

PaFPC1 of Periplaneta americana (American cockroach) (Blatta americana)

Putative two component voltage-gated Na⁺ channel, subunit 1 of 1149 aas and subunit 2 of 958 aas.  Decreased expression of these genes, encoding this system, gives rise to mortality of the peach-potato aphid, Myzus persicae (Tariq et al. 2019).

NaV of Myzus persicae.

Ca²⁺-regulated heart Na⁺ channel, Na_v1.5, SCN5A or INa channel of 2016 aas. The COOH terminus functions in the control of channel inactivation and in pathologies caused by inherited mutations that disrupt it (Glaaser et al., 2006); regulated by ProTx-II Toxin (Smith et al. 2007), telethonin, the titin cap protein (167aas; secreted protein; O15273) (Mazzone et al., 2008), and the Mog1 protein, a central component of the channel complex (Wu et al., 2008). Na_v1.5, the principal Na⁺ channel in the heart, possesses an ankyrin binding site, and direct interaction with ankyrin-G is required for the expression of Na_v1.5 at the cardiomyocyte cell surface (Bennett and Healy, 2008; Lowe et al., 2008). Mutations cause type 3 long QT syndrome and type 1 Brugada syndrome, two distinct heritable arrhythmia syndromes (Mazzone et al., 2008; Kapplinger et al. 2010; Wang et al. 2015). SCN5A mutations causing arrhythmic dilated cardiomyopathy, commonly localized to the voltage-sensing mechanism, and giving rise to gating pore currents (currents that go through the voltage sensor) have been identified (McNair et al., 2011; Moreau et al., 2014).  Patients with Brugada syndrome are prone to develop ventricular tachyarrhythmias that may lead to syncope, cardiac arrest or sudden cardiac death (Sheikh and Ranjan 2014) and (Kapplinger et al. 2015). Mutations causing disease have been identified (Qureshi et al. 2015). These give rise to arrhythias and cardiomyopathies (Moreau et al. 2015).  Mutations that cause relative resistance to slow inactivation have been identified (Chancey et al. 2007).  Green tea catechins are potential anti-arrhythmics because of the significant effect of Epigallocatechin-3-Gallate (E3G) on  cardiac sodium channelopathies that display a hyperexcitability phenotype (Boukhabza et al. 2016). A mutatioin, R367G, causes the familial cardiac conductioin disease (Yu et al. 2017). The C-terminal domain of calmodulin (CaM) binds to an IQ motif in the C-terminal tail of the alpha-subunit of all NaV isoforms, and contributes to calcium-dependent pore-gating in some (Isbell et al. 2018). Ventricular fibrillation in patients with Brugada syndrome (BrS) is often initiated by premature ventricular contractions, and the presence of SCN5A mutations increases the risk upon exposure to sodium channel blockers in patients with or without baseline type-1 ECG (Amin et al. 2018). A mutation (R367G) is associated with familial cardiac conduction disease (Yu et al. 2017).

Na_v1.5 of Homo sapiens (Q14524)

The skeletal muscle Na⁺ channel, Na_V1.4 of 1836 aas and 24 TMSs. Mutations in charged residues in the S4 segment cause hypokalemic periodic paralysis (HypoPP)) due to sustained sarcolemmal depolarization (Struyk and Cannon 2007; Sokolov et al., 2007; Groome et al. 2014). Also causes myotonia; regulated by calmodulin which binds to the C-terminus of Na_v1.4 (Biswas et al., 2008). Na_V1.4 gating pores are permeable to guanidine as well as Na⁺ and H⁺ (Sokolov et al., 2010). The R669H mutation allows transmembrane permeation of protons, but not larger cations, similar to the conductance displayed by histidine substitution at Shaker K⁺ channel S4 sites (Struyk and Cannon 2007).  The mechanism of inactivation involves transient interactions between intracellular domains resulting in direct pore occlusion by the IFM motif and concomitant extracellular interactions with the beta1 subunit (Sánchez-Solano et al. 2016). Potassium-sensitive hypokalaemic and normokalaemic periodic paralysis are inherited skeletal muscle diseases in humans, characterized by episodes of flaccid muscle weakness. They are caused by single mutations in positively charged residues ('gating charges') in the S4 transmembrane segment of the voltage sensor of the voltage-gated sodium channel Nav1.4 or the calcium channel Cav1.1. Mutations of the outermost gating charges (R1 and R2) cause hypokalaemic periodic paralysis by creating a pathogenic gating pore in the voltage sensor through which cations leak in the resting state. Mutations of the third gating charge (R3) cause normokalaemic periodic paralysis owing to cation leak in both activated and inactivated states (Jiang et al. 2018). The neurotoxic cone snail peptide μ-GIIIA specifically blocks skeletal muscle voltage-gated sodium (Na_V1.4) channels (Leipold et al. 2017). the cryo-electron microscopy structure of the human Na_v1.4-β1 complex at 3.2-Å resolution. Accurate model building was made for the pore domain, the voltage-sensing domains, and the β1 subunit (Pan et al. 2018) provided insight into the molecular basis for Na⁺ permeation and kinetic asymmetry of the four repeats. Structural analysis of reported functional residues and disease mutations corroborates an allosteric blocking mechanism for fast inactivation of Na_v channels.

Na_V1.4 of Homo sapiens (P35499)

Voltage-sensitive Na channel, type 9, α-subunit, Nav1.7 or SCN9A (orthologous to 1.A.1.10.1). Loss of function, resulting from point mutations, results in a channelopathy called Congenital Insensitivity to Pain (CIP) (He et al. 2018), that causes the congenital inability to experience pain (Cregg et al., 2010; Kleopa, 2011). An S241T mutation causes inherited erythromelalgia IEM; erythermalgia, an autosomal dominant neuropathy characterized by burning pain in the extremities in response to mild warmth (due to altered gating) (Lampert et al., 2006; Drenth and Waxman, 2007). Gain-of-function mutations in the Na(v)1.7 channel lead to DRG neuron hyperexcitability associated with severe pain, whereas loss of the Na(v)1.7 channel in patients leads to indifference to pain (Dib-Hajj et al., 2007). Blocked by 1-benzazepin-2-one (Kd = 1.6 nM) (Williams et al., 2007). Mutations in the Nav1.7 Na channel α-subunit give rise to familial pain syndromes called chronic non-paoxysmal neuropathic pain (Catterall et al., 2008; Fischer and Waxman, 2010; Dabby et al. 2011 ). It interacts with the sodium channel beta3 (Scn3b), rather than the beta1 subunit, as well as the collapsing-response mediator protein (Crmp2) through which the analgesic drug lacosamide regulates Nav1.7 current (Kanellopoulos et al. 2018). The R1488 variant is totally inactive (He et al. 2018). Nav1.7 is inhibited by knottins (see TC# 8.B.19.2) (Agwa et al. 2018). Nav1.7 interacts with the following proteins: syn3b (TC# 8.a.17.1.2; the β3 subunit), Crmp2, Syt2 (Q8N9I0) and Tmed10 (P49755), and it also regulates opioid receptor efficacy (Kanellopoulos et al. 2018). Mutations in TRPA1 and Nav1.7 to  insensitivity to pain-promoting algogens such as capsaicin, acid, and allyl isothiocyanate (AITC), have been documented (Eigenbrod et al. 2019).

Nav1.7 of Homo sapiens (Q15858)

Tetrodotoxin-resistant voltage-gated Na⁺ channel of dorsal ganglion sensory neurons, Na_v1.8 (Akopian et al., 1996). Essential for pain at low temperatures (Zimmermann et al., 2007). Na_v1.8 is the sole electrical impulse generator in a nociceptor that transmits information to the central nervous system.  Bark scorpion venom induces pain in many mammals (house mice, rats, humans) by activating Nav1.7 but has no effect on Nav1.8. Grasshopper mice Nav1.8 has amino acid variants that bind bark scorpion toxins and inhibit Na⁺ currents, blocking action potential propagation and inducing analgesia.  These mice thereby can use scorpions as a food source (Zhu et al. 2013; Rowe et al. 2013). Nav1.8 is involved in bull spermatozoa dynamics including motility, membrane integrity, acrosome integrity, capacitation and mitochondrial transmembrane potential (Chauhan et al. 2017).

Nav1.8 of Rattus norvegicus
(Q62968)

Voltage-sensitive Na⁺ channel, Na_v1.1 or SCN1A (causes epilepsy when mutated) (Rusconi et al., 2007).  Mutations are associated with a wide range of mild to severe epileptic syndromes with phenotypes ranging from the relatively mild generalized epilepsy with febrile seizures to other severe epileptic encephalopathies (Colosimo et al. 2007), including myoclonic epilepsy in infancy (SMEI), cryptogenic focal epilepsy (CFE), cryptogenic generalized epilepsy (CGE) and a distinctive subgroup termed as severe infantile multifocal epilepsy (SIMFE) (Ben Mahmoud et al. 2015). Mutations can give rise to familial sporadic hemiplegic migranes (Prontera et al. 2018).

Na_v1.1 of Homo sapiens (P35498)

The Voltage-gated Na⁺ channel α-subunit, Na_v1.6, encoded by the Scn8a gene which when defective gives rise to the ENU-induced neurological mutant ataxia3 which gives rise to ataxia, tremors, and juvenile lethality.   75% identical to 1.A.1.10.7. Nav1.6 is the dendritic, voltage-gated sodium channel (responsible for dendritic excitability (Lorincz and Nusser, 2010)). Nav1.6 (SCN8A) interacts with microtubule-associated protein (O'Brien et al., 2012). Scorpion alpha toxins bind at receptor site-3 and inhibit channel inactivation, whereas beta toxins bind at receptor site-4 and shift the voltage-dependent activation toward more hyperpolarizing potentials (Gurevitz, 2012).  Mutations give rise to epileptic encephalopathy and intellectual disability (O'Brien and Meisler 2013).  A gain-of-function mutation gave rise to increased channel activation and infantile epileptic encephalopathy (Estacion et al. 2014). Benign familial infantile seizures (BFIS), paroxysmal kinesigenic dyskinesia (PKD), and their combination - known as infantile convulsions and paroxysmal choreoathetosis (ICCA) - are related autosomal dominant diseases involving SCN8A (Gardella et al. 2015).  Mutations can lead to chronic movement disorder in the mouse (Jones et al. 2016), and loss of function mutations in humans can lead to intellectual disability without seizures (Wagnon et al. 2017). Nav1.6 has been quantitated in mouse brain and proved to be present in 2-fold decreased amounts in epileptic mice (Sojo et al. 2019). SCN8A developmental and epileptic encephalopathy results in intractable seizures including spasms, focal seizures, neonatal status epilepticus, and nonconvulsive status epilepticus (Kim et al. 2019).

Na_v1.6 of Homo sapiens (Q9UQD0)

The voltage-gated Na⁺ channel α-subunit, Na_v1.9. It is present in excitable membranes and is resistant to tetrodotoxin and saxitoxin (Bosmans et al., 2011).  The mutation, S360Y, makes NaV1.9 channels sensitive to tetrodotoxin and saxitoxin, and the unusual slow open-state inactivation of Na_V1.9 is mediated by the isoleucine-phenylalanine-methionine inactivation motif located in the linker connecting domains III and IV (Goral et al. 2015).  Gain-of function mutations can lead to heritable pain disorders, and painful small-fibre neuropathy (Han et al. 2016). It is a threshold channel that regulates action potential firing, and is preferentially expressed in myenteric neurons, the small-diameter dorsal root ganglion (DRG) and trigeminal ganglion neurons including nociceptors. There is a monogenic Mendelian link of Nav1.9 to human pain disorders including episodic pain due to a N816K mutation (Huang et al. 2019).

.

Na_v1.9 of Homo sapiens (Q9UI33)

Plasma membrane voltage-gated, high affinity Ca²⁺ channel, Cch1/Mid1; activated by mating pheromones and environmental stresses; required for growth in low Ca²⁺ (Locke et al., 2000; Paidhungat and Garrett, 1997). Also essential for tolerance to cold stress and iron toxicity (Peiter et al., 2005). Ecm7, (448aas; 4 TMS; TC# 1.H.1.4.6), a member of the PMP-22/EMP/MP20 Claudin superfamily of transmembrane proteins that includes gamma-subunits of voltage-gated calcium channels appears to interact with Mid1 TC# 8.A.41.1.1) and regulate the activity of the Cch1/Mid1 channel (Martin et al., 2011). Ecm7p is related to members of TC families 1.H.1, 1.H.2 and 1.A.81. The two indispensable subunits, Cch1 and Mid1 are equivalent to the mammalian pore-forming α₁ and auxiliary α₂ /δ subunits, respectively. Cho et al. 2016 screened candidate proteins that interact with Mid1 and identified the plasma membrane H⁺-ATPase, Pma1 (TC#3.A.3.3.6). Mid1 co-immunoprecipitated with Pma1. At the nonpermiss, and Mid1-EGFP colocalized with Pma1-mCherry at the plasma membrane.  Using a temperature-sensitive mutant, pma1-10, the membrane potential was less negative, and Ca²⁺ uptake was lower than in wild-type cells. Thus, Pma1 interacts physically with Cch1/Mid1 Ca²⁺ channels to enhance their activity via its H⁺-pumping activity (Cho et al. 2016).

Cch1/Mid1 of Saccharomyces cerevisiae
Cch1 (P50077)
Mid1 (P41821)
Ecm7p (Q06200) 

The Ca_v1.4 Ca²⁺ channel (gene CACNA1F). Mutations resulting in increased activity cause x-linked incomplete congenital stationary night blindness (CSNB2) (Hemara-Wahanui et al., 2005; Peloquin et al., 2007).  Aland Island eye disease (AIED), also known as Forsius-Eriksson syndrome, is an X-linked recessive retinal disease characterized by a combination of fundus hypopigmentation, decreased visual acuity, nystagmus, astigmatism, protan color vision defect, progressive myopia, and defective dark adaptation. Since the clinical picture of AIED is quite similar to CSNB2, these disorders are allelic or form a single entity. Thus, AIED is also caused by CACNA1F gene mutations (Jalkanen et al. 2007).

Ca_v1.4 of Homo sapiens
(O60840)

T-type Ca²⁺ channel (CACNA1G; Ca_v3.1d), (σ1G T-type Ca²⁺ channel) in developing heart (fetal myocardium (Cribbs et al., 2001)) and elsewhere. Both Ca_v3.1 and Ca_v3.2 are permeated by divalent metal ions, such as Fe²⁺ and Mn²⁺, and possibly Cd²⁺ (Thévenod, 2010).  Ca_V3.1 channels are activated at low votage and regulate neuronal excitability in the spinal cord (Canto-Bustos et al. 2014).  It is regulated by protein kinase C (PKC) and the RanBPM protein (Q96S59) (Kim et al. 2009).  T-type calcium channels belong to the "low-voltage activated (LVA)" group and are strongly blocked by mibefradil. A particularity of this type of channel is an opening at quite negative potentials and voltage-dependent inactivation. T-type channels serve pacemaking functions in both central neurons and cardiac nodal cells, and support calcium signaling in secretory cells and vascular smooth muscle (Coutelier et al. 2015). The human ortholog is 85% identical to the mouse protein. These channels also determines the angiogenic potential of pulmonary microvascular endothelial cells (Zheng et al. 2019).

Ca_v3.1d of Mus musculus (Q9WUT2)

Neuronal nonselective cation channel, NALCN (forms background leak conductance and controls neuronal excitability; Lu et al., 2007).  It is also found in the pancreatic β-cell (Swayne et al. 2010). NALCN serves as a variable sensor that responds to calcium or sodium ion flux, depending on whether the total cellular current density is generated more from calcium-selective or sodium-selective channels (Senatore and Spafford 2013). It functions in a complex with Unc80 (3258 aas; Q8N2C7) and Unc79 (2635 aas; Q9P2D8) (Bramswig et al. 2018). Heterozygous de novo NALCN missense variants in the S5/S6 pore-forming segments lead to congenital contractures of the limbs and face, hypotonia, and developmental delay (Bramswig et al. 2018). Overexpression of the NALCN gene ablates allyl isothiocyanate-promoting pain reception by nociceptors (Eigenbrod et al. 2019).

NALCN of Homo sapiens (Q6P2S6)

The high affinity Ca²⁺ channel; associates with elongation factor 3 (EF3) to target Cch1/Mid1 to the plasma membrane (Liu and Gelli, 2008).

Cch1/Mid1 of Cryptococcus neoformans
Cch1 (Q1HHN2)
Mid1 (Q5KM96) 

The nicotinic acid adenine dinucleotide phosphate (NAADP)- dependent two pore Ca²⁺- channel, TPC3 (Brailoiu et al., 2010).

Two pore Ca²⁺ channel 3, TPC3 of Bos taurus (C4IXV8)

The phosphoinositide (PI(3,5)P₂)-activated Na⁺ two pore channel-2, TPC2, in endosomes and lysosomes (Wang et al. 2012). Previously thought, incorectly, according to Wang et al. 2012, to be a nicotinic acid adenine dinucleotide phosphate (NAADP)-dependent two pore Ca²⁺ channel.  TPC2, like TPC1, has a 12 TMS topology (two channel units) (Hooper et al., 2011). The two domains of human TPCs can insert into the membrane independently (Churamani et al., 2012).  Cang et al. (2013), showed that TPC1 and TPC2 together form an ATP-sensitive two-pore Na⁺ channel that senses the metabolic state of the cell.  The channel complex detects nutrient status, becomes constitutively open upon nutrient removal, and controls the lysosome's membrane potential, pH stability, and amino acid homeostasis.  Essential for Ebola virus (EBOV) host entry. Several inhibitors of TPC2 that act in the nM (tetrandrine) or μM (verapamil; Ned19) range block channel activity, prevent Ebola Virus from escaping cell vesicles and may be used to treat the disease (Sakurai et al. 2015).  TPC2 may transport both Na⁺ and Ca²⁺ (Sakurai et al. 2015).

TPC2 of Homo sapiens (Q8NHX9)

Muscle plasmalemma, voltage-gated, L-type dihydropyridine receptor Ca²⁺ channel, α-1 subunit (DHPR) (Ba²⁺ > Ca²⁺), Cav1.1, CACNA1S,CACH1 CDCN1, CACNL1A3 of 1873 aas in the human orthologue.  Distinc voltage sensor domains control voltage sensitivity and kinetics of current activation (Tuluc et al. 2016).

DHPR of Oryctolagus cuniculus

The voltage-gated Ca²⁺ channel, L-type α-subunit, Eg1-19 regulated by Macoilin (8.A.38.1.2)

Eg1-19 of Caenorhabditis elegans (A8PYS5)

Voltage-gated Ca²⁺ channel, Egl-19, isoform a

Egl-19 of C. elegans (G5EG02)

The phosphoinositide (PI(3,5)P₂)-activated Na⁺ two pore channel-1, TPC1 of endosomes and lysosomes (Wang et al. 2012). Previously thought, incorectly, according to Wang et al. (2012), to be an NAADP-activated two pore voltage-dependent calcium channel protein.  However, Cang et al. (2013), showed that TPC1 and TPC2 (TC# 1.A.1.11.19) together form an ATP-sensitive two-pore Na⁺ channel that senses the metabolic state of the cell.  The channel complex detects nutrient status, becomes constitutively open upon nutrient removal, and controls the lysosome's membrane potential, pH stability, and amino acid homeostasis.  May be regulated by the HCLS-associated X-1 (HAX-1) protein (Lam et al. 2013). The cryoEM 3-D structure has been ellucidated (She et al. 2018). This voltage-dependent, phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P₂)-activated Na⁺ channel was solved in both the apo closed state and ligand-bound open state. The channel has a coin-slot-shaped ion pathway in the filter that defines the selectivity of mammalian TPCs. Only the voltage-sensing domain from the second 6-TMS domain confers voltage dependence while endolysosome-specific PtdIns(3,5)P₂ binds to the first 6-TMS domain and activates the channel under conditions of depolarizing membrane potential. Structural comparisons between the apo and PtdIns(3,5)P2-bound structures show the interplay between voltage and ligand activation. These MmTPC1 structures reveal lipid binding and regulation in a 6-TMS voltage-gated channel (She et al. 2018).

Tpcn1 of Mus musculus

Cch1 calcium channel, alpha subunit; acts with Mid1 (8.A.41.1.7) which is required for function.

Mid1 of Schizosaccharomyces pombe

Voltage-sensitive calcium channel of 2693 aas (Docampo et al. 2013). Found to be essential for bloodstream-form Trypanosoma brucei through a genome-wide RNAi screen (Schmidt et al. 2018).

Calcium channel of Trypanosoma brucei

Endosomal/lysosomal, two pore Na⁺and Ca²⁺-release channel  (Na⁺> Ca²⁺) protein of 816 aas and 12 TMSs, TPC1 or TPCN1 (Guo et al. 2017).  Endosomes and lysosomes are electrically excitable organelles (Cang et al. 2014). In a subpopulation of endolysosomes, a brief electrical stimulus elicits a prolonged membrane potential depolarization spike. The organelles have a depolarization-activated, non-inactivating Na⁺ channel (lysoNaV). The channel is formed by a two-repeat six-transmembrane-spanning (2x6 TMS) protein, TPC1, which represents the evolutionary transition between 6 TMS and 4x6 TMS voltage-gated channels. Luminal alkalization also opens lysoNaV by markedly shifting the channel's voltage dependence of activation toward hyperpolarization. Thus, TPC1 is a voltage-gated Na⁺ channel that senses pH changes and confers electrical excitability to organelles (Cang et al. 2014). Essential for Ebola virus (EBOV) host entry.  Several inhibitors act in the nM (tetrandrine) or μM (verapamil; Ned19) range to block Na⁺ and Ca²⁺ channel activity, inhibit virus escape from membrane vesicles and may possibly be used to treat the disease (Sakurai et al. 2015). A cluster of arginine residues in the first domain required for selective voltage-gating of TPC1 map not to the voltage-sensing fourth transmembrane region (S4) but to a cytosolic downstream region (S4-S5 linker). These residues are conserved between TPC isoforms suggesting a generic role in TPC activation. Accordingly, mutation of residues in TPC1 but not the analogous region in the second domain prevents Ca²⁺ release by NAADP in intact cells (Patel et al. 2017). Dramatic conformational changes in the cytoplasmic domains communicate directly with the VSD during activation (Kintzer et al. 2018).

TPC1 of Homo sapiens

Two pore Ca²⁺ > Na⁺, Li⁺ or K⁺ (non-selective for these three monovalen caions) channel protein of 733 aas and 12 TMSs, TPC1 (Guo et al. 2017). The crystal structure of this vacuolar two-pore channel, a homodimer, has been solved (Guo et al. 2015) (Kintzer and Stroud 2016). Activation requires both voltage and cytosolic Ca²⁺. Ca²⁺ binding to the cytosolic EF-hand domain triggers conformational changes coupled to the pair of pore-lining inner helices from the first 6-TMS domains, whereas membrane potential only activates the second voltage-sensing domain, the conformational changes of which are coupled to the pair of inner helices from the second 6-TMS domains. Luminal Ca²⁺ or Ba²⁺ modulates voltage activation by stabilizing the second voltage-sensing domain in the resting state and shift voltage activation towards more positive potentials. The basis for understanding ion permeation, channel activation, the location of voltage-sensing domains and regulatory ion-binding sites is partially explained by the 3-d structure (Kintzer and Stroud 2016). Only the second Shaker domain senses voltage (Jaślan et al. 2016). It has a selectivity filter that is passable by hydrated divalent cations (Demidchik et al. 2018).

TPC1 of Arabidopsis thaliana

Voltage-dependent P/Q-type Ca²⁺ channel subunit α1A, CACNA1A (CACH4; CACN3; CACNL1A4) of 2,505 aas.  90% identical to 1.A.1.11.8.  Associated with four neurological phenotypes: familial and sporadic hemiplegic migraine type 1 (FHM1, SHM1), episodic ataxia type 2 (EA2), spinocerebellar ataxia type 6 (SCA6) and epileptic encephalopathu with nerve atrophy (Reinson et al. 2016). A gain of function mutation gave symptoms of congenital ataxia, abnormal eye movements and developmental delay with severe attacks of hemiplegic migraine (García Segarra et al. 2014). Mutations can cause F/SHM with high penitrance (Prontera et al. 2018).

CACNA1A Ca²⁺ channel of Homo sapiens

Voltage-dependent L-type calcium channel subunit α, VDCC, CCA-1 or CaACNa1S, of 1873 aas and 24 TMSs. Ca²⁺ channels containing the alpha-1S subunit play an important role in excitation-contraction coupling in skeletal muscle.  They are regulated by dystrophin-1 (Zhan et al. 2014).

VDCC of Caenorhabditis elegans

Voltage-gated calcium channel (VDCC) of 3097 aas and 24 TMSs, Cav7 (Wheeler and Brownlee 2008). 

Cav7 of Chlamydomonas reinhardtii

Voltage-dependent R-type Ca²⁺ channel, α-1E subunit (Cav2.3) (brain Ca²⁺ channel type II) (Ca²⁺ > Ba²⁺). Interacts with V-type ATPases (3.A.2), specifically, the G1-subunit, to regulate its activity (Radhakrishnan et al., 2011).  Syntaxin-3 (Syn-3) interacts directly with Cav2.3 to regulate its activity (Xie et al. 2016).

R-type Ca²⁺ channel of Mus musculus

TPC calcium channel protein with two transmembrane domains of 6 TMSs each (720 aas and 12 TMSs) (Wheeler and Brownlee 2008).

TPC of Physcomitrella patens

Pore-forming, alpha-1S subunit of the voltage-gated calcium channel of 1873 aas and 24 TMSs, Cav1.1; CACNA1S; CACN1; CACH1; CACNL1A3, that gives rise to L-type calcium currents in skeletal muscle. Calcium channels containing the alpha-1S subunit play an important role in excitation-contraction coupling in skeletal muscle via their interaction with RYR1, which triggers Ca²⁺ release from the sarcplasmic reticulum and ultimately results in muscle contraction. Long-lasting (L-type) calcium channels belong to the 'high-voltage activated' (HVA) group (Jiang et al. 2018). The 3-d structure of a bacterial homologue has been solved (Jiang et al. 2018).

Cav1.1 of Homo sapiens

Calcium channel protein of 2556 aas and 24 TMSs.  Inhibited by 1,4-dihydrophyridines such as nifedipine (Tempone et al. 2009).

Ca²⁺ channel of Leishmania donovani

Calcium channel of 913 aas and 12 TMSs. Ca²⁺ channels in trophozoites are inhibited by Amlodipine (Baig et al. 2013).

Calcium channel of Acanthamoeba castellanii

Calcium channel of 2725 aas and 24 TMSs.  T. cruzi calcium channels are inhibited by fendiline and bepridil (Reimão et al. 2011).

Ca²⁺ channel of Trypanosoma cruzi

The voltage-dependent L-type Ca²⁺ channel α-subunit-1C (L-type Ca_v1.2), CACNA1C (CACH2, CACN2, CACNL1A1, CCHL1A1) of 2221 aa. Mutations cause Timothy's syndrome, a disorder associated with autism (Splawski et al., 2006). The C-terminus of Ca_v1.2 encodes a transcription factor (Gomez-Ospina et al., 2006). Ca_v1.2 associates with the α-2, δ-1, β and γ subunits (Yang et al., 2011). The CRAC channel activator STIM1 binds and inhibits L-type voltage-gated calcium channel, Ca_v1.2 (Park et al., 2010).  This channel appears to function as the molecular switch for synaptic transmission (Atlas 2013). Intramembrane signalling occurs with syntaxin 1A for catecholamine release in chromaffin cells (Bachnoff et al. 2013).  miR-153 intron RNA is a negative regulator of both insulin and dopamine secretion through its effect on Cacna1c expression, suggesting that IA-2beta and miR-153 have opposite functional effects on the secretory pathway (Xu et al. 2015).   Co-localizes with Syntaxin-1A in nano clusters at the plasma membrane (Sajman et al. 2017). It is a high voltage-activated Ca²⁺ channel in contrast to Ca_v3.3 which is a low voltage-activated Ca²⁺ channel (Sanchez-Sandoval et al. 2018). Nifedipine blocks and potentiates this and other L-type VIC Ca²⁺ channels (Wang et al. 2018).  Cav1.2 is upregulated when STIM1 is deficient (Pascual-Caro et al. 2018). CaV1.2 regulates chondrogenesis during limb development (Atsuta et al. 2019).

CACNA1C of Homo sapiens (2221 aas; Q13936)

The voltage-dependent L-type Ca²⁺ channel α-subunit-1H (T-type Ca_v3.2), CACNA1H (mutations can cause an increased propensity for autism spectrum disorders (ASD) characterized by impaired social interactions, communication skills and restricted and repetitive behaviors) (Splawski et al., 2006). Also called Cav3.2 or VSCC. Involved in a variety of calcium-dependent processes, including muscle contraction, hormone or neurotransmitter release, gene expression, cell motility, cell division and cell death. The isoform alpha-1H gives rise to T-type calcium currents, ''low-voltage activated'' currents blocked by nickel and mibefradil. Defective in Childhood Absence Epilepsy. Are permeated by divalent metal ions, such as Fe²⁺ and Mn²⁺ , and possibly Cd²⁺ (Thévenod, 2010).  Patented inhibitors of T-type calcium channels have been reviewed (Giordanetto et al. 2011).  Regulated by Syntaxin-1A (Xie et al. 2016).

CACNA1H of Homo sapiens (2353 aas; Q95180)

Voltage-dependent L-type Ca²⁺ channel subunit α-1C (αCa_v1.2) of cardiac muscle [A C-terminal fragment of Ca_v1.2 translocates to the nucleus and regulates transcription, explaining how a channel can directly activate transcription and differentiation of excitable cells.] (Gomez-Ospina et al., 2006). Ca_v1.2 associates with the α-2, δ-1, β and γ subunits (Yang et al., 2011).

α-Ca_v1.2 of Mus muscultus (2139 aas; Q01815)

The voltage-dependent Ca²⁺ channel subunit α-1I, Cav3.3, CACNA1I (isoform CRA_c (2223 aas and 24 TMSs)) (Hamid et al. 2006). It is a low voltage-activated Ca²⁺ channel in contrast to Ca_v1.2 (TC# 1.A.1.11.4) which is a high voltage-activated Ca²⁺ channel (Sanchez-Sandoval et al. 2018).

Ca²⁺ channel CRA_c of Homo sapiens (Q9P0X4)

Voltage-dependent Ca²⁺ channel α-1A subunit (2212 aas), Cav2.1 (P/Q-type) (when mutated in humans, leads to a human channelopathy (episodic ataxia type-2 (EA2)) due to protein misfolding and retention in the E.R. (Mezghrani et al., 2008; Kleopa, 2011).  Mutations give rise to familial and sporadic hemiplegic migraine type 1 (FHM1, SHM1), episodic ataxia type 2 (EA2), and spinocerebellar ataxia type 6 (SCA6) (García Segarra et al. 2014).  Syntaxin 1A (Sx1A), SNAP-25 and synaptotagmin (Syt1), either alone or in combination, modify the kinetic properties of voltage-gated Ca²⁺ channels (VGCCs) including Cav2.1 (Cohen-Kutner et al. 2010).

Cav2.1 of Rattus norvegicus (P54282)

Voltage-dependent Ca²⁺ channel -subunit 1B (2339 aas), Cav2.2 (N-type) or NCC receptor of 2237 aas. Anchorin B interacts with Cav2.2 in the loop between TMSs 2 and 3.  TSPAN-13 specifically interacts with the α-subunit and modulates the efficiency of coupling between voltage sensor activation and pore opening of the channel while accelerating the voltage-dependent activation and inactivation of the Ba²⁺ current through CaV2.2 (Mallmann et al. 2013). The structure of the closed state in the pore forming domains have been modeled (Pandey et al. 2012). Amlodipine, cilnidipine and nifedipine compounds are potent channel antagonists.  Ca_V2.2 also interacts with reticulon 1 (RTN1) (TC# 8.A.102), member 1 of solute carrier family 38 (SLC38, TC#2.A.18), prostaglandin D2 synthase (PTGDS) and transmembrane protein 223 (TMEM223; TC#8.A.115). Of these, TMEM223 and, to a lesser extent, PTGDS, negatively modulate Ca²⁺ entry, required for transmitter release and/or for dendritic plasticity under physiological conditions (Mallmann et al. 2019).

Cav2.2 of Mus musculus (O55017)

Paramecium bursaria Chlorella virus 1 (PBCV-1) K⁺ channel, Kcv1. (The viral-encoded K⁺ channel inserts into the green algal host membrane to aid ejection of DNA from the viral particle into the cytoplasm (Neupartl et al., 2007)). It may mediate host cell membrane depolarization and K⁺ loss (Agarkova et al., 2008; Balss et al., 2008).  It is inhibited by Ba²⁺ and amantidine. (Reviewed by Thiel et al., 2010). The presence of charged amino acids which form dynamic inter- and intra- subunit salt bridges is crucial for channel activity (Hertel et al. 2010).

Kcv1 K⁺ channel of Chlorella virus PBCV-1

Acanthocystis turfacea chlorella virus cation, K⁺-preferring, channel, ATCV1 (82aas) (Gazzarrini et al., 2009; Siotto et al. 2014). The difference in open probability between close isoforms is caused by one long closed state in Kcv_S versus Kcv_NTS. This state is structurally created in the tetrameric channel by a transient, Ser mediated, intrahelical hydrogen bond. The resulting kink in the inner transmembrane domain swings the aromatic rings from downstream Phenylalanines in the cavity of the channel, which blocks ion flux.

ATCV1 (KCVS/KCVNTS) of Acanthocystis turfacea chlorella virus (A7K9J5)

The viral K⁺ channel, Kesv of 124 aas.  It is inhibited by Ba²⁺ and amantidine.  It is important for infection and replication in marine brown algae (Chen et al. 2005; Balss et al., 2008; Siotto et al. 2014).

Kesv of Ectocarpus siliculosus virus 1 (Q8QN67)

Viral K⁺ channel of 96 aas and 2 TMSs, Kcv (Siotto et al. 2014).  Mechanical perturbation of the N-terminus can be transmitted to the C-terminal channel gates (Hoffgaard et al. 2015).

Kcv of Paramecium bursaria Chlorella virus

Potassium ion channel protein of 86 aas and 2 TMSs (Greiner et al. 2018).

K⁺ channel protein of Micromonas pusilla virus SP1

6TMS K⁺ channel (Munsey et al. 2002; Kuo et al., 2003).  The kch gene, the only known potassium channel gene in E. coli, has the property to express both full-length Kch and its cytosolic domain (RCK) due to a methionine at position 240. The RCK domains form an octameric ring structure and regulate the gating of the potassium channels after having bound certain ligands. Several different gating ring structures have been reported for the soluble RCK domains.  The octameric structure of Kch may be composed of two tetrameric full-length proteins through RCK interaction (Kuang et al. 2013).  The RCK domains face the solution, and an RCK octameric gating ring arrangement does not form under certain conditions (Kuang et al. 2015).

Kch of E. coli

2 TMS ( P-loop) Ca²⁺-gated K⁺ channel, MthK (see Jiang et al., 2002 for the crystal structure, and Parfenova et al., 2006 for mutations affecting open probability). For the studies of ion permeation and Ca²⁺ blockage, see Derebe et al., 2011. (structures: 3LDD_A and 2OGU_A.). Voltage-dependent K⁺ channels including MthK which lacks a canonical voltage sensor can undergo a gating process known as C-type inactivation, which involves entry into a nonconducting state through conformational changes near the channel's selectivity filter (Thomson and Rothberg, 2010). C-type inactivation may involve movements of transmembrane voltage sensor domains.

MthK of Methanothermobacter thermoautotrophicus (O27564)

TuoK of Thermoplasma volcanium (Q979Z2)

The Biofilm-inducing putative K⁺ channel, BikC or YugO (Prindle et al. 2015). BikC has an N-terminal 2 TMS + P-loop channel domain and a C-terminal NADB_Rossman superfamily domain (TrkA domain). YugO is in a two cistronic operon where Mistic (MstX; 9.A.66; Debnath et al., 2011; Roosild et al., 2005) is encoded by the gene that precedes yugO. Both play a role in biofilm formation, probably by functioning together (Lundberg et al. 2013; Marino et al. 2015). These K⁺ channels in bacterial biofilms provide an active, long-range electrical signalling for cellular communities (Prindle et al. 2015).  Metabolic co-dependency gives rise to collective electrical oscillations in biofilms (Liu et al. 2015). This oscillatory electrical signalling, due to periodic release of K⁺, giving rise to K⁺ gradients, increasing as swimming cells approach the biofilm that generates the gradiens, allows cells of the same and different speices to find and then incorporate themselves into existing biofilms (Humphries et al. 2017).

BikC of Bacillus subtilis (Q795M8)

Putative 2 TMS ion channel protein (N-terminus) with C-terminal TrkA_N (NADB Rossman) domain.

Ion channel protein of Streptomyces coelicolor

Ca²⁺-activated K⁺ channel, SynCaK. Functions in the regulation of photosynthesis (Checchetto et al. 2013; Checchetto et al. 2013).

K⁺ channel of Synechocystis PCC6803

Putative K⁺ channel, TrkA1, of 365 aas and 2 N-terminal TMSs, with a C-terminal NAD binding domain.

K⁺ channel of Synechocystis PCC6803

Potassium channel protein, MjK1 of 333 aas and 6 TMSs. Seems to conduct potassium at low membrane potentials (Hellmer and Zeilinger 2003). Also called TrkA3, a Trk channel with a C-terminal NAD-binding domain.

MjK1 of Methanocaldococcus jannaschii (Methanococcus jannaschii)

K⁺ channel of 387 aas and 2 TMSs, KchA.  KchA is essential for growth at low concentrations of K⁺. This K⁺ uptake system is essential for gastric colonization and the persistence of H. pyloriin the stomach (Stingl et al. 2007). This protein is of the two-transmembrane RCK (regulation of K⁺ conductance) domain family (Stingl et al. 2007).

KchA of Helicobacter pylori

Voltage-activated, Ca²⁺  channel blocker-inhibited, Na channel, NaChBac (Ren et al., 2001; Zhao et al., 2004; Nurani et al, 2008; Charalambous and Wallace, 2011). Transmembrane and extramembrane contributions to thermal stability have been studied (Powl et al., 2012). Deprotonation of arginines in S4 is involved in NaChBac gating (Paldi, 2012). Hinge-bending motions in the pore domain of NaChBac have been reported (Barber et al., 2012).  The C-terminal coiled-coli stabilizes subunit interactions (Mio et al. 2010).  Within the 4 TMS voltage sensor, coupling between residues in S1 and S4 determines its resting conformation (Paldi and Gurevitz 2010). The conserved asparagine was changed to aspartate, N225D, and this substitution shifted the voltage-dependence of inactivation by 25 mV to more hyperpolarized potentials. The mutant also displays greater thermostability (O'Reilly et al. 2017).  Possibly, the side-chain amido group of asn 225 forms one or more hydrogen bonds with different channel elements and that these interactions are important for normal channel function.

NaChBac of Bacillus halodurans

Voltage-gated Na⁺ channel, VGSC (Koishi et al., 2004; McCusker et al., 2011) Changing the selectivity filter from LESSM to LDDWSD yielded a Calcium-selective channel (Shaya et al., 2011).

VGSC of Silicibacter pomeroyi (56676695)

Voltage-gated Na⁺ channel, Na_vCh or Na_vAb. The 3d-structure is known (3ROW; Payandeh et al., 2011; 4MW3A-D; 4+ selectivity through partial dehydration of Na⁺ via its direct interaction with conserved glutamate side chains). The pore is preferentially occupied by two ions, which can switch between different configurations by crossing low free-energy barriers (Furini and Domene, 2012). Jiang et al. 2018 presented high-resolution structures of NavAb with the analogous gating-charge mutations that have similar functional effects as in the human channels that cause hypokalaemic and normokalaemic periodic paralysis.  Wisedchaisri et al. 2019 presented a cryo-EM structure of the resting state and a complete voltage-dependent gating mechanism via the voltage sensor (VS). The S4 segment of the VS is drawn intracellularly, with three gating charges passing through the transmembrane electric field. This movement forms an elbow connecting S4 to the S4-S5 linker, tightens the collar around the S6 activation gate, and prevents its opening. This structure supports the classical "sliding helix" mechanism of voltage sensing and provides a complete gating mechanism for voltage sensor function, pore opening, and activation-gate closure based on high-resolution structures of a single sodium channel protein (Wisedchaisri et al. 2019).

(see also TC#s 1.A.1.10.4 and 1.A.1.11.32).

The transport reaction catalyzed by Na_vCh is:

Na⁺ (in) ⇌ Na⁺ (out)

Na_vCh of Arcobacter butzleri (A8EVM5)

Bacterial voltage-gated sodium channel, Nav.  3-d crystal structures of vaious conformations are known (4P_3A A-D; 4PA7_A-D; 4P9P_A-D. etc.)  (McCusker et al. 2012).  It has its internal cavity accessible to the cytoplasmic surface as a result of a bend/rotation about a central residue in the carboxy-terminal TMS that opens the gate to allow entry of hydrated sodium ions. The molecular dynamics of ion transport through the open conformation has been analyzed (Ulmschneider et al. 2013).  The C-terminal four helix coiled coil bundle domain couples inactivation with channel opining, depedent on the negatively charged linker region (Bagnéris et al. 2013).

Nav of Magnetococcus marinus (also called sp. strain MC-1)

Tetrameric 6 TMS subunit Na⁺ channel protein, Na_V.  Two low resolution cyroEM structures revealed two conformations reconstituted in lipid bilayers (Tsai et al. 2013). Despite a voltage sensor arrangement identical with that in the activated form, Tsai et al. 2013 observed two distinct pore domain structures: a prominent form with a relatively open inner gate, and a closed inner-gate conformation similar to the first prokaryotic Na_v structure. Structural differences, together with mutational and electrophysiological analyses, indicated that widening of the inner gate was dependent on interactions among the S4-S5 linker, the N-terminal part of S5 and its adjoining part in S6, and on interhelical repulsion by a negatively charged C-terminal region subsequent to S6 (Tsai et al. 2013).

Nav of Caldalkalibacillus thermarum

Voltage-gated Na⁺ channel, Nsv, of 277 aas and 6 TMSs with a structually defined C-terminal regulatory domain (Miller et al. 2016). Voltage-gated sodium channels (Na_Vs) are activated by transiting the voltage sensor from the deactivated to the activated state. Tang et al. 2017 identified peptide toxins stabilizing the deactivated VSM of bacterial Na_Vs. A cystine knot toxin, called JZTx-27, from the venom of the tarantula Chilobrachys jingzhao proved to be a high-affinity antagonist. JZTx-27 stabilizes the inactive form of the voltage sensor, thereby inhibiting channel activity (Tang et al. 2017).

Nsv of Bacillus alcalophilus

Bacterial type voltage-activated sodium channel of 718 aas, NaV.

NaV of Phaeodactylum tricornutum

6 TMS basolateral tracheal epithelial cell/voltage-gated, small conductance, K⁺ α-chain) [acts with the KCNE3 β-chain]. Mutations in human K_v7 genes lead to severe cardiovascular and neurological disorders such as the cardiac long QT syndrome and neonatal epilepsy (Haitin and Attali, 2008). KCNE3 can co-assemble with KCNQ1 (1.A.1.15.6) (Kang et al., 2010). KCNQ1 regulates insulin secretion in  the MIN6 beta-cell line (Yamagata et al., 2011).  The S4-S5 linker of KCNQ1 forms a scaffold with S6 controlling gate closure (Labro et al. 2011).  The KCNQ1 channel is differentially regulated by KCNE1 and KCNE2 (Li et al. 2014.  Slow-activating channel complexes formed by KCNQ1 and KCNE1 are essential for human ventricular myocyte repolarization, while constitutively active KCNQ1-KCNE3 channels are important in the intestine. Inherited sequence variants in human KCNE1 and KCNE3 cause cardiac arrhythmias but by different mechanisms, and each is important for hearing in unique ways (Abbott 2015). The topology and dynamics of the voltage sensor domain of KCNQ1 reconstituted in a lipid bilayer environment has been studied (Dixit et al. 2019).

KCNQ1 K⁺ channel of Mus musculus

    6 TMS voltage-gated K channel, KCNQ2 or Kv7.2.  Mutations cause benign familial neonatal convulsions (BNFC; epilepsy; Maljevic et al. 2016; Soldovieri et al. 2019).  It forms homotetramers or heterotetramers with KCNQ3/Kv7.3) (Soldovieri et al., 2006; Uehara et al., 2008)). Like all other Kv7.2 channels, it is activated by phosphatidyl inositol-4,5-bisphosphate and hence can be regulated by various neurotransmitters and hormones (Telezhkin et al. 2013).  Gating pore currents that go through the gating pores in TMSs1-4 (the voltage sensor) may give rise to peripheral nerve hyperexcitability (Moreau et al. 2014). Retigabine and ICA73, two anti-epileptic drugs, act via distinct mechanisms due to interactions with specific residues that underlie subtype specificity of KCNQ channel openers (Wang et al. 2016). A  tight spatial and functional relationship between the DAT/GLT-1 transporters and the Kv7.2/7.3 potassium channel immediately readjusts the membrane potential of the neuron, probably to limit the neurotransmitter-mediated neuronal depolarization (Bartolomé-Martín et al. 2019). E-2-dodecenal from cilantro (Coriandrum sativum) is a potent activator and anticonvulsant that binds with an affinity of 60 nM to TMS5 in several KCNQ channels including KCNQ2 and 3 (Manville and Abbott 2019).

KCNQ2 K⁺ channel of Homo sapiens (O43526)

6 TMS voltage-gated K⁺ channel, KCNQ3 or K_v7.3.  Mutations cause benign familial neonatal convulsions (BNFC; epilepsy; Maljevic et al. 2016). Forms homotetramers or heterotetramers with KCNQ2 (Soldovieri et al., 2006; Uehara et al., 2008).  Retigabine and ICA73, two anti-epileptic drugs, act via distinct mechanisms due to interactions with specific residues that underlie subtype specificity of KCNQ channel openers (Wang et al. 2016). Gabapentin at low concentrations is a activator of KCNQ3, KCNQ2/3 and KCNQ5 but not KCNQ2 or KCNQ4 (Manville and Abbott 2018). At high concentrations it can be inhibitory.  A  tight spatial and functional relationship between the DAT/GLT-1 transporters and the Kv7.2/7.3 potassium channel immediately readjusts the membrane potential of the neuron, probably to limit the neurotransmitter-mediated neuronal depolarization (Bartolomé-Martín et al. 2019). E-2-dodecenal from cilantro (Coriandrum sativum) is a potent activator and anticonvulsant that binds with an affinity of 60 nM to TMS5 in several KCNQ channels including KCNQ2 and 3 (Manville and Abbott 2019).

KCNQ3 K⁺ channel of Homo sapiens (O43525)

6 TMS cell volume sensitive, voltage-gated K⁺ channel, KCNQ4 or Kv7.4 (mutations cause DFNA2, an autosomal dominant form of progressive hearing loss) (forms homomers or heteromers with KCNQ3) (localized to the basal membrane of cochlear outer hair cells and in several nuclei of the central auditory pathway in the brainstem). Four splice variants form heterotetramers; each subunit has different voltage and calmodulin-sensitivities (Xu et al., 2007).  Autosomal dominant mutant forms leading to progressive hearing loss (DFNA2) have been characterized (Kim et al. 2011). Phosphatidylinositol 4,5-bisphosphate (PIP2) and polyunsaturated fatty acids (PUFAs) impact ion channel function (Taylor and Sanders 2016). This channel may be present in mitochondria (Parrasia et al. 2019).

KCNQ4 K⁺ channel of Homo sapiens

The KCNQ5 K⁺ channel (modulated by Zn²⁺ , pH and volume change) (Jensen et al., 2005).  A triple cysteine module within M-type K⁺ channels mediates reciptrocal channel modulation by nitric oxide and reactive oxygen species (Ooi et al. 2013). Gabapentin at low concentrations is a activator of KCNQ3, KCNQ2/3 and KCNQ5 but not of KCNQ2 or KCNQ4 (Manville and Abbott 2018). At high concentrations, it can be inhibitory.

KCNQ5 of Mus musculus
(Q9JK45)

K⁺ voltage-gated channel, LQT-like subfamily; Kv7.1; KvLQT1. KCNQ1 (regulated by KCNE peptides (TC# 8.A.58) affect voltage sensor equilibrium (Rocheleau and Kobertz, 2007). Almost 300 mutations of KCNQ1 have been identified in patients, and most are linked to the long QT syndrome (LQT1), some in the voltage sensor (Peroz et al., 2008; Eldstrom et al. 2010; Qureshi et al. 2013; Ikrar et al. 2008). KCNQ1-KCNE1 complexes may interact intermittently with the actin cytoskeleton via the C-terminal region (Mashanov et al., 2010). The stoichiometry of the KCNQ1 - KCNE1 complex is flexible, with up to four KCNE1 subunits associating with the four KCNQ1 subunits of the channel (Nakajo et al., 2010). A familial mutation in the voltage-sensor of the KCNQ1 channel results in a cardiac phenotype (Henrion et al., 2012). KCNQ1 regulates insulin secretion in the MIN6 beta-cell line (Yamagata et al., 2011; Gofman et al., 2012).  Electrostatic interactions of S4 arginines with E1 and S2 contribute to gating movements of S4, but coupling requires the lipid phosphatidylinositol 4,5-bisphosphate (PIP2) as voltage-sensing domain activation failed to open the pore in the absence of PIP2 (Zaydman et al. 2013). The D242N mutation causes impaired action potential adaptation to exercise and an increase in heart rate. Moreover, the D242 amino acylposition is involved in the KCNE1-mediated regulation of the voltage dependence of activation of the K_V7.1 channel (Moreno et al. 2017). The KCNQ1 channel interacts with MinK (KCNE1) to cause pore constriction, generating the slow delayed rectifier (IKs) current in the heart (Jalily Hasani et al. 2018). KCNQ1 rescues TMC1 plasma membrane expression but not mechanosensitive channel activity (Harkcom et al. 2019). Activation of the neuronal Kv7/KCNQ/M-current represents an attractive therapeutic strategy for treatment of hyperexcitability-related neuropsychiatric disorders such as epilepsy, pain, and depression, and channel openers for treatment of antiepilepsy have been developed (Zhang et al. 2019). The relationship between mutation locations in KCNQ1, which is a major gene in long QT syndrome (LQTS), and phenotype has been analyzed and used for risk stratification (Yagi et al. 2018). The proximal C-terminal regions of KCNQ1 and KCNE1 participate in a physical and functional interaction during channel opening that is sensitive to perturbation (Chen et al. 2019).

KCNQ1 of Homo sapiens (P51787)

The small conductance Ca²⁺-activated K⁺ channel, SkCa2, Sk2 or Kcnn2 (not inhibited by arachidonate) (activated by three small organic molecules, the 1-EBIO and N5309 channel enhancers and the DCEBIO channel modulation (Pedarzani et al., 2005)).  It is inhibited by protonation of outer pore histidine residues (Goodchild et al., 2009). The same is true for SK3 (K(Ca) 2.3 (Q9UGI6)). Regulates endothelial vascular function (Sonkusare et al., 2012).  Distinct subcellular mechanisms enhance the surface membrane expression by its interacting proteins, α-actinin 2 (TC# 8.A.66.1.3) and filamin A (TC# 8.A.66.1.4) (Zhang et al. 2016). SK channel activators can compensate for age-related changes of the autorhythmic functions of the cerebellum (Karelina et al. 2017). SK2 proteins are more abundant in Purkinje cells than in the ventricular myocytes of normal rabbit ventricles (Reher et al. 2017). Apamin inhibits and isoproterenol activates this and other SK (KCNN) channels, and activation by isoproterenol is sex-dependent (Chen et al. 2018).  Diverse interactions between KCa and TRP channels integrate cytoplasmic Ca²⁺, oxidative, and electrical signaling affecting cardiovascular physiology and pathology (Behringer and Hakim 2019). This channel may be present in mitochondria (Parrasia et al. 2019).

SkCa2 of Homo sapiens

The intermediate conductance, Ca²⁺-activated K⁺ channel, Kcnn4, SK4, Sk4, Smik, Ik1 hIK1, IKCa or KCa3.1, also called the Gardos channel (inhibited by 1 μM arachidonate which binds in the pore (Hamilton et al., 2003)). Nucleoside diphosphate kinase B (NDPK-B) activates KCa3.1 via histidine phosphorylation, resulting in receptor-stimulated Ca²⁺ flux and T cell activation (Di et al., 2010). Regulates endothelial vascular function (Sonkusare et al., 2012).  Tissue-specific expression of splice variants of the orthologous rat KCNN4 protein have been reported (Barmeyer et al. 2010).  Residues involved in gating have been identified (Garneau et al. 2014). Also present in the inner mitochondrial membrane where increases of mitochondrial matrix [Ca²⁺] cause mtKCa3.1 opening, thus linking inner membrane K⁺ permeability and transmembrane potential to Ca²⁺ signalling (De Marchi et al. 2009). KCa3.1 (IKCa) channels are expressed in CA1 hippocampal pyramidal cells and contribute to the slow afterhyperpolarization that regulates spike accommodation (Turner et al. 2016). SK channel activators can compensate for age-related changes of the autorhythmic functions of the cerebellum (Karelina et al. 2017). The activation mechanism has been revealed by the cryoEM structure of the SK4-calmodulin complex (Lee and MacKinnon 2018).  It is responsible for hyperpolarization in some tumor cells (Lazzari-Dean et al. 2019). Mutations are linked to dehydrated hereditary stomatocytosis (xerocytosis) (Andolfo et al. 2015). This channel may be present in mitochondria (Parrasia et al. 2019).

hIK1 of Homo sapiens (AAC23541)

Small conductance calcium-gated potassium (SK) channel. Three charged residues in TMS S6 of SK channels near the inner mouth of the pore collectively control the conductance and rectification through an electrostatic mechanism (Li and Aldrich, 2011).

SK of Drosophila melanogaster (Q7KVW5)

Small conductance Ca2+-activated K+ channel, KCNL-2 of 672 aas.  Plays a role in the rate of egg laying (Chotoo et al. 2013).

KCNL-2 of Caenorhabditis elegans

Plasma membrane small conductance calcium-activated K⁺ channel of 396 aas, TSKCa; probably involved in immunoregulation (Cong et al. 2009).

TSKCa of Psetta maxima (Turbot) (Pleuronectes maximus)

Small conductance calcium-activated K⁺ channel of 543 aas. KCNN1 or SK

SK of Homo sapiens

Small conductance calcium-activated potassium channel protein 3 of 736 aas and 6 TMSs, SK3 or KCNN3.

KCNN3 of Homo sapiens

The archaeal voltage-regulated K channel, KvAP (Ruta et al., 2003). X-ray and solution structures are available. The latter shows phospholipid interactions with the isolated voltage sensor domain (Butterwick and MacKinnon 2010; Li et al. 2014). The gating-charge arginine in TMS4 of the voltage sensor forms part of the helical hairpin "paddle", and it moves 15-20 Å through the membrane to open the pore (Ruta et al., 2005). The orientation and depth of insertion of the voltage-sensing S4 helix has been determined (Doherty et al., 2010). A synthetic S6 segment derived from the KvAP channel self-assembles, permeabilizes lipid vesicles, and exhibits ion channel activity in bilayer lipid membrane (Verma et al., 2011).  Thus the gating mechanism combines structural rearrangements and electric-field remodeling ( Li et al. 2014).  KvAP has been reconstituted in Giant Unilamellar Vesicles (GUVs) (Garten et al. 2015).  TMS4 (S4) which senses voltage also promotes membrane insertion of the voltage-sensor domain (Mishima et al. 2016). KvAP has a configuration consistent with a water channel, possibly underlying the conductance of protons, and other cations, through voltage-sensor domains (Freites et al. 2006). The structural dynamics of the paddle motif loop in the activated conformation of the KvAP voltage sensor have been studied from biophysical standpoints (Das et al. 2019).

KvAP of Aeropyrum pernix (Q9YDF8) 

Kv of Listeria monocytogenes (Q8Y5K1)

The two-pore domain potassium channel, TRESK-1 (Czirjak et al., 2004) (provides the background K⁺ current in mouse DRG neurons (Dobler et al., 2007)) TRESK (TWIK-related spinal cord K⁺ channel) is reversibly activated by the calcium/calmodulin-dependent protein phosphatase, calcineurin. Czirjak et al. 2008 reported that 14-3-3 proteins directly bind to the intracellular loop to TRESK and control the kinetics of the calcium-dependent regulation. Cloxyquin (5-chloroquinolin-8-ol) is an activator (Wright et al. 2013).  A cytoplasmic loop binds tubulin (Enyedi et al. 2014). Channel activity is modified by phosphorylation (inactive) and dephosphorylation (active) of the unusually long intracellular loop between the 2nd and 3rd TMS (Lengyel et al. 2018).

TRESK-1 of Mus musculus (AAQ91836)

Alkalinizatioin-activated Ca²⁺-selective channel, sperm-associated cation channel, CatSper, required for male fertility and the hyperactivated motility of spermatozoa (Kirichok et al. 2006). These channels require auxiliary subunits, CatSperβ, γ and δ for activity (Chung et al., 2011).  The primary channel protein is CatSper1 (Liu et al., 2007).  CatSper1 may be a target for immunocontraception (Li et al. 2009). CatSper channels regulate sperm motility (Vicente-Carrillo et al. 2017). Sperm competition is selective for a disulfide-crosslinked macromolecular architecture. CatSper channel opening occurs in response to pH, 2-arachidonoylglycerol, and mechanical force. A flippase function is hypothesized, and a source of the concomitant disulfide isomerase activity is found in CatSper-associated proteins beta, delta and epsilon (Bystroff 2018).

CatSper of Homo sapiens
CatSper1 (Q96P76)
CatSper3 (Q86XQ3)
CatSper4 (Q7RTX7)
CatSperβ (Q9H7T0)
CatSperγ (Q6ZRH7)
CatSperδ (Tmem146) (Q86XM0)
CatSperε (B1AQM6)

Alkalinization-activated Ca²⁺-selective channel, Cation channel of sperm 1, CatSper1, required for male fertility and the hyperactivated motility of spermatozoa. These channels require auxiliary subunits, CatSper β, γ and δ for activity (Chung et al., 2011; Liu et al., 2007).

CatSper of Mus musculus
CatSper1 (Q91ZR5)
CatSper3 (Q86XQ3)
CatSper4 (Q8BVN3)
CatSperβ (Q8C0R2)
CatSperγ (C6KI89)
CatSperδ (Tmem146) (E9Q9F6) 

Voltage-sensitive K⁺ channel (P_Na⁺/P_K⁺ ≈ 0.1) Shaker and Shab K⁺ channels are blocked by quinidine (Gomez-Lagunas, 2010).

Shab11 of Drosophila melanogaster

Voltage-gated K⁺ channel, chain A, Shaker-related, K_v1.2 (Crystal structure known, Long et al., 2007; Chen et al. 2010). Functions with the auxiliary subunit, Ivβ1.2; 8.A.5.1.1) (Peters et al. 2009).  Delemotte et al. (2010) described the effects of sensor domain mutations on molecular dynamics of Kv1.2.  The Sigma 1 receptor (Q99720; Sigma non-opioid intracellular receptor 1) interacts with Kv1.2 to shape neuronal and behavioral responses to cocaine (Kourrich et al. 2013).  Amino acid substitutions cause Shaker to become heat-sensing (opens with increasing temperature as for TrpV1) or cold-sensing (opens with decreasing temperature as for TrpM8) (Chowdhury et al. 2014).  The Shaker Kv channel was truncated after the 4th transmembrane helix S4 (Shaker-iVSD) which showed altered gating kinetics and formed a cation-selective ion channel with a strong preference for protons (Zhao and Blunck 2016).  Direct axon-to-myelin linkage by abundant K_V1/Cx29 (TC# 1.A.24.1.12) channel interactions in rodent axons supports the idea of an electrically active role for myelin in increasing both the saltatory conduction velocity and the maximal propagation frequency in mammalian myelinated axons (Rash et al. 2016). A cryoEM structure (3 - 4 Å resolution; paddle chimeric channel; closed form) in nanodiscs has been determined (Matthies et al. 2018). Possible gating mechanisms have been discussed (Kariev and Green 2018; Infield et al. 2018).

K_v1.2 of Homo sapiens (P16389)

Voltage-gated K⁺ channel, Shab-related, K_v2.1 or KCNB1 (858aas) The crystal structure is known (Long et al., 2007). Rat K_v2.1 and K_v2.2 (long) are colocalized in the somata and proximal dendrites of cortical pyramidal neurons and are capable of forming functional heteromeric delayed rectifier channels. The delayed rectifer currents, which regulate action potential firing, are encoded by heteromeric K_v2 channels in cortical neurons (Kihira et al., 2010). Phosphorylation by AMP-activated protein kinase regulates membrane excitability (Ikematsu et al., 2011). Functional interactions between residues in the S1, S4, and S5 domains of Kv2.1 have been identified (Bocksteins et al., 2011).  Missense variants in the ion channel domain and loss-of-function variants in this domain and the C-terminus cause neurodevelopmental disorders, sometimes with seizures (de Kovel et al. 2017).  Kv2.1 channels consist of two types of alpha-subunits: (1) electrically-active Kcnb1 alpha-subunits and (2) silent or modulatory alpha-subunits plus beta-subunits that, similar to silent alpha-subunits, regulate electrically-active subunits (Jędrychowska and Korzh 2019). It plays a role  in neurodevelopmental disorders, such as epileptic encephalopathy. The N- and C-terminal domains of the alpha-subunits interact to form the cytoplasmic subunit of hetero-tetrameric potassium channels. Kcnb1-containing channels are involved in brain development and reproduction.

K_v2.1/K_v2.2 of Homo sapiens
(Q14721)
(Q92953)

Voltage-gated K⁺ channel, Kv1.1 or KCNA1. It is palmitoylated, modulating voltage sensing (Gubitosi-Klug et al. 2005). It is regulated by syntaxin through dual action on channel surface expression and conductance (Feinshreiber et al., 2009).  Defects cause episodic ataxia type 1 (EA1), an autosomal dominant K⁺ channelopathy accompanied by short attacks of cerebellar ataxia and dysarthria (D'Adamo et al. 2014). Direct axon-to-myelin linkage by abundant K_V1/Cx29 channel interactions in rodent axons supports the idea of an electrically active role for myelin in increasing both the saltatory conduction velocity and the maximal propagation frequency in mammalian myelinated axons (Rash et al. 2016). Kv1.1 is present in bull sperm where it is necessary for normal sperm progressive motility, per cent capacitated spermatozoa (B-pattern) and the acrosome reaction (Gupta et al. 2018). Gating induces large aqueous volumetric remodeling (Díaz-Franulic et al. 2018).

Kv1.1 of Homo sapiens (Q09470)

Voltage-gated K⁺ channel subfamily C member 3, Kv3.3. Negatively modulated by protein kinase C (Desai et al., 2008). Phosphorylation of Kv3.3 by PKC may allow neurons to maintain action potential height during stimulation at high frequencies, and therefore contributes to stimulus-induced changes in the intrinsic excitability of neurons such as those of the auditory brainstem (Desai et al., 2008).  N-glycosylation impacts the sub-plasma membrane localization and activity of Kv3.1b-containing channels, and N-glycosylation processing of Kv3.1b-containing channels contributes to neuronal excitability (Hall et al. 2017).

Kv3.3 of Homo sapiens (Q14003)

Voltage-gated delayed rectifier K⁺ channel, Kv1 of the octopus. RNA editing underlies adaption (Garrett and Rosenthal, 2012).  94% identical to the squid giant axon delayed rectifier voltage-dependent potassium channel, SqKv1A (Q25376).

Kv1 of Octopus vulgaris (H2EZS9)

Potassium voltage-gated channel (KCNH) subfamily G member 3 (Voltage-gated potassium channel subunit Kv10.1) (Voltage-gated potassium channel subunit Kv6.3).  Splice variants have different properties and can activate cyclin-dependent protein kinases (Ramos Gomes et al. 2015). Control of transport (pore) function  by the voltage sensor may involve more than one mechanism (Tomczak et al. 2017).  The silent (non transporting) behaviour of Kv6.3 in the ER is caused by the C-terminal part of its sixth transmembrane domain that causes ER retentio (Ottschytsch et al. 2005).

Kcng3 or K_v10.1of Rattus norvegicus

The voltage-gated K⁺ channel subfamily D member 3, KCND3 or K_v4.3.  Mutations cause spinocerebellar ataxia type 19 (Duarri et al. 2012). Positively charged residues in S4 contribute to channel inactivation and recovery (Skerritt and Campbell 2007). The crystal structure of Kv4.3 with its regulatory subunit, Kchip1, has been solved (2NZ0) (Wang et al. 2007).

KCND3 of Homo sapiens

Voltage-sensitive K⁺ channel

Shaw2 of Drosophila melanogaster

Shaker K+ channel, Shk-1, Shk1, Kv1 of 536 aas and 6 TMSs.  Mediates the voltage-dependent potassium ion permeability of excitable membranes. Has an important role in repolarization and in regulating the pattern of action potential firing. Isoform a expresses currents in a more depolarized voltage range than isoform d (Liu et al. 2011).

Shk-1 of Caenorhabditis elegans

Shal (SHL-1, Kv4) K⁺ channels of 578 aas and 6 TMSs are the predominant transient outward current in C. elegans muscle. SHL-1 expression occurs in a subset of neurons, body wall muscle and in male-specific diagonal muscles (Fawcett et al. 2006) and control the excitability of neurons and cardiac myocytes by conducting rapidly activating-inactivating currents. Activity is modulated by three K⁺ channel interacting (KChIP) soluble auxiliary subunits, NCS-4, NCS-5, and NCS-7. All three ceKChIPs alter electrical characteristics of SHL-1 currents by slowing down inactivation kinetics and shifting voltage dependence of activation to more hyperpolarizing potentials. Native SHL-1 current is completely abolished in cultured myocytes of Triple KO worms in which all three KChIP genes are deleted (Chen et al. 2015).

Shal of Caenorhabditis elegans

K⁺ channel, jShak1 of 487 aas and 6 TMSs. Intramolecular interactions control voltage sensitivity (Sharmin and Gallin 2016).

jShak1 of Polyorchis penicillatus (Hydromedusa; jellyfish)

Voltage-gated potassium channel subunit Kv8.2, KCNC2, of 545 aas and 6 TMSs. Mutation causes central ellipsoid loss which involves cone dystrophy with supernormal rod electroretinogram. It is a monogenic disease due to KCNV2 gene mutations that affect KCNC2 channel function in rod and cone photoreceptors (Xu et al. 2017).

KCNC2 of Homo sapiens

Voltage-gated K⁺ channel, KCNC1 (K_v3.1) of 511 aas and 6 TMSs.  It plays an important role in the rapid repolarization of fast-firing brain neurons. The channel opens in response to the voltage difference across the membrane, forming a potassium-selective channel through which potassium ions pass in accordance with their electrochemical gradient (Muona et al. 2015). Can form functional homotetrameric channels and heterotetrameric channels that contain variable proportions of KCNC2 (TC# 1.A.1.2.23), and possibly other family members as well. Contributes to fire sustained trains of very brief action potentials at high frequency in pallidal neurons. Causes various genetic neurological disorders when functioning abnormally such as attention deficit/hyperactivity (Yuan et al. 2017), myoclonus epilepsy and ataxia (Oliver et al. 2017) and intellectual disability (Poirier et al. 2017). The lipid environment, including 7-ketocholesterol (7KC), 24S-hydroxycholesterol (24S-OHC) and tetracosanoic acid (C24:0) affects Kv3.1b channel expression/functionality (Bezine et al. 2018).

KCNC1 of Homo sapiens

Potassium voltage-gated channel subfamily B member 2, K_v2.2 or KCNB2 of 911 aas and 6 TMSs.

Kv2.2 of Homo sapiens

Potassium voltage-gated channel subfamily G member 4, KCNG4, of 519 aas and 6 TMSs.  Potassium channel subunit that does not form functional channels by itself, but can form functional heterotetrameric channels with KCNB1; modulates the delayed rectifier voltage-gated potassium channel activation and deactivation rates of KCNB1 (Mederos Y Schnitzler et al. 2009).

KCNG4 of Homo sapiens

Potassium voltage-gated channel subfamily G member 1, KCNG1, of 513 aas and 6 TMSs. Expressed in brain and placenta, and at much lower levels in kidney and pancreas (Su et al. 1997). This potassium channel subunit does not form functional channels by itself, but can form functional heterotetrameric channels with KCNB1. It modulates the delayed rectifier voltage-gated potassium channel activation and deactivation rates of KCNB1 (Mederos Y Schnitzler et al. 2009).

KCNG1 of Homo sapiens

Potassium voltage-gated channel subfamily D member 1, K_v4.1 of 647 aas and 6 TMSs.  Pore-forming α-subunit of a voltage-gated rapidly inactivating A-type potassium channel. It may contribute to I(To) current in the heart and I(Sa) current in neurons. Channel properties are modulated by interactions with other α-subunits and with regulatory subunits, KChIP-1 and DPPX-S. The complex voltage-dependent gating rearrangements are not limited to the membrane-spanning core but must include the intracellular T1-T1 tetramerization domains interface (Wang and Covarrubias 2006).

Kv4.1 of Homo sapiens

K⁺ channel of 529 aas and 6 TMSs, Kv1.6 or KCNA6. It can form functional homotetrameric channels and heterotetrameric channels that contain variable proportions of KCNA1, KCNA2, KCNA4, KCNA6, and possibly other family members (). channel properties depend on the type of alpha subunits that are part of the channel. Channel properties are modulated by cytoplasmic beta subunits that regulate the subcellular location of the alpha subunits and promote rapid inactivation (By similarity). Homotetrameric channels display rapid activation and slow inactivation (Grupe et al. 1990). It is inhibited by 0.6 μM β-defensin 3 (BD3) (Zhang et al. 2018) as well as by neurotoxic cone snail peptide μ-GIIIA and other conotoxins (Leipold et al. 2017).

Kv1.6 of Homo sapiens

Voltage-sensitive fast transient outward current K⁺ channel in neurons and muscle of flies and worms (Fawcett et al., 2006)

Shal2 of Drosophila melanogaster

Potassium channel protein of 542 aas and 10 TMSs in a 2 + 2 + 6 TMS toplogy, where the last 6 TMSs comprise the voltage-gated K⁺ channel.

K⁺ channel of Haloplasma contractile

The KCNA4 OR K_v1.4 K⁺ channel of 653 aas and 6 TMSs (potassium voltage-gated channel subfamily A member 4).  The channel alternates between opened and closed conformations in response to the voltage difference across the membrane (Ramaswami et al. 1990); Po et al. 1993).It can form functional homotetrameric channels and heterotetrameric channels that contain variable proportions of KCNA1, KCNA2, KCNA4, KCNA5, and possibly other family members; channel properties depend on the type of alpha subunits that are part of the channel (Po et al. 1993). Channel properties are modulated by cytoplasmic beta subunits that regulate the subcellular location of the alpha subunits and promote rapid inactivation. In vivo, membranes probably contain a mixture of heteromeric potassium channel complexes. The Molecular basis involved in the blocking effect of the antidepressant, metergoline, on C-type inactivation has been reported (Bai et al. 2018). The molecular basis for the inactivation of the channel by the antidepressant, metergoline, has been presented (Bai et al. 2018).

KCNA4 of Homo sapiens

Margatoxin-sensitive voltage-gated K⁺ channel, Kv1.3 (in plasma and mitochondrial membranes of T lymphocytes) (Szabò et al., 2005). K_v1.3 associates with the sequence similar (>80%) K_v1.5 protein in macrophage forming heteromers that like K_v1.3 homomers are r-margatoxin sensitive (Vicente et al., 2006). However, the heteromers have different biophysical and pharmacological properties. The K_v1.3 mitochondrial potassium channel is involved in apoptotic signalling in lymphocytes (Gulbins et al., 2010). Interactions between the C-terminus  of K_v1.5 and K_vβ regulate pyridine nucleotide-dependent changes in channel gating (Tipparaju et al., 2012).  Intracellular trafficking of the K_V1.3 K⁺ channel is regulated by the pro-domain of a matrix metalloprotease (Nguyen et al. 2013).  Direct binding of caveolin regulates K_v1 channels and allows association with lipid rafts (Pérez-Verdaguer et al. 2016). Addtionally, Na_vBeta1 interacts with the voltage sensing domain (VSD) of K_v1.3 through W172 in the transmembrane segment to modify the gating process (Kubota et al. 2017). During insertion of Kv1.3, the extended N-terminus of the second α-helix, S2, inside the ribosomal tunnel is converted into a helix in a transition that depends on the nascent peptide sequence at specific tunnel locations (Tu and Deutsch 2017). The microRNA, mmumiR449a, reduced the mRNA expression levels of transient receptor potential cation channel subfamily A member 1 (TRPA1), and calcium activated potassium channel subunit alpha1 (KCNMA1) and increased the level of transmembrane phosphatase with tension homology (TPTE) in the DRG cells (Lu et al. 2017). This channel is regulation by sterols (Balajthy et al. 2017). Loss of function causes atrial fibrillation, a rhythm disorder characterized by chaotic electrical activity of cardiac atria (Olson et al. 2006). The N-terminus and S1 of Kv1.5 can attract and coassemble with the rest of the channel (i.e. Frag(304-613)) to form a functional channel independently of the S1-S2 linkage (Lamothe et al. 2018). This channel may be present in mitochondria (Parrasia et al. 2019).

Kv1.3 homomers and Kv1.3/Kv1.5 heteromers of Homo sapiens (P22001)
Kv1.3 (P22001)
Kv1.5 (P19024)

Voltage-gated K⁺ channel subfamily D, member 2, Kv4.2 or KCND2, in neurons and muscle; forms complexes with auxiliary subunits and scaffolding proteins via its N-terminus, influencing trafficking, temperature-sensitivity and gating (Radicke et al. 2013).These subunits are (1) dipeptidyl-peptidase-like type II transmembrane proteins typified by DPPX-S (e.g., protein 6, P42658; 865 aas), and (2) cytoplasmic Ca²⁺ binding proteins known as K⁺ channel interacting proteins (KChIPs; TC#5.B.1.1.7; Q6PIL6) (Seikel and Trimmer 2009).  The C-terminus interacts with KChIP2 to influence gating, surface trafficking and gene expression (Han et al., 2006; Schwenk et al., 2008). KChIPs (250 aas for mouse KChIP4a; Q6PHZ8) are homologous to domains in NADPH oxidases (5.B.1). Heteropoda toxin 2 (P58426; PDB 1EMX; TC#8.B.5.2.2) interactions with Kv4.3 and Kv4.1 give rise to differences in gating modifications (DeSimone et al., 2011).  Mutations cause autism and seizures due to a slowing of channel inactivation (Lee et al. 2014).  The stoichiometry of Kv4.2 and DPP6 is 4:4 (Soh and Goldstein 2008). Neferine, an isoquinoline alkaloid from plants, inhibits Kv4.3 channels, probably by blocking the open state (Wang et al. 2015). SUMOylating (derivatizing with small ubiquitin-like modifier) two distinct sites on Kv4.2, increases surface expression and decreases current amplitude (Welch et al. 2019).

Kv4.2 of Homo sapiens (Q9NZV8)

Voltage-gated K⁺ channel, Shaker. Shaker and Shab K⁺ channels are blocked by quinidine (Gomez-Lagunas, 2010). Also regulated by unsaturated fatty acids (Börjesson and Elinder, 2011).  TMSs 3 and 4 comprise the voltage sensor paddle (Xu et al. 2013).  Partially responsible for action potential repolarization during synaptic transmission (Ford and Davis 2014). Shaker K⁺ channels can be mutated in S4 to create an analogous "omega" pore (Held et al. 2018). The NMR structure of the isolated Shaker voltage-sensing domain in LPPG micelles has been reported (Chen et al. 2019). Substituting the first S4 arginine with a smaller amino acid opens a high-conductance pathway for solution cations in the Shaker K⁺ channel at rest. The cationic current does not flow through the central K⁺ pore and is influenced by mutation of a conserved residue in S2, suggesting that it flows through a protein pathway within the voltage-sensing domain (Tombola et al. 2005). The current can be carried by guanidinium ions, suggesting that this is the pathway for transmembrane arginine permeation. Tombola et al. 2005 proposed that when S4 moves, it ratchets between conformations in which one arginine after another occupies and occludes to ions in the narrowest part of this pathway.

Shaker of Drosophila melanogaster (CAA29917)

Electrically silent lens epithelium K⁺ channel (Delayed rectifier K⁺ channel α-subunit, Kv9.1 (Shepard & Rae, 1999))

Kv9.1 of Homo sapiens
(Q96KK3)

Voltage gated K⁺ channel/MiNK related peptide (MiRP) complex, KVS1(α)/MPS-1/MiRPβ (expressed in chemo- and mechano-sensory neurons. Involved in chemotaxis, mechanotransduction and locomotion (Bianchi et al., 2003)). KVS-1 and KVS-2 are homologous; MPS-1 is member of the MiNK family (8.A.10). KVS-1/MPS-1 association involves hydrophobic forces (Wang and Sesti, 2007).

KVS-1 (α)/ MPS-1 (MiRPβ) of Caenorhabditis elegans
KVS-1 (α) (Q86GI9)
MPS-1 (MiRPβ) (Q86GJ0)

Brain-specific regulatory α-chain homologue that coassembles with other α-subunits to form active heteromultimeric K⁺ channels of unique kinetic properties, Kv2.3r. The functional expression of this regulatory α-subunit represents a novel mechanism without precedents in voltage-gated channels, which contributes to the functional diversity of K⁺ channels (Castellano et al., 1997). Beta subunits regulate the response of human Kv4.3 to protein kinae C phosphorylation and provide a potential mechanism for modifying the response of ion conductance to alpha-adrenergic regulation in vivo (Abbott 2017).

Kv2.3r of Rattus norvegicus (P97557)

K⁺ voltage-gated ether-a-go-go-related channel, H-ERG (KCNH2; Erg; HErg; Erg1, Kv11.1) subunit K_v11.1 (long QT syndrome type 2) (Gong et al., 2006; Chartrand et al. 2010; McBride et al. 2013).  Forms a heteromeric K⁺ channel regulating cardiac repolarization, neuronal firing frequency and neoplastic cell growth (Szabó et al., 2011). Oligomerization is due to N-terminal interactions between two splice variants, hERG1a and hERG1b (Phartiyal et al., 2007). Structure function relationships of ERG channel activation and inhibition have been reviewed (Durdagi et al., 2010). Interactions between the N-terminal domain and the transmembrane core modulate hERG K channel gating (Fernández-Trillo et al., 2011). The marine algal toxin azaspiracid is an open state blocker (Twiner et al., 2012). Verapamil blocks channel activity by binding to Y₆₅₂ and F₆₅₆ in TMS S₆ (Duan et al. 2007).  Hydrophobic interactions between the voltage sensor and the channel domain mediate inactivation (Perry et al. 2013), but voltage sensing by the S4 segment can be transduced to the channel gate in the absence of physical continuity between the two modules (Lörinczi et al. 2015).  Mutations give rise to long QT syndrome (Osterbur et al. 2015).  Polyphenols such as caffeic acid, phenylethyl ester (CAPE) and curcumin inhibit by modification of gating, not by blocking the pore (Choi et al. 2013). Potassium ions can inhibit tumorigenesis through inducing apoptosis of hepatoma cells by upregulating potassium ion transport channel proteins HERG and VDAC1 (Xia et al. 2016).  Incorrectly folded hERG can be degraded by Bag1-stimulated Trc-8-dependent proteolysis (Hantouche et al. 2016). The S1 helix regulates channel activity. Thus, S1 region mutations reduce both the action potential repolarizing current passed by Kv11.1 channels in cardiac myocytes, as well as the current passed in response to premature depolarizations that normally helps protect against the formation of ectopic beats (Phan et al. 2017). Interactions of beta1 integrins with hERG1 channels in cancer cells stimulate distinct signaling pathways that depended on the conformational state of hERG1 (Becchetti et al. 2017). ERG1 is sensitive to the alkaloid, ginsenoside 20(S) Rg3 which alters the gating of hERG1 channels by interacting with and stabilizing the voltage sensor domain in an activated state (Gardner et al. 2017). Channels split at the S4-S5 linker, at the intracellular S2-S3 loop, and at the extracellular S3-S4 loop are fully functional channel proteins (de la Peña et al. 2018). IKr is the rapidly activating component of the delayed rectifier potassium current, the ion current largely responsible for the repolarization of the cardiac action potential. Inherited forms of long QT syndrome (LQTS) in humans are linked to functional modifications in the Kv11.1 (hERG) ion channel and potentially life threatening arrhythmias. hERG1b affects the generation of the cardiac Ikr and plays an important role in cardiac electrophysiology (Perissinotti et al. 2018). X-ray crystallography and cryoEM have revealed features of the "nonswapped" transmembrane architecture, an "intrinsic ligand," and small hydrophobic pockets off a pore cavity. Drug block and inactivation mechanisms are discussed (Robertson and Morais-Cabral 2019).

H-ERG of Homo sapiens (Q12809)

Erg3 (Kv11.3) K⁺ channel with a large transient current at positive potentials (expressed only in the nervous system) (Shi et al., 1997). Erg3-mediated suppression of neuronal intrinsic excitability prevents seizure generation (Xiao et al. 2018).

Erg3 of Rattus norvegicus
(O54852)

K⁺ voltage-gated channel, rEAG1; K_v10.1; rat ether a go-go channel 1 (962 aas). Blocked by Cs⁺, Ba²⁺ and quinidine (Schwarzer et al., 2008). Cysteines control the N- and C-linker-dependent gating of KCNH1 potassium channels  (Sahoo et al., 2012).  The 3-d structure has been determined at 3.8 Å resolution using single-particle cryo-EM with calmodulin bound. The structure suggests a novel mechanism of voltage-dependent gating. Calmodulin binding closes the potassium pore (Whicher and MacKinnon 2016). Eag1 has three intracellular domains: PAS, C-linker, and CNBHD. Whicher and MacKinnon 2019 demonstrated that the Eag1 intracellular domains are not required for voltage-dependent gating but likely interact with the VS to modulate gating. Specific interactions between the PAS, CNBHD, and VS domains modulate voltage-dependent gating, and VS movement destabilizes these interactions to promote channel opening. Mutations affecting these interactions render Eag1 insensitive to calmodulin inhibition (Whicher and MacKinnon 2019). The structure of the calmodulin insensitive mutant in a pre-open conformation suggests that channel opening may occur through a rotation of the intracellular domains, and calmodulin may prevent this rotation by stabilizing interactions between the VS and the other intracellular domains. Intracellular domains likely play a similar modulatory role in voltage-dependent gating of the related Kv11-12 channels.

EAG1 of Rattus norvegicus (Q63472)

Potassium voltage-gated channel subfamily H member 3 (Brain-specific eag-like channel 1) (BEC1) (Ether-a-go-go-like potassium channel 2) (ELK channel 2) (ELK2) (Voltage-gated potassium channel subunit Kv12.2). A selective inhibitor is ASP2905 (Takahashi et al. 2017).

KCNH3 of Homo sapiens

Cyclic nucleotide-binding, voltage-gated, Mg²⁺-dependent, CaMKII-regulated K⁺ channel, Eag.  Eag recruits CASK (TC# 9.B.106.3.2) to the plasma membrane; forms a heterotetramer (Liu et al. 2010).

Eag of Drosophila melanogaster

Cyclic nucleotide-gated K⁺ channel, CNGC or CNG1 of 894 aas and 6 TMSs (Wheeler and Brownlee 2008).

CNG1 of Chlamydomonas reinhardtii

Aureochrome 1-like protein of 370 aas and a probable C-terminal 2 TMS ion channel domain.

Aureochrome 1 of Chattonella antiqua

K⁺- and Na⁺-conducting NaK channel (3-D structure solves with Na⁺ and K⁺) (Shi et al., 2006).  Exhibits tight structural and dynamic coupling between the selectivity filter and the channel scaffold (Brettmann et al. 2015).

NaK channel of Bacillus cereus (2AHYB) (Q81HW2)

Two pore domain potassium channel family protein of 122 aas and 2 TMSs.

K+ channel of Anaerolineales bacterium

Two pore domain potassium channel family protein of 140 aas and 2 TMSs.

K⁺ channel of Methanosarcina mazei

The pea symbiosis protein, essential for nodulation, mycorrhization, and Nod-factor-induced calcium spiking, SYM8 or DMI1 (Does not make infections 1). (Most similar to 1.A.1.13.2; 894aas; 4 TMSs between residues 136 and 339) (Edwards et al., 2007).

SYM8 of Pisum sativum
(Q4VY51)

CASTOR of Lotus japonicus (Q5H8A6)

POLLUX of Lotus japonicus (Q5H8A5)

putative ion channel (N-terminal domain) protein with C-terminal TrkA-N domain (DUF1012); NAD-binding lipoprotein.

Ion channel protein of Streptomyces coelicolor

K⁺ channel protein homologue 

K⁺ channels protein homologue of Stigmatella aurantiaca (Q08U57)

Putative 6 TMS potassium channel

Potassium ion channel of Myxococcus xanthus

Putative K⁺ channel

K channel of Cyanotheca (Synechococcus) sp PCC8801

Cyclic nucleotide-gated K⁺ channel of 459 aas.

Channel of Labenzia aggregata

Uncharacterized ion channel protein of 276 aas and 6 TMSs

UP of Flavobacterium psychrolimnae

The 6TMS bacterial cyclic nucleotide-regulated, voltage independent channel, MlotiK1 or MloK1 (Clayton et al., 2008). Gating involves large rearrangements of the cyclic nucleotide-binding domains (Mari et al., 2011).  The electron crystalographic structure is available (PDB 4CHW)revealing ligand-induced structural changes (Schünke et al. 2011; Scherer et al. 2014; Kowal et al. 2014). Such changes may be lipid dependent (McCoy et al. 2014).

MlotiK1 of Mesorhizobium loti (Q98GN8)

Putative 4 TMS ion channel protein.  TMSs 1-2 may not be homologous to TMSs 3-4 which probably form the channel.

Hypothetic VIC family member of Streptomyces coelicolor

Putative 4 TMS potassium ion channel protein.  TMSs 1-2 may not be homologous to TMSs 3-4 which probably form the channel.

Putative ion channel of Streptomyces coelicolor

Uncharacterized protein of 114 aas

UP of Rhizobium meliloti

Uncharacterized protein of 148 aas and 3 or 4 TMSs

UP of Marinobacter hydrocarbonoclasticus

Putative K⁺ channel

Putative K⁺ channel of Klebsiella varicola (D3RJS6)

Putative K⁺ channel

Putative K⁺ channel of Pseudomonas fluorescens (C3K1P0) 

Thylakoid membrane 6 TMS voltage-sensitive K⁺ channel, SnyK; important for photosynthesis (Checchetto et al. 2012).

SynK of Synechocystis sp.

Putative voltage-dependent K+ channel

K⁺ channel of Vibrio alginolytcus

Putative voltage-dependent K⁺ channel

K⁺ channel of E. coli

Putative voltage-dependent K⁺ channel

K⁺ channel of Acinetobacter baumannii

Uncharacterized protein of 228 aas and 6 TMSs

UP of Methanoculleus bourgensis (Methanogenium bourgense)

Two pore domain potassium channel family protein of 246 aas and 6 TMSs.

Putative K⁺ channel of Planctomycetes bacterium

The 2 - 4 TMS K⁺ channel, LctB (Wolters et al. 1999).

LctB of Bacillus stearothermophilus

Uncharacterized protein of 481 aas and 2 TMSs.  (Pfam CL0030)

UP of Pyrococcus furiosus

Uncharacterized protein of 326 aas and 2 TMSs

UP of Pseudoalteromonas luteoviolacea

C-terminal 2 TMS channel protein of 723 aas with 5 N-terminal pentapeptide repeats in a YjbI domain of unknown function

Channel protein of Natrinema altunense

Ion transport 2 domain-containing protein of 345 aas and 2 TMSs

Ion transport 2 domain-containing protein of Halococcus salifodinae

Putative cation transporting channel-2 of 288 aas with 2 N-terminal TMSs (Hug et al. 2016).

Channel-2 of Candidatus Peribacter riflensis

Large conductance, voltage- and Ca²⁺-activated K⁺ calcium-dependent potassium (BK) channel. Four pairs of RCK1 and RICK2 domains form the Ca²⁺-sensing apparatus known as the "gating ring" in Big Potassium (BK) channel proteins (Savalli et al., 2012). Gating of BK channels does not seem to require a physical gate. Instead, changes in the pore shape and surface hydrophobicity in the Ca²⁺-free state allow the channel to readily undergo hydrophobic dewetting transitions, giving rise to a large free energy barrier for K⁺ permeation (Jia et al. 2018).  Voltage-dependent dynamics of the BK channel cytosolic gating ring are coupled to the membrane-embedded voltage sensor (Miranda et al. 2018).

Ca²⁺-activated K⁺ channel of Drosophila melanogaster

Calcium-, Magnesium- and voltage-activated K⁺ channel, Slo1 (Kcma1; KCNMA) of 1236 aas and 6 N-terminal TMSs. Its activation dampens the excitatory events that elevate the cytosolic Ca²⁺ concentration and/or depolarize the cell membrane. It therefore contributes to repolarization of the membrane potential, and it plays a key role in controlling excitability in a number of systems. Ethanol and carbon monoxide-bound heme increase channel activation while heme inhibits channel activation (Tang et al. 2003). The molecular structures of the human Slo1 channel in complex with beta4 has been solved revealing four beta4 subunits, each containing two transmembrane helices, encircling Slo1, contacting it through helical interactions inside the membrane. On the extracellular side, b4 forms a tetrameric crown over the pore. Structures with high and low Ca²⁺ concentrations show that identical gating conformations occur in the absence and presence of beta4, implying that beta4 serves to modulate the relative stabilities of 'pre-existing' conformations rather than creating new ones (Tao and MacKinnon 2019). BK channels show increased activities in Angelman syndrome due to genetic defects in the ubiquitin protein ligase E3A (UBE3A) gene (Sun et al. 2019).

Kcma1 of Homo sapiens

Large conductance Ca²⁺ - and voltage-activated K⁺ channel, α-subunit (subunit α1), BK, BK_Ca, Kca1.1, Slowpoke, Slo1, KCNMA1 or MaxiK (functions with four β-subunits (TC# 8.A.14) encoded by genes KCNMB1-4 and the γ subunit (TC# 8.A.43) in humans (Toro et al. 2013; Li et al. 2016); the positions of beta2 and beta3 have been determined (Wu et al. 2013).  The KB channel is inhibited by 3 scorpion toxins, charybda toxin, iberiotoxin and slotoxin.  It forms a ''Ca²⁺  nanodomain'' complex with Cav1.2 (L-type; 1.A.1.11.4), Cav2.1 (P/Q-type; 1.A.1.11.5) and Cav2.2 (N-type; 1.A.1.11.6) where Ca²⁺ influx through the Cav channel activates BK_Ca (Berkefeld et al., 2006; Romanenko et al., 2006). The RCK2 domain is a Ca²⁺ sensor (Yusifov et al., 2008). Binding of Ca²⁺ to D367 and E535 changes the conformation around the binding site and turns the side chain of M513 into a hydrophobic core, explaining how Ca²⁺ binding opens the activation gate of the channel (Zhang et al., 2010). A structural motif in the C-terminal tail of Slo1 confers carbon monoxide sensitivity to human BK_Ca channels (Williams et al., 2008; Hou et al., 2008). These channels are present in the inner mitochondrial membrane of rat brain (Douglas et al., 2006).The Stress-Axis Regulated Exon (STREX) is responsible for stretch sensitivity. Ca²⁺ binds to two sites. Ca²⁺ binding to the RCK1 site is voltage dependent, but Ca²⁺ binding to the Ca²⁺ bowl is not (Sweet and Cox et al., 2008). Type 1 IP3 receptors activate BKCa channels via local molecular coupling in arterial smooth muscle cells (Zhao et al., 2010). The open structure is known (Yuan et al., 2012). BK_Ca is essential for ER calcium uptake in neurons and cardiomyocytes (Kuum et al., 2012) and link Ca²⁺ signaling to action potential firing and neurotransmitter release via serotonin receptors in many types of neurons (Rothberg 2012). The molecular mechanism of pharmacological activation of BK channels has been discussed by Gessner et al. (2012). The first TMS of the β2-subunit binds to TMS S1 of the α-subunit (Morera et al., 2012).  Mutations in Cav1.2 give rise to Timothy syndrome (Dixon et al. 2012).  Exhibits low voltage activation by interaction with Cav3 (Rehak et al. 2013) as well as Ca²⁺-gating (Berkefeld and Fakler 2013). Single-channel kinetics have been reported (Geng and Magleby 2014). The γ-subunit has TC# 8.A.43.1.8.  RBK channels regulate myogenesis in vascular smooth muscle cells (Krishnamoorthy-Natarajan and Koide 2016). Latorre et al. 2017 reviewed molecular, physiological and pathological aspects of Slo1. The microRNA, mmumiR449a, reduced the mRNA expression levels of transient receptor potential cation channel subfamily A member 1 (TRPA1), and calcium activated potassium channel subunit alpha1 (KCNMA1) and increased the level of transmembrane phosphatase with tension homology (TPTE) in the DRG cells (Lu et al. 2017), thereby reducing pain. The N-terminal sequence determines its modification by β-subunits (Lorca et al. 2017). Inhibition of BKCa negatively alters cardiovascular function (Patel et al. 2018). BKCa may be the target of verteporfin, a benzoporphyrin photosensitizer that alters membrane ionic currents (Huang et al. 2019). Globotriaosylceramide (Gb3) accumulates due to mutations in the gene encoding alpha-galactosidase A. Gb3 deposition in skin fibroblasts impairs KCa1.1 activity and activate the Notch1 signaling pathway, resulting in an increase in pro-inflammatory mediator expression, and thus, contributing to cutaneous nociceptor sensitization as a potential mechanism of FD-associated pain (Rickert et al. 2019). This channel may be present in mitochondria (Parrasia et al. 2019).

BK_Ca or MaxiK channel of Rattus norvegicus (Q62976)

Ca²⁺-activated K⁺ channel Slo-1 (Maxi K; BK channel) (ethanol-activated; responsible for intoxication) (Davies et al., 2003); tyrosyl phosphorylation regulates BK channels via cortactin (Tian et al. 2008a), but palmitoylation gates phosphorylation-dependent regulation of BK potassium channels (Tian et al., 2008b). Also regulated by Mg²⁺ which mediates interaction between the voltage sensor and cytosolic domain to activate BK channels (Yang et al., 2007). Modulated by the ss2 subunit (Lee et al., 2010). The structure of the gating ring from the human large-conductance Ca²⁺-gated K⁺ channel has been reported (Wu et al., 2010). Four pairs of RCK1 and RICK2 domains form the Ca²⁺-sensing apparatus known as the "gating ring" in BK channel proteins (Savalli et al., 2012).  The dystrophin (Q9TW65) dystrobrevin (Q9Y048) complex controls BK channel localization and muscle activity as well as neurotroansmitter release (Kim et al. 2009, Chen et al. 2011).  Syntrophin (Q93646) links various receptors and transporters to the actin cytoskeleton and the dystrophin glycoprotein complex (DGC), and α-catulin (CTN-1; 759 aas, 0 TMSs) facilitates targeting.

BK K⁺ channel of Caenorhabditis elegans (Q95V25)

The one or two component intracellularly Na⁺ and Cl^--activated delayed rectifier K⁺ channel, rSlo2.2 (Slack; KCNT1)/r Slo2.1 (Slick; KCNT2; TC# 1.A.1.3.6) provides protection against ischemia (Yuan et al., 2003). The N-terminal domain of Slack determines the formation and trafficking of Slick/Slack heteromeric sodium-activated potassium channels (Chen et al., 2009). Slick and Slack can also form separate homooligomeric channels. These channels are widely distributed in the mammalian CNS and they play roles in slow afterhyperpolarization, generation of depolarizing afterpotentials and in setting and stabilizing the resting potential (Rizzi et al. 2015). The small cytoplasmic protein beta-synuclein TC# 1.C.77.1.2) and the transmembrane protein 263 (TMEM 263; TC# 8.A.101.1.1) are interaction partners of both Slick and Slack channels. The inactive dipeptidyl-peptidase (DPP 10) and the synapse associated protein 102 (SAP 102) are constituents of the Slick and Slack channel complexes (Rizzi et al. 2015).

Slo2 of Rattus norvegicus (Q9Z258)

Sperm Slo3 high conductance K⁺ channel, activated by voltage and intracellular alkalinization. In sperm, it gives rise to pH-dependent outwardly rectifying K⁺ currents. (required for the ensuing acrosome reaction; activated by phosphatidylinositol 4,5-bisphosphate (PIP(2)) (Tang et al., 2010).

Slo3 of Mus musculus (O54982)

Human outward rectifying potassium channel, Slo2.1 (also called KCNT2 and Slick) of 1135 aas. Produces rapidly activating outward rectifier K⁺ currents. Activated by high intracellular sodium and chloride levels. Channel activity is inhibited by ATP and by inhalation of anesthetics such as isoflurane.  Inhibited upon stimulation of G-protein coupled receptors such as CHRM1 and GRIA1.  Orthologous to 1.A.1.3.4 (Garg et al. 2013) and can form a heteromeric complex with it and several other proteins (see TC# 1.A.1.3.4).  Hydrophobic interactions between residues in S5 and the C-terminal end of the pore helix stabilize Slo2.1 channels in a closed state (Suzuki et al. 2016).

Slo2.1 of Homo sapiens

Slo2.2 sodium-activated potassium channel subfamily T member 1 of 1217 aas and 6 putative N-terminal TMSs plus a P-loop.  Cryo electron microscopic structures of the chicken orthologue at 4.5 Å resolution has been solved, revealing a large cytoplasmic gating ring in which resides the Na⁺-binding site and a transmembrane domain that closely resembles voltage-gated K⁺ channels (Hite et al. 2015).

Slo2.2 of Homo sapiens

SLOwpoke K⁺ channel, SLO-2 or Slo2, of 1107 aas and 6 TMSs, present in motor neurons.  It has six putative TMSs with a K⁺-selective pore and a large C-terminal cytosolic domain (Lim et al. 1999). Its requirements for both Cl^- and Ca²⁺ are synergistic and associated with the same functional domain (Yuan et al. 2000) which serves to counteract hypoxia stress when cytoplasmic Cl^- and Ca²⁺ concentrations increase (Yuan et al. 2003; Santi et al. 2003).  SLO2 protects from hypoxic injury by increasing the permeability of the mitochondrial inner membrane to K⁺ (Wojtovich et al. 2011). SLO-2 is functionally coupled with CaV1 and regulates neurotransmitter release (Liu et al. 2014).  Partially responsible for action potential repolarization during synaptic transmission (Ford and Davis 2014).

Slo-2 of Caenorhabditis elegans

Voltage-gated calcium-activated potassium channel of 862 aas and 6 or 7 TMSs.

VIC protein of Entamoeba histolytica

Uncharacterized putative chloride channel protein of 219 aas and 2 TMSs.

UP of Vibrio phage 1.081.O._10N.286.52.C2

Uncharacterized VIC superfamily member of 230 aas and 6 or 7 TMSs (Anwar and Samudrala 2018).

UP of Entamoeba histolytica

K⁺ channel, AKT1; may form heteromeric channels with KC1 (TC # 1.A.1.4.9) (Geiger et al., 2009).  Required for seed development and postgermination growth in low potassium (Pyo et al. 2010).  Functions optimally with intermediate potassium concentrations (~1 mM) (Nieves-Cordones et al. 2014).

AKT1 of Arabidopsis thaliana

Akt1 of Physcomitrella patens (A5PH36)

Potassium channel, KCN11.  The UniProt entry included here is not complete. The correct gene ID is Cre06.g278111 in the Chlamydomonas genome database Phytozome. The complete sequence and description of its function are published by Xu et al. (2016).  KCN11 is a 6 TMS organelle K⁺ channel found exclusively in the contractile vacuole. It is required for osmoregulation under hypotonic conditions (Xu et al. 2016).

KCN11 of Chlamydomonas reinhardtii (Chlamydomonas smithii)

Synthetic light-sensitive K⁺ channel, BLINK2, of 406 aas and 2 TMSs with a P-loop between the two TMSs (residues 158 - 233). Residues 8 - 142 are derived from residues 403 - 537 of NPH1-1, a light-sensitive ser/thr protein kinase of the oat plant, Avena sativa ( acc # AAC05083); residues 143 - 234 are derived from residues 3 - 94 of the Paramecium bursaria Chlorella virus 1 (PBCV-1) K⁺ channel, Kcv1 (TC# 1.A.1.12.1); residues 235 - 404 derive from residues 506 - 675 of another K⁺ channel protein, KAT1 (TC# 1.A.1.12.1). BLINK1 has been used to manipulate stomatal kinetics to improve carbon assimilation, water use, and growth of A. thaliana (Papanatsiou et al. 2019).

Synthetic light-sensitive K⁺ channel, BLINK2.

KDC1 of Daucus carota

Guard cell outward rectifying K⁺ out channel, GORK, controls leaf stomatal pore opening (by increasing solute content) and closing (by decreasing solute content), which in turn controls gas and water loss (Schroeder, 2003).

GORK of Arabidopsis thaliana (CAC17380)

Root stelar K⁺ outward rectifying channel, SKOR (involved in K⁺ release into the xylem sap; part of the plant water stress response) (Gaymard et al., 1998). SKOR is an outwardly rectifying K⁺ channel that mediates the delivery of K⁺ from stelar cells to the xylem in the roots, a critical step in the long-distance distribution of K⁺ from roots to the upper parts of the plant. Liu et al. 2006 and Johansson et al. 2006 reported that SKOR channel activity is strictly dependent on intracellular and extracellular K⁺ concentrations. Activation by K⁺ did not affect the kinetics of voltage dependence, indicating that a voltage-independent gating mechanism underlies K⁺ sensing. The C-terminal non-transmembrane region is required for sensing. The intracellular K⁺ sensing mechanism couples SKOR activity to the K⁺ status of the 'source cells', thereby establishing a supply-based unloading system for the regulation of K⁺ distribution (Liu et al. 2006; Johansson et al. 2006).

SKOR of Arabidopsis thaliana (AAF26975)

Heterotetrameric K⁺ channel, KAT2/AKT2/KCT2 (Nieves-Cordones et al. 2014). Forms heteromeric channels (2:2 stoichiometry) with KAT1 (1.A.1.4.7) (Lebaudy et al., 2010) (Properties differ from those of homomeric channels; Xicluna et al., 2007).  KAT2 also forms homomeric channels in the plasma membrane (Nieves-Cordones et al. 2014). AKT2 functions in phloem loading and unloading and operates as an inward-rectifying channel that allows H⁺-ATPase-energized K⁺ uptake. Through reversible post-translational modifications, it can also function as an open, K⁺-selective channel, providing energy for transmembrane transport processes. It is present in a complex of several proteins in which it interacts with the receptor-like kinase, MRH1/MDIS2 (Sklodowski et al. 2017). The ortholog in Brassica rapa (Chinese cabbage), KCT2, is induced by stress. It has a TxxTxGYGD motif in the P-domain and a putative cyclic nucleotide-binding-like domain within a long C-terminal region (Zhang et al. 2006).

AKT2/KAT2 of Arabidopsis thaliana
AKT2 (Q38898)
KAT2 (Q38849)

The voltage-sensitive inward rectifying K⁺ channel, KAT1 (similar to 1.A.1.4.3; activated by protein 14-3-3 (AAF87262)) (Sottocornola et al., 2006). May also transport Na⁺ and Cs⁺ (Nakamura and Gaber, 2009). Forms heterotetrameric channels with KAT2 with a stoichiometry of 2:2 (Lebaudy et al., 2010). The pH-sensor is built of a sensory cloud rather than of single key amino acids (Gonzalez et al., 2011). The transmembrane core region of KAT1 is important for its activity in S. cerevisiae, and this involves not only the pore region but also parts of its voltage-sensor domain (Saito et al. 2017).

KAT1 of Arabidopsis thaliana (Q39128)

The KC1 (KAT3) potassium channel-like subunit; regulates other channels such as AKT1 (1.A.1.4.1) and KAT1 (1.A.1.4.7) (Duby et al., 2008); may form heteromeric channels with AKT1 (Geiger et al., 2009). It forms a tripartite SNARE-K⁺ channel complex which regulates KAT3 channel opening (Honsbein et al., 2009).  Tripartite interactions with SNARE (SYP121; SYR1; PEN1) and AKT1 control gating (Grefen et al. 2010).  ZKC1 also forms homoleric channels in the endoplasmic reticulum (Nieves-Cordones et al. 2014).

KC1 of Arabidopsis thaliana (P92960)

Orthologue K⁺/Na⁺ pacemaker channel, Hcn4 (Scicchitano et al., 2012).  Hyperpolarization-activated cyclic nucleotide-regulated HCN channels underlie the Na⁺-K⁺ permeable IH pacemaker current. As with other voltage-gated members of the 6-transmembrane KV channel superfamily, opening of HCN channels involves dilation of a helical bundle formed by the intracellular ends of S6, but this is promoted by inward, not outward, displacement of S4. Direct agonist binding to a ring of cyclic nucleotide-binding sites, one of which lies immediately distal to each S6 helix, imparts cAMP sensitivity to HCN channel opening. At depolarized potentials, HCN channels are further modulated by intracellular Mg²⁺ which blocks the open channel pore and blunts the inhibitory effect of outward K⁺ flux. Lyashchenko et al. 2014 showed that cAMP binding to the gating ring enhances not only channel opening but also the kinetics of Mg²⁺ block.  Mutations in HCN4 cause sick sinus and the Brugada syndrome, cardiac abnormalities. Associated withfamiial sinus bradycardia (Boulton et al. 2017).

Hcn4 of Homo sapiens (Q9Y3Q4)

Hyperpolarization-activated cyclic nucleotide-gated (HCN) inward current carrying cationic channel, I(f), (HCN2/HCN4) (Ye and Nerbonne, 2009).

HCN2/HCN4 channels of Homo sapiens 
HCN2 (Q9UL51)
HCN4 (Q9Y3Q4) 

Cyclic nucleotide-gated cation channel α3 (CNGA3 or CNG3); photoreceptor cGMP-gated channel α-subunit. Also possibly expressed in inner ear cell cells where it binds to an intracellular C-terminal domain of EMILIN1 (Selvakumar et al., 2012).  Elastic network model analysis of the CNGA3 channel supports a modular model of allosteric gating, according to which protein domains are quasi-independent: they can move independently but are coupled to each other allosterically (Gofman et al. 2014).

CNGA3 of Homo sapiens (Q16281) 

Trout cyclic nucleotide-gated cation channel α3 (CNGA3 or CNG3). Expressed in inner ear cell cells where it binds to an intracellular C-terminus domain of EMILIN1 (Selvakumar et al., 2012). 

CNGA3 of Oncorhynchus mykiss (G9BHJ0)

Cyclic nucleotide gated K⁺ channel of 650 aas

Channel of Naegleria gruberi

Cyanobacterial cyclic nuceotide K⁺ channel of 454 aas (Brams et al. 2014).

Channel of Trichodesmium erythraeum

Cyclic nucleotide-gated K⁺channel of 430 aas.  Activated by cAMP, not by cGMP.  Highly specific for K+ over Na+.  Has a C-terminal cAMP-binding domain linked to the 6 TMS channel domain (Brams et al. 2014).

Channel of Spirochaeta thermophila

Cyclic nucleotide-gated cation (CNG) channel of 665 aas.

CNG of Drosophila melanogaster

TAX-2 cyclic nucleotide-gated cation channel-B (CNGB) of 800 aas (Wojtyniak et al. 2013).

TAX-2 CNGB of Caenorhabditis elegans

Hyperpolarization-activated and cyclic nucleotide-gated K⁺ channel, HCN (bCNG-1) (P_Na⁺/P_K⁺ ≈ 0.3). The human orthologue (O88703) is 863 aas in length and also catalyzes mixed monovalent cation currents K⁺:Na⁺= 4:1 (Lyashchenko and Tibbs et al., 2008). Biel et al. (2009) presented a detailed review of hyperpolarization-activated cation-channels. They are inhibited by nicotine and epibatidine which bind to the inner pore (Griguoli et al., 2010). They control cardiac and neuronal rhythmicity. HCN channels contain cyclic nucleotide-binding domains (CNBDs) in their C-terminal regions, linked to the pore-forming transmembrane segment with a C-linker. The C-linker couples the conformational changes caused by the direct binding of cyclic nucleotides to the HCN pore opening. Cyclic dinucleotides antagonize the effect of cyclic nucleotides in HCN4 but not in HCN2 channels. Interaction of the C-linker/CNBD with other parts of the channel appears to be necessary for cyclic-dinucleotide binding in HCN4 channels (Hayoz et al. 2017).

HCN of Mus musculus

TAX-4 cyclic nucleotide-gated cation channel A (CNGA) of 733 aas (Wojtyniak et al. 2013).  Li et al. 2017 determined the 3.5 Å resolution single-particle electron cryo-microscopy structure  in the cyclic guanosine monophosphate (cGMP)-bound open state. The channel has an unusual voltage-sensor-like domain, accounting for its deficient voltage dependence. A carboxy-terminal linker connecting S6 and the cyclic-nucleotide-binding domain interacts directly with both the voltage sensor-like domain and the pore domain, forming a gating ring that couples conformational changes triggered by cyclic nucleotide binding to the gate. The selectivity filter is lined by the carboxylate side chain of a functionally important glutamate and three rings of backbone carbonyls (Li et al. 2017).

TAX-4 CNGA of Caenorhabditis elegans

K⁺ channel protein, PAK2.1 of 543 aas.  Contains a cyclic nucleotide-binding domain (Ling et al. 1998; Jegla and Salkoff 1995).

PAK2.1 of Paramecium tetraurelia

K⁺ channel protein, PAK11-MAC of 772 aas.  Contains a cyclic nucleotide-binding domain (Ling et al. 1998; Jegla and Salkoff 1995).

PAK11-MAC of Paramecium tetraurleia

Cyclic nuceotide-gated Na⁺ channel of 729 aas and 6 putative TMSs, CNGC19.  Constitutively expressed in roots but Induced in leaves and shoots under conditions of salt (NaCl) stress (Kugler et al. 2009).

CNGC19 of Arabidopsis thaliana

Cyclic nucleotide-gated Na⁺ channel of 764 aas and 6 putative TMSs, CNGC20.  Induced in shoots in response to salt (NaCl) stress (Kugler et al. 2009).

CNGC20 of Arabidopsis thaliana

Putative cyclic AMP-activated K⁺ channel protein, annotated in GenBank as a Crp homologue with a CAP_ED domain N-terminal and a helix-turn-hexix (HTH) domain C-terminal. This protein has two poorly hydrophobic peaks at ~ residue # 85 and 165, with a potential "P-loop" between these two peaks.  All of the cyclic nucleotide regulated channels in subfamily 2.A.1.5 have the two TMSs separated by about 80 residues with the P-loop inbetween.  Maybe the cAMP binding domains in the transcription factors and the channel proteins are derived from a common origin, and are therefore homologous.

Putative K⁺ channel homologue of Bifidobacterium longum

Cyclic nuceotide gated channel of 706 aas, CNGC18.  It is the essential Ca²⁺ channel for pollen tube guidance (Gao et al. 2016).

CNGC18 of Arabidopsis thaliana (Mouse-ear cress)

CNGC15 of 678 aas and 6 TMSs.  In Medicago truncatula, three such channels, CNGC15a, b and c, are required for nuclear calcium oscillations, spiking and subsequent symbiotic responses. These three channels form a complex with the potassium permeable channel, DMI1 (TC# 1.A.1.23.1), in the nuclear envelope.  They are expressed in flowers and pods, and mutants in these channels have decreased fertilization rates (Charpentier et al. 2016). 

CNG15 of Arabidopsis thaliana

The cyclic nucleotide-gated cation channel, CNG-1 of 661 aas and 6 TMSs. CNG-1 functions in multiple capacities to link nutritional information with behavioral output (He et al. 2016).

CNG-1 of Caenorhabditis elegans

spHCN1 is a hyperpolarization-activated cyclic nucleotide-gated (HCN) non-selective cation channel of 767 aas and 6 TMSs that opens due to inward movement of the positive charges in the fourth transmembrane domain (S4).  These channels open after only two S4s have moved, and S4 motion is rate limiting during voltage activation of spHCN channels (Bruening-Wright et al. 2007). HCN channels regulate electrical activity in the heart and brain. Distinct from mammalian isoforms, the sea urchin (spHCN) channel exhibits strong voltage-dependent inactivation in the absence of cAMP (Idikuda et al. 2018). The voltage sensor undergoes a large downward motion during hyperpolarization (Dai et al. 2019).

HPN1 of Strongylocentrotus purpuratus (Purple sea urchin)

Heterotetrameric (3A:1B) rod photoreceptor cyclic GMP-gated cation channel, CNGA1 (Zhong et al., 2002). Cyclic nucleotides are required to open the channel. Gating is proposed to be initiated by an anticlockwise rotation of the N-terminal region of the C-linker, which is then, transmitted through the S6 transmembrane helices to the P-helix, and in turn from this to the pore lumen, which opens from 2 to 5Å, thus allowing for ion permeation (Giorgetti et al. 2005). Defects produce channelopathies (Biel & Michalakis, 2007). A ring of four Glutamate residues (Glu363) in the outer vestibule, and a ring of four Threonines (Thr360) in the inner vestibule of the pore of CNGA1 channels constitute binding sites for permeating ions (Marchesi et al., 2012).  The tetraspanning peripherin-2 (TC# 8.A.40.1.2) links rhodopsin to this cyclic nucleotide-dependent channel in the outer segments of rod photoreceptors.  The G266D retinitis pigmentosa mutation in TMS4 of rhodopsin abolishes binding of peripherin-2 and prevents association with the CNGA1/CNGB1a subunits present in the complex (Becirovic et al. 2014).  External protons cause inactivation (Marchesi et al. 2015). CNG transmembrane domains have dynamic structure, undergoing conformational rearrangements (Maity et al. 2015).  Moreover, structural heterogeneity of CNGA1 channels has been demonstrated (Maity et al. 2016).

CNG of Homo sapiens
Subunit A1 (CNGA1)
Subunit B1 (CNGB1)

Hyperpolarization-activated cyclic nucleotide-modulated cation channel splice variant ABs-II of 682 aas and probably 6 TMSs, Ih channel encoded by the PIIH gene (Ouyang et al. 2007). 

Ih channel of Panulirus interruptus (California spiny lobster) (Palinurus interruptus)

Multi-domain cation channel with a C-terminal cyclic nucleotide-binding domain; of 465 aas and 6 TMS, LliK.  Cyclic nucleotide-gated (CNG) and hyperpolarization-activated cyclic nucleotide-regulated (HCN) channels play roles in phototransduction, olfaction, and cardiac pace making.  James et al. 2017 used cryoEM to determine the structure of the intact LliK CNG channel. A short S4-S5 linker connects voltage-sensing and pore domains to produce a non-domain-swapped transmembrane architecture. The conformation of the LliK structure may represent a functional state of this channel family not seen before (James et al. 2017).

LliK of Leptospira licerasiae

HCN1 is a hyperpolarization-activated cyclic nucleotide-gated (HCN) channel of 890 aas and 6 TMSs that opens due to inward movement of the positive charges in the fourth transmembrane domain (S4).  These channels open after only two S4s have moved, and S4 motion is rate limiting during voltage activation of spHCN channels (Bruening-Wright et al. 2007). HCN1 exhibits weak selectivity for potassium over sodium ions.  It's structure (3.5 Å resolution) is known (Lee and MacKinnon 2017).  It contributes to the native pacemaker currents in heart and neurons. It may also mediate responses to sour stimuli. It is inhibited by Cs⁺, zatebradine, capsazepine and ZD7288 (Gill et al. 2004). HCN1 mutational variants include epileptic encephalopathy and common generalized epilepsy. HCN1 has a pivotal function in brain development and control of neuronal excitability (Marini et al. 2018).

HCN1 of Homo sapiens

Hyperpolarization-gated and cyclic nucleotide regulated K⁺ channel of 638 aas and 6 TMSs, HCN2, present in the flagellum of sea urchin sperm (Galindo et al. 2005)

HCN2 of Strongylocentrotus purpuratus (Purple sea urchin)

The cyclic nucleotide- and voltage-gated ion (K⁺, Rb⁺, Cs⁺) channel, CNGC1 (inward rectifying) (functions in heavy metal and cation transport, as does CNGC10) (Dreyer and Uozumi, 2011; Zelman et al., 2012).

CNGC1 of Arabidopsis thaliana (O65717)

The cyclic nucleotide-gated ion (K⁺, Rb⁺, Cs⁺) channel, CNGC2 (functions in plant defense responses, as does CNGC4) (Zelman et al., 2012).

CNGC2 of Arabidopsis thaliana (O65718)

The cyclic nucleotide-gated ion (K⁺, Rb⁺, Cs⁺) channel, HLM1 (CNGC4) (mediates the hypersensitive response (HR) of plants in programmed cell death.) Mutants show abnormal cell death and resistance to infection by Pseudomonas syringae (Balagué et al., 2003; Zelman et al., 2012).

HLM1 of Arabidopsis thaliana
(Q94AS9)

The non-selective cation transporter involved in germination, CNGC3 (Gobert et al., 2006; Zelman et al., 2012).

CNG3 of Arabidopsis thaliana (Q9SKD7)

K⁺ channel, MthK or MVP of 209 aas and 6 TMSs.  Voltage-gated potassium-selective channel opened by hyperpolarization (Hellmer and Zeilinger 2003).  Mediates K⁺ uptake and sensitivity.  The structure and local dynamics of the closed activation gate (lower S6 region) of MVP have been reported (Randich et al. 2014).

MthK channel protein of Methanocaldococcus jannaschii (Methanococcus jannaschii)

Tok1 twin (dual) barrel outward rectifying K⁺ channel with exterme assymmetry which includes an extra 4 N-terminal TMSs for a total of 16 TMSs.  (Transports K⁺ and Cs⁺) (Bertl et al., 2003; Roller et al. 2008).

Tok1 outward rectifier K⁺ channel of Saccharomyces cerevisiae

AtTPK4 two-pore K⁺ channel 4 (Becker et al., 2004). Asp86 and Asp200 are essential for K⁺ permeation as well as inward rectification (Marcel et al., 2010).  Reviewed by González et al. 2014.

AtTPK4 of Arabidopsis thaliana (AAP82009)

The 2-pore (4TMS) outward rectifying K⁺ channel, KCO1 or TPK1. Possesses two tandem Ca²⁺-binding EF-hand motifs, and cytosolic free Ca²⁺ (~300 nM) activates (Czempinski et al., 1997).  Reviewed by González et al. 2014 and Basu and Haswell 2017.

KCO1 of Arabidopsis thaliana
(Q8LBL1)

Two-pore potassium channel 5 (AtTPK5) (Calcium-activated outward-rectifying potassium channel 5, chloroplastic) (AtKCO5).  Reviewed by González et al. 2014.

TPK5 of Arabidopsis thaliana

Chloroplast thylakoid two-pore calcium and proton-activated K⁺ channel, TPK3 of 436 aas and 4 TMSs.  Mediates ion counterbalancing, influencing photosynthetic llight utilization (Carraretto et al. 2013).  Reviewed by González et al. 2014.

TPK3 of Arabidopsis thaliana

Putative K⁺ channel of 96 aas nd 2 TMSs.

K⁺ channel of Yellowstone lake phycodnavirus 2

TWIK-1 (KCNK1, HOHO1, KCNO1) inward rectifier K⁺ channel (Enyedi and Czirják, 2010) expressed in the distal nephron segments (Orias et al. 1997).  Lipid tails from both the upper and lower leaflets can partially penetrate into the pore (Aryal et al. 2015). The lipid tails do not sterically occlude the pore, but there is an inverse correlation between the presence of water within the hydrophobic barrier and the number of lipids tails within the lining of the pore (Aryal et al. 2015).

TWIK-1 of Mus musculus

TASK-2 (KCNK5) two-pore domain, pH-sensitive, voltage-insensitive, outward rectifying K⁺ channel (K⁺ > Rb⁺ >> Cs⁺ > NH₄⁺ > Na⁺ ≈ Li⁺), present in renal epithelia.  Regulated [inhibited] via group 1 metabolotropic glutamate receptors and by inositol phosphates (Chemin et al., 2003).  TASK-2 gating is controlled by changes in both extra- and intracellular pH through separate sensors: arginine 224 and lysine 245, located at the extra- and intracellular ends of transmembrane domain 4, respectively. TASK-2 is inhibited by a direct effect of CO2 and is regulated by and interacts with G protein subunits. TASK-2 takes part in regulatory adjustments and is a mediator in the chemoreception process in neurons of the retrotrapezoid nucleus where its pH_i sensitivity could be important in regulating excitability and therefore signalling of the O₂/CO₂ status. Extracellular pH increases, brought about by HCO₃^- efflux from proximal tubule epithelial cells may couple to TASK-2 activation to maintain electrochemical gradients favourable to HCO₃^- reabsorption. TASK-2 is expressed at the basolateral membrane of proximal tubule cells (López-Cayuqueo et al. 2014). Mutations are associated with the Balkan Endemic Nephropathy (BEN) chronic tubulointerstitial renal disease (Reed et al. 2016). pH sensing in TASK2 channels is conferred by the combined action of several charged residues in the large extracellular M1-P1 loop (Morton et al. 2005).

TASK-2 of Homo sapiens

TREK-1 (KCNK2) K⁺ channel subunit (Regulated by group 1 metabotropic glutamate receptors and by diacylglycerols and phosphatidic acids) (Chemin et al., 2003). TREK-1, TREK-2 and TRAAK are all regulated by lysophosphatidic acid, converting these mechano-gated, pH voltage-sensitive channels into leak conductances (Chemin et al., 2005). The mammalian K2P2.1 potassium channel (TREK-1, KCNK2) is highly expressed in excitable tissues, where it plays a key role in the cellular mechanisms of neuroprotection, anaesthesia, pain perception and depression (Cohen et at., 2008). Alternative translation initiation in rat brain yields K2P2.1 potassium channels permeable to sodium (Thomas et al. 2008). The crystal structure of the human 2-pore domain K⁺ channel, K2P1 has been solved (Miller and Long, 2012). Multiple modalities converge on a common gate to control K2P channel function (Bagriantsev et al., 2011).  TREK-1 mediates fast and slow glutamate release in astrocytes upon GPCR activation (Woo et al. 2012). It is a mechanosensitive K⁺ channel, present in rat bladder myocytes, which is activated by swelling and arachidonic acid (Fukasaku et al. 2016). The M2-hinges of TREK-1 and TREK-2 channels control their macroscopic current, subcellular localization and gating (Zhuo et al. 2017). The human ortholog has acc # O95069 and has an additional N-terminal 15 aas. BL-1249, a compound from the fenamate class of nonsteroidal anti-inflammatory drugs, is known to activate K2P2.1(TREK-1), the founding member of the thermo- and mechanosensitive TREK subfamily (Pope et al. 2018).

TREK-1 of Mus musculus (P97438)

pH-dependent, voltage-insensitive, background potassium channel protein involved in maintaining the membrane potential, KCNK9, K2P9.1 or TASK3 (TASK-3) of 374 aas  (Huang et al. 2011). Terbinafine is a selective activator of TASK3 (Wright et al. 2017). The response of the tandem pore potassium channel TASK-3 to voltage involves gating at the cytoplasmic mouth (Ashmole et al., 2009).  TASK-3 is involved in several physiological and pathological processes including sleep/wake control, cognition and epilepsy (Tian et al. 2019). N-(2-((4-nitro-2-(trifluoromethyl)phenyl)amino)ethyl)benzamide (NPBA) is an activator (Tian et al. 2019). KCC2 regulates neuronal excitability and hippocampal activity via interaction with Task-3 channels (Goutierre et al. 2019). A  biguanide compound, CHET3, is a highly selective allosteric activator, and TASK-3 is a druggable target for treating pain (Liao et al. 2019). This channel may be present in mitochondria (Parrasia et al. 2019).

KCNK9 or TASK3 of Homo sapiens

KCNK15 of Homo sapiens

The Kcnk10a (TREK-2A) K⁺ channel of 569 aas and 6 TMSs. It localizes in the brain and seems to regulate reproduction (Loganathan et al. 2017).

TREK-2A of Danio rerio (Zebrafish) (Brachydanio rerio)

Open rectifier K⁺ channel 1, isoform D of 1001 aas and 6 TMSs, Ork1.

ORK1 of Drosophila melanogaster (Fruit fly)

KCNK3 K⁺ channel (TASK1, OAT1, TBAK1) (the K⁺ leak conductance). TASK1 and 3 may play a role in nontumorigenic primary hyperaldosteronism (Davies et al., 2008).  KCNK3/9/15 expression limits membrane depolarization and depolarization-induced secretion at least in part by maintaining intracellular K⁺ (Huang et al. 2011).

KCNK3 of Homo sapiens (AAG29340)

Neuronal 2-P (4 TMS) domain K⁺ membrane tension-gated channel, TRAAK (stimulated by arachidonic acid and polyunsaturated fatty acids (Fink et al., 1998). The crystal structures of conductive and nonconductive human K2P TRAAK K⁺ channel has been solved (Brohawn et al., 2012; Brohawn et al. 2014). Regulated by mechanical deformation of the membrane and temperature as well as polyunsaturated fatty acids (Brohawn et al., 2012). Multiple modalities converge on a common gate to control K2P channel function (Bagriantsev et al., 2011).  In the non-conductive state, a lipid acyl chain accesses the channel cavity through a 5 Å-wide lateral opening in the membrane inner leaflet and physically blocks ion passage. In the conductive state, rotation of transmembrane helix 4 about a central hinge seals the intramembrane opening, preventing lipid block of the cavity and permitting ion entry. Additional rotation of a membrane interacting TM2-TM3 segment, unique to mechanosensitive K2Ps, against TM4 may further stabilize the conductive conformation. Comparison of the structures provodes a biophysical explanation for TRAAK mechanosensitivity--an expansion in cross-sectional area up to 2.7 nm² in the conductive state is expected to create a membrane-tension-dependent energy difference between conformations that promotes force activation (Brohawn et al. 2014).  TM helix straightening and buckling may underlie channel activation (Lolicato et al. 2014). A lipid chain blocks the conducting path in the closed state (Rasmussen 2016).

TRAAK of Mus musculus (O88454)

Outward rectifying mechanosensitive 2-P (4 TMS) domain K⁺ channel, TREK-2 (KCNKA; KCNK10; K2P10). Activated by membrane stretch, acidic pH, arachidonic acid and unsaturated fatty acids.  Dong et al. 2015 described crystal structures of the human TREK-2 channel (up to 3.4 angstrom resolution) in two conformations and in complex with norfluoxetine, the active metabolite of fluoxetine (Prozac) and a state-dependent blocker of TREK channels. Norfluoxetine binds within intramembrane fenestrations found in only one of these two conformations. Channel activation by arachidonic acid and mechanical stretch involves conversion between these states through movement of the pore-lining helices. This provides an explanation for TREK channel mechanosensitivity, regulation by diverse stimuli, and possible off-target effects of the serotonin reuptake inhibitor Prozac (Dong et al. 2015).  The unique gating properties of TREK-2 and the mechanisms by which extracellular and intracellular stimuli harness pore gating allosterically have been studied (Zhuo et al. 2016). TREK-2 moves from the """"down"""" to """"up"""" conformation in direct response to membrane stretch. Aryal et al. 2017 showed how state-dependent interactions with lipids affect the movement of TREK-2, and how stretch influences both the inner pore and selectivity filter. They also demonstrated why direct pore block by lipid tails does not represent theprincipal mechanism of mechanogating (Aryal et al. 2017). The M2-hinges of TREK-1 and TREK-2 channels control their macroscopic current, subcellular localization and gating (Zhuo et al. 2017). TREK-2 responds to a diverse range of stimuli. Two states, termed """"up"""" and """"down,"""" are known from x-ray structural crystallographic studies and have been suggested to differ in conductance. Brennecke and de Groot 2018 found that the down state is less conductive than the up state. The introduction of membrane stretch in the simulations shifts the state of the channel toward an up configuration. Membrane pressure changes the conformation of the transmembrane helices directly and consequently also influences the channel conductance (Brennecke and de Groot 2018). 3-d structures are known (PDB 4XDJ_!-D. Phosphatidyl-(3,5)-bisphosphate (PI(3,5)P2) activates (Kirsch et al. 2018).

TREK-2 of Rattus norvegicus
(Q9JIS4)

The pH-sensitive 2 pore (4 TMS) K⁺ channel, TASK-4 (Expressed in liver, lung, pancreas and other tissues; Decher et al., 2001). The response of the tandem pore potassium channel TASK-3 (TC# 1.A.1.9.11) (K(2P)9.1) to voltage involves gating at the cytoplasmic mouth (Ashmole et al., 2009). Models have revealed the convergence of amino acid regions that are known to modulate anesthetic activity onto a common three-dimensional cavity that forms a putative anesthetic binding site in tandem pore potassium channels (Bertaccini et al. 2014).

TASK-4 of Homo sapiens
(Q96T54)

Sup-9 K⁺ channel of 329 aas and 6 TMSs. It is involved in coordination of muscle contraction (de la Cruz et al. 2003). Activity is regulated by Sup-18 (de la Cruz et al. 2014) and by Unc-93 (TC# 2.A.1.5.8).  It may also be regulated by Sup-10 (Q17374); it may be a suppressor of Unc-93 (de la Cruz et al., 2003). Mutation of a single residue promotes gating of  this channel and of several vertebrate and invertebrate two-pore domain potassium channels (Ben Soussia et al. 2019).

Sup-9 of Caenorhabditis elegans (O17185)

Twk-12 of Caenorhabditis elegans

AMPA-selective glutamate ionotropic channel receptor (GIC; AMPAR), kainate-subtype, GluR-K1; GluR1; GluR-A; GluA1; Gria1 of 906 aas (preferentially monovalent cation selective). A mature complex contains GluR1, TARPs, and PSD-95 (Fukata et al. 2005). The receptor contributes to amygdala-dependent emotional learning and fear conditioning (Humeau et al., 2007). Transmembrane AMPAR regulatory protein (TARP) gamma-7 (TC#8.A.16.2.5) selectively enhances the synaptic expression of Ca²⁺-permeable (CP-AMPARs) and suppresses calcium-impermeable (CI-AMPAR) activities (Studniarczyk et al. 2013).  Thus, TARPs modulate the pharmacology and gating of AMPA-type glutamate receptors (Soto et al. 2014).  TARPs interact with the N-terminal domain of the AMPAR and control channel gating; residues in the receptor and the TARP involved in this interaction have been identified (Cais et al. 2014).  The auxilary protein, Shisa9 or CKAMP44 (UniProt acc# B4DS77), has a C-terminal PDZ domain that allows interaction with scaffolding proteins and AMPA glutamate receptors (Karataeva et al. 2014).  The transmembrane domain alone can tetramerize (Gan et al. 2016). The most potent and well-tolerated AMPA receptor inhibitors, used to treat epilepsy, act via a noncompetitive mechanism.  The crystal structures of the rat AMPA-subtype GluA2 receptor in complex with three noncompetitive inhibitors have been solved. The inhibitors bind to a binding site, completely conserved between rat and human, at the interface between the ion channel and linkers connecting it to the ligand-binding domains (Yelshanskaya et al. 2016). The endogenous neuropeptide, cyclopropylglycine, at a physiological concentration of 1 μM, enhances the transmembrane AMPA currents in rat cerebellar Purkinje cells (Gudasheva et al. 2016). The energetics of glutamate binding have been estimated (Yu and Lau 2017). The TMEM108 protein (Q6UXF1 of 575 aas and 2 TMSs, N- and C-terminal, is required for surface expression of AMPA receptors (Jiao et al. 2017). CERC-611 is a selective antagonist of AMPA receptors containing transmembrane AMPA receptor regulatory protein (TARP; TC# 8.A.16) gamma-8 (Witkin et al. 2017). Drug effects, regulatory protein modulators and positive allosteric modulators have been reviewed (Fu et al. 2019). Herguedas et al. 2019 presented a cryo-EM structure of the heteromeric GluA1/2 receptor associated with two transmembrane AMPAR regulatory protein (TARP) gamma8 auxiliary subunits, the principal AMPAR complex at hippocampal synapses.  The native heterotetrameric AMPA-R adopts various conformations, which reflect a variable separation of the two dimeric extracellular amino-terminal domains; members of the stargazin/TARP family of transmembrane proteins co-purify with AMPA-Rs and contribute to the density representing the transmembrane region of the complex. Glutamate and cyclothiazide altered the conformational equilibrium of the channel complex, suggesting that desensitization is related to separation of the N-terminal domains (Nakagawa et al. 2005).

GluR-K1 of Rattus norvegicus

The homo- and heteromeric glutamate receptor, GLR3.3/3.4 (Desensitized in 3 patterns: (1) by Glu alone; (2) by Ala, Cys, Glu or Gly; (3) by Ala, Cys, Glu, Gly, Ser or Asn (Stephens et al., 2008). A regulatory mechanism underlies Ca²⁺ homeostasis by sorting and activation of AtGLRs by AtCNIHs (see for example, 8.A.61.1.9) (Wudick et al. 2018).  May be responsible in part for Cd²⁺ uptake (Chen et al. 2018). GLR3.3 and GLC3.6 (TC# 1.A.10.1.24) (but not GLR3.4) play different roles in nervous system-like signaling in plant defense by a mechanism that differs substantially from that in animals (Toyota et al. 2018).

GLR3.3/GLR3.4 receptor of Arabidopsis thaliana
GLR3.3 (Q9C8E7)
GLR3.4 (Q8GXJ4)

GriK2; GluK2; GluR6 glutamate receptor, ionotropic kainate 2. The 3-d structure is known (2XXY_A). The domain organization and function have been analyzed by Das et al. (2010).  Two auxiliary subunits, Neto1 and Neto2 (Neuropilin and tolloid-like proteins) alter the trafficking, channel kinetics and pharmacology of the receptors (Howe 2014).  They reduce inward rectification without altering calcium permeability (Fisher and Mott 2012). Interactions between the pore helix (M2) and adjacent segments of the transmembrane inner (M3) and outer (M1) helices may be involved in gating (Lopez et al. 2013). Mutations in the human GRIK2 (GLUR6) cause moderate-to-severe nonsyndromic autosomal recessive mental retardation (Motazacker et al. 2007). Kainate receptors regulate KCC2 (TC# 1.A.10.1.11) expression in the hippocampus (Pressey et al. 2017). GluR6, carrying the pore loop plus adjacent transmembrane domains of the prokaryotic, glutamate-gated, K⁺-selective GluR0 (TC# 1.A.10.2.1), adopted several electrophysiological properties of the donor pore uponpore transplantation (Hoffmann et al. 2006).

Grik2 of Rattus norvegicus (P42260)

The NMDA receptor. The crystal structure of the N-terminal domains (GluN1 and GluN2) have been determined (PDB#3QEL; Talukder and Wollmuth, 2011). The ligand-specific deactivation time courses of GluN1/GluN2D NMDA receptors have been measured (Vance et al., 2011).  NMDA receptors are Hebbian-like coincidence detectors, requiring binding of glycine and glutamate in combination with the relief of voltage-dependent magnesium block to open an ion conductive pore. Lee et al. 2014 presented X-ray structures of the Xenopus laevis GluN1-GluN2B NMDA receptor with the allosteric inhibitor, Ro25-6981, partial agonists and the ion channel blocker, MK-801. Receptor subunits are arranged in a 1-2-1-2 fashion, demonstrating extensive interactions between the amino-terminal and ligand-binding domains. The 3-TMS transmembrane domains harbour a closed-blocked ion channel, a pyramidal central vestibule lined by residues implicated in binding ion channel blockers and magnesium, and a approximately twofold symmetric arrangement of ion channel pore loops. GRIN2D mediates developmental and epileptic encephalopathy (XiangWei et al. 2019).

NMDA receptor of Xenopus laevis (Q91977)

Glu2 AMPA receptor (GluR-2; GluR2-flop; CX614; GluA2).  The 3-d structure is known at 3.6 Å resolution.  It shows a 4-fold axis of symmetry in the transmembrane domain, and a 2-fold axis of symmetry overall, although it is a homotetramer (Sobolevsky et al. 2009). A structure showing an agoniar-bound form of the rat GluA2 receptor revealed conformational changes that occur during gating (Yelshanskaya et al. 2014). GluR2 interacts directly with β3 integrin (Pozo et al., 2012).  In general, integrin receptors form macromolelcular complexes with ion channels (Becchetti et al. 2010).  TARPS are required for AMP receptor function and trafficking, but seven other auxiliary subunits have also been identified (Sumioka 2013). For example, AMPA receptors are regulated by S-SCAM through TARPs (Danielson et al. 2012).  The C-terminal domains of various TARPs (TC#8.A.16.2) play direct roles in the regulation of GluRs (Sager et al. 2011).  Whole-genome analyses have linked multiple TARP loci to childhood epilepsy, schizophrenia and bipolar disorders (Kato et al. 2010). Thus, TARPs emerge as vital components of excitatory synapses that participate both in signal transduction and in neuropsychiatric disorders. The architecture of a fully occupied GluR2-TARP complex has been illucidated by cryoEM, showing the homomeric GluA2 AMPA receptor saturated with TARP Υ2 subunits, showing how the TARPs are arranged with four-fold symmetry around the ion channel domain, making extensive interactions with the M1, M2 and M4 TMSs (Zhao et al. 2016). The binding mode and sites for prototypical negative allosteric modulators at the GluA2 AMPA receptor revealing new details of the molecular basis of molulator binding and mechanisms of action (Stenum-Berg et al. 2019). Drug effects, regulatory protein modulators and positive allosteric modulators have been reviewed (Fu et al. 2019). TARP γ2 converts the desensitized state to the high-conductance state which exhibits tighter coupling between subunits in the extracellular parts of the receptor (Carrillo et al. 2020).

GluR-2 of Homo sapiens (P42262) Drug effects, regulatory protein modulators and positive allosteric modulators have been reviewed (Fu et al. 2019).

Ionotropic receptor 25a, Ir25a.  Not involved in salt sensing (Zhang et al. 2013).  It resets the circadian clock in response to temperature (Chen et al. 2015).

Ir25a of Drosophila melanogaster

Glutamate ionotropic receptor homologue

Glutamate receptor in Daphnia pulex (water flea)

Olfactory ionotropic receptor, Ir93a of 842 aas

Ir93a of Panulirus argus (spiny lobster)

Ionotropic sodium channel; attractive, sodium gustatory sensory receptor for positive salt taste.  Not involved in salt avoidance which uses a distinct receptor (Zhang et al. 2013). 

Ir76b of Drosophila melanogaster

Calcium channel of 551 aas, Glr1 (Wheeler and Brownlee 2008).

Glr1 of Chlamydomonas reinhardtii

Olfactory glutamate-like ionotropic receptor, kainate 2 isoform X1 of 754 aas and 4 TMSs. Chen et al. 2017 identify 102 putative IR genes, (dubbed AalbIr genes) in the mosquito Aedes albopictus (Skuse), 19 of which showed expression in the female antenna. These putative olfactory AalbIRs share four conservative hydrophobic domains similar to the transmembrane and ion channel pore regions found in conventional iGluRs. To determine their potential functions in host-seeking, Chen et al. 2017 compared their transcript expression levels in the antennae of blood-fed females with that of non-blood-fed females. Three AalbIr genes showed downregulation when the mosquito finished a bloodmeal.

Olfactory receptor of Aedes albopictus (Asian tiger mosquito) (Stegomyia albopicta)

Glutamate receptor 4, GIC, AMPA-subtype, GluR4, GRIA4 or GluR-D (preferentially monovalent cation selective). Binding of the excitatory neurotransmitter, L-glutamate, induces a conformation change, leading to the opening of the cation channel, thereby converting the chemical signal to an electrical impulse. The receptor then desensitizes rapidly and enters a transient inactive state, characterized by the presence of bound agonist. In the presence of CACNG4, CACNG7 or CACNG8, GluR4 shows resensitization characterized by a delayed accumulation of current flux upon continued application of glutamate (Gill et al. 2008; Birdsey-Benson et al. 2010). De novo variants in GRIA4 lead to intellectual disability with or without seizures, gait abnormalities, problems of social behavior, and other variable features (Martin et al. 2017).

GluR-D of Rattus norvegicus

Glutamate receptor 1, GluR1; Glr-1 of 962 aas and 5 TMSs.  Plays a role in controlling movement in response to environmental cues such as food availability and mechanosensory stimulation such as the nose touch response (Campbell et al. 2016). Regluated by SOL1 (TC# 8.A.47.2.1) (Walker et al. 2006).

Glr-1 of Caenorhabditis elegans

NMDA-like glutamate receptor, NR1, of 964 aas and 4 TMSs.  It functions in the organization of feeding, locomotory and defensive behaviors. Two are present, NR1-1 and NR1-2 in nurrons (Ha et al. 2006).

NR1 of Aplysia californica (California sea hare)

Ionotropic glutamate receptor, GluR1 (GluR-1, GluR1-flip; GRIA1; GluH1; CTZ) of 906 aas and 4 - 6 TMSs. L-glutamate acts as an excitatory neurotransmitter at many synapses in the central nervous system. Binding of the excitatory neurotransmitter, L-glutamate, induces a conformational change, leading to the opening of the cation-specific channel, thereby converting the chemical signal to an electrical impulse upon entry of Na⁺ and Ca²⁺. The receptor then desensitizes rapidly and enters a transient inactive state, characterized by the presence of bound agonist. In the presence of CACNG4 or CACNG7 or CACNG8, it shows resensitization characterized by a delayed accumulation of current flux upon continued application of glutamate (Kato et al. 2010). The polyamines, spermine, spermidine and putrescine can be drawn into the permeation pathway and get stuck, blocking the movement of other ions. The degree of this polyamine-mediated channel block is highly regulated by processes that control the free cytoplasmic polyamine concentration, the membrane potential, and the iGluR subunit composition (Bowie 2018).

GluR-1 of Homo sapiens

Glutamate-gated receptor 3.6 of 903 aas, GLR3.6.  It probably acts as non-selective cation channel, transporting Ca²⁺ into the cell. It mediates leaf-to-leaf wound signaling. GLR3/6 may be involved in light-signal transduction and calcium homeostasis via the regulation of calcium influx into cells (Mousavi et al. 2013). Together with GLR3.3 (TC# 1.A.10.1.10), it plays a roles in nervous system-like signaling in plant defense. GLR3.3 and GLR3.6 play different roles by a mechanism that differs substantially from that in animals (Toyota et al. 2018).

GLR3.6 of Arabidopsis thaliana

NMDA receptor subtype 1, NMDAR1, of glutamate-gated ion channels with high calcium permeability and voltage-dependent sensitivity to magnesium. This protein plays a key role in synaptic plasticity, synaptogenesis, excitotoxicity, memory acquisition and learning. It mediates neuronal functions in glutamate neurotransmission and is involved in cell surface targeting of NMDA receptors. It plays a role in associative learning and in long-term memory consolidation (Xia et al. 2005). F654A and K558Q mutations affect ethanol-induced behaviors in Drosophila.(Troutwine et al. 2019).

NMDAR of Drosophila melanogaster (Fruit fly)

GIC, NMDA-subtype, Grin C2 (highly permeable to Ca²⁺ and monovalent cations). A single residue in the GluN2 subunit controls NMDA receptor channel properties via intersubunit interactions (Retchless et al., 2012). Memantine (Namenda) is prescribed as a treatment for moderate to severe Alzheimer's Disease. Memantine functions by blocking the NMDA receptor, and the sites of interaction have been identified (Limapichat et al. 2013).  Genetic mutations in multiple NMDAR subunits cause various childhood epilepsy syndromes (Li et al. 2016). NMDA receptor gating is complex, exhibiting multiple closed, open, and desensitized states, but the structure-energy landscape of gating for the rat homologue has been mapped (Dolino et al. 2017). NMDARs are tetrameric complexes consisting of two glycine-binding GluN1 and two glutamate-binding GluN2 subunits. Four GluN2 subunits encoded by different genes can produce up to ten different di- and triheteromeric receptors.  These heteromeric systems have been modeled (Gibb et al. 2018). A conserved glycine associated with diseases permits NMDA receptors to acquire high Ca²⁺ permeability (Amin et al. 2018). The ND2 protein (see TC# 3.D.1.6.1), a component of the NMDAR complex, enables Src tyrosine protein kinase (TC# 8.A.23.1.12) regulation of NMDA receptors (Scanlon et al. 2017). Drug effects, regulatory protein modulators and positive allosteric modulators have been reviewed (Fu et al. 2019).

NMDA receptor, Grin C2, of Homo sapiens

AMPA glutamate receptor 3 (GluR3, GluA3. GRIA3. LLUR3. GLURC) (non-selective monovalent cation channel and Ca²⁺  channel) (Ayalon et al., 2005; Midgett et al., 2012). Regulated by AMPA receptor regulatory proteins (TARPs) including stargazin and CNIH auxiliary subunits (Kim et al., 2010; Straub and Tomita, 2011; Jackson and Nicoll, 2011; Bats et al., 2012; Rigby et al. 2015). The domain architecture of a calcium-permeable AMPA receptor in a ligand-free conformation has been solved (Midgett et al., 2012). The TARP, stargazin, is elevated in the somatosensory cortex of Genetic Absence Epilepsy Rats (Kennard et al. 2011). TARPs alter the conformation of pore-forming subunits and thereby affect antagonist interactions (Cokić and Stein 2008).  The structural basis of AMPAR regulation by TARP gamma2, or stargazin (STZ) involves variable interaction stoichiometries of the AMPAR-TARP complex, with one or two TARP molecules binding one tetrameric AMPAR (Twomey et al. 2016).  The ion channel extracellular collar plays a role in gating and represents a hub for powerful allosteric modulation of AMPA receptor function (Yelshanskaya et al. 2017). The A653T mutation stabilizes the closed configuration of the channel and affects duration of sleep and awake periods in both humans and mice (Davies et al. 2017). The tetramer exhibits 4 distinct conductase leves due to independent subunit activation.  Perampanel is an anticonvulsant drug that regulates gating (Yuan et al. 2019).

GluR3 of Homo sapiens (P42263)

The homomeric cation channel/glutamate receptor/kainate 1, GluR5 (weakly responsive to glutamate) (expressed in the developing nervous system) (Bettler et al., 1990).  The 3-d structures of the protein have been determined with agonists and antagonists.  The agonist, domoic acid, stabilizes the ligand-binding core of the iGluR5 complex in a conformation that is 11 degrees more open than the conformation observed when the full agonist, (S)-glutamate, is bound (Hald et al. 2007). Kainate receptors regulate KCC2 expression in the hippocampus (Pressey et al. 2017).

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GluR5 of Rattus norvegicus
(P22756)

The heteromeric monovalent cation/Ca²⁺ channel/glutamate (NMDA) receptor NMDAR1/NMDAR2A/NMDAR2B/NMDAR2C) (Monyer et al., 1992). Note: NR2B is the same as NR3, GluN2A, GRIN2A or subunit epsilon (Schüler et al., 2008). Mediates voltage- and Mg²⁺-dependent control of Na⁺ and Ca²⁺ permeability (Yang et al., 2010).  Mutations in the subunit, GRIN1, a 1464 aa protein, identified in patients with early-onset epileptic encephalopathy and profound developmental delay, are located in the transmembrane domain and the linker region between the ligand-binding and transmembrane domains (Yuan et al. 2014; Ohba et al. 2015).  Karakas and Furukawa 2014 determined the crystal structure of the heterotetrameric GluN1-GluN2B NMDA receptor ion channel at 4 Å resolution. The receptor is arranged as a dimer of GluN1-GluN2B heterodimers with the twofold symmetry axis running through the entire molecule composed of an amino terminal domain, a ligand-binding domain, and a transmembrane domain.  The GluN2 subunit regulates synaptic trafficking of AMPA in the neonatal mouse brain (Hamada et al. 2014).  GRIN1 and GRIN2A mutations are associated with severe intellectual disability with cortical visual impairment, epilepsy and oculomotor and movement disorders being discriminating phenotypic features (Lemke et al. 2016; Chen et al. 2017).The cryoEM structure of a triheteromeric receptor including GluN1 (glycine binding), GluN2A and GluN2B (both glutamate binding)has been solved with and without a GluN2B the allosteric antagonist, Ro 25-6981 (Lü et al. 2017). Ogden et al. 2017 implicated the pre-M1 region in gating, provided insight into how different subunits contribute to gating, and suggested that mutations in the pre-M1 helix, such as those that cause epilepsy and developmental delays, can compromise neuronal health. The severity of GRIN2A (Glu2A)-related disorders can be predicted based on the positions of the mutations in the encoding gene (Strehlow et al. 2019). Knock-in mice expressing an ethanol-resistant GluN2A NMDA receptor subunit show altered responses to ethanol (Zamudio et al. 2019).

NR1/NR2A or NR2B or NR2C of Rattus norvegicus
NR1 (Q05586)
NR2A (O08948)
NR2B (Q00960)
NR2C (Q62644)

The mouse glutamate receptor δ-2 subunit precursor (GluR δ-2, GluR delta subunit, or GluD2) (Uemura et al., 2004).  The 3-d structure in the synaptic junctional complex with presynaptic β-neurexin 1 (β-NRX1 or NRXN1A; Q9ULB1 = the human homologue) and the C1q-like synaptic organizer, cerebellin-1 (Cbln1; 193 aas, 1 or 2 TMSs; Q9R171 = the human homolgue) has been solved (Elegheert et al. 2016).

GluR δ2 of Mus musculus (Q61625)

Glutamate-gated ionotropic K⁺ channel receptor, GluR0 (5TMSs). X-ray structures are available (PDB: 1IIT) (Lee et al. 2005; Lee et al. 2008)  GluR6 (TC# 1.A.10.1.11), carrying the pore loop plus adjacent transmembrane domains of this prokaryotic, glutamate-gated, K⁺-selective GluR0, adopted several electrophysiological properties of the donor pore upon pore transplantation (Hoffmann et al. 2006).

GluR0 of Synechocystis sp. PCC6803

Probable Ionotropic glutamate receptor (GluR)

GluR homologue of Algoriphagus sp. PR1 (A3I049)

Probably Ionotropic glutamate receptor (GluR) 

GluR homologue of Chlorobium luteolum (Q3B5G3)

Probable Ionotropic glutamate receptor (GluR)

GluR homologue of Vibrio fischeri (B5FDH7)

Uncharacterized protein of 1003 aas and 5 - 7 TMSs

UP of Chlamydomonas reinhardtii (Chlamydomonas smithii)

Ionotropic ligand (glutamate) receptor of 433 aas and 3 or 4 TMSs (Greiner et al. 2018). 

GluR of Paramecium bursaria Chlorella virus IL-3A

The Wongabel virus U5 protein (putative viroporin) of 127 aas and 1 or 2 TMSs.

U5 of Wongabel virus

Putative viroporin of 118 aas and 1 or 2TMSs

Putative Viroporin of Joinjakaka virus

Putative viroporin of 90 aas and 1 TMS

Putative viroporin of Kotonkan virus

Pex11 of 236 aas and possibly 3 TMSs (Mindthoff et al. 2015).

Pex11 of Saccharomyces cerevisiae

Pex11 of 247 aas.

Pex11 of Homo sapiens

Pex11 of 248 aas

Pex11 of Arabidopsis thaliana

Pex11 of 222 aas

Pex11 of Leishmainia major

Pex11 of 234 aas

Pex 11 of Drosophila melanogaster (Fruit fly)

Pex11C-like protein of 199 aas

Pex11C-like protein of Mombyx mori

Uncharacterized glycosomal protein of 220 aas

UP of Leishmania major

Putative Pex11 of 355 aas

Pex11 homologue of Aspergillus niger

Putative Pex11 homologue of 315 aas

Pex11 homologue of Cryptococcus neoformans (Filobasidiella neoformans)

Uncharacterized protein of 327 aas

UP of Bipolaris oryzae

Pex11 homologue of 290 aas

Pex11 homologue of Sphaerulina musiva (Poplar stem canker fungus) (Septoria musiva)

Uncharacterized protein of 188 aas

UP of Aphanomyces astaci

Uncharacterized protein of 317 aas

UP of Wallemia mellicola (Wallemia sebi

Uncharacterized protein of 410 aas

UP of Serpula lacrymans (Dry rot fungus)

Uncharacterized protein of 334 aas

UP of Rhodosporidium toruloides (Yeast) (Rhodotorula gracilis)

Uncharacterized protein of 228 aas

UP of Mucor circinelloides (Mucormycosis agent) (Calyptromyces circinelloides)

Uncharacterized glycosomal protein of 225 aas

UP of Leishmania braziliensis

Uncharacterized protein of 253 aas

UP of Leishmania braziliensis

Uncharacterized protein of 247 aas

UP of Trypanosoma cruzi

Uncharacterized protein of 252 aas

UP of Emiliania huxleyi

Uncharacterized protein of 252 aas

UP of Saprolegnia diclina

Uncharacterized protein of 421 aas

UP of Emiliania huxleyi

Pex11 of 240 aas

Pex11 of Tetrahymena thermophila

Uncharacterized protein of 289 aas

UP of Paramecium tetraurelia

Pex11 domain containing protein of 235 aas

Pex11 of Oxytricha trifallax

Peroxin Pex11 of 243 aas

Pex11 of Physcomitrella patens (Moss)

Uncharacterized protein of 244 aas

UP of Kazachstania africana (Yeast) (Kluyveromyces africanus)

Pex25 of 294 aas

Pex25 of Saccharomyces cerevisiae

Pex27 of 376 aas and 2 predicted TMSs.

Pex27 of Saccharomyces cerevisiae

Uncharacterized protein of 206 aas

UP of Thalassiosira oceanica (Marine diatom)

Uncharacterized protein of 360 aas

UP of Phaeodactylum tricornutum

Uncharacterized protein of 252 aas

UP of Pythium ultimum

The PB1-F2 protein of 90 aas, and possibly a weakly hydrophobic C-terminal TMS, exhibits viroporin activity, transporting monovalent cations and Ca²⁺ (Hyser and Estes 2015).

PB1-F2 of Influenza virus A

Influenza A virus PB1-F2 protein of 57 aas

PB1-F2 of INfluenza A virus

PB1-F2 protein of 57 aas and 0 TMSs

PB1-F2 of Influenza A virus (A/chicken/Dongguan)

The Parainfluenza Virus 5 (Simian Virus 5) viroporin, SH, of 44 aas and 1 C-terminal TMS. Transports monovalent cations (Hyser and Estes 2015).

SH of Parainfluenza Virus 5 (Simian Virus 5)

SH protein of 44 aas and 1 C-terminal TMS. Transports monovalent cations (Hyser and Estes 2015).

SH of Parainfluenza Virus 5

The FlhA flagellar biosyththesis energizer Na⁺ channel of 692 aas and 8 TMSs.  Evidence for the Na⁺ channel activity of FlhA has been presented (Minamino et al. 2016).

FlhA of E. coli

The mixed lineage kinase domain-like (MLKL) protein of 471 aas and 5 TMSs; forms Mg²⁺-selective channels in the presence of Na⁺ and K⁺ (Xia et al. 2016).

MLKL of Homo sapiens

Mixed lineage kinase domain-like protein, MLKL, of 711 aas and 4 N-terminal TMSs.

MLKL of Balaenoptera acutorostrata scammoni (whale)

Mixed lineage kinase domain-like protein, MLKL, isoform X1 of 503 aas.

MLKL of Acanthaster planci

The Transmembrane and coiled-coil domains protein 1 (TMCO1, TMCC4, PNAS-10, PNAS-136, UNQ155) of 188 aas and 3 TMSs. It is an ER transmembrane protein that actively prevents Ca²⁺ stores from overfilling, acting as a ""Cacium Load-activated Calcium channel"" or ""CLAC"" channel. TMCO1 undergoes reversible homotetramerization in response to ER Ca²⁺ overloading and disassembly upon Ca²⁺ depletion to form a Ca²⁺-selective ion channel as demonstrated in liposomes (Wang et al. 2016). TMCO1 knockout mice reproduce the main clinical features of human cerebrofaciothoracic (CFT) dysplasia spectrum, a developmental disorder linked to TMCO1 dysfunction, and exhibit severe mishandling of ER Ca²⁺ in cells. Thus, TMCO1 provides a protective mechanism to prevent overfilling of ER stores with calcium ions (Wang et al. 2016). It regulates Ca²⁺ stores in granulosa cells (Sun et al. 2018).  TMCO1-mediated Ca²⁺ leak underlies osteoblast functions via CaMKII signaling (Li et al. 2019).

TMCO1, a CLAC channel of Homo sapiens

Uncharacterized CLAC channel of 176 aas and 3 TMSs.  It appears to be a calcium-selective channel required to prevent calcium stores from overfilling.

CLAC channel of Entamoeba histolytica

TMCO1 of 183 aas and 3 TMSs

TMCo1 of Hydra vulgaris (Hydra) (Hydra attenuata)

TMCO1 or Anon-37B-2 of 177 aas and 3 TMSs.

TMCO1 of Drosophila melanogaster (Fruit fly)

TMCO1 of 177 aas and 3 TMSs

TMCO1 of Schistosoma japonicum (Blood fluke)

TMCO1 homologue of 201 aas and 3 TMSs

TMCO1 homologue of Toxoplasma gondii

Uncharacterized protein of 199 aas and 3 TMSs

UP of Zea mays (Maize)

Uncharacterized protein of 219 aas and 3 TMSs

UP of Eimeria tenella (Coccidian parasite)

Uncharacterized protein of 196 aas and 3 TMSs

UP of Arabidopsis thaliana

Uncharacterized TMCO1 homologue of 192 aas and 3 TMSs.

UP of Chlamydomonas reinhardtii

TMCO1 homologue of 167 aas and 3 TMSs

TMCO1 homologue of Korarchaeum cryptofilum

Uncharacterized protein of 174 aas and 3 TM

UP of Thermococcus nautili

Uncharacterized protein of 191 aas and 3 TMSs.

UP of Methanobrevibacter smithii

Uncharacterized protein of 301 aas and 4 TMSs

UP of Halorubrum saccharovorum

Uncharactized protein of 193 aas and 3 or 4 TMSs

UP of Ferroglobus placidus

Pore-forming Hemoglobin-α of 142 aas (Morrill and Kostellow 2016).

Hemoglobin-α of Homo sapiens

Pore-forming hemoglobin-β of 147 aas (Morrill and Kostellow 2016).

Hemoglobin-β of Homo sapiens

Pore-forming myoglobin of 154 aas (Morrill and Kostellow 2016).

Myoglobin of Homo sapiens

Pore-forming neuroglobin of 151 aas (Morrill and Kostellow 2016).

Neuroglobin of Homo sapiens

Pore-forming cytoglobin of 154 aas (Morrill and Kostellow 2016).

Cytoglobin of Homo sapiens

Fibroblast growth factor 2, FGF2, of 288 aas.  Forms oligomeric pores in the plasma membrane, and these are involved in its secretion to the external medium where it exerts its action (La Venuta et al. 2016; see family description). The ATP1A1 (TC# 3.A.3.1.1) serves as the receptor for membrane insertion (Zacherl et al. 2015).

FGF2 of Homo sapiens

Fibroblast growth factor 23, FGF23 of 251 aas.  Functions through FGF receptor 4 to regulate the level of the calcium/inorganic cation channel, TrpC6 or TRP6 (TC# 1.A.4.1.5) (Smith et al. 2017).

FGF23 of Homo sapiens

Interleukin 1β, IL1β, IL1beta, IL1B, IL1F2, of 269 aas.  Its release depends on its insertioin into the membrane with the formation of a transmembrane pore (Martín-Sánchez et al. 2016).

IL1beta of Homo sapiens

Ammonia transporter and regulatory sensor, AmtB (Blauwkamp and Ninfa, 2003; Khademi et al., 2004).  It has a cleavable N-terminal signal peptide, and while Amt proteins in Gram-negative bacteria appear to utilize a signal peptide, the homologous proteins in Gram-positive organisms do not (Thornton et al. 2006).

AmtB of E. coli (P69681)

AMT of 514 aas and 11 TMSs. Trypanosoma cruzi, the etiologic agent of Chagas disease, undergoes drastic metabolic changes when it transits between a vector and mammalian hosts. Amino acid catabolism leads to the production of NH₄⁺, which must be detoxified. Cruz-Bustos et al. 2018 identified an intracellular ammonium transporter of T. cruzi (TcAMT) that localizes to acidic compartments (reservosomes, lysosomes). TcAMT possesses all conserved and functionally important residues that form the pore in other ammonium transporters. Functional expression in Xenopus oocytes followed by a two-electrode voltage clamp showed an inward current that is NH₄⁺ dependent at a resting membrane potential lower than -120 mV and is not pH dependent, suggesting that TcAMT is an NH₄⁺or NH3/H⁺ transporter. Ablation of TcAMT resulted in defects in epimastigote and amastigote replication, differentiation, and resistance to starvation and osmotic stress (Cruz-Bustos et al. 2018). 

Amt of Trypanosoma cruzi

High-affinity electrogenic ammonia/methylammonia transporter (allosterically activated by the C-terminus (Loqué et al., 2009).  NH_4⁺ is stable in the AmtB pore, reaching a binding site from which it can spontaneously transfer a proton to a pore-lining histidine residue (His168). The substrate diffuses down the pore in the form of NH₃, while the proton is cotransported through a highly conserved hydrogen-bonded His168-His318 pair (Wang et al. 2012).

Amt1 of Arabidopsis thaliana (P54144)

Ammonium transporter, AmtB or Amt1 of 463 aas and 9 TMSs.  Regulated by direct interaction with GlnK (Pedro-Roig et al. 2013).

AmtB of Haloferax mediterranei (Halobacterium mediterranei)

Ammonium uptake transporter, Amt1 of 458 aas and 11 TMSs.  62% identical to Amt1 of Pyropia yezoensis (Rhodophyta) which is 483 aas long with 11 TMSs and is induced by nitrogen deficiency (Kakinuma et al. 2016).

Amt1 of Chondrus crispus (Carrageen Irish moss) (Polymorpha crispa)

High affinity (~50 mμM) ammonium transporter, Amt1.3 of 498 aas and 10 TMSs (Loqué et al. 2006). The tobacco orthologue, of 464 aas and 10 TMSs, NtAMT1.3, is present in roots and leaves and faciltates NH₄⁺ entry. It is up regulated upon nitrogen starvation (Fan et al. 2017).

Ant1.3 of Arabidopsis thaliana (Mouse-ear cress)

Putative ammonia/ammonium transporter of 439 aas and 11 TMSs.

NH₃ transporter of Ostreococcus tauri virus RT-2011

Ammonia-specific uptake carrier, Amt2. For AMT2 from Arabidopsis thaliana NH₄⁺ is the recruited substrate, but the uncharged form NH₃ is conducted.  AtAMT2 partially co-localizes with electrogenic AMTs and conducts methylamine with low affinity (Neuhäuser et al., 2009). This may explain the different capacities of AMTs to accumulate ammonium in the plant cell.

Amt2 of Arabidopsis thaliana

Ammonium/methyl ammonium uptake permease, AmtB (may need AmtB to concentrate [¹⁴C]methyl ammonium (Paz-Yepes et al., 2007))

AmtB of Synechococcus sp CC9311 (Q0IDE4)

Amt1;4 of Arabidopsis thaliana (Q9SVT8)

Amt2 NH₄⁺/CH₃-NH₃⁺ transporter, subject to allosteric activation by a C-terminal region (Loqué et al., 2009).

Amt2 of Archaeoglobus fulgidus (O28528)

Amt1;1, a proposed NH₄⁺:H⁺ sumporter (Ortiz-Ramirez et al., 2011)

Amt1;1 of Phaseolus vulgaris (E2CWJ2)

Ammonium transporter 2, AmtB

AmtB of Dictyostelium discoideum

Mep2 of Candida albicans (Q59UP8)

Rhesus (Rh) type C glycoprotein NH₃/NH₄⁺ transporter, RhCG (also called tumor-related protein DRC2) (Bakouh et al., 2004; Worrell et al., 2007). Zidi-Yahiaoui et al. (2009) have described characteristics of the pore/vestibule. The structure is known to 2.1 Å resolution (Gruswitz et al., 2010). Each monomer contains 12 transmembrane helices, one more than in the bacterial homologs. Reconstituted into proteoliposomes, RhCG conducts NH₃ to raise the internal pH. Models of the erythrocyte Rh complex based on the RhCG structure suggest that the erythrocytic Rh complex is composed of stochastically assembled heterotrimers of RhAG, RhD, and RhCE (Gruswitz et al., 2010).

RhCG of Homo sapiens (Q9UBD6)

RH (Rhesus) antigen-related protein, Rhr-1 or Rh1, of 463 aas and 12 TMSs.  CeRh1 is abundantly expressed in all developmental stages of C. elegans, with highest levels in adults, whereas CeRh2 shows a differential and much lower expression pattern. It is required for passage throung the late stages of C. elegans embryonic development and hypodermal function (Ji et al. 2006).

Rhr-1 of Caenorhabditis elegans

Rh protein of 478 aas and 11 TMSs. It is a primary contributor to ammonia excretion, and poor expression changes the expression levels of many enzymes (Si et al. 2018).

Rh protein of Portunus trituberculatus (the swimming crab)

Rhesus (Rh) type B glycoprotein NH₃/NH₄⁺ transporter, RhBG (~50% identical to type C) (Lopez et al., 2005; Worrell et al., 2008). Electrogenic NH₄⁺ transport is stimulated by alkaline pH_(out) but inhibited by acidic pH_(out) (Nakhoul et al., 2010). Regulated by Wnt/β-catenin signalling, a pathway frequently deregulated in many cancers and associated with tumorigenesis (Merhi et al. 2015).

RhBG of Homo sapiens (Q9H310)

Rhesus (Rh) complex (tetramer: RhAG₂, RhCE₁, RhD₁) of 409 aas and 12 TMSs. Exports ammonia from human red blood cells (Conroy et al., 2005). RhAG is also called RH50.  RhAG variants (I61R, F65S), associated with overhydrated hereditary stomatocytosis (OHSt), a disease affecting erythrocytes, are alterred for bidirectional ammonium transport (Deschuyteneer et al. 2013).  The system transports ammonia, methylammonia, ethylammonia and fluoroethylamine.  ¹⁹F-fluoroethylamine has been used to study rapid transport as its NMR spectra are different inside and outside of human red blook cells (Szekely et al. 2006).

The RhAG/RhCE/RhD, complex of Homo sapiens
RhAG (Q02094)
RhCE (P18577)
RhD (Q02161)

The RH50 NH₃ channel (most like human Rh proteins TC# 1.A.11.4.1 and 2; 36-38% identity) (Cherif-Zahar et al., 2007). The Rh CO₂ channel protein (3-D structure ± CO₂ available) (3B9Z_A; 3B9Y_A) (Li et al., 2007; Lupo et al., 2007) (also transports methyl ammonia) (Weidinger et al., 2007).

RH50 of Nitrosomonas europaea (Q82X47)

Kidney rhesus glycoprotein p2 (Rhp 2). Transports NH₃ and methylammonium (Nakada et al., 2010).

Rhp2 of Triakis scyllium (D0VX38)

Rhesus-like glycoprotein A (Rh50-like protein RhgA)

RhgA of Dictyostelium discoideum

Ammonium transporter of 391 aas and 12 TMSs.  Shows limited seqences similarity with 9.B.124.1.7 (e^-5) (residues 1-5 align with residues 4 - 8 in 9.B.124.1.7).

Ammonium transporter of [Clostridium] papyrosolvens

NH₃ (NH₄⁺) transporting Rhesus glycoprotein, Rhag, of 437 aas and 11 TMSs.  Induced by ammonia exposure in the apical membrane of gill epithelia (Chen et al. 2017).

Rhag of Anabas testudineus (climbing perch)

NH₃ (NH₄⁺) transporting Rhesus glycoprotein, Rhcg2, of 482 aas and 11 TMSs.  Induced by ammonia exposure in the basolateral membrane of gill epithelia (Chen et al. 2017).

Rhcg2 of Anabas testudineus (climbing perch)

OTOP1 of 600 aas and 12 TMSs in a 5 + 5 + 2 arrangement.  OTOP1 is a proton-selective ion channel enriched in acid-detecting taste receptor cells and is required for their zinc-sensitive proton conductance (Tu et al. 2018). Two related murine genes, Otop2 and Otop3, and a Drosophila ortholog also encode proton channels. Evolutionary conservation of the gene family and its widespread tissue distribution suggest a broad role for proton channels in physiology and pathophysiology (Tu et al. 2018).

OTOP1 of Mus musculus

Otop3 of 600 aas and 12 TMSs.  The high resolution 3-d structure has been determined by cryoEM (Saotome et al. 2019) (see family description).

Otop3 of Gallus gallus (chicken)

OTOP1 H⁺ channel of 612 aas and 10 TMSs (Tu et al. 2018).

OTOP1 of Homo sapiens

OTOP2 H⁺ channel of 562 aas and 10 TMSs in a 5 + 5 + 2 arrangement (Tu et al. 2018).

OTOP2 of Homo sapiens

OTOP3 of 596 aas and 10 TMSs in a 5 + 5 + 2 TMS arrangement (Tu et al. 2018).

OTOP3 of Homo sapiens

Zinc-sensitive proton channel, OTOP1, of 586 aas and 10 TMSs in a 5 + 5 + 2 TMS arrangement (Tu et al. 2018). It inhibits P2Y purinoceptors and modulates calcium homeostasis and influx of calcium in response to extracellular ATP. It is essential for the formation of otoliths in the inner ear of developing larvae and for the perception of gravity and acceleration (Söllner et al. 2004; Hughes et al. 2004). The 3-d structure has been determined by cryoEM (Saotome et al. 2019) (see family description).

OTOP1 of Danio rerio (Zebrafish) (Brachydanio rerio)

OtoPetrin-like (Otpl6) of 581 aas and 12 TMSs.

Otpl6 of Caenorhabditis elegans

OToPetrin-like protein, isoform D, of 647 aas and 12 or 13 TMSs in a 5 +7 or 8 TMS arrangement.

OTOP, isoform D of Drosophila melanogaster (Fruit fly)

Putative otopetrin of 747 aas and 11 or 12 TMSs in a 1 + 3 + 7 or 8 TMS arrangement.

Putative Otop protein of Schistosoma mansoni (Blood fluke)

The OTOP3 protopn channel protein of 681 aas and 12 TMSs. The cryo-EM structure along with functional characteristics have been described (Chen et al. 2019). XtOTOP3 forms a homodimer with each subunit containing 12 transmembrane helices that can be divided into two structurally homologous halves; each half assembles as an alpha-helical barrel that could serve as a proton conduction pore.

OTOP3 of Xenopus tropicalis

Uncharacterized protein of 165 aas and 2 TMSs.

UP of Theileria annulata

Uucharacterized putative reactive oxygen species modulator 1 protein of 168 aas and 2 C-terminal TMSs.

UP of Plasmodium falciparum

Magnesium and cobalt efflux protein, CorC or MpfA (Magnesium Protection Factor A) of 449 aas and 4 N-terminal TMSs, apparently in a 2 + 2 TMS arrangement. Evidence has been presented that this protein catalyzes active Mg²⁺ extrusion from the cell (Armitano et al. 2016).  If so, It must be an active transporter, either a secondary carrier (TC subclass 2.A) or an ATP hydrolysis-driven exporter (TC subclass 3.A).

MpfA of  Staphylococcus aureus

Mg²⁺ and Co²⁺ transporter, CorB, of 413 aas and 3 TMSs. It contains DUF21, CBS pair, and CorC-HlyC domainsin succession.

CorB of Pseudomonas bauzanensis

HlyC/CorC family transporter of 354 aas and 4 TMSs.

CorC domain protein of Micromonospora peucetia

Uncharacterized protein of 329 aas and 4 TMSs.

UP of Verrucomicrobia bacterium]

Small integral membrane protein 18 of 111 aas and 1 TM

Protein 18 of Phascolarctos cinereus

Uncharacterized protein of 112 aas and 1 TMS.

UP of Empidonax traillii

Uncharacterized protein of 108 aas and 1 TMS.

UP of Electrophorus electricus (Electric eel) (Gymnotus electricus)

Uncharacterized protein of 109 aas and 1 TMS.

UP of Apis mellifera

Uncharacterized protein of 77 aas and 1 TMS.

UP of Anopheles gambiae (African malaria mosquito)

Uncharacterized protein of 84 aas and 1 TMS.

UP of Harpegnathos saltator (Jerdon's jumping ant)

Uncharacterized protein of 81 aas and 1 TMS

UP of Helicoverpa armigera

Uncharacterized protein of 97 aas and 1 TMS

UP of Nasonia vitripennis (Parasitic wasp)

Uncharacteerized protein of 84 aas and 1 TMS

UP of Nicrophorus vespilloides (Boreal carrion beetle)

Uncharacterized protein of 87 aas and 1 TMS

UP of Laodelphax striatella (small brown planthopper)

Uncharacterized protein of 192 aas and 2 putative TMSs.

UP of Folsomia candida (Springtail)

Plasma membrane proton-activated chloride channel, PACC1, or acid-sensitive outwardly-rectifying anion channel PAORAC/ASOR or TMEM206, of 350 aas and 2 TMSs.  Ion permeation-changing mutations along the length of TMS2 and at the end of TMS1 suggest that these segments line the pore. TMEM206 probably has orthologs in all vertebrates (Ullrich et al. 2019).  Knockout of mouse Pac abolished I _Cl,H in neurons and attenuated brain damage after ischemic stroke (Yang et al. 2019).

PACC1 of Homo sapiens

Proton-activated chloride channel of 298 aas and 2 TMSs. It mediates import of chloride ion in response to extracellular acidic pH (Yang et al. 2019) and displays channel activity with kinetic properties distinct from that of the human ortholog.

TMEM206 of Danio rerio (Zebrafish) (Brachydanio rerio)

TMEM206 of 469 aas and 2 TMSs.

TMEM206 of Amphimedon queenslandica

TMEM206-like protein of 382 aas and 2 TMSs.

TMEM206 of Saccoglossus kowalevskii

TMEM206 of 254 aas and 2 TMSs.

TMEM206 of Callorhinchus milii

The pore-forming NADPH-dependent 1-acyldihydroxyacetone phosphate reductase, Ayr1.  Ayr1 consists of α-helicies rather than β-strands (Krüger et al. 2017).

Ayr1 of Saccharomyces cerevisiae

Nuclear chloride channel-27, NCC27 or CLIC1 (Br^- > Cl^- > I^-) (241 aas).  CLIC1 has two charged residues, K37 and R29, in its single TMS which are important for the biophysical properties of the channel (Averaimo et al. 2013).  A putative Lys37-Trp35 cation-pi interaction stabilizes the active dimeric form of the CLIC1 TMS in membranes (Peter et al. 2013).  This channel may play a role in cancer (Peretti et al. 2014).  A positively charged motif at the C-terminus of the single TMS enhances membrane partitioning and insertion via electrostatic contacts.  It also functions as an electrostatic plug to anchor the TMS in membranes and is involved in orientating the TMS with respect to the cis and trans faces of the membrane (Peter et al. 2014).

CLIC1 or NCC27 of Homo sapiens

CLIC-5A of Homo sapiens (Q53G01)

The Janus protein, CLIC2. The 3-D structure of its water soluble form has been determined at 1.8 Å resolution (Cromer et al., 2007). CLIC2 interacts with the skeletal ryanodine receptor (RyR1) and modulates its channel activity (Meng et al., 2009).

CLIC2 of Homo sapiens (O15247)

Chloride intracellular channel protein 4, CLIC4.  Regulates the histamine H3 receptor (Maeda et al., 2008)) Discriminates poorly between anions and cations (Singh and Ashley, 2007). 76% identical to CLIC5.  May play a role in cancer (Peretti et al. 2014).

CLIC4 of Homo sapiens (Q9Y696)

Intracellular Cl^- channel-3 (CLIC3). The 3-d structure is known (3FY7). This protein is associated with pregnancy disorders (Murthi et al., 2012). 

CLIC3 of Homo sapiens (O95833)

Putative Glutathione S-transferase.  Pore formation has not been demonstrated in prokaryotes.

Probable glutathione S-transferase of Leptospira interrogans

The bacterial CLIC homologue, stringent starvation protein A, SspA (212 aas; 0 TMSs) [N-terminal Trx domain; C-terminal glutathione S-transferase (GST) domain].  May be involved in acid (Hansen et al. 2005) and sodium ion tolerance (Wu et al. 2014).

Stringent starvation protein A of E. coli (P0ACA3)

Glutathione S-transferase, YfcF of 214 aas.  Pore formation has not been demostrated.

YfcF of E. coli

Voltage-gated bovine epithelial Cl^- channel protein (Ca²⁺-activated), bEClC. In rats, two possible paralogues (rbCLCA1 and A2) are expressed in the CNS and peripheral organs (Yoon et al., 2006). CLCA1 may play a role in inflammatory airway diseases (Sala-Rabanal et al. 2015). It is called the von Willebrand factor type A, the DUF1973 protein.

EClC of Bos taurus (NP_001070824)

The Ca-activated chloride channel-6 (Lee et al., 2011).

Ca-CLC-6 of Xenopus laevis (F7IYU6)

Putative lipoprotein of 1054 aas and 1-3 TMSs.

Putative lipoprotein of Leptospira biflexa

Ca⁺²-activated Cl^- channel, CLCA1 or CACC1 or 914 aas.  CLCA1 may play a role in inflammatory airway diseases (Sala-Rabanal et al. 2015).

CLCA1 of Homo sapiens

xCLCA3; xCLCA2 of 942 aas and 7 TMSs.  xCLCA3 contains a predicted signal sequence, multiple sites of N-linked glycosylation, N-myristoylation, PKA, PKC, and casein kinase II phosphorylation sites, five putative hydrophobic segments, and the HExxH metalloprotease motif. Additionally, the transmembrane prediction server yielded a preserved N-terminal CLCA domain and a von Willebrand factor type A domain with one transmembrane domain in the C-terminal region (Lee and Jeong 2016). xCLCA3 is expressed in a number of tissues, with strong expression in the brain, colon, small intestine, lung, kidney, and spleen, and poor expression in the heart and liver. xCLCA3 may be a candidate CLCA family member as well as a metalloprotease, rather than just an ion channel accessory protein.

xCLCA3 of Xenopus laevis (African clawed frog)

Hypothetical protein, HP

HP of Oryza sativa (B8AFH9)

Sll0103

Sll0103 of Synechocystis (Q55874)

The YfbK/CaClC homologue of 575aas and 0 TMSs.  YfbK has amyloidogenic regions due to asparagine- and glutamine-rich regions which is a common feature of many known amyloid proteins. This correlates with detergent-induced denaturation resistance (Antonets et al. 2016).

YfbK of E. coli (P76481)

Von Willebrand factor type A protein, vWFA. (905 aas; 2 N-terminal and 1 C-terminal TMSs)

vWFA of Chloroflexus aurantiacus (A9WIT9)

Bacterial homologue, BatB, of mammalian Ca-CLC channels (N- and C-terminal TMSs)

BatB of Myxococcus fulvus (F8CM01)

Uncharacterized protein of 252 aas and 2-3 TMSs

UP of E. coli

The Bax Inhibitor-1, BI-1 of 311 aas and 6 TMSs.  Also called the testis-enhanced gene transcript (TEGT) protein or the transmembrane BAX inhibitor motif-containing protein 6 (TMBIM6).  It forms a pH-sensitive, cation-selective, Ca²⁺-permeable leak channel (Bultynck et al., 2011; Chang et al. 2014).  Residues that contribute to the ion-conducting pore and affect apoptosis, cell adhesion and migration independently have been identified (Carrara et al. 2015).

BI-1 or TEGT of Homo sapiens (P55061)

Uncharacterized protein of 304 aas and 7 TMSs.

UP of Trypanosoma cruzi marinkellei

Uncharacterized protein of 238 aas and 7 TMSs

UP of Babesia microti

Uncharacterized protein of 317 aas and 7 TMSs

UP of Emiliania huxleyi

Growth hormone-inducible membrane protein of 345 aas and 8 putative TMSs

GH-inducible membrahe protein of Anas platyrhynchos (Mallard) (Anas boschas)

Putative Bax inhibitor of 213 aas and 7 TMSs

Putative Bax inhibitor of Entamoeba histolytica

The YccA protein, an inhibitor of FtsH. May share a similar mechanism of action as BI-1 in regulation apoptsis upon prolonged secretion stress (van Stelten et al., 2009).

YccA of E. coli (P0AAC6)

The YbhL (AceP) protein. Possibly a pmf-dependent acetate uptake transporter. [¹⁴C]Acetate uptake was inhibited by CCCP as well as cold acetate, serine, α-ketoglutarate, lactate, and succinate (M. Inouye, personal communication).

YbhL of E. coli (P0AAC4)

The 7 TMS proton-sensitive Ca²⁺ leak channel, YetJ.  The activity and high resolution 3-d structure have been determined (Chang et al. 2014).

YetJ of Bacillus subtilis (O31539)

Uncharacterized protein, YbhM, of 237 aas and 7 TMSs.  The ybhM gene is adjacent to the homologous ybhL gene (TC# 1.A.14.2.2), and another  homologous gene is inbetween these two; these two or three genes could function together as a single transporter. 

YbhM of E. coli

Protein of 234 aas and 7 TMSs encoded by a gene between and homologous to YbhL and YbhM. In the Pfam family Bax1-I.

Bax1-I protein of E. coli

Uncharacterized protein of 227 aas and 7 TMSs

UP of Streptococcus sanguinis

The NMDA receptor glutamate binding chain. Also called Protein lifeguard-1, putative MAPK-activating protein PMO2, and transmembrane BAX inhibitor motif-containing protein 3 (TMBIM3, GRINA, LFG1, NMDARA1). The human orthologue is Q7Z429.  Stimulation and block by spermine involve separate binding sites and distinct mechanisms (Jin et al. 2008).

NMDA receptor glutamate binding chain of Rattus sp. (Q63863)

The Protein Lifeguard 3 (LFG3, RECS1, Tmbim1) of 311 aas and 7 TMSs. LFG3 is a multivesicular bidy regulator that protects agains non-alcoolic fatty liver by targeting the lysosomal degradation of Tlr4 (Zhao et al. 2017). TMBIM1 protects against pathological cardiac hypertrophy through promoting the lysosomal degradation of activated TLR4 (Deng et al. 2018).

LFG3 of Homo sapiens

Uncharacterized protein of 638 aas and 7 N-terminal TMSs

UP of Drosophila eugracilis

Viroporin, pUS21, of 243 aas and 8 TMSs.  It modulates calcium ion homeostasis and protects cells against apoptosis (Luganini et al. 2018).  pUS21 of HCMV constitutes a TMBIM-derived viroporin that may contribute to HCMV's overall strategy to counteract apoptosis of infected cells.

pUS21 of Human cytomegalovirus (strain Merlin) (HHV-5) (Human herpesvirus 5)

Lifeguard 2, LFG2, or the Brz-insensitive-long hypocotyl4 mutant ,BIL4,  of 239 aas and 7 TMSs. BIL4 regulates cell elongation and Brassinosteroid (BRs; plant steroid hormones) signaling, in part  via the regulation of BRI1 localization (Yamagami et al. 2017).

BIL4 of Arabidopsis thaliana

GRINA  (Lifguard 1, LFG1, NMDARA1, TMBIM3) has 371 aas and 7 C-terminal TMSs.  It is expressed in 218 organ(s) with highest expression in the right hemisphere of the cerebellum.  It is involved in ER Ca²⁺ ion homeostasis and regulates apoptosis. The expression of the pro-apoptotic protein Bax is upregulated, whereas the anti-apoptotic protein Bcl-2 is downregulated in GRINA silenced cells (Xu et al. 2018).  Grina/TMBIM3 modulates NMDA receptors and voltage-gated Ca_V2.2 Ca²⁺ channels (TC# 1.A.1.11.9) in a G-protein-like manner (Mallmann et al. 2019).

GRINA of Homo sapiens

Glutamate Receptor Gr2

Gr2 of Capsaspora owczarzaki (E9CCY6)

Golgi anti-apoptotic protein, GAAP of 237 aas.  Forms cation-selective channels; residues that contribute to ion-conduction and affect apoptosis, cell adhesion and migration independently have been identified (Carrara et al. 2015).

GAAP of Vaccinia virus, VacV (A2VCJ6)

Ionotropic glutamate receptor; N-methyl-D-aspartate-associated protein 1 (glutamate-binding).

Gr1 of Salmo salar (B5X2N0)

The BH3-only protein, Ynl205c (Büttner et al., 2011)

Ynl305c of Saccharomyces cerevisiae (P48558)

Protein lifeguard 4 (LFG4), also called Golgi anti-apoptotic protein (GAAP), Protein S1R, CGI-119, Transmembrane BAX inhibitor motif-containing protein 4, (TMBIM4) and Z-protein.  Forms cation-selective ion channels. Residues that contribute to the ion-conducting pore and affect apoptosis, cell adhesion and migration independently of each other have been identified (Carrara et al. 2015).  It's functions have been reviewed (Carrara et al. 2017).

TMBIM4 of Homo sapiens

7 TMS integral membrane protein

Uncharacterized membrane protein of Rhodopirellula baltica

Uncharacterized protein of 242 aas and 7 TMSs.

UP of Synechococcus elongatus (Anacystis nidulans R2)

Lifeguard homologue, CG3814 of 244 aas and 7 TMSs.  Knockdown of CG3814/LFG in Ddc-Gal4-expressing neurons diminishes its neuroprotective ability, and results in a shortened lifespan and loss of climbing ability, phenotypes that are improved upon overexpression of the pro-survival Buffy (M'Angale and Staveley 2016).

CG3814 of Drosophila melanogaster (Fruit fly)

Viral protein HWLF3 (342 aas; 7 TMSs)

HWLF3 of human cytomegalovirus, HHV-5 (Q03307)

Viral membrane protein US14 of 286 aas and 7 TMSs.

US14 of Panine herpesvirus 2 (Chimpanzee cytomegalovirus)

Viral US18 protein of 274 aas and 7 TMSs

US18 of Human cytomegalovirus (HHV-5) (Human herpesvirus 5)

Membrane protein US12A of 250 aas and 7 TMSs

US12A of Simian cytomegalovirus

Membrane protein US19 of 240 aas and 7 TMSs.

US19 of Human cytomegalovirus (HHV-5) (Human herpesvirus 5)

Sec62 of 274 aas and 2, 3 or 4  putative TMSs (Lyman and Schekman 1997).

Sec62 of Saccharomyces cerevisiae

Sec62 protein of 348 aas and 4 putative TMSs.

Sec62 of Paramecium tetraurelia

Translocation protein Sec62 of 276 aas and 3 or 4 putative TMSs.

Sec62 of Plasmodium vivax

Translocation protein Sec62 of 264 aas and 4 putative TMSs.

Sec62 of Arabidopsis thaliana (Mouse-ear cress)

Protein translocation protein, Sec62, of 265 aas and 2 - 4 putative TMSs.

Sec62 of Chlorella variabilis (Green alga)

NS channel protein-1 or Sec62 protein of 399aas and 2 TMSs.  Efficient secretion of small proteins in mammalian cells relies on Sec62-dependent posttranslational translocation (Lakkaraju et al. 2012).

NS channel translocation protein-1 or Sec62 of Homo sapiens

Formate uptake/efflux permease, FocA.  It catalyzes bidirectional transport, has a pentameric aquaporin-like (TC# 1.A.8) structure, and may function by a channel-type mechanism (Falke et al., 2009; Wang et al. 2009). The structure at 2.25 Å resolution has been determined (Wang et al., 2009).  The protein is encoded in an operon with pyruvate-formate lyase, PflB.  A pyruvate:formate antiport mechanism has been suggested (Moraes and Reithmeier 2012). The C-terminal 6 aas are required for formate transport, but not for homopentamer formation (Hunger et al. 2017).  The N-terminus of FocA interacts with PflB, and this interaction is essential for optimal formate translocation (Doberenz et al. 2014). In fact, the GREs, TdcE and PflB, interact with the FNT channel protein, probably to control formate translocation by FocA (Falke et al. 2016).

FocA of E. coli (P0AC23)

Probable formate transporter 2 (Formate channel 2), FocB (Andrews et al. 1997).

FocB of Escherichia coli

Formate channel, FocA. Competition of formate by Thr90 from the Ω loop may open the channel (Waight et al., 2010).

FocA of Vibrio cholerae (F9A868)

Formate-specific channel protein, FdhC of 280 aas (Nölling and Reeve 1997).

FdhC of Methanobacterium thermoformicium

Nitrite uptake porter, NitA (Unkles et al., 1991; 2011)

NitA of Aspergillus (Emericella) nidulans

Probable formate uptake permease (Wood et al., 2003).

FdhC of Methanococcus maripaludis

Nitrite uptake porter of 355 aas, Nar1.

Nar1 of Chlamydomonas reinhardtii (Chlamydomonas smithii)

Nitrite channel transporter, NirC, of 382 aas. Structure/function studies including the x-ray structure of the Salmonella orthologue have been reported (Rycovska-Blume et al. 2015).

NirC of Thermofilum pendens

Nitrite/Nitrate exporter of 476 aas, Nar1 (Cabrera et al. 2014).

Nar1 of Pichia angusta (Yeast) (Hansenula polymorpha)

FNT protein of 313 aas and 6 TMSs that transports L-lactacte (Wiechert et al. 2017).  Trophozoites are inhibited by drugs such as MMVOO7839 (Golldack et al. 2017, Hapuarachchi et al. 2017).

PfFNT of Plasmodium falciparum

Formate/nitrite (FNT) transporter of 356 aas and 6 TMSs.

FNT of Entamoeba histolytica

Nitrite uptake/efflux channel (Jia et al. 2009).

NirC of E. coli (P0AC26)

Uncharacterized transporter YwcJ

YwcJ of Bacillus subtilis

Hydrosulfide (hydrogen sulfide; HS^-), Fnt3 (Hsc) channel.  Also probably transports chloride, formate and nitrite. The 3-d crystal structure (2.2Å resolution in the closed state) is known (PDB# 3TE2) (Czyzewski and Wang, 2012). The Fnt3 gene is linked to the asrABC operon encoding the sulfite (SO₃^2-) reductase that gives HS^- as the product (Czyzewski and Wang 2012).

Hsc or Fnt3 HS^- channel of Clostridium difficile (Q186B7)

Nitrite transporter, NirC, of 268 aas and 6 TMSs (Park et al. 2008).

NirC of Klebsiella oxytoca

Formate channel of 283 aas and 6 TMSs, Fnt or FdhC.  Its function has been veritifed (Helmstetter et al. 2019).

FdhC of Bacillus thuringiensis

Inner membrane protein, YfdC (310aas; 6 TMSs).  May be involved in surfactant resistance (Nakata et al. 2010).

YfdC of E. coli (P37327)

Putative FNT transporter of 346 aas

FNT transporter of Psychrobacter arcticus

FNT homologue  of 313 aas

FNT homologue of Salinarchaeum sp. Harcht-Bsk1

FNT homologue of 230 aas

FNT homologue of Acholeplasma palmae

FNT homologue of 213 aas

FNT homologue of Acholeplasma laidlawii

The plasma membrane Ca² -activated chloride (ICl_Ca) channel, TMEM16A (Anoctamin 1a; ANO1a) (Huang et al., 2012; Chen et al. 2011). The mouse orthologue (Q8BHY3), TMEM16A (956aas), is localized to the apical membranes of epithelia as well as intracellular membranes in many cell types. Knockout mice show diminished rhythmic contraction of gastric smooth muscle (Huang et al., 2009). ANO1 is also required for normal tracheal development (Ousingsawat et al., 2009). Expression is upregulated by epidermal growth factor (Mroz and Keely, 2012). Novel 5-substituted benzyloxy-2-arylbenzofuran-3-carboxylic acids are inhibitors (Kumar et al., 2012). TMEM16A channels contribute to the myogenic response in cerebral arteries (Bulley et al., 2012). Membrane stretch activates arterial myocyte TMEM16A channels, leading to membrane depolarization and vasoconstriction. A local Ca²⁺  signal generated by nonselective cation channels stimulates TMEM16A channels to induce myogenic constriction (Bulley et al., 2012).  Ca²⁺/calmodulin activates bicarbonate (anion) transport (Jung et al. 2012).  The protein exists in the membrane as a homodimer where the cytoplasmic N-terminus functions in dimerization (Tien et al. 2013).  TMSs 5-6 of the 8 TMSs may comprise parts of the pore-loop that controls Cl^- conductance (Adomaviciene et al. 2013).  ANO1 confers ICl_Ca in retinal neurons and acts as an intrinsic regulator of the presynaptic membrane potential during synaptic transmission (Jeon et al. 2013).  TMEM16A may be a primary driver of the """"""""Grow"""""""" (tumor proliferation) or """"""""Go"""""""" (metastasis) model for cancer progression, in which TMEM16A expression acts to balance tumor proliferation and metastasis via its promoter methylation (Shiwarski et al. 2014).  Regulation of TMEM16A/16B by Ca²⁺ is mediated by preassociated apo-calmodulin (Yang et al. 2014).as well asCaMKIIδ (Gui et al. 2015).  Because the Cl^- channel is the only active ion-selective conductance with a reversal potential that lies within the dynamic range of spiral ganglion neurons (SGN) action potentials, developmental alteration of [Cl^-], and hence the equilibrium potential for Cl^- (ECl), transforms the pre- to the posthearing phenotype (Zhang et al. 2015).  Four basic residues involved in ion selectivity and pore blocker sensitivity have been identified (Peters et al. 2015).  Channel activity is required for mucus secretion induced by interleukin-13 (Lin et al. 2015; Zhang et al. 2015). May interact cooperatively with TrpV1 (TC# 1.A.4.2.1) to form a thermal sensor (Kanazawa and Matsumoto 2014). Inhibitors have been described (Boedtkjer et al. 2015).  The first intracellular loop serves as a Ca²⁺ binding site and includes D439, E444 and E447 (Pang et al. 2015).  Inhibited by various 4-Aryl-2-amino thiazoles at concentrations as low as 1 mμM (Piechowicz et al. 2016).  ANO1 and TRPC6 (1.A.4.1.5) are present in the same macromolecular complex and localize in close spatial proximity in the myocyte plasma membrane.  TRPC6 channels probably generate a local intracellular Ca²⁺ signal that activates nearby ANO1 channels in myocytes to stimulate vasoconstriction (Wang et al. 2016).  ANO1 transports bicarbonate which functions in the regulation of pancreatic acinar cell pH (Han et al. 2016).  TMEM16A contains two ion conduction pores that are independently activated by Ca²⁺ binding to sites that are embedded within the transmembrane part of each subunit (Lim et al. 2016).  Interactions between the carboxy- terminus and the first intracellular loop in the TMEM16A homo-dimer regulate channel activity (Scudieri et al. 2016).  A STAT6-TMEM16A-ERK1/2 signal pathway and TMEM16A channel activity are required for the Interleukin-13 (IL-13)-induced TMEM16A mediated mucus production (Qin et al. 2016). Angiotensin II elicits a TMEM16A-mediated current, and TMEM16A participates in Ang II-induced basilar constriction via the RhoA/ROCK signaling pathway (Li et al. 2016). 2-acylamino-cycloalkylthiophene-3-carboxylic acid arylamides (AACTs) are inhibitors of TMEM16A, and 48 synthesized analogs (10ab-10bw) of the original AACT compound (10aa) have been synthesized and studied.  The most potent compound (10bm), which contains an unusual bromodifluoroacetamide at the thiophene 2-position, had an IC50 ~ 30 nM (Truong et al. 2017).  Plays a role in asthma (Wang et al. 2017). The E143A mutant showed reduced sensitivity to Ca²⁺ but not to high temperatures, whereas the E705V mutant exhibited reduced sensitivity to both Ca²⁺ and noxious heat (Choi et al. 2018). Voltage modulation of TMEM16A involves voltage-dependent occupancy of calcium- and anion-binding site(s) within the membrane electric field as well as a voltage-dependent conformational change intrinsic to the channel protein. These gating modalities all critically depend on the sixth transmembrane segment (Peters et al. 2018). TMEM16A in myocytes plays a major functional role in contraction (Mohanakumar et al. 2018). Bile acids activate TMEM16A and thereby increase cholangiocyte fluid secretion (Li et al. 2018). TMEM16A participates in H₂O₂-induced apoptosis via modulation of mitochondrial membrane permeability (Zeng et al. 2018). Glioma-associate oncogene proteins, Gli1 and Gli2, bind to the promoter and repress ANO1 transcription, dependent on Gli binding to a site close to the ANO1 transcriptional start site (Mazzone et al. 2019). Clarithromycin suppresses IL-13-induced goblet cell metaplasia via the TMEM16A-dependent pathway in guinea pig airway epithelial cells (Hara et al. 2019).  TMEM16A is involved in gastric emptying, and TMEM16A inhibition may be effective in treating disorders of accelerated gastric emptying, such as dumping syndrome (Cil et al. 2019).  Phosphatidylinositol (4,5)-bisphosphate (PIP2) regulates TMEM16A channel activation and desensitization by binding to a binding site, possibly at the cytosolic interface of TMSs 3-5. The ion-conducting pore of TMEM16A consists of two functionally distinct modules: a Ca²⁺-binding module formed by TMSs 6-8 and a PIP2-binding regulatory module formed by TMs 3-5, which mediate channel activation and desensitization, respectively (Le et al. 2019).