1.A.1 The Voltage-gated Ion Channel (VIC) Superfamily
Proteins of the VIC family are ion-selective channel proteins found in a wide range of bacteria, archaea, eukaryotes and viruses. They are often homo- or heterooligomeric structures with several dissimilar subunits (e.g., α1-α2-δ-β Ca2+ channels, αβ1β2 Na+ channels or (α)4-β K+ channels), but the channel and the primary receptor is usually associated with the α (or α1) subunit. Functionally characterized members are specific for K+, Na+ or Ca2+. The K+ channels usually consist of homotetrameric structures with each α-subunit possessing six transmembrane spanners (TMSs). Many voltage-sensitive K+ channels function with β-subunits that modify K+ channel gating. These nonintegral β-subunits are oxidoreductases that coassemble with the tetrameric α-subunits in the endoplasmic reticulum and remain tightly adherent to the α-subunit tetramer. The high resolution β-subunit structure is available (Gulbis et al., 1999). Non-homologous β-subunits of Na+ and Ca2+ channels function in regulation (Hanlon and Wallace, 2002). Voltage-gated Ca2+ (Cav) channels have 4 subunits which have all been examined phylogenetically from evolutionary standpoints (Moran and Zakon 2014). Members of the VIC (1.A.1), RIR-CaC (2.A.3) and TRP-CC (1.A.4) Families have similar transmembrane domain structures, but very different cytosolic doman structures (Mio et al. 2008). How membrane proteins sense voltage (the membrane potential) has been reviewed (Bezanilla 2008). The involvement of glycosylation in the function and expression of these channels has also been reviewed (Lazniewska and Weiss 2017). Ion channel disfunction in semen may account for male infertility (Carkci et al. 2017). The spatial expression of K+ channels in mammalian cells has been reviewed (Capera et al. 2019). Large-conductance Ca2+- and voltage-gated K+ channels form and break interactions with membrane lipids during each gating cycle (Tian et al. 2019). Pulsed electric fields can create pores in the voltage sensors of voltage-gated ion channels (Rems et al. 2020). Curcumin, a multi-ion channel blocker preferentially blocks late Na+ current and prevents I/R-induced arrhythmias (Song et al. 2020). The contribution of ion channels to multiple stages of tumorigenesis has been reviewed (Fan and Huang 2020). Structures of K+ channels have revealed aspects of ion selectivity, conduction, channel gating, and modulation (Jiang 2021).
The α-subunits of the Ca2+ and Na+ channels are usually four times as large as the K+ channel α-subunits and possess 4 units, each with 6 TMSs separated by a hydrophilic loop, for a total of 24 TMSs. These large channel proteins form heterotetrameric-unit structures equivalent to the homotetrameric structures of most K+ channels. All four units of the Ca2+ and Na+ channels are homologous to the single unit in the homotetrameric K+ channels. Some Na+ and Ca2+ channels are half sized with two 6 TMS units, forming dimers (see subfamily 1.A.1.11). Ion flux via the eukaryotic channels is generally controlled by the transmembrane electrical potential (hence the designation, voltage-sensitive) although some are controlled by ligand or receptor binding. The 6 TMS VIC family members have a gating charge transfer center in the voltage sensors (Tao et al., 2010). Structural aspects of the calcium channels, revealing the architectural features that underlie their feedback regulatory mechanisms have been reviewed (Minor and Findeisen 2010). The evolution of VIC superfamily channels with a special emphasis on the metazoan lineage has been reviewed (Moran et al. 2015). Evolutioin of the 4 TMS voltage sensor has also been reviewed (Freites and Tobias 2015). Blockade of Na+ channels (NaVs) enables control of pathological firing patterns that occur in a diverse range of conditions such as chronic pain, epilepsy, and cardiac arrhythmias (Bagal et al. 2015). Crotonoside regulates sodium and calcium channels in rabbit ventricular myocytes, diminishing arrhythmias (Liu et al. 2020). The structures and pharmacology of voltage-gated sodium and calcium channels, including the basis for their voltage-dependent activation, fast and slow inactivation, ion conductance and selectivity have been reviewed (Catterall et al. 2020).
Voltage-gated sodium channels (VGSCs) are heteromeric transmembrane protein complexes. Nine homologous members, SCN1A-11A, make up the VGSC gene family. Sodium channel isoforms display a wide range of kinetic properties endowing different neuronal types with distinctly varied firing properties. Among the VGSCs isoforms, Nav1.7, Nav1.8 and Nav1.9 are preferentially expressed in the peripheral nervous system. These isoforms are known to be crucial in the conduction of nociceptive stimuli with mutations in these channels thought to be the underlying cause of a variety of heritable pain disorders (Kanellopoulos and Matsuyama 2016). Na+ channels are associated with neuropathic pain (Devor 2006). A 4 x 6 TMS template is shared among voltage-gated sodium (Nav1 and Nav2) and calcium channels (Cav1, Cav2, and Cav3) and leak channel (NALCN) plus homologs from yeast, different single-cell protists (heterokont and unikont) and algae (green and brown) (Fux et al. 2018). The asymmetrically arranged pores of 4x6 TMS channels allows for a changeable ion selectivity via a single lysine residue change in the high field strength site of the ion selectivity filter in Domains II or III. Modeling has provided clues for rational drug design (Montini et al. 2018). Mexiletine, a class Ib antiarrhythmic drug, is used clinically to reduce or prevent myotonia and is neuroprotective. It binds to the upper part of the pore in the open state and lower part in the closed state. High-affinity binding in the open states of Nav1.4 and Nav1.5 is caused by a pi-pi interactions with Phe (Nakagawa et al. 2019). VGSCs are involved in a variety of diseases, including epilepsy, cardiac arrhythmias, and neuropathic pain, and therefore are therapeutic targets for the development of anticonvulsant, antiarrhythmic, and local anesthetic drugs. Xu et al. 2019 reviewed advances in understanding the structures and biological functions of VGSCs and summarized eight pharmacologically distinct ligand-binding sites in VGSCs and representative isoform-selective VGSC modulators. They also review studies on molecular modeling and computer-aided drug design for VGSCs.
There are four known K+ channel families in mammals (humans): (1) The voltage dependent K+ channels designated as Kv channels, which consist of twelve subfamilies. (2) The two pore domain channels, the K2P, which consist of fourteen subfamilies. (3) The calcium activated K+ channels, KCa channels, which consist of five subfamilies. (4) The inward rectifier K+ channels, the Kir, which include seven subfamilies, designated Kir 1 - Kir 7 with fifteen members. The diversity of voltage-dependent K+ channels in human pulmonary artery smooth muscle cells have been tabulated and reviewed (Platoshyn et al. 2004). G-protein coupled receptors (GPCRs) modulate a number of K+ channels. The most intensively studied and characterized are the K+ inward rectifier Kir 3 subfamily (Kir3.1-Kir3.4) (Gohar, 2006). The Kv channels' voltage dependences are set in part by charged amino-acid residues of the extracellular linkers which electrostatically affect the charged amino-acid residues of the voltage sensor, S4 (Elinder et al. 2016). Kv-type channels can be consdered to be allosteric machines in which gating may be dynamically influenced by some long-range interactional/allosteric mechanisms (Barros et al. 2019). Molecular dynamics simulations directly predict the response of a voltage-gated K+ channels within a phospholipid bilayer membrane to applied transmembrane voltages (Tronin et al. 2019). Distinct lipid bilayer compositions have general and protein-specific effects on K+ channel function (Winterstein et al. 2021). A diversity of voltage-dependent K+ channels play roles in human pulmonary artery smooth muscle cells (Platoshyn et al. 2004).
BK-type Ca2+ channels and lipid phosphatases have a transmembrane voltage sensor domain (VSD) that moves in response to physiological variations of the membrane potential to control their activities. However, VSD movements and coupling to the channel or phosphatase activities may differ depending on various interactions between the VSD and its host molecules (Cui 2010). BK-type voltage, Ca²+ and Mg²+ activated K+ channels contain the VSD and a large cytosolic domain (CTD) that binds Ca²+and Mg²+. VSD movements are coupled to BK channel opening with a unique allosteric mechanism and are modulated by Ca²+ and Mg²+ binding via interactions between the channel pore, VSD and CTD. It is energetically advantageous for the pore to be controlled by multiple stimuli (Cui 2010). Insight regarding four types of tetrameric channels with 6 TMSs architectures, Eag1, SK2/SK4, TRPV5/TRPV6 and KCNQ1-5, and their regulation by calmodulin (CaM) have been described structurally (Núñez et al. 2020). Different CaM regions, N-lobe, C-lobe and the EF3/EF4-linker, play prominent signaling roles in different complexes, leading to the realization that crucial non-canonical interactions between CaM and its target channel proteins are apparent in the full-channel structures. Different mechanisms to control gating are used, including direct and indirect mechanical actuation over the pore, allosteric control, indirect effects through lipid binding, as well as direct plugging of the pore. Although each CaM lobe engages through apparently similar α-helices, they do so using different docking strategies (Núñez et al. 2020).
The erg or Kv11 channel is a subfamily of the voltage-dependent K+ channel superfamily and includes three members: Kv11.1 (erg1), Kv11.2 (erg2) and Kv11.3 (erg3) channels. The most studied member of this subfamily is Kv11.1 that regulates the duration of the cardiac action potential. Mutations in this channel have been associated with cardiac arrhythmias and sudden death (Bronstein-Sitton, 2006). All charge-translocating proteins follow two molecular coupling mechanisms: direct- or indirect-coupling, depending on whether the translocated charge is involved in the driving reaction. Calisto et al. 2021 explored these two coupling mechanisms by thoroughly examining the different types of charge-translocating membrane proteins.
Five types of Ca2+ channels are expressed in the CNS of mammals: The L-type (Cav1), N-type (Cav2.2), P/Q-type (Cav2.1), R-type (Cav2.3), and the T-type (Cav3). Each Cav channel is a multimeric protein composed of a pore forming α1 subunit and the auxiliary β (Cavβ), α2δ and γ subunits. There are four known Cavβ subunits, in addition to four α2δ subunits and eight γ subunits. The best characterized Ca2+ channels that are regulated by GPCRs are the N-type and the P/Q-type which have significant roles in neuronal communication. This mechanism is the basis of synaptic modulation caused by endogenous hormones as well as exogenously applied agents (such as analgesia caused by morphine). The identification of the types of Ca2+ channels that are modulated by GPCRs was enabled by the use of specific toxins: ω-Conotoxin GVIA for the N-type channels and ω-Agatoxin-IVA for the P/Q-type channels. Many Ca2+ channels are regulated by GPCRs (Gohar, 2006). Endodgenous membrane phosphatidylinositol 4,5-biphosphate, PIP2, activates high voltage activated L-, N- and P/Q type Ca2+ channels, and PIP2 depletion inhibits these Ca2+ channels (Suh et al., 2010).
In type-2 diabetes, the tight link between glucose sensing and insulin secretion is impaired due to mutations in a KATP channel. K+ channels that are sensitive to ATP are plasma membrane protein complexes composed of four Kir6.2 (KCNJ11) pore-forming subunits surrounded by four SUR1 (sulphanylurea receptor, of the ABC superfamily) auxiliary subunits. These protein complexes sense the amount of glucose entering a beta cell in the pancreas since the activity of KATP channels depends on the amount of ATP in the cytoplasm, which in turn depends on the amount of glucose absorbed by the beta cell. The activity of KATP channels is negatively correlated to the amount of ATP. KATP channels are the main channels that are open during resting conditions. Closure of KATP channels by increased ATP concentrations leads to membrane depolarization, which causes opening of voltage dependent Ca2+ (Cav) channels, leading to Ca2+ influx. The main Cav channels that control insulin secretion are L-type channels of the Cav1 subfamily (Cav1.2 and/or Cav1.3) (Cherki et al., 2006).
Ion channelopathies are inherited diseases in which alterations in control of ion conductance through the central pore of ion channels impair cell function, leading to periodic paralysis, cardiac arrhythmia, renal failure, epilepsy, migraine and ataxia (Kullmann and Waxman, 2010). However, Sokolov et al. (2007) have shown that, in contrast with this well-established paradigm, three mutations in gating-charge-carrying arginine residues in an S4 segment of NaV1.4 (TC #1.A.1.10.4) that cause hypokalaemic periodic paralysis induce a hyperpolarization-activated cationic leak through the voltage sensor of the skeletal muscle NaV1.4 channel. This 'gating pore current' is active at the resting membrane potential and closed by depolarizations that activate the voltage sensor. It has similar permeability to Na+, K+ and Cs+, but the organic monovalent cations tetraethylammonium and N-methyl-D-glucamine are much less permeant. The inorganic divalent cations Ba2+, Ca2+ and Zn2+ are not detectably permeant and block the gating pore at millimolar concentrations. The results reveal gating pore current in naturally occurring disease mutations of an ion channel and show a clear correlation between mutations that cause gating pore current and hypokalemic periodic paralysis. The involvement of channel protein in neurodegenerative disorders has been reviewed (Kumar et al. 2016).
Several putative K+-selective channel proteins of the VIC family have been identified in prokaryotes. The structures of two of them, the 2 TMS voltage-insensitive KcsA K+ channel of Streptomyces lividans and the 6 TMS KvAP voltage-sensitive K+ channel of Aeropyrum pernix, have been solved to 3.2 Å resolution (TC #1.A.1.1.1 and 1.A.1.17.1, respectively) (Cuello et al., 2004; Doyle et al., 1998; Jiang et al., 2003a,b; Ruta et al., 2003). Both proteins possess four identical subunits, each with two transmembrane helices, arranged in the shape of an inverted teepee or cone, forming the channel. The cone cradles the 'selectivity filter' P domain in its outer end. The narrow selectivity filter is only 12 Å long, whereas the remainder of the channel is wider and lined with hydrophobic residues. The first TMS (S1) is at the contact interface between the voltage sensing and pore domains (Cuello et al., 2004). A large water-filled cavity and helix dipoles stabilize K+ in the pore. The selectivity filter has two bound K+ ions about 7.5 Å apart from each other. Ion conduction is proposed to result from a balance of electrostatic attractive and repulsive forces. Evolutionary relationships between K+ channels and certain K+:cation symporters has been reviewed and discussed (Durell et al., 1999).
KcsA channels twist around the axis of the pore. Conformational changes are prevented by an open-channel blocker, tetrabuthylammonium. Random clockwise and counterclockwise twisting in the range of several tens of degrees originate in the transmembrane domain and are transmitted to the cytoplasmic domain. This twisting motion may play a role in gating (Shimizu et al., 2008). This coupling suggests a mechanical interplay between the transmembrane and cytoplasmic domains. Artificial cell membrane systems provide a platform for reconstituting ion channels (Komiya et al. 2020).
The open-state conformation of the KcsA K+ channel has been studied using the Monte Carlo normal mode following simulations. Gating involves rotation and unwinding of the TM2 bundle, lateral movement of the TM2 helices away from the channel axis, and disappearance of the TM2 bundle. The gating transition is intrinsically multidimensional and described by a rough free-energy landscape (Delemotte et al. 2015). The open-state conformation of KcsA exhibits a wide inner vestibule, with a radius approximately 5-7 Å and inner helices bent at the A98-G99 hinge. Computed conformational changes demonstrate that spin labeling and X-ray experiments illuminate different stages in gating: transition begins with clockwise rotation of the TM2 helices ending at a final state with the TM2 bend hinged near residues A98-G99. The concordance between the computational and experimental results provides atomic-level insight into the structural rearrangements of the channel's inner pore (Miloshevsky and Jordan, 2007).
Interconversion between conductive and non-conductive forms of the K+ channel selectivity filter underlies a variety of gating events. Cuello et al. (2010) reported the crystal structure of the Streptomyces lividans K+ channel, KcsA, in its open-inactivated conformation. They investigated the mechanism of C-type inactivation gating at the selectivity filter from channels 'trapped' in a series of partially open conformations. Five conformer classes were identified with openings ranging from 12 Å in closed KcsA to 32 Å when fully open. A correlation was observed between the degree of gate opening and the conformation and ion occupancy of the selectivity filter. A gradual filter backbone reorientation leads first to a loss of the S2 ion binding site and a subsequent loss of the S3 binding site, presumably abrogating ion conduction. The S4 helix may undergo a transition from an alpha-helical conformation to a short-lived different secondary structure transiently before reaching the active state in the activation process (Bassetto et al. 2019).
The archaeal voltage-dependent K+ channel (TC #1.A.1.17.1) has been characterized (Ruta et al., 2003). It exhibits the properties of a classical neuronal K+ channel including structural conservation in the voltage sensor as revealed by specific high affinity tarantula venom toxin binding. This toxin evolved to inhibit animal Kv channels. The first four transmembrane helices (S1-S4) of any 6 TMS VIC family member, undergoes the first conformational changes in response to membrane voltage variations, and the S4 segment of each domain, which contains several positively charged residues interspersed with hydrophobic amino acids, is located within the membrane electric field and plays an essential role in voltage sensing (Miceli et al. 2015).
Three other bacterial VIC family channels have been characterized functionally. One is the 2 TMS LctB channel of Bacillus stearothermophilus (TC #1.A.1.1.2; Wolters et al., 1999), the second is the 6 TMS Kch channel of E. coli (TC #1.A.1.13.1; Ungar et al., 2001), and the third is the Bacillus halodurans 6 TMS voltage-gated Na+ channel (TC #1.A.1.14.1; Ren et al., 2001). This last-mentioned protein, called NaChBac, is most similar in sequence to voltage-gated Ca2+ channels (TC #1.A.1.11.1-3). A family of these 6 TMS voltage-gated Na+ channels (22-54% identical) is widespread in bacteria, suggesting a fundamental function (Koishi et al., 2004). These three proteins are all distantly related to KcsA of S. lividans, particularly LctB. Kch has been shown to form tetramers that may function to maintain the membrane potential in the early stationary phase of growth (Ungar et al., 2001).
Prokaryotic voltage-gated sodium channels form homotetramers with each subunit contributing six transmembrane α-helices (S1-S6). Helices S5 and S6 form the ion-conducting pore, and helices S1-S4 function as the voltage sensor with helix S4 thought to be the essential element for voltage-dependent activation. The crystal structures have provided insight into voltage-gated K channels, revealing a characteristic domain arrangement in which the voltage sensor domain of one subunit is close to the pore domain of an adjacent subunit in the tetramer. Shimomura et al. (2011) showed that the domain arrangement in NaChBac, (TC# 1.A.1.14.1), is similar to that in voltage-gated K+ channels. The domain arrangement and vertical mobility of helix S4 in NaChBac indicated that the structure and mechanism of voltage-dependent activation in prokaryotic Na+ channels are similar to those in canonical voltage-gated K+ channels (Shimomura et al., 2011).
In eukaryotes, each VIC family channel type has several subtypes based on pharmacological and electrophysiological data. Thus, there are six types of Ca2+ channels (L, N, P, Q, R and T). There are at least ten types of K+ channels, each responding in different ways to different stimuli: voltage-sensitive [Ka, Kv, Kvr, Kvs and Ksr], Ca2+-sensitive [BKCa, IKCa and SKCa] and receptor-coupled [KM and KACh+ channels (I, II, III, μ1, H1 and PN3). Cyclic nucleotide-responsive channels (families 1.A.1.4 and 1.A.1.5) contain centrally located CAP_ED domains, although the cyclic nucleotide regulatory properties have only been reported for family 1.A.5, not 1.A.4. Tetrameric channels from both prokaryotic and eukaryotic organisms are known in which each α-subunit possesses 2 TMSs rather than 6, and these two TMSs are homologous to TMSs 5 and 6 of the 6 TMS unit found in the voltage-sensitive channel proteins. KcsA of S. lividans is an example of such a 2 TMS channel protein. These channels may include the KNa (Na+-activated) and KVol (cell volume-sensitive) K+ channels, as well as distantly related channels such as the Tok1 K+ channel of yeast. The TWIK-1 and -2, TREK-1, TRAAK, and TASK-1 and -2 K+ channels all exhibit a duplicated 2 TMS unit and may therefore form a homodimeric channel. About 50 of these 4 TMS proteins are encoded in the C. elegans genome. Because of insufficient sequence similarity with proteins of the VIC family, inward rectifier K+ IRK channels (ATP-regulated; G-protein-activated) which possess a P domain and two flanking TMSs are placed in a distinct family (TC #1.A.2). However, substantial sequence similarity in the P region suggests that they are homologous. The β, γ, and δ subunits of VIC family members, when present, frequently play regulatory roles in channel activation/deactivation.
The function of voltage-dependent K+ channels is dependent on the negatively charged phosphodiester of phospholipid molecules. A non-voltage-dependent K+ channel does not exhibit the same dependence. It was proposed that the phospholipid membrane, by providing stabilizing interactions between positively charged voltage-sensor arginine residues and negatively charged lipid phosphodiester groups, provides an appropriate environment for the energetic stability and operation of the voltage-sensing machinery. The usage of arginine residues in voltage sensors is an adaptation to the phospholipid composition of cell membranes (Schmidt et al., 2006). The X-ray structure of a voltage-dependent K+ channel (Kv) can explain charge stabilization within the membrane and thus suggests the mechanism for coupling voltage-sensor movements to pore gating (Long et al., 2007).
Voltage-gated ion channels derive their voltage sensitivity from the movement of specific charged residues in response to a change in transmembrane potential. Several studies on mechanisms of voltage sensing in ion channels support the idea that these gating charges move through a well-defined permeation pathway. This gating pathway in a voltage-gated ion channel can also be mutated to transport free cations, including protons (Chanda and Chanda and Bezanilla, 2008). The discovery of proton channels homologous to voltage-sensing domains suggests that the same gating pathway is used by voltage-dependent proton transporters. The voltage sensor depends on the movement of charges in an electric field. Gating currents of the voltage sensor depend on the movements of positively charged arginines through the hydrophobic plug of a voltage sensor domain. Transient movements of these permanently charged arginines, caused by a change in the transmembrane potential further drag the S4 segment and induce opening/closing of the ion conduction pore by moving the S4-S5 linker. Thus, moving permanent charge induces capacitive current flow (Horng et al. 2018). Salt bridge interactions between S4-arginines and the negatively charged residues in other TMSs in the voltage sensor domain appear to contribute more to stabilizing the energy than the van der Waals interactions between nonpolar residues (Boonamnaj et al. 2021).
The voltage-sensing domains (VSDs) of K+ channels have been shown to undergo large rearrangements during gating, whereas the S4 segment may remain positioned between the central pore and the remainder of the VSD in both states (Grabe et al., 2007). In the Shaker K+ channel (1.A.1.2.6), mutation of the first arginine residue of the S4 helix to a smaller uncharged residue makes the VSD permeable to ions in the resting conformation ('S4 down'). There are four omega pores per channel, consistent with one conduction path per VSD. Permeating ions from the extracellular medium enter the VSD at its peripheral junction with the pore domain, and then plunge into the core of the VSD in a curved conduction pathway (Tombola et al. 2007).
Amongst the nine voltage-gated K(+) channel (Kv) subunits expressed in Arabidopsis, AtKC1 does not seem to form functional Kv channels. Co-expression of AtKC1 (1.A.1.4.9), AKT1 (1.A.1.4.1) and/or KAT1 (1.A.1.4.7) genes in tobacco mesophyll protoplasts showed that AtKC1 remains in the endoplasmic reticulum unless it is co-expressed with AKT1 (Duby et al., 2008). Heteromeric AtKC1-AKT1 channels display functional properties different from those of homomeric AKT1 channels. In particular, the activation threshold voltage of the former channels is more negative than that of the latter ones preferred to AKT1-AKT1 homodimers during the process of tetramer assembly. Thus, AtKC1 is a Kv subunit, which downregulates the physiological activity of other Kv channel subunits (Duby et al., 2008).
Shaker-type K+ channels in plants display distinct voltage-sensing properties despite sharing sequence and structural similarity. For example, an Arabidopsis K+ channel (SKOR) and a tomato K+ channel (LKT1) share high amino acid sequence similarity and identical domain structures; however, SKOR conducts outward K+ current and is activated by positive membrane potentials (depolarization), whereas LKT1 conducts inward current and is activated by negative membrane potentials (hyperpolarization). The structural basis for the 'opposite' voltage-sensing properties of SKOR and LKT1 was determined in SKOR channel single amino acid mutations that converted the outward-conducting channel into an inward-conducting channel. Domain-swapping and random mutagenesis produced similar results, suggesting functional interactions between several regions of the SKOR protein that lead to specific voltage-sensing properties. Thus, dramatic changes in rectifying properties can be caused by single amino acid mutations.
The structure of the transmembrane regions of the bacterial cyclic nucleotide-regulated channel MlotiK1 (TC# 1.A.1.25.1), a non-voltage-gated 6 TM channel, has been determined (Clayton et al., 2008). The S1-S4 domain and its associated linker serve as a clamp to constrain the gate of the pore and possibly function in concert with ligand-binding domains to regulate the opening of the pore. Motions of the S6 inner helices can gate the ion conduction pathway at a position along the pore closer to the selectivity filter than the canonical helix bundle crossing.
Carbon monoxide (CO) is a lethal gas, but it is also a physiological signaling molecule capable of regulating a variety of proteins. Among them, large-conductance Ca2+- and voltage-gated K+ (Slo1 BK) channels, important in vasodilation and neuronal firing, have been suggested to be directly stimulated by CO. In fact, CO activates Slo1 BK channels (Hou et al, 2008) in the absence of Ca2+ in a voltage-sensor-independent manner. The stimulatory action of CO requires an aspartic acid and two histidine residues located in the cytoplasmic RCK1 domain. CO probably acts as a partial agonist for the high-affinity divalent cation sensor in the RCK1 domain of the Slo1 BK channel (1.A.1.3.2).
Ca2+-activated BK channels (e.g., 1.A.1.3.3) modulate neuronal activities, including spike frequency adaptation and synaptic transmission. Ca2+-binding sites and the activation gate are spatially separated in the channel protein. By studying an Asp-to-Gly mutation (D434G) associated with human syndrome of generalized epilepsy and paroxysmal dyskinesia (GEPD), Yang et al. (2010) showed that a cytosolic motif immediately following the activation gate S6 helix, known as the AC region, mediates the allosteric coupling between Ca2+ binding and channel opening. The GEPD mutation inside the AC region increases BK channel activity by enhancing this allosteric coupling. Ca2+ sensitivity is enhanced by increases in solution viscosity that reduce protein dynamics. The GEPD mutation alters such a response, suggesting that a less flexible AC region may be more effective in coupling Ca2+ binding to channel opening.
The voltage sensors in VIC family cation channels use a sliding-helix mechanism for electromechanical coupling in which outward movement of gating charges in the S4 transmembrane segments catalyzed by sequential formation of ion pairs pulls the S4-S5 linker, bends the S6 segment, and opens the pore (Catterall, 2010). Impairment of voltage-sensor function by mutations in Na+ channels contributes to several ion channelopathies, and gating pore current conducted by mutant voltage sensors in Na(V)1.4 channels is the primary pathophysiological mechanism in hypokalemic periodic paralysis. Confinement of water within a hydrophobic cavity can drive a cooperative dewetting transition. For a nanometer-scale pore, the dewetting transition leads to a stable dry state that is physically open but impermeable to ions (Yazdani et al. 2020). This phenomenon is often referred to as hydrophobic gating. Numerous ion channels utilize hydrophobic gating in their activation and regulation. Yazdani et al. 2020 reviewed recent theoretical, simulation, and experimental studies that together establish the principles of hydrophobic gating and discuss how channels of various sizes, topologies, and biological functions can utilize these principles to control the thermodynamic properties of water within their interior pores for gating and regulation.
In animals, calcium regulates heartbeat, muscle contraction, neuronal communication, hormone release, cell division, and gene transcription. Major entryways for Ca2+ in excitable cells are high-voltage activated (HVA) Ca2+ channels, Cav (Buraei and Yang, 2010). These are plasma membrane proteins composed of several subunits, including α1, α2δ, β, and γ. Although the principal α1 subunit contains the channel pore, gating machinery and most drug binding sites, the cytosolic auxiliary β subunit plays an essential role in regulating the surface expression and gating properties of HVA Ca2+ channels. Cavβ is also crucial for the modulation of HVA Ca2+ channels by G proteins, kinases, and the Ras-related RGK GTPases. Additional proteins modulate HVA Ca2+ channels by binding to Cavβ, and it may carry out Ca2+ channel-independent functions, including directly regulating gene transcription. All four subtypes of Cavβ, encoded by different genes, have a modular organization, consisting of three variable regions, a conserved guanylate kinase (GK) domain, and a conserved Src-homology 3 (SH3) domain, placing them into the membrane-associated guanylate kinase (MAGUK) protein family. Crystal structures of Cavβs reveal how they interact with Cavα1 (Buraei and Yang, 2010).
Regulator of K+ conductance (RCK) domains control the activity of a variety of K+ transporters and channels, including the human large conductance Ca2+-activated K+ channel that is important for blood pressure regulation and control of neuronal firing, and MthK, a prokaryotic Ca2+-gated K+ channel that has yielded structural insight toward mechanisms of RCK domain-controlled channel gating. In MthK, a gating ring of eight RCK domains regulates channel activation by Ca2+. Pau et al. (2011) showed that each RCK domain contributes to three different regulatory Ca2+-binding sites, two of which are located at the interfaces between adjacent RCK domains. The additional Ca2+-binding sites, resulting in a stoichiometry of 24 Ca2+ions per channel, is consistent with the steep relation between [Ca2+] and MthK channel activity. Comparison of Ca2+-bound and unliganded RCK domains suggests a physical mechanism for Ca2+-dependent conformational changes that underlie gating in this class of channels.
The mechanism of ion channel voltage gating - how channels open and close in response to voltage changes - has been debated since Hodgkin and Huxley's seminal discovery that the crux of nerve conduction is ion flow across cellular membranes. Using all-atom molecular dynamics simulations, Jensen et al. (2012) showed how a voltage-gated potassium channel (KV) switches between activated and deactivated states. On deactivation, pore hydrophobic collapse rapidly halts ion flow. Subsequent voltage-sensing domain (VSD) relaxation, including inward, 15-angstrom S4-helix motion, completes the transition. On activation, outward S4 motion tightens the VSD-pore linker, perturbing linker-S6-helix packing. Fluctuations allow water, then potassium ions, to reenter the pore; linker-S6 repacking stabilizes the open pore. Jensen et al. (2012) proposed a mechanistic model for the sodium/potassium/calcium voltage-gated ion channel superfamily that reconciles apparently conflicting experimental data.
In yeast and filamentous fungi, the Ca2+ channel, Cch1 forms a complex with an auxiliary subunit Mid1 to form the active complex (1.A.1.11.10). Mid1 was originally reported to have Ca2+ channel activity because when produced in Chinese hamster ovary cells, it produced channel activity (Kanzaki et al., 1999). However, it is now clear from many studies that Mid1 is required for Cch1-mediated Ca2+ flux and probably has no inherent channel activity (Ma et al., 2011; Martin et al., 2011; Cavinder and Trail, 2012). Mid1 was originally assigned to TC family: 1.A.16, The Yeast Stretch-Activated Cation-selective Ca2+ Channel, Mid1 (Mid1) Family, but this assignment has been deleted from TCDB, and Mid1 proteins have been incorporated into TC subfamily 1.A.1.11.
Hyperpolarization activated and cyclic nucleotide-gated (HCN) ion channels as well as cyclic nucleotide-gated (CNG) ion channels are essential for the regulation of cardiac cells, neuronal excitability, and signaling in sensory cells (Börger et al. 2014). Both classes are composed of four subunits. Each subunit comprises a transmembrane region, intracellular N- and C-termini, and a C-terminal cyclic nucleotide-binding domain (CNBD). Binding of cyclic nucleotides to the CNBD promotes opening of both CNG and HCN channels. In the case of CNG channels, binding of cyclic nucleotides to the CNBD is sufficient to open the channel. In contrast, HCN channels open upon membrane hyperpolarization and their activity is modulated by binding of cyclic nucleotides, shifting the activation potential to more positive values. Several high-resolution structures of CNBDs from HCN and CNG channels are available. Börger et al. 2014 reported the complete backbone and side chain resonance assignments of the murine HCN2 CNBD with part of the C-linker.
Plant Shaker channels are members of the 6 transmembrane-1 pore (6TM-1P) cation channel superfamily as are the animal Shaker (Kv) and HCN channels. All these channels are voltage-gated K+ channels: Kv channels are outward-rectifiers, opened at depolarized voltages, and HCN channels are inward-rectifiers, opened by membrane hyperpolarization. Among plant Shaker channels, are outward-rectifiers, inward-rectifiers and weak-rectifiers with weak voltage dependence (Nieves-Cordones and Gaillard 2014). Despite the absence of crystal structures of plant Shaker channels, functional analyses coupled to homology modeling, mostly based on Kv and HCN crystals, have permitted the identification of several regions contributing to plant Shaker channel gating. In a recent mini-review, Nieves-Cordones and Gaillard 2014 updated information on the voltage-gating mechanism of plant Shaker channels which seem to be comparable to that proposed for HCN channels.
The membrane dipole potential (Psid) constitutes one of three electrical potentials generated by cell membranes. Psid arises from the unfavorable parallel alignment of phospholipid and water dipoles, and varies in magnitude both longitudinally and laterally across the bilayer according to membrane composition and phospholipid packing density. Pearlstein et al. 2016 proposed that dynamic counter-balancing between Psid and the transmembrane potential (Δψ) governs the conformational state transitions of voltage-gated ion channels.
In the cell membrane, ion channels and enzymes are able to sense voltage. Sodium, Ca2+ and K+ voltage-dependent channels of the VIC superfamily have a conserved positively charged transmembrane (S4) segment that moves in response to changes in membrane voltage.S4 forms part of a domain that crystallizes as a well-defined structure consisting of the first four transmembrane (S1-S4) segments of the protein, the voltage sensor domain (VSD). VSD movements are allosterically coupled to pore opening to various degrees, depending on the type of channel. How many charges are moved during channel activation, how much they move, and which are the molecular determinants that mediate the electromechanical coupling between the VSD and the pore domains are discussed by Gonzalez et al. 2012.
The family of P-loop channels is characterized by four membrane re-entering extracellular P-loops that connect eight transmembrane helices. X-ray and cryo-EM structures of the open- and closed-state channels show conserved state-dependent folding despite the fact that the sequences are diverse. In homologous sodium, calcium, TRPV and two-pore channels, the pore-lining helices contain conserved asparagines that may or may not include pi-helix bulges. Comparison of the sequence- and 3D-alignemnts suggests that the asparagines appeared in evolution as insertions that are accommodated in two ways: by pi-helix bulges, which preserve most of inter-segment contacts, or by twists of the C-terminal thirds and switch of inter-segment contacts (Tikhonov and Zhorov 2017).
Several VIC superfamily K+ channels are affected by molybdenum disulfide nanoflakes (MoS2) (Gu et al. 2017). For example, MoS2 binds to the extracellular loops of KcsA, which indirectly destroys the delicate structure of the selectivity filter, causing a strong leak of K+ ions. In the binding mode with Kir3.2, a MoS2 nanoflake completely covers the entrance to the channel pore, affecting ion conduction. For the Kv1.2 chimera, the MoS2 nanoflake prefers to bind into a crevice located at the extracellular side of the Voltage Sensor Domain (VSD). This crevice involves the N-terminal segment of S4, which directly controls the gating process of the Kv1.2 chimera channel by electromechanical coupling of the VSD to the transmembrane electric field (Gu et al. 2017).
Many potassium-channel openers (agonists) share a distinct biaryl-sulfonamide motif. The negatively charged variants of these compounds bind to the top of the voltage-sensor domain, between transmembrane segments 3 and 4, to open the channel. Although biaryl-sulfonamide compounds open some potassium channels, they have also been reported to block sodium and calcium channels (Liin et al. 2018). The biaryl-sulfonamide motif seems to be a general ion-channel activator motif.
Voltage-dependent activation of voltage-gated cation channels results from the outward movement of arginine-bearing helices within proteinaceous voltage sensors. The arginine side chains in the voltage-sensing residues in potassium channels may make electrostatic or steric contributions to voltage sensing. Infield et al. 2018 functionally characterized engineered Shaker K+ channels, and observed effects on both voltage sensitivity and gating kinetics following substitution of the fourth S4 charged arginine with neutral citrulline, which caused substantial changes in the conductance-voltage relationship and channel kinetics. This suggested that a positive charge is required at this position for efficient voltage sensor deactivation and channel closure.
Toxins of voltage-gated ion channels are broadly divided into two categories—pore blockers that physically occlude the channel pore and gating modifiers that alter channel gating by interfering with the voltage sensor domains (VSDs). Whereas small-molecule neurotoxins such as tetrodotoxin (TTX) and saxitoxin (STX) function as pore blockers, most peptidic Nav channel toxins are gating modifiers that trap the channel in a particular stage of the gating cycle through interactions with one or more VSDs. Shen et al. 2018 determined the structure of NavPaS, the Na+ channel from the American cockroach, bound to a peptide toxin, Dc1a, from the venom of the desert bush spider, Diguetia canitries that specifically binds VSDII of insect Navs to promote chanell opeining, as well as TTX or STX that bind to and block the pore. Dc1a binds in a cleft between VSDII and hte pore region, causing structural rearrangements (see 8.B.30 for the Dc1a toxin descrption.
Calcium channels play roles in tumorigenesis and progression. Zhong et al. 2019 reviewed the evidence for a linkage between calcium channels and major characteristics of tumors such as multi-drug resistance (MDR), metastasis, apoptosis, proliferation, evasion of immune surveillance, and the alterations of tumor microenvironment. Ion channels also play active roles in phagocytosis. The participation of some channels in cell proliferation during interphase and mitosis has been discussed (Rosendo-Pineda et al. 2020). Pozdnyakov et al. 2018 characterized the functional determinants (selectivity filter, voltage sensor, Nav-like inactivation gates, Cavbeta-interaction motifs, and calmodulin-binding regions) of 277 eukaryotic VIC family members and constructed a phylogenetic tree. This allowed them to uncovere lineage-specific structural gains and losses in the course of evolution and suggest the ancient structural features of these channels.
Voltage-gated sodium channels are targets for a range of pharmaceutical drugs developed for the treatment of neurological diseases. Cannabidiol (CBD), the non-psychoactive compound isolated from cannabis plants, has been approved for treatment of two types of epilepsy associated with sodium channel mutations. CBD binds at a site at the interface of the fenestrations and the central hydrophobic cavity of the channel (Sait et al. 2020). Binding at this site blocks the TM sodium ion translocation pathway, providing a molecular mechanism for channel inhibition. The closely-related psychoactive tetrahydrocannabinol seems not to have the same effects on these channels. The TRPV2 channel may also be a target site for CBD.
K2P K+ channels contribute to many processes including anesthesia, pain, arrythmias, ischemia, hypertension, migraine, intraocular pressure regulation, and lung injury responses. Structural studies of six homomeric K2Ps have established the basic architecture of this channel sub-family, revealed key moving parts involved in K2P function, uncovered the importance of asymmetric pinching and dilation motions in the K2P selectivity filter (SF) C-type gate, and defined two K2P structural classes based on the absence or presence of an intracellular gate (Natale et al. 2021). Further, a series of structures characterizing K2P:modulator interactions have revealed a striking polysite pharmacology housed within an approximately 70 kDa channel. Binding sites for small molecules or lipids that control channel function are found at every layer of the channel structure, starting from its extracellular side through the portion that interacts with the membrane bilayer inner leaflet. This framework provides the basis for understanding how gating cues sensed by different channel parts control function, and how small molecules and lipids modulate K2P activity (Natale et al. 2021).
The generalized transport reaction catalyzed by members of the VIC family is:
cation (out) ⇌ cation (in).