1.A.23 The Small Conductance Mechanosensitive Ion Channel (MscS) Family
The MscS family (the SwissProt UPF0003 family) is a group of topographically diverse proteins, some of which are functionally characterized. They exhibit homology in only a restricted region. Early electrophysiological studies with E. coli suggested the presence of two distinguishable mechanosensitive ion channels, one with large conductance (MscL; TC #1.A.22) and one with small conductance (MscS; this family) (Martinac et al., 1987, 1990). Cytoplasmic beta domains appear to be new gating elements in MscS channels (Koprowski et al., 2011). Open and closed conformational states of the heptameric channel have been proposed and discussed (Pliotas et al. 2012). Evolutionary considerations have been reported (Booth et al. 2015). Some MscS channels may function in amino acid efflux, Ca2+ regulation and cell division (Cox et al. 2015).
Application of a ramp of negative pressure to a patch excised from an E. coli giant spheroplast gave (1) a small conductance (MscS; ~1 nS in 400 mM salt) with a sustained open state, and (2) a large conductance (MscL; ~3 nS) with faster kinetics, activated at higher pressure. MscS was reported to exhibit a weak anionic preference and a voltage dependency, tending to open upon depolarization. Activation by membrane-intercalating amphipathic compounds suggested that the MscS channel is sensitive to mechanical perturbations in the lipid bilayer. It was suggested that MscS plays a role in fast osmoregulatory responses. How these channels may respond to change in the mechanical environment the lipid bilayer provides is discussed by Kung et al. (2010).
Mechanosensitive channels function as electromechanical switches with the capability to sense the physical state of lipid bilayers. The X-ray crystal structures of MscL and MscS allow identification of the types of protein motions associated with the opening and closing of these structurally unrelated channels, while providing the framework to address a mechanism of tension sensing that is defined by channel-lipid interactions (Perozo and Rees, 2003). Functional, structural and dynamic data offer fresh insights into the molecular basis of gating for these membrane proteins. Members of the MscS family are found in bacteria, archaea, fungi, and plants (Wilson et al. 2013).
Mutations in the genes encoding the KefA (AefA) and YggB proteins of E. coli block the MscS mechanosensitive channel activity. The principal one is affected by knockouts of YggB, while the minor one is affected by knockouts of KefA. These two channels open in response to pressure changes during osmotic downshift just below those that cause cell disruption and death (Biggin and Sansom, 2003; Pivetti et al., 2003). The C-termini of the YggB heptameric channel move apart upon channel opening and may serve as the gate (Koprowski and Kubalski, 2003). High resolution 3-D structures are available (Bass et al., 2002; Lai et al. 2013). Crosslinking studies indicate that a large conformational change accompanies the open to the closed configuration (Miller et al., 2003b).
Homologues of YggB are found in Gram-negative, Gram-positive and cyanobacteria, in archaea, in yeast and in plants, but not in animals (Booth and Louis, 1999). One archaeon, Haloferax volcanii, exhibits mechanosensitive channels similar in conductance and mass to YggB of E. coli, but the sequences of these channel proteins are not available (Le Dain et al., 1998). Two sequenced MscS homologues have been functionally characterized from Methanococcus jannaschii (Kloda and Martinac, 2001a,b).
MscS family homologues vary in length between 248 and 1120 amino acyl residues, but the homologous region that is shared by most of them is only 200-250 residues long, exhibiting 4-5 TMSs (Miller et al., 2003b). The topologies of these proteins differ drastically. YggB (286 aas; spP11666) exhibits 4-5 putative transmembrane α-helical spanners (TMSs); KefA (AefA) (1120 aas; spP77338) exhibits 11 TMSs; YjeP of E. coli (1107 aas; spP39285) exhibits 11 TMSs; YbiO of E. coli (741 aas; spP75783) exhibits 10 TMSs; and YbdG of E. coli (415 aas; spP39455) exhibits 5 TMSs. Moreover, of the M. jannaschii homologues, MJ0170 (350 aas; spQ57634) exhibits 4-5 TMSs; MJ0700 (324 aas; spQ58111) exhibits 4 TMSs; and MJ1143 (361 aas; spQ58543) exhibits 5-6 TMSs. This topological variability is an unusual characteristic of a family of homologous transport proteins, and its functional significance cannot be evaluated at this time. It is possible that transport mechanisms will vary in accordance with topology. On the other hand, only 2 TMSs, common to all of these proteins may comprise the channel (Booth and Louis, 1999).
The E. coli and Synechocystis genomes include five recognized paralogues of the MscS family while the Bacillus subtilis and Methanococcus jannaschii genomes include three paralogues. KefA is multidomain and may be multifunctional. It has a large (470 amino acyl residues) N-terminal extracytoplasmic domain that may interact with the peptidoglycan cell wall, a central hydrophobic region including the 11 TMSs (residues 480-940), and a C-terminal cytoplasmic domain (residues 941-1120). Only the last four TMSs and the C-terminal hydrophilic domain are homologous to the much shorter YggA protein. One of the Synechocystis homologues (slr1575) possesses a C-terminal domain homologous to cyclic AMP-dependent protein kinaseA regulatory subunits (Ochoa de Alde and Houmard, 2000). It may therefore be a cyclic nucleotide-regulated channel.
A homologue of MscS channels in Erwinia chrysanthemi, BspA, is encoded within an operon that includes the psd gene encoding phosphatidyl serine decarboxylase. In high salts medium, glycine betaine initially is taken up normally in a bspA mutant, but uptake is followed by reduced glucose uptake and release of glycine betaine without loss of viability. It was suggested that BspA is not a channel but instead senses the intracellular glycine betaine and the extracellular salt concentrations, and thus serves as a receptor for osmoadaptation (Touzé et al., 2001).
In bacterial and animal systems, mechanosensitive (MS) ion channels are thought to mediate the perception of pressure, touch, and sound. Ten MscS-Like (MSL) proteins are encoded within the genome of Arabidopsis thaliana. MSL2 and MSL3, along with MSC1, a MscS family member from green algae, are implicated in the control of organelle morphology. Haswell et al. (2008) characterized MSL9 and MSL10, two MSL proteins found in the plasma membrane of root cells. MSL9 and MSL10, along with three other members of the MSL family, are required for MS channel activities detected in protoplasts derived from root cells.
The Escherichia coli mechanosensitive channel, MscS, opens to allow rapid ion efflux, relieving the turgor pressure that would otherwise destroy the cell. Wang et al. (2008) described a 3.45 angstrom-resolution structure in an open conformation. MscS has a pore diameter of ~13 angstroms created by substantial rotational rearrangement of the three transmembrane helices. The structure suggests a molecular mechanism that underlies MscS gating and its decay of conductivity during prolonged activation (Wang et al., 2008). The levels of both MscL and MscS channels in Bacillus subtilis are high during exponential phase growth, very low in stationary phase and non-detectable in spores (Wahome et al., 2009).
The E. coli MscS (EcMscS) has been extensively studied, but it may display characteristics not widely conserved in this protein family. With numerous members now electrophysiologically characterized, these channels displays a breadth of ion selectivity with both anion and cation selective members. The selectivities of these channels may be relatively weak in comparison to voltage-gated channels. Residues important for selectivity in MscS homologs suggest different selectivity mechanisms than those employed by voltage gated K+, Na+, Ca2+ and Cl- channels whose selectivity filters are housed within their transmembrane pores. Cox et al. 2013 attempted to unravel the potential physiological relevance of these differences.
Mechanosensitive (MS) channels provide protection against hypo-osmotic shock in bacteria whereas eukaryotic MS channels fulfill a multitude of important functions in addition to osmoregulation. Interactions with the membrane lipids are responsible for the sensing of mechanical force for most known MS channels. Not only prokaryotic, but also eukaryotic, MS channels are able to directly sense the tension in the membrane bilayer without an additional cofactor. Sensitivity towards tension changes can be explained as result of the hydrophobic coupling between membrane and transmembrane (TM) regions of the channel. Molecular interactions of lipids with the channels may play an important role in mechanosensation. Pockets in between TM helices were identified in MscS and YnaI (TC# 1.A.23.4.3) that are filled with lipids. Fewer lipids are present in the open state of MscS than the closed. Thus, exclusion of lipid fatty acyl chains from these pockets, as a consequence of increased tension, may trigger gating. Similarly, in the eukaryotic MS channel TRAAK it was found that a lipid chain blocks the conducting path in the closed state (Rasmussen 2016).
The generalized transport reaction proposed for MscS channels is:
osmolytes (in) and ions (in) osmolytes (out) and ions (out).
Minor K+-dependent MscS-type mechanosensitive channel protein, designated KefA, AefA or MscK, (Edwards et al. 2012).
Bacteria, archaea, yeast, plants
KefA (AefA) of E. coli
Mini conductance (300 pS) mechanosensitive channel, YjeP or MscM (1107aas; 13 TMSs in a 1 + 12 TMS arrangement). Encoded in an operon with phosphatidyl serine decarboxylase (Moraes and Reithmeier 2012). Protects against hypoosmotic shock (Edwards et al. 2012).
YjeP of E. coli (P39285)
Uncharacterized protein of 571 aas and 6 TMSs.
UP of Bdellovibrio exovorus
Major MscS channel protein, YggB. Seven residues, mostly hydrophobic, in the first and second transmembrane helices are lipid-sensing residues (Malcolm et al., 2011). X-ray structures are available (Lai et al. 2013). The cytoplasmic cage domain senses macromolecular crowding (Rowe et al. 2014). A gating mechanism has been proposed (Malcolm et al. 2015). The thermodynamics of K+ leak have been studied (Koprowski et al. 2015). In the MscS crystal structure (PDB 2OAU ), a narrow, hydrophobic opening is visible in the crystal structure, and a vapor lock, created by hydrophobic seals consisting of L105 and L109, is the barrier to water and ions (Rasmussen et al. 2015). The voltage dependence of inactivation occurs independently of the positive charges of R46, R54, and R74 (Nomura et al. 2016).
Protobacteria; homologues are found in archaea, yeast and plants.
YggB of E. coli (P0C0S1)
MscS protein. The x-ray structure at 4.2 Å is available (Lai et al. 2013).
MscS of Helicobacter pylori
MscS mechanosensitive channel of 462 aas and 5 TMSs.
MscS channel of Candidatus Peribacter riflensis
Putative small conductance mechanosensetive channel protein of 261 aas and 3 TMSs
MscS homologue of Aureococcus anophagefferens virus
Mechanosensitive NaCl-inducible RpoS-dependent channel (1,000 pS), YbiO (741 aas; 10TMSs). Protects agains hypoosmotic shock (Edwards et al. 2012).
YbiO of E. coli (P75783)
Mechanosensitive channel, small conductance, YggB or MscCG (533 aas; 6-7 TMSs). Mediates glutamate efflux (Becker et al. 2013). The pore domain is in the N-terminus. The C-terminus includes three subdomains, the periplasmic loop, the fourth transmembrane segment, and the cytoplasmic loop, all of which are important for MscCG function, in particular for glutamate excretion (Becker and Krämer 2015).
YggB or MscCG of Corynebacterium glutamicum (P42531)
MscCG2 of 334 aas and 4 TMSs in a 3 + 1 arrangement. It functions as an L-glutamate exporter and an osmotic safety valve (Wang et al. 2018). It is 23% identical to MscCG (TC# 1.A.23.3.3) in the same organism. MscCG2-mediated L-glutamate excretion was activated by biotin limitation or penicillin treatment, and constitutive L-glutamate excretion was triggered by a gain-of-function mutation (A151V). It was not induced by glutamate producing conditions (Wang et al. 2018).
MscCG2 of Corynebacterium glutamicum
Uncharacterized MscS homologue
MscS homologue of Helicobacter pylori
MscS-like channel, MSL1. Mechanosensitive ion channel protein 1, mitochondrial.
MSL1 of Arabidopsis thaliana
Uncharacterized MscS channel of 351 aas and 4 N-terminal TMSs.
UP of Bdellovibrio bacteriovorus
MscS channel of 553 aas and 6 TMSs.
MscS of Entamoeba histolytica
Mechanosensitive channel-like 10, Msl10 of 734 aas and 5 or more TMSs. Functions in triggering cell death in a process that is independent of its channel activity (Maksaev et al. 2018).
Mscl10 of Arabidopsis thaliana (Mouse-ear cress)
Plasma membrane small conductance mechanosensitive channel, MSL4, of 881 aas and 5 putative TMSs (Hamilton et al. 2015).
MSL4 of Arabidopsis thaliana
Mechanosensative cation-selective channel with a conductance of 100 pS, YnaI (344aas; 4TMSs). Protects against hypoosmotic shock (Edwards et al. 2012). The structure has been solved by cryo-electron microscopy to a resolution of 13 A (Böttcher et al. 2015). While the cytosolic vestibule is structurally similar to that in MscS, additional density is seen in the transmembrane region, consistent with the presence of two additional TMSs predicted for YnaI. The location of this density suggests that the extra TMSs are tilted, which could induce local membrane curvature extending the tension-sensing paddles seen in MscS. Off-center lipid-accessible cavities are seen that resemble gaps between the sensor paddles in MscS. The conservation of the tapered shape and the cavities in YnaI suggest a mechanism similar to that of MscS (Böttcher et al. 2015). The voltage dependence of inactivation occurs independently of the positive charges of R46, R54, and R74 (Nomura et al. 2016). A 3.8 Å structure by cryoEM revealed a heptamer structural fold similar to previously studied MscS channels. The ion-selective filter is formed by seven hydrophobic methionines (Met158) in the transmembrane pore (Yu et al. 2017). Details of the gating transition for MscS have been predicted (Zhu et al. 2018).
YnaI of E. coli (P0AEB5)
Plant plastid mechanosensitive channel MscS-like-2 (Msl2) (controls plastid organellar morphology, as does Msl3) (Haswell and Meyerowitz, 2006; Haswell et al., 2008). It functions as do the bacterial homologues, but is essential for leaf growth, chloroplast integrity and normal starch accumulation (Jensen and Haswell 2012). msl2 msl3 double mutant seedlings exhibit several hallmarks of drought or environmental osmotic stress, including solute accumulation, elevated levels of the compatible osmolyte proline (Pro), and accumulation of the stress hormone abscisic acid (ABA). Furthermore, msl2 msl3 mutants expressed Pro and ABA metabolism genes in a pattern normally seen under drought or osmotic stress. Pro accumulation in the msl2 msl3 mutant was suppressed by conditions that reduce plastid osmotic stress leading to the conclusion that these channels function like their bacterial homologues (Wilson et al. 2014).
Msl2 of Arabidopsis thaliana (Q56X46)
MscM (YbdG) is a distant member of the MscS family. It displays miniconductance (MscM) activity (Schumann et al., 2010; Edwards et al. 2012).
MscM (YbdG) of E. coli (P0AAT4)
Mechanosensitive channel, MscS
MscS of Sulfolobus islandicus (C4KE93)
Mechanosensitive ion channel protein 8 (Mechanosensitive channel of small conductance-like 8) (MscS-Like protein 8) is a pollen-specific, membrane tension-gated ion channel required for pollen to survive the hypoosmotic shock of rehydration and for full male fertility. It negatively regulates pollen germination but is required for cellular integrity during germination and tube growth. MSL8 thus senses and responds to changes in membrane tension associated with pollen hydration and germination (Hamilton et al. 2015).
MSL8 of Arabidopsis thaliana
Putative small conductance mechanosensitive channel; Calcium channel, MacS
MacS of Mycosphaerella graminicola (Zymoseptoria tritici)
The cyclic nucleotide-binding MscS homologue, MT2508 (the C-terminal domain is the CAP_ED domain CD00038). It lacks mechanosensitivity but is ligand-gated by cyclic nucleotides (Caldwell et al., 2010).
MscS homologue, MT2508 of Mycobacterium tuberculosis (P71915)
Msc1 of Chlamydomonas reinhardtii (A3KE12)
MscS homologue of Streptomyces coelicolor
MscS of Myxococcus xanthus
CmpX of 274 aas and 5 TMSs in a 1 + 4 arrangement. CmpX regulates virulence and controls biofilm formation in P. aeruginosa (Bhagirath et al. 2017). It also modulates intra-cellular c-di-GMP levels. A cmpX knockout showed decreased promoter activity of exoS (PA1362) and increased activity of the small RNA, RsmY. As compared to the wild-type PAO1, the cmpX mutant had elevated intracellular c-di-GMP levels as well as increased expression of wspR (PA3702), a c-di-GMP synthase. Transcription of the major outer membrane porin gene oprF (PA1777) and sigma factor sigX (PA1776) was decreased in the cmpX mutant. The cmpX knockout mutant had increased sensitivity to membrane detergents and antibiotics such as lauryl sulfobetaine, tobramycin, and vancomycin (Bhagirath et al. 2017).
CmpX of Pseudomonas aeruginosa
CmpX protein of 227 aas and 5 TMSs
CmpX of Candidatus Wolfebacteria bacterium
Uncharacterized protein of 439 aas and 9 TMSs in a 5 + 4 arrangement.
UP of Brevundimonas viscosa
Mechanosensitive ion channel protein MscS of 254 aas and 5 TM
MscS of Haloterrigena daqingensis
Uncharacterized protein of 486 aas and 11 TMSs.
UP of Hydrogenophaga taeniospiralis