Two types of mechanosensitive channels in the Escherichia coli cell envelope: solubilization and functional reconstitution

The effects of parabens on the mechanosensitive channels of E. coli

Parabens are alkyl esters of p-hydroxybenzoic acid used as preservatives in a wide range of food, pharmaceutical, and cosmetic products. Despite their common use for over 50 years, their mechanism of action is still unclear. In this study we examined the effects of ethyl and propyl paraben, on gating of the E. coli mechanosensitive channel of large conductance (MscL) reconstituted into azolectin liposomes. We found that propyl and ethyl paraben spontaneously activate MscL. Moreover, the addition of propyl paraben caused an increase in MscL activity and the lowering of p(1/2), the pressure at which the MscL was opened 50% of the time, the DeltaG(o), the free energy required to open the MscL, and the parameter alpha, which describes the channel sensitivity to pressure. In addition, in silico studies showed that propyl paraben binds to the channel gate of the MscL. The mechanosensitive channel of small conductance was also found to be spontaneously activated by parabens. In summary, our study indicates that one of the previously unidentified mechanisms of action of parabens as antimicrobial agents is via an interaction with the mechanosensitive channels to upset the osmotic gradients in bacteria.

Mutations in the Pore Region Modify Epithelial Sodium Channel Gating by Shear Stress

Previous studies have shown that epithelial Na+ channels (ENaCs) are activated by laminar shear stress (LSS). ENaCs with a high intrinsic open probability because of a mutation (betaS518K) or covalent modification of an introduced Cys residue (alphaS580C) in the pre-second transmembrane domain (pre-M2) were not activated by LSS, suggesting that the pre-M2 region participates in conformational rearrangements during channel activation. We examined the role of the pore region of the alpha-subunit in channel gating by studying the kinetics of activation by LSS of wild-type ENaC and channels with Cys mutations in the tract Ser576-Ser592. Whole cell Na+ currents were monitored in oocytes expressing wild-type or mutant ENaCs prior to and following application of LSS. Following a 2.2-s delay, a monoexponential increase in Na+ currents was observed with a time constant (tau) of 8.1 s in oocytes expressing wild-type ENaC. Cys substitutions within the alpha-subunit in the tract Ser580-Ser589 resulted in: (i) a reduction (Ser580-Trp585, Gly587) or increase (Ser589) in delay times preceding channel activation by LSS, (ii) an increase (Gln581, Leu584, Trp585, Phe586, Ser588) or decrease (Ser589) in the rate of channel activation, or (iii) a decrease in the magnitude of the response (Ser583, Gly587, Leu584). Cys substitutions at a putative amiloride-binding site (alphaSer583 or betaGly525) or within the selectivity filter (alphaGly587) resulted in a reduction in the LSS response, and exhibited a multiexponential time course of activation. The corresponding gamma-subunit mutant (alphabetagammaG542C) had a minimal response to LSS and exhibited a high intrinsic open probability. These data suggest that residues in the pore region participate in the sensing and/or transduction of the mechanical stimulus that results in channel activation and are consistent with the hypothesis that the ENaC pore region has a key role in modulating channel gating.

Gating transitions in bacterial ion channels measured at 3 microns resolution

Ion channels of high conductance (>200 pS) are widespread among prokaryotes and eukaryotes. Two examples, the Escherichia coli mechanosensitive ion channels Ec-MscS and Ec-MscL, pass currents of 125–300 pA. To resolve temporal details of conductance transitions, a patch-clamp setup was optimized for low-noise recordings at a time resolution of 3 μs (10–20 times faster than usual). Analyses of the high-resolution recordings confirm that Ec-MscL visits many subconductance states and show that most of the intersubstate transitions occur more slowly than the effective resolution of 3 μs. There is a clear trend toward longer transition times for the larger transitions. In Ec-MscS recordings, the majority of the observed full conductance transitions are also composite. We detected a short-lived (∼20 μs) Ec-MscS substate at 2/3 of full conductance; transitions between 2/3 and full conductance did not show fine structure and had a time course limited by the achieved resolution. Opening and closing transitions in MscS are symmetrical and are not preceded or followed by smaller, rapid currents (“anticipations” or “regrets”). Compared with other, lower-conductance channels, these measurements may detect unusually early states in the transitions from fully closed to fully open. Increased temporal resolution at the single-molecule level reveals that some elementary steps of structural transitions are composite and follow several alternative pathways, while others still escape resolution. High-bandwidth, low-noise single-channel measurements may provide details about state transitions in other high-conductance channels; and similar procedures may also be applied to channel- and nanopore-based single-molecule DNA measurements.

Pivotal role of the glycine-rich TM3 helix in gating the MscS mechanosensitive channel

The crystal structure of an open form of the Escherichia coli MscS mechanosensitive channel was recently solved. However, the conformation of the closed state and the gating transition remain uncharacterized. The pore-lining transmembrane helix contains a conserved glycine- and alanine-rich motif that forms a helix-helix interface. We show that introducing 'knobs' on the smooth glycine face by replacing glycine with alanine, and substituting conserved alanines with larger residues, increases the pressure required for gating. Creation of a glycine-glycine interface lowers activation pressure. The importance of residues Gly104, Ala106 and Gly108, which flank the hydrophobic seal, is demonstrated. A new structural model is proposed for the closed-to-open transition that involves rotation and tilt of the pore-lining helices. Introduction of glycine at Ala106 validated this model by acting as a powerful suppressor of defects seen with mutations at Gly104 and Gly108.

TRPC1 forms the stretch-activated cation channel in vertebrate cells

The mechanosensitive cation channel (MscCa) transduces membrane stretch into cation (Na(+), K(+), Ca(2+) and Mg(2+)) flux across the cell membrane, and is implicated in cell-volume regulation, cell locomotion, muscle dystrophy and cardiac arrhythmias. However, the membrane protein(s) that form the MscCa in vertebrates remain unknown. Here, we use an identification strategy that is based on detergent solubilization of frog oocyte membrane proteins, followed by liposome reconstitution and evaluation by patch-clamp. The oocyte was chosen because it expresses the prototypical MscCa (>or=10(7)MscCa/oocyte) that is preserved in cytoskeleton-deficient membrane vesicles. We identified a membrane-protein fraction that reconstituted high MscCa activity and showed an abundance of a protein that had a relative molecular mass of 80,000 (M(r) 80K). This protein was identified, by immunological techniques, as the canonical transient receptor potential channel 1 (TRPC1). Heterologous expression of the human TRPC1 resulted in a >1,000% increase in MscCa patch density, whereas injection of a TRPC1-specific antisense RNA abolished endogenous MscCa activity. Transfection of human TRPC1 into CHO-K1 cells also significantly increased MscCa expression. These observations indicate that TRPC1 is a component of the vertebrate MscCa, which is gated by tension developed in the lipid bilayer, as is the case in various prokaryotic mechanosensitive (Ms) channels.

Surface changes of the mechanosensitive channel MscS upon its activation, inactivation, and closing

MscS is a bacterial mechanosensitive channel that shows voltage dependence. The crystal structure of MscS revealed that the channel is a homoheptamer with a large chamber on the intracellular site. Our previous experiments indicated that the cytoplasmic chamber of the channel is not a rigid structure and changes its conformation upon the channel activation. In this study, we have applied various sized cosolvents that are excluded from protein surfaces. It is well known that such cosolvents induce compaction of proteins and prevent thermal fluctuations. It is also known that they shift channel equilibrium to the state of lower volume. We have found that large cosolvents that cannot enter the channel interior accelerate channel inactivation when applied from the cytoplasmic side, but they slow down inactivation when applied from the extracellular side. We have also found that small cosolvents that can enter the channel cytoplasmic chamber prevent the channel from opening, unlike the large ones. These data support our idea that the channel cytoplasmic chamber shrinks upon inactivation but also give new clues about conformational changes of the channel upon transitions between its functional states.

Lactococcus lactis uses MscL as its principal mechanosensitive channel

The functions of the mechanosensitive channels from Lactococcus lactis were determined by biochemical, physiological, and electrophysiological methods. Patch-clamp studies showed that the genes yncB and mscL encode MscS and MscL-like channels, respectively, when expressed in Escherichia coli or if the gene products were purified and reconstituted in proteoliposomes. However, unless yncB was expressed in trans, wild type membranes of L. lactis displayed only MscL activity. Membranes prepared from an mscL disruption mutant did not show any mechanosensitive channel activity, irrespective of whether the cells had been grown on low or high osmolarity medium. In osmotic downshift assays, wild type cells survived and retained 20% of the glycine betaine internalized under external high salt conditions. On the other hand, the mscL disruption mutant retained 40% of internalized glycine betaine and was significantly compromised in its survival upon osmotic downshifts. The data strongly suggest that L. lactis uses MscL as the main mechanosensitive solute release system to protect the cells under conditions of osmotic downshift.

Gating of the large mechanosensitive channel in situ: estimation of the spatial scale of the transition from channel population responses

Physical expansion associated with the opening of a tension-sensitive channel has the same meaning as gating charge for a voltage-gated channel. Despite increasing evidence for the open-state conformation of MscL, the energetic description of its complex gating remains incomplete. The previously estimated in-plane expansion of MscL is considerably smaller than predicted by molecular models. To resolve this discrepancy, we conducted a systematic study of currents and dose-response curves for wild-type MscL in Escherichia coli giant spheroplasts. Using the all-point histogram method and calibrating tension against the threshold for the small mechanosensitive channel (MscS) in each patch, we found that the distribution of channels among the subconducting states is significantly less dependent on tension than the distribution between the closed and conducting states. At -20 mV, all substates together occupy approximately 30% of the open time and reduce the mean integral current by approximately 6%, essentially independent of tension or P(o). This is consistent with the gating scheme in which the major rate-limiting step is the transition between the closed state and a low-conducting substate, and validates both the use of the integral current as a measure of P(o), and treatment of dose-response curves in the two-state approximation. The apparent energy and area differences between the states deltaE and deltaA, extracted from 29 independent dose-response curves, varied in a linearly correlated manner whereas the midpoint tension stayed at approximately 10.4 mN/m. Statistical modeling suggests slight variability of gating parameters among channels in each patch, causing a strong reduction and correlated spread of apparent deltaE and deltaA. The slope of initial parts of activation curves, with a few channels being active, gave estimates of deltaE = 51 +/- 13 kT and deltaA = 20.4 +/- 4.8 nm(2), the latter being consistent with structural models of MscL, which predict deltaA = 23 nm(2).

Loss-of-function mutations at the rim of the funnel of mechanosensitive channel MscL

MscL is a bacterial mechanosensitive channel that is activated directly by membrane stretch. Although the gene has been cloned and the crystal structure of the closed channel has been defined, how membrane tension causes conformational changes in MscL remains largely unknown. To identify the site where MscL senses membrane tension, we examined the function of the mutants generated by random and scanning mutagenesis. In vitro (patch-clamp) and in vivo (hypoosmotic-shock) experiments showed that when a hydrophilic amino acid replaces one of the hydrophobic residues that are thought to make contact with the membrane lipid near the periplasmic end of the M1 or M2 transmembrane domain, MscL loses the ability to open in response to membrane tension. Hydrophilic (asparagine) substitution of the other residues in the lipid-protein interface did not impair the channel's mechanosensitivity. These observations suggest that the disturbance of the hydrophobic interaction between the membrane lipid and the periplasmic rim of the channel's funnel impairs the function of MscL.

Genetic Models of Mechanotransduction: The NematodeCaenorhabditis elegans

Mechanotransduction, the conversion of a mechanical stimulus into a biological response, constitutes the basis for a plethora of fundamental biological processes such as the senses of touch, balance, and hearing and contributes critically to development and homeostasis in all organisms. Despite this profound importance in biology, we know remarkably little about how mechanical input forces delivered to a cell are interpreted to an extensive repertoire of output physiological responses. Recent, elegant genetic and electrophysiological studies have shown that specialized macromolecular complexes, encompassing mechanically gated ion channels, play a central role in the transformation of mechanical forces into a cellular signal, which takes place in mechanosensory organs of diverse organisms. These complexes are highly efficient sensors, closely entangled with their surrounding environment. Such association appears essential for proper channel gating and provides proximity of the mechanosensory apparatus to the source of triggering mechanical energy. Genetic and molecular evidence collected in model organisms such as the nematode worm Caenorhabditis elegans, the fruit fly Drosophila melanogaster, and the mouse highlight two distinct classes of mechanically gated ion channels: the degenerin (DEG)/epithelial Na+channel (ENaC) family and the transient receptor potential (TRP) family of ion channels. In addition to the core channel proteins, several other potentially interacting molecules have in some cases been identified, which are likely parts of the mechanotransducing apparatus. Based on cumulative data, a model of the sensory mechanotransducer has emerged that encompasses our current understanding of the process and fulfills the structural requirements dictated by its dedicated function. It remains to be seen how general this model is and whether it will withstand the impiteous test of time.

Gating-by-tilt of mechanically sensitive membrane channels

We propose an alternative mechanism for the gating of biological membrane channels in response to membrane tension that involves a change in the slope of the membrane near the channel. Under biological membrane tensions we show that the energy difference between the closed (tilted) and open (untilted) states can far exceed k(B)T and is comparable to what is available under simple dilational gating. Recent experiments demonstrate that membrane leaflet asymmetries (spontaneous curvature) can strongly affect the gating of some channels. Such a phenomenon would be easier to explain under gating-by-tilt, given its novel intrinsic sensitivity to such asymmetry.

Mechanosensitive channels: multiplicity of families and gating paradigms

Mechanosensitive ion channels are the primary transducers that convert mechanical force into an electrical or chemical signal in hearing, touch, and other mechanical senses. Unlike vision, olfaction, and some types of taste, which all use similar kinds of primary heterotrimeric GTP-binding protein-coupled receptors, mechanosensation relies on diverse types of transducer molecules. Unrelated types of channels can be used for the perception of various mechanical stimuli, not only in distant groups of organisms, but also in separate locations of the same organism. The extreme sensitivity of the transduction mechanism in the auditory system, which relies on an elaborate structure of rigid cilia, filamentous links, and molecular motors to focus force on transduction channels, contrasts with that of the bacterial channel MscL, which is opened by high lateral tension in the membrane and fulfills a safety-valve rather than a sensory function. The spatial scales of conformational movement and force in these two systems are described, and are shown to be consistent with a general physical description of mechanical channel gating. We outline the characteristics of several types of mechanosensitive channels and the functional contexts in which they participate in signaling and cellular regulation in sensory and nonsensory cells.

Investigating lipid composition effects on the mechanosensitive channel of large conductance (MscL) using molecular dynamics simulations

Previous experimental work has shown that the functional properties of the mechanosensitive channel of large conductance (MscL) are affected by variations in lipid composition. Here, we utilize molecular dynamics simulations of Mycobacterium tuberculosis MscL to investigate such lipid composition effects on a molecular level. In particular, two sets of simulations were performed. In the first, trajectories using lipids with different headgroups (phosphatidylcholine and phosphatidylethanolamine) were compared. Protein-lipid interactions were clearly altered by the headgroup changes, leading to conformational differences in the C-terminal region of M. tuberculosis MscL. In the second set of simulations, lipid tails were gradually shortened, thinning the membrane over a molecular dynamics trajectory. These simulations showed evidence of hydrophobic matching between MscL and the lipid membrane, as previously proposed. For all simulations, protein-lipid interaction energies in the second transmembrane region were correlated to mutagenic data, emphasizing the importance of lipid interactions for proper MscL function.

The closed structure of the MscS mechanosensitive channel. Cross-linking of single cysteine mutants

Mechanosensitive channels must make a large conformational change during the transition from the closed to the open state. The crystal structure of the open form of the Escherichia coli MscS channel was recently solved and depicts a homoheptamer (1). In this study, cross-linking of site-specific cysteine substitutions demonstrates that residues up to 10-33 A apart in the crystal structure readily form disulfide bridges in the closed form and can also be cross-linked by a 10-A linker. Cross-linking between adjacent subunits stabilizes the heptameric form of the channel providing biochemical evidence to support the crystal structure. The data are consistent with the published model (1) in that the membrane domain is highly flexible and that the closed to open transition may involve a significant displacement of transmembrane helices 1 and 2, possibly by as much as 30 A. The data are also consistent with significant flexibility of the cytoplasmic domain.

Structure and mechanism in prokaryotic mechanosensitive channels

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 offer a unique opportunity to identify 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. Recent functional, structural and dynamic data offer fresh insights into the molecular basis of gating for these membrane proteins.

On the conformation of the COOH-terminal domain of the large mechanosensitive channel MscL

COOH-terminal (S3) domains are conserved within the MscL family of bacterial mechanosensitive channels, but their function remains unclear. The X-ray structure of MscL from Mycobacterium tuberculosis (TbMscL) revealed cytoplasmic domains forming a pentameric bundle (Chang, G., R.H. Spencer, A.T. Lee, M.T. Barclay, and D.C. Rees. 1998. SCIENCE: 282:2220-2226). The helices, however, have an unusual orientation in which hydrophobic sidechains face outside while charged residues face inside, possibly due to specific crystallization conditions. Based on the structure of pentameric cartilage protein, we modeled the COOH-terminal region of E. coli MscL to better satisfy the hydrophobicity criteria, with sidechains of conserved aliphatic residues all inside the bundle. Molecular dynamic simulations predicted higher stability for this conformation compared with one modeled after the crystal structure of TbMscL, and suggested distances for disulfide trapping experiments. The single cysteine mutants L121C and I125C formed dimers under ambient conditions and more so in the presence of an oxidant. The double-cysteine mutants, L121C/L122C and L128C/L129C, often cross-link into tetrameric and pentameric structures, consistent with the new model. Patch-clamp examination of these double mutants under moderately oxidizing or reducing conditions indicated that the bundle cross-linking neither prevents the channel from opening nor changes thermodynamic parameters of gating. Destabilization of the bundle by replacing conservative leucines with small polar residues, or complete removal of COOH-terminal domain (Delta110-136 mutation), increased the occupancy of subconducting states but did not change gating parameters substantially. The Delta110-136 truncation mutant was functional in in vivo osmotic shock assays; however, the amount of ATP released into the shock medium was considerably larger than in controls. The data strongly suggest that in contrast to previous gating models (Sukharev, S., M. Betanzos, C.S. Chiang, and H.R. Guy. 2001a. NATURE: 409:720-724.), S3 domains are stably associated in both closed and open conformations. The bundle-like assembly of cytoplasmic helices provides stability to the open conformation, and may function as a size-exclusion filter at the cytoplasmic entrance to the MscL pore, preventing loss of essential metabolites.

Two families of mechanosensitive channel proteins

Mechanosensitive (MS) channels that provide protection against hypoosmotic shock are found in the membranes of organisms from the three domains of life: bacteria, archaea, and eucarya. Two families of ubiquitous MS channels are recognized, and these have been designated the MscL and MscS families. A high-resolution X-ray crystallographic structure is available for a member of the MscL family, and extensive molecular genetic, biophysical, and biochemical studies conducted in many laboratories have allowed postulation of a gating mechanism allowing the interconversion of a tightly closed state and an open state that controls transmembrane ion and metabolite fluxes. In contrast to the MscL channel proteins, which are of uniform topology, the much larger MscS family includes protein members with topologies that are predicted to vary from 3 to 11 alpha-helical transmembrane segments (TMSs) per polypeptide chain. Sequence analyses reveal that the three C-terminal TMSs of MscS channel proteins are conserved among family members and that the third of these three TMSs exhibits a 20-residue motif that is shared by the channel-forming TMS (TMS 1) of the MscL proteins. We propose that this C-terminal TMS in MscS family homologues serves as the channel-forming helix in a homooligomeric structure. The presence of a conserved residue pattern for the putative channel-forming TMSs in the MscL and MscS family proteins suggests a common structural organization, gating mechanism, and evolutionary origin.

Gravitaxis and graviperception in flagellates

There is strong evidence that gravitactic orientation in flagellates and ciliates is mediated by an active physiological gravireceptor rather than by passive alignment of the cells in the water column. In flagellates the threshold for graviorientation was found to be at 0.12 x g on a slow rotating centrifuge during the IML-2 mission on the Shuttle Columbia and a subsequent parabolic rocket flight (TEXUS). During the IML-2 mission no adaptation to microgravity was observed over the duration of the space flight, while gravitaxis was lost in a terrestrial closed environmental system over the period of almost two years. Sedimenting statoliths are not likely to be involved in graviperception because of the small size of the cells and their rotation around the longitudinal axis during forward locomotion. Instead the whole cytoplasmic content of the cell, being heavier than the surrounding aqueous medium (1.05 g/ml), exerts a pressure on the lower membrane. This force activates stretch-sensitive calcium specific ion channels which can be inhibited by the addition of gadolinium which therefore abolishes gravitaxis. The channels seem to mainly allow calcium ions to pass since gravitaxis is blocked by the addition of the calcium ionophore A23187 and by vanadate which blocks the Ca-ATPase in the cytoplasmic membrane. Recently, a gene for a mechanosensitive channel, originally sequenced for Saccharomyces, was identified in Euglena by PCR. The increase in intracellular free calcium during reorientation can be visualized by the fluorophore Calcium Crimson using laser excitation and image intensification. This result was confirmed during recent parabolic flights. The gated calcium changes the membrane potential across the membrane which may be the trigger for the reorientation of the flagellum. cAMP plays a role as a secondary messenger. Photosynthetic flagellates are suitable candidates for life support systems since they absorb CO2 and produce oxygen. Preliminary experiments are discussed.

Effect of microgravity on the swimming behaviour of larvae of the starfishPisaster ochraceus

Swimming larvae of the starfish Pisaster ochraceus rotate around their animal–vegetal axis every 2–5 s ( 12–30 rpm) and exhibit two patterns of swimming behaviour. They may swim with the animal pole forward in any direction or orient vertically (animal pole upward) and remain relatively stationary. Vertically oriented larvae adjust the pattern of rotation so that they present a larger surface area to gravity (holding behaviour). To determine how gravity is involved in the development and pattern of larval swimming behaviour, Pisaster larvae were raised in the aquatic research facility aboard the NASA space shuttle Endeavour (Mission STS 77). Control larvae raised in 1 g appeared to orient themselves along the gravity vector and to exhibit holding behaviour. Tracks of the larval swimming-pattern studies with a motion-analysis system demonstrated that larvae raised in microgravity swam in randomly oriented straight lines or broad arcs. Some of the tracks exhibited oscillations with a period of 2–5 s, while others did not. The results suggest that the holding behaviour, which normally serves as a response to gravity, develops despite the absence of gravitational clues. Possible mechanisms that the larvae may use to orient to gravity are discussed.

Crystal structure of Escherichia coli MscS, a voltage-modulated and mechanosensitive channel

The mechanosensitive channel of small conductance (MscS) responds both to stretching of the cell membrane and to membrane depolarization. The crystal structure at 3.9 angstroms resolution demonstrates that Escherichia coli MscS folds as a membrane-spanning heptamer with a large cytoplasmic region. Each subunit contains three transmembrane helices (TM1, -2, and -3), with the TM3 helices lining the pore, while TM1 and TM2, with membrane-embedded arginines, are likely candidates for the tension and voltage sensors. The transmembrane pore, apparently captured in an open state, connects to a large chamber, formed within the cytoplasmic region, that connects to the cytoplasm through openings that may function as molecular filters. Although MscS is likely to be structurally distinct from other ion channels, similarities in gating mechanisms suggest common structural elements.

Functional design of bacterial mechanosensitive channels. Comparisons and contrasts illuminated by random mutagenesis

MscS and MscL are mechanosensitive channels found in bacterial plasma membranes that open large pores in response to membrane tension. These channels function to alleviate excess cell turgor invoked by rapid osmotic downshock. Although much is known of the structure and molecular mechanisms underlying MscL, genes correlating with MscS activity have only recently been identified. Previously, it was shown that eliminating the expression of Escherichia coli yggB removed a major portion of MscS activity. YggB is distinct from MscL by having no obvious structural similarity. Here we have reconstituted purified YggB in proteoliposomes and have successfully detected MscS channel activity, confirming that purified YggB protein encodes MscS activity. Additionally, to define functional regions of the channel protein, we have randomly mutagenized the structural gene and isolated a mutant that evokes a gain-of-function phenotype. Physiological experiments demonstrate that the mutated channel allows leakage of solutes from the cell, suggesting inappropriate channel opening. Interestingly, this mutation is analogous in position and character to mutations yielding a similar phenotype in MscL. Hence, although MscS and MscL mechanosensitive channels are structurally quite distinct, there may be analogies in their gating mechanisms.

Purification of the small mechanosensitive channel of Escherichia coli (MscS): the subunit structure, conduction, and gating characteristics in liposomes

The small mechanosensitive channel, MscS, is a part of the turgor-driven solute efflux system that protects bacteria from lysis in the event of osmotic downshift. It has been identified in Escherichia coli as a product of the orphan yggB gene, now called mscS (Levina et al., 1999, EMBO J. 18:1730). Here I show that that the isolated 31-kDa MscS protein is sufficient to form a functional mechanosensitive channel gated directly by tension in the lipid bilayer. MscS-6His complexes purified in the presence of octylglucoside and lipids migrate in a high-resolution gel-filtration column as particles of approximately 200 kDa. Consistent with that, the protein cross-linking patterns predict a hexamer. The channel reconstituted in soybean asolectin liposomes was activated by pressures of 20-60 mm Hg and displayed the same asymmetric I-V curve and slight anionic preference as in situ. At the same time, the single-channel conductance is proportional to the buffer conductivity in a wide range of salt concentrations. The rate of channel activation in response to increasing pressure gradient across the patch was slower than the rate of closure in response to decreasing steps of pressure gradient. Therefore, the open probability curves were recorded with descending series of pressures. Determination of the curvature of patches by video imaging permitted measurements of the channel activity as a function of membrane tension (gamma). Po(gamma) curves had the midpoint at 5.5 +/- 0.1 dyne/cm and gave estimates for the energy of opening DeltaG = 11.4 +/- 0.5 kT, and the transition-related area change DeltaA = 8.4 +/- 0.4 nm(2) when fitted with a two-state Boltzmann model. The correspondence between channel properties in the native and reconstituted systems is discussed.

Molecular dynamics simulations of wild-type and mutant forms of the Mycobacterium tuberculosis MscL channel

The crystal structure of the Mycobacterium tuberculosis homolog of the bacterial mechanosensitive channel of large conductance (Tb-MscL) provides a unique opportunity to consider mechanosensitive signal transduction at the atomic level. Molecular dynamics simulations of the Tb-MscL channel embedded in an explicit lipid bilayer and of its C-terminal helical bundle alone in aqueous solvent were performed. C-terminal calculations imply that although the helix bundle structure is relatively unstable at physiological pH, it may have been stabilized under low pH conditions such as those used in the crystallization of the channel. Specific mutations to the C-terminal region, which cause a similar conservation of the crystal structure conformation, have also been identified. Full channel simulations were performed for the wild-type channel and two experimentally characterized gain-of-function mutants, V21A and Q51E. The wild-type Tb-MscL trajectory gives insight into regions of relative structural stability and instability in the channel structure. Channel mutations led to observable changes in the trajectories, such as an alteration of intersubunit interactions in the Q51E mutant. In addition, interesting patterns of protein-lipid interactions, such as hydrogen bonding, arose in the simulations. These and other observations from the simulations are relevant to previous and ongoing experimental studies focusing on characterization of the channel.

Site-directed spin-labeling analysis of reconstituted Mscl in the closed state

The mechanosensitive channel from Escherichia coli (Eco-MscL) responds to membrane lateral tension by opening a large, water-filled pore that serves as an osmotic safety valve. In an attempt to understand the structural dynamics of MscL in the closed state and under physiological conditions, we have performed a systematic site-directed spin labeling study of this channel reconstituted in a membrane bilayer. Structural information was derived from an analysis of probe mobility, residue accessibility to O(2) or NiEdda and overall intersubunit proximity. For the majority of the residues studied, mobility and accessibility data showed a remarkable agreement with the Mycobacterium tuberculosis crystal structure, clearly identifying residues facing the large water-filled vestibule at the extracellular face of the molecule, the narrowest point along the permeation pathway (residues 21-26 of Eco-MscL), and the lipid-exposed residues in the peripheral transmembrane segments (TM2). Overall, the present dataset demonstrates that the transmembrane regions of the MscL crystal structure (obtained in detergent and at low pH) are, in general, an accurate representation of its structure in a membrane bilayer under physiological conditions. However, significant differences between the EPR data and the crystal structure were found toward the COOH-terminal end of TM2.

Chemically charging the pore constriction opens the mechanosensitive channel MscL

MscL is a bacterial mechanosensitive channel that protects the cell from osmotic downshock. We have previously shown that substitution of a residue that resides within the channel pore constriction, MscL's Gly-22, with all other 19 amino acids affects channel gating according to the hydrophobicity of the substitution (). Here, we first make a mild substitution, G22C, and then attach methanethiosulfonate (MTS) reagents to the cysteine under patch clamp. Binding MTS reagents that are positively charged ([2-(trimethylammonium)ethyl] methanethiosulfonate and 2-aminoethyl methanethiosulfonate) or negatively charged (sodium (2-sulfonatoethyl)methanethiosulfonate) causes MscL to gate spontaneously, even when no tension is applied. In contrast, the polar 2-hydroxyethyl methanethiosulfonate halves the threshold, and the hydrophobic methyl methanethiolsulfonate increases the threshold. These observations indicate that residue 22 is in a hydrophobic environment before gating and in a hydrophilic environment during opening to a substate, a finding consistent with our previous study. In addition, we have found that cysteine 22 is accessible to reagents from the cytoplasmic side only when the channel is opened whereas it is accessible from the periplasmic side even in the closed state. These results support the view that exposure of hydrophobic surfaces to a hydrophilic environment during channel opening serves as the barrier to gating.

Structural and functional differences between two homologous mechanosensitive channels of Methanococcus jannaschii

We report the molecular cloning and characterization of MscMJLR, a second type of mechanosensitive (MS) channel found in the archaeon Methanococcus jannaschii. MscMJLR is structurally very similar to MscMJ, the MS channel of M.jannaschii that was identified and cloned first by using the TM1 domain of Escherichia coli MscL as a genetic probe. Although it shares 44% amino acid sequence identity and similar cation selectivity with MscMJ, MscMJLR exhibits other major functional differences. The conductance of MscMJLR of approximately 2 nS is approximately 7-fold larger than the conductance of MscMJ and rectifies with voltage. The channel requires approximately 18 kT for activation, which is three times the amount of energy required to activate MscMJ, but is comparable to the activation energy of Eco-MSCL: Our study indicates that a multiplicity of conductance-wise and energetically well-tuned MS channels in microbial cell membranes may provide for cell survival by the sequential opening of the channels upon challenge with different osmotic cues.

Molecular basis of mechanotransduction in living cells

The simplest cell-like structure, the lipid bilayer vesicle, can respond to mechanical deformation by elastic membrane dilation/thinning and curvature changes. When a protein is inserted in the lipid bilayer, an energetic cost may arise because of hydrophobic mismatch between the protein and bilayer. Localized changes in bilayer thickness and curvature may compensate for this mismatch. The peptides alamethicin and gramicidin and the bacterial membrane protein MscL form mechanically gated (MG) channels when inserted in lipid bilayers. Their mechanosensitivity may arise because channel opening is associated with a change in the protein's membrane-occupied area, its hydrophobic mismatch with the bilayer, excluded water volume, or a combination of these effects. As a consequence, bilayer dilation/thinning or changes in local membrane curvature may shift the equilibrium between channel conformations. Recent evidence indicates that MG channels in specific animal cell types (e.g., Xenopus oocytes) are also gated directly by bilayer tension. However, animal cells lack the rigid cell wall that protects bacteria and plants cells from excessive expansion of their bilayer. Instead, a cortical cytoskeleton (CSK) provides a structural framework that allows the animal cell to maintain a stable excess membrane area (i.e., for its volume occupied by a sphere) in the form of membrane folds, ruffles, and microvilli. This excess membrane provides an immediate membrane reserve that may protect the bilayer from sudden changes in bilayer tension. Contractile elements within the CSK may locally slacken or tighten bilayer tension to regulate mechanosensitivity, whereas membrane blebbing and tight seal patch formation, by using up membrane reserves, may increase membrane mechanosensitivity. In specific cases, extracellular and/or CSK proteins (i.e., tethers) may transmit mechanical forces to the process (e.g., hair cell MG channels, MS intracellular Ca(2+) release, and transmitter release) without increasing tension in the lipid bilayer.

Induced membrane hypo/hyper-mechanosensitivity: a limitation of patch-clamp recording

Practical limitations of the patch-clamp technique when recording mechanogated membrane ion channels are considered. Mechanical overstimulation of the patch or the cell from excessive suction/pressure protocols induces morphological and functional changes. In particular, the plasma membrane becomes decoupled from the underlying cytoskeleton to form either membrane blebs (cell-attached) or ghosts (whole cell). As a consequence, a membrane ion channel may show either a decrease or an increase in its native mechanosensitivity or even acquire mechanosensitivity. The effect varies with ion channel and cell type and presumably arises because of a disruption of membrane-cytoskeleton interactions. We consider that such disruptions are a pathological consequence of excessive mechanical stress, either during or after seal formation, rather than an immutable consequence of patch-clamp recording. By careful attention to the suction/pressure protocols during sealing and throughout recording, such artifacts can be avoided.

Molecular identification of a mechanosensitive channel in archaea

The TM1 domain of the large conductance mechanosensitive (MS) channel of Escherichia coli was used as a genetic probe to search the genomic database of the archaeon Methanoccoccus jannashii for MscL homologs. We report that the hypothetical protein MJ0170 of M. jannashii exhibited 38.5% sequence identity with the TM1 domain of Eco-MscL. Moreover, MJ0170 was found to be a conserved homolog of MscS, the second type of E. coli MS channel encoded by the yggB gene. Furthermore, we identified a cluster of charged residues KIKEE in the C-terminus of MJ0170 that strikingly resembled the charged C-terminal amino acid cluster present in Eco-MscL (RKKEE). We cloned and expressed MJ0170 in E. coli, which when reconstituted into liposomes or expressed in the cell membrane of giant E. coli spheroplasts, exhibited similar activity to the bacterial MS channels. Our study suggests that the M. jannashii MS channel and its homologs evolved as a result of gene duplication of the ancestral MscL-like molecule with the TM1 domain remaining the most conserved structural motif among prokaryotic MS channels.

Graviperception and gravitaxis in algae

Photosynthetic flagellates are among the most intensely studied unicellular organisms in the field of graviperception and gravitaxis. While the phenomenon of graviorientation has been known for many decades, only recently was the molecular mechanism unveiled. Earlier hypotheses tried to explain the precise orientation by a passive buoy mechanism assuming the tail end to be heavier than the front. In the photosynthetic flagellate Euglena gracilis, the whole cell body is denser than the surrounding medium, pressing onto the lower cell membrane where it seems to activate mechanosensitive ion channels specific for calcium. The calcium entering the cells during reorientation can be visualized by the fluorescence probe, Calcium Crimson. Cyclic AMP is likewise involved in the molecular pathway. Inhibitors of calcium channels and ionophores impair gravitaxis while caffeine, a blocker of the phosphodiesterase, enhances the precision of orientation.

Correlating a protein structure with function of a bacterial mechanosensitive channel

MscL, a mechanosensitive channel found in many bacteria, protects cells from hypotonic shock by reducing intracellular pressure through release of cytoplasmic osmolytes. First isolated from Escherichia coli, this protein has served as a model for how a protein senses and responds to membrane tension. Recently the structure of a functionally uncharacterized MscL homologue from Mycobacterium tuberculosis was solved by x-ray diffraction to a resolution of 3.5 A. Here we demonstrate that the protein forms a functional MscL-like mechanosensitive channel in E. coli membranes and azolectin proteoliposomes. Furthermore, we show that M. tuberculosis MscL crystals, when re-solubilized and reconstituted, yield wild-type channel currents in patch clamp, demonstrating that the protein does not irreversibly change conformation upon crystallization. Finally, we apply functional clues acquired from the E. coli MscL to the M. tuberculosis channel and show a mechanistic correlation between these channels. However, the inability of the M. tuberculosis channel to gate at physiological membrane tensions, demonstrated by in vivo E. coli expression and in vitro reconstitution, suggests that the membrane environment or other additional factors influence the gating of this channel.

Mechanical control of cyclic AMP signalling and gene transcription through integrins

This study was carried out to discriminate between two alternative hypotheses as to how cells sense mechanical forces and transduce them into changes in gene transcription. Do cells sense mechanical signals through generalized membrane distortion or through specific transmembrane receptors, such as integrins? Here we show that mechanical stresses applied to the cell surface alter the cyclic AMP signalling cascade and downstream gene transcription by modulating local release of signals generated by activated integrin receptors in a G-protein-dependent manner, whereas distortion of integrins in the absence of receptor occupancy has no effect.

Effects of organic antagonists of Ca(2+), Na(+), and K(+) on chemotaxis and motility of escherichia coli

Various Ca(2+) antagonists used in animal research, many of them known to be Ca(2+) channel blockers, inhibited Escherichia coli chemotaxis (measured as entry of cells into a capillary containing attractant). The most effective of these, acting in the nanomolar range, was omega-conotoxin GVIA. The next most effective were gallopamil and verapamil. At concentrations around 100-fold higher than that needed for inhibition of chemotaxis, each of these antagonists inhibited motility (measured as entry of cells into a capillary lacking attractant). Various other Ca(2+) antagonists were less effective, though chemotaxis was almost always more sensitive to inhibition than was motility. Cells treated with each of these Ca(2+) antagonists swam with a running bias, i.e., tumbling was inhibited. Similarly, some Na(+) antagonists used in animal research inhibited bacterial chemotaxis. E. coli chemotaxis was inhibited by saxitoxin at concentrations above 10(-7) M, while more than 10(-4) M was needed to inhibit motility. Cells treated with saxitoxin swam with a tumbling bias. In the case of other Na(+) antagonists in animals, aconitine inhibited bacterial chemotaxis 10 times more effectively than it inhibited motility, and two others inhibited chemotaxis and motility at about the same concentration. In the case of K(+) antagonists used in animal research, 4-aminopyridine blocked E. coli chemotaxis between 10(-3) M and, totally, 10(-2) M, while motility was not affected at 10(-2) M; on the other hand, tetraethylammonium chloride failed to inhibit either chemotaxis or motility at 10(-2) M.

The positive role of voids in the plasma membrane in growth and energetics of Escherichia coli

Bacterial respiration, endogenous as well as induced respiration by glucose, lactose and glycine betaine, was found to be sensitive to external solute concentration. Permeability of hydrogen peroxide, a non-electrolyte of molecular size between water and urea, through the bacterial membranes changed directly with the rate of respiration (an activity residing in the bacterial plasma membrane) in E. coli and the enhanced permeability and respiratory activity were highly correlated. Hydrogen peroxide permeability and induction of voids (spaces in the matrix of the bilayer into which hydrophobic fluorescent probes partition, which in turn were used to assess the modulation of these cavities) were shown to be a direct and excellent measure of leak conductance. Fluorescence intensity and anisotropy of the extrinsic fluorescent probes (incorporated by growing bacteria in their presence) decreased with increased respiration in bacteria, consistent with lowered molecular restriction and enhanced hydration in the membrane phase for these probes as seen in dimyristoylphosphatidylcholine bilayers due to phase transition. The physical basis of osmotic phenomena, as a relevant (thermodynamic) volume, could relate to water exchange or compression, depending on the osmotic domain. In the domain of compression in bacteria, i.e. well above the isotonic range, the computed activation volume was consistent with voids in the membrane. This study emphasises a major role of leak conductance in bacterial physiology and growth.

Hydrophilicity of a single residue within MscL correlates with increased channel mechanosensitivity

Mechanosensitive channel large (MscL) encodes the large conductance mechanosensitive channel of the Escherichia coli inner membrane that protects bacteria from lysis upon osmotic shock. To elucidate the molecular mechanism of MscL gating, we have comprehensively substituted Gly(22) with all other common amino acids. Gly(22) was highlighted in random mutagenesis screens of E. coli MscL (, Proc. Nat. Acad. Sci. USA. 95:11471-11475). By analogy to the recently published MscL structure from Mycobacterium tuberculosis (, Science. 282:2220-2226), Gly(22) is buried within the constriction that closes the pore. Substituting Gly(22) with hydrophilic residues decreased the threshold pressure at which channels opened and uncovered an intermediate subconducting state. In contrast, hydrophobic substitutions increased the threshold pressure. Although hydrophobic substitutions had no effect on growth, similar to the effect of an MscL deletion, channel hyperactivity caused by hydrophilic substitutions correlated with decreased proliferation. These results suggest a model for gating in which Gly(22) moves from a hydrophobic, and through a hydrophilic, environment upon transition from the closed to open conformation.

Bacterial mechanosensitive channels: integrating physiology, structure and function

When confronted with hypo-osmotic stress, many bacterial species are able rapidly to adapt to the increase in cell turgor pressure by jettisoning cytoplasmic solutes into the medium through membrane-tension-gated channels. Physiological studies have confirmed the importance of these channels in osmoregulation. Mutagenesis of one of these channels, combined with structural information derived from X-ray crystallography, has given the first clues of how a mechanosensitive channel senses and responds to membrane tension.

Mechanosensitive channels in bacteria as membrane tension reporters

The purpose of this short review is to discuss recent data on the molecular structure and mechanism of gating of MscL, a mechanosensitive channel of large conductance from Escherichia coli. MscL is the first isolated molecule shown to convert mechanical stress of the membrane into a simple response, the opening of a large aqueous pore. The functional complex appears to be a stable homo-pentamer of 15-kDa subunits, the gating transitions in which are driven by stretch forces conveyed through the lipid bilayer. We have measured the open probability of MscL and the kinetics of transitions as a function of membrane tension. The parameters extracted from the single-channel current recordings and dose-response curves such as the energy difference between the closed, open, and intermediate conducting states, and the transition-related changes in protein dimensions suggest a large conformational rearrangement of the channel complex. The estimations show that in native conditions MscL openings could be driven primarily by forces of osmotic nature. The thermodynamic and spatial parameters reasonably correlate with the available data on the structure of a single MscL subunit and multimeric organization of the complex. Combined with the functional analysis of mutations, these data give grounds to hypotheses on the nature of the channel mechanosensitivity.

Channel gate! Tension, leak and disclosure

The crystal structure of a bacterial MscL shows how this homopentameric channel protein is held tightly shut to prevent leakage whilst at rest. By inference, the structure also shows how a stretch force in the lipid bilayer causes the channel to open. We now have a concrete picture as to how a stimulus 'gates' an ion channel.

Managing hypoosmotic stress: aquaporins and mechanosensitive channels in Escherichia coli

When Escherichia coli cells are subject to hypoosmotic shock they are subject to substantial flows of water that can be equivalent to a 4-5-fold increase in the pressure exerted from the cytoplasm on the membrane and peptidoglycan wall. The recently described aquaporin that facilitates rapid water movement across the cytoplasmic membrane is repressed during growth at high osmolarity. This may enable the cell to reduce the rate of pressure build up during transitions from high to low osmolarity. The presence of multiple mechanosensitive channels in the E. coli cell membrane is well documented. The recent identification of genes that inactivate the MscL and MscS channels has established their role in releasing the pressure built up by hypoosmotic shock. The isolation of specific mutations and the structural studies on MscL now pave the way to a molecular understanding of the mechanism of activation of mechanosensitive channels.

Mechanosensitivity as an integrative system in heart: an audit

This review examines a manifold of apparently loosely linked observations and mechanisms, from membrane to man, and assembles them to support the notion that mechanoelectric transduction is an integrative regulatory system in the heart. For this, the assemblage has to satisfy, at least to some extent, criteria that apply to other integrative regulatory systems such as the endocrine and nervous systems. The integrative effectors in the endocrine system are chemical linkages, circulating hormones: in the nervous system the linkage is a network of cables, nerve conduction and neurotransmitters. Mechanical integration is would be effected through mechanical machinery, cardiac contractile and hydraulic function with attendant stress and strain transmitted via "tensegrity". This can, through the cytoskeleton, begin with membrane integrins and transmit intracellularly for example via F actins to reach the rest of the membranous integrins. Further transmission to the organ is via cell-to-cell adhesion complexes and the extracellular matrix. This tensegrity facilitates integration of force and strain changes from area to area. In consequence, and analogous to the neurendocrine system, mechanoelectric transduction should, and does (1) operate at the molecular or membrane level--this would be via mechanotransducers affecting transmembrane ionic flow; (2) operate in the cell--to influence electrophysiology; (3) have a multicellular expression--e.g. mechanical distortion of one cell can raise intracellular calcium of an adjacent cell; (4) express in the intact organ--e.g. an increase in venous return hydraulically distends the sinoatrial node, steepening its pacemaker potential, thus increasing heart rate. It should also (5) demonstrate elements of a feedback system--"mechanoelectric feedback", and (6) interact with other systems--the cytoskeleton incorporates cell signalling complexes intersecting with other signal cascades. Finally, (7) it can malfunction to produce clinical abnormality--it contributes electrophysiologically to lethal cardiac arrhythmia. This anatomical and functional behaviour of mechanoelectric transduction could sanction the prospect of viewing it as analogous to the other integrative physiological systems.

Negative gravitactic behavior of Euglena gracilis can not be described by the mechanism of buoyancy-oriented upward swimming

Gravitactic behavior of microorganisms has been known for more than a hundred years. Euglena gracilis serves as a model system for gravity-triggered behavioral responses. Two basic mechanisms are discussed for gravitaxis: one is based on a physical mechanism where an asymmetric mass distribution pulls the cell passively in the correct orientation and, in contrast, the involvement of an active sensory system. A recently developed high-resolution motion-tracking system allows the analysis of single tracks during reorientation. The results are compared to a model developed by Fukui and Asai (1985) which describes gravitaxis of Paramecium caudatum on the basis of a physical mechanism. Taking into account the different size, different density, different mass distribution as well as the different velocity, results of the adapted model description of Paramecium were applied to measured data of Euglena. General shapes as well as the time scale of the predicted reorientational movement compared to measurements were different. The analysis clearly rules out the possibility that gravitaxis of Euglena gracilis is based on a pure physical phenomenon, and gives further support to the involvement of an active reorientational system. In addition, it could be shown that cell form changes during reorientation, even in an initial period where no angular change was observed.

Mechanosensitive ion channels in putative aortic baroreceptor neurons

Cell-attached patch-clamp experiments were performed on dissociated neurons from nodose ganglia of adult rats. Putative aortic baroreceptor neurons were identified by labeling nerve endings in the adventitia of the aortic arch with the carbocyanine dye DiI. Whereas previous experiments demonstrated the presence of mechanosensitive (MS) whole cell currents, these experiments studied single MS ion channels and examined the influence of culture conditions on their expression. Single MS channels were activated by applying negative pressure through the recording pipette. Channel openings became more frequent as the negative pressure was increased, with open probability increasing significantly above 30 mmHg. MS channels had a slope conductance of 114 pS and a reversal potential of approximately 0 mV, consistent with a nonspecific cation conductance. Channels were not affected by antagonists of voltage-gated conductances but were blocked by 20 microM gadolinium, a known blocker of MS ion channels. When nodose neurons were cocultured with aortic endothelial cells, but not aortic smooth muscle cells, the percentage of patches exhibiting MS ion channels increased significantly, suggesting that aortic endothelial cells secrete a diffusible factor that increases channel expression.

One face of a transmembrane helix is crucial in mechanosensitive channel gating

MscL is a mechanosensitive channel in bacteria that responds directly to membrane tension by opening a large conductance pore. To determine functionally important residues within this molecule, we have randomly mutagenized mscL, expressed the genes in living bacteria, and screened for gain-of-function mutants with hampered growth. Expression of these genes caused leakage of cytoplasmic solutes on little or no hypo-osmotic stress. In excised patches, the mutant channels gated at membrane tensions that are less than that required for the gating of the wild-type MscL. Hence, the data suggest that the slowed or no-growth phenotype is caused by solute loss because of inappropriate gating of the channel. Most of the mutations mapped to the first transmembrane domain. When this domain is modeled as an alpha-helix, the most severe mutations are substitutions of smaller amino acids (three glycines and one valine) on one facet, suggesting an important role for this structure in MS channel gating.

Intracellular ionic strength regulates the volume sensitivity of a swelling-activated anion channel

Cell swelling activates an outwardly rectifying anion channel termed VSOAC (volume-sensitive organic osmolyte/anion channel). Regulation of VSOAC by intracellular electrolytes was characterized in Chinese hamster ovary cells by whole cell patch clamp. Elevation of intracellular CsCl concentration from 40 to 180 mM resulted in a concentration-dependent decrease in channel activation. Activation of VSOAC was insensitive to the salt gradient across the plasma membrane, the intracellular concentration of specific anions or cations, and the total intracellular concentration of cations, anions, or electrolytes. Comparison of cells dialyzed with either CsCl or Na2SO4 solutions demonstrated directly that VSOAC activation is modulated by intracellular ionic strength (microi). The relative cell volume at which VSOAC current activation was triggered, termed the channel volume set point, decreased with decreasing ionic strength. At microi = 0.04, VSOAC activation occurred spontaneously in shrunken cells. The rate of VSOAC activation was nearly 50-fold higher in cells with microi = 0.04 vs. those with microi = 0.18. We propose that microi modulates the volume sensor responsible for channel activation.