Proteoliposomes reconstituted with human aquaporin-1 reveal novel single-ion-channel properties.

Human aquaporin 1 (hAQP1) forms homotetrameric channels that facilitate fluxes of water and small solutes across cell membranes. In addition to water channel activity, hAQP1 displays non-selective monovalent cation-channel activity gated by intracellular cyclic GMP. Dual water and ion-channel activity of hAQP1, thought to regulate cell shape and volume, could offer a target for novel therapeutics relevant to controlling cancer cell invasiveness. This study probed properties of hAQP1 ion channels using proteoliposomes, which, unlike conventional cell-based systems such as Xenopus laevis oocytes, are relatively free of background ion channels. Histidine-tagged recombinant hAQP1 protein was synthesized and purified from the methylotrophic yeast, Pichia pastoris, and reconstituted into proteoliposomes for biophysical analyses. Osmotic water channel activity confirmed correct folding and channel assembly. Ion-channel activity of hAQP1-Myc-His6 was recorded by patch-clamp electrophysiology with excised patches. In symmetrical potassium, the hAQP1-Myc-His6 channels displayed coordinated gating, a single-channel conductance of approximately 75 pS, and multiple subconductance states. Applicability of this method for structure-function analyses was tested using hAQP1-Myc-His6 D48A/D185A channels modified by site-directed mutations of charged Asp residues estimated to be adjacent to the central ion-conducting pore of the tetramer. No differences in conductance were detected between mutant and wild-type constructs, suggesting the open-state conformation could differ substantially from expectations based on crystal structures. Nonetheless, the method pioneered here for AQP1 demonstrates feasibility for future work defining structure-function relationships, screening pharmacological inhibitors, and testing other classes in the broad family of aquaporins for previously undiscovered ion-conducting capabilities.

"Force-From-Lipids" Dependence of the MscCG Mechanosensitive Channel Gating on Anionic Membranes.

Mechanosensory transduction in Corynebacterium glutamicum plays a major role in glutamate efflux for industrial MSG, whose production depends on the activation of MscCG-type mechanosensitive channels. Dependence of the MscCG channel activation by membrane tension on the membrane lipid content has to date not been functionally characterized. Here, we report the MscCG channel patch clamp recording from liposomes fused with C. glutamicum membrane vesicles as well as from proteoliposomes containing the purified MscCG protein. Our recordings demonstrate that mechanosensitivity of MscCG channels depends significantly on the presence of negatively charged lipids in the proteoliposomes. MscCG channels in liposome preparations fused with native membrane vesicles exhibited the activation threshold similar to the channels recorded from C. glutamicum giant spheroplasts. In comparison, the activation threshold of the MscCG channels reconstituted into azolectin liposomes was higher than the activation threshold of E. coli MscL, which is gated by membrane tension close to the bilayer lytic tension. The spheroplast-like activation threshold was restored when the MscCG channels were reconstituted into liposomes made of E. coli polar lipid extract. In liposomes made of polar lipids mixed with synthetic phosphatidylethanolamine, phosphatidylglycerol, and cardiolipin, the activation threshold of MscCG was significantly reduced compared to the activation threshold recorded in azolectin liposomes, which suggests the importance of anionic lipids for the channel mechanosensitivity. Moreover, the micropipette aspiration technique combined with patch fluorometry demonstrated that membranes containing anionic phosphatidylglycerol are softer than membranes containing only polar non-anionic phosphatidylcholine and phosphatidylethanolamine. The difference in mechanosensitivity between C. glutamicum MscCG and canonical MscS of E. coli observed in proteoliposomes explains the evolutionary tuning of the force from lipids sensing in various bacterial membrane environments.

Role of the Arginine Cluster in the Disordered Domain of Herpes Simplex Virus 1 UL34 for the Recruitment of ESCRT-III for Viral Primary Envelopment.

During the nuclear export of nascent nucleocapsids of herpesviruses, the nucleocapsids bud through the inner nuclear membrane (INM) by acquiring the INM as a primary envelope (primary envelopment). We recently reported that herpes simplex virus 1 (HSV-1) nuclear egress complex (NEC), which consists of UL34 and UL31, interacts with an endosomal sorting complex required for transport III (ESCRT-III) adaptor ALIX and recruits ESCRT-III machinery to the INM for efficient primary envelopment. In this study, we identified a cluster of six arginine residues in the disordered domain of UL34 as a minimal region required for the interaction with ALIX, as well as the recruitment of ALIX and an ESCRT-III protein CHMP4B to the INM in HSV-1-infected cells. Mutations in the arginine cluster exhibited phenotypes similar to those with ESCRT-III inhibition reported previously, including the mislocalization of NEC, induction of membranous invagination structures containing enveloped virions, aberrant accumulation of enveloped virions in the invaginations and perinuclear space, and reduction of viral replication. We also showed that the effect of the arginine cluster in UL34 on HSV-1 replication was dependent primarily on ALIX. These results indicated that the arginine cluster in the disordered domain of UL34 was required for the interaction with ALIX and the recruitment of ESCRT-III machinery to the INM to promote primary envelopment. IMPORTANCE Herpesvirus UL34 homologs contain conserved amino-terminal domains that mediate vesicle formation through interactions with UL31 homologs during primary envelopment. UL34 homologs also comprise other domains adjacent to their membrane-anchoring regions, which differ in length, are variable in herpesviruses, and do not form distinguished secondary structures. However, the role of these disordered domains in infected cells remains to be elucidated. In this study, we present data suggesting that the arginine cluster in the disordered domain of HSV-1 UL34 mediates the interaction with ALIX, thereby leading to the recruitment of ESCRT-III machinery to the INM for efficient primary envelopment. This is the first study to report the role of the disordered domain of a UL34 homolog in herpesvirus infections.

Corynebacterium glutamicum Mechanosensing: From Osmoregulation to L-Glutamate Secretion for the Avian Microbiota-Gut-Brain Axis.

After the discovery of Corynebacterium glutamicum from avian feces-contaminated soil, its enigmatic L-glutamate secretion by corynebacterial MscCG-type mechanosensitive channels has been utilized for industrial monosodium glutamate production. Bacterial mechanosensitive channels are activated directly by increased membrane tension upon hypoosmotic downshock; thus; the physiological significance of the corynebacterial L-glutamate secretion has been considered as adjusting turgor pressure by releasing cytoplasmic solutes. In this review, we present information that corynebacterial mechanosensitive channels have been evolutionally specialized as carriers to secrete L-glutamate into the surrounding environment in their habitats rather than osmotic safety valves. The lipid modulation activation of MscCG channels in L-glutamate production can be explained by the "Force-From-Lipids" and "Force-From-Tethers" mechanosensing paradigms and differs significantly from mechanical activation upon hypoosmotic shock. The review also provides information on the search for evidence that C. glutamicum was originally a gut bacterium in the avian host with the aim of understanding the physiological roles of corynebacterial mechanosensing. C. glutamicum is able to secrete L-glutamate by mechanosensitive channels in the gut microbiota and help the host brain function via the microbiota-gut-brain axis.

Membrane stiffness is one of the key determinants of E. coli MscS channel mechanosensitivity.

Mechanosensitive (MS) channels have an intimate relationship with membrane lipids that underlie their mechanosensitivity. Membrane lipids may influence channel activity by directly interacting with MS channels or by influencing the global properties of the membrane such as elastic area expansion modulus or bending rigidity. Previous work has implicated membrane stiffness as a potential determinant of the mechanosensitivity of E. coli (Ec)MscS. Here we systematically tested this hypothesis using patch fluorometry of azolectin liposomes doped with lipids of increasing elastic area expansion modulus. Increasing dioleoylphosphatidylethanolamine (DOPE) content of azolectin liposomes made it more difficult to activate EcMscS by membrane tension (i.e. increased gating threshold). This effect was exacerbated by stiffer forms of phosphatidylethanolamine such as the branched chain lipid diphytanoylphosphoethanolamine (DPhPE) or the fully saturated lipid distearoyl-sn-glycero-3-phosphoethanolamine (DSPE). Furthermore, a comparison of the branched chain lipid diphytanoylphosphocholine (DPhPC) to the stiffer DPhPE indicated again that it was harder to activate EcMscS in the presence of the stiffer DPhPE. We show that these effects are not due to changes in membrane bending rigidity as the membrane tension threshold of EcMscS in membranes doped with PC18:1 and PC18:3 remained the same, despite a two-fold difference in their bending rigidity. We also show that after prolonged pressure application sudden removal of force in softer membranes caused a rebound reactivation of EcMscS and we discuss the relevance of this phenomenon to bacterial osmoregulation. Collectively, our data suggests that membrane stiffness (elastic area expansion modulus) is one of the key determinants of the mechanosensitivity of EcMscS.

Cell membrane mechanics and mechanosensory transduction.

The rapid progress in mechanobiology has brought together many scientific and engineering disciplines to work hand in hand toward better understanding of the role that mechanical force plays in functioning and evolution of different forms of life. New tools designed by engineers helped to develop new methods and techniques for investigation of mechanical properties of biological cells and tissues. This multidisciplinary approach made it clear that cell mechanics is tightly linked to intracellular signaling pathways, which directly regulate gene expression in response to mechanical stimuli originating outside or inside the cells. Mechanical stimuli act on mechanoreceptors which convert these stimuli into intracellular signals. In this chapter, we review the current knowledge about cell mechanics and the role cell mechanics plays for the function of mechanosensitive ion channels as a special class of mechanoreceptors functioning as molecular transducers of mechanical stimuli on a millisecond timescale.

"Force-From-Lipids" mechanosensation in Corynebacterium glutamicum.

Since the mechanosensitive channel MscCG has been identified as the major glutamate efflux system in Corynebacterium glutamicum, studies of mechanotransduction processes in this bacterium have helped to unpuzzle a long-unresolved mystery of glutamate efflux that has been utilised for industrial monosodium glutamate production. The patch clamp recording from C. glutamicum giant spheroplasts revealed the existence of three types of mechanosensitive (MS) channels in the cell membrane of this bacterium. The experiments demonstrated that the MS channels could be activated by membrane tension, indicating that the channel gating by mechanical force followed the "Force-From-Lipids (FFL)" principle characteristic of ion channels inherently sensitive to transbilayer pressure profile changes in the mechanically stressed membrane bilayer. Mechanical properties of the C. glutamicum membrane are characteristics of very soft membranes, which in the C. glutamicum membrane are due to negatively charged lipids as its exclusive constituents. Given that membrane lipids are significantly altered during the fermentation process in the monosodium glutamate production, MS channels seem to respond to changes in force transmission through the membrane bilayer due to membrane lipid dynamics. In this review, we describe the recent results describing corynebacterial FFL-dependent mechanosensation originating from the particular lipid composition of the C. glutamicum membrane and unique structure of MscCG-type channels.

Corynebacterium glutamicum mechanosensitive channels: towards unpuzzling "glutamate efflux" for amino acid production.

Corynebacterium glutamicum has been utilized for industrial amino acid production, especially for monosodium glutamate (MSG), the food-additive for the "UMAMI" category of taste sensation, which is one of the five human basic tastes. Glutamate export from these cells is facilitated by the opening of mechanosensitive channels in the cell membrane within the bacterial cell envelope following specific treatments, such as biotin limitation, addition of Tween 40 or penicillin. A long-unsolved puzzle still remains how and why C. glutamicum mechanosensitive channels are activated by these treatments to export glutamate. Unlike mechanosensitive channels in other bacteria, these channels are not simply osmotic safety valves that prevent these bacteria from bursting upon a hypo-osmotic shock. They also function as metabolic valves to continuously release glutamate as components of a pump-and-leak mechanism regulating the cellular turgor pressure. Recent studies have demonstrated that the opening of the mechanosensitive channel, MscCG, mainly facilitates the efflux of glutamate and not of other amino acids and that the "force-from-lipids" gating mechanism of channels also applies to the MscCG channel. The bacterial types of mechanosensitive channels are found in cell-walled organisms from bacteria to land plants, where their physiological functions have been specialized beyond their basic function in bacterial osmoregulation. In the case of the C. glutamicum MscCG channels, they have evolved to function as specialized glutamate exporters.

Tuning ion channel mechanosensitivity by asymmetry of the transbilayer pressure profile.

Mechanical stimuli acting on the cellular membrane are linked to intracellular signaling events and downstream effectors via different mechanoreceptors. Mechanosensitive (MS) ion channels are the fastest known primary mechano-electrical transducers, which convert mechanical stimuli into meaningful intracellular signals on a submillisecond time scale. Much of our understanding of the biophysical principles that underlie and regulate conversion of mechanical force into conformational changes in MS channels comes from studies based on MS channel reconstitution into lipid bilayers. The bilayer reconstitution methods have enabled researchers to investigate the structure-function relationship in MS channels and probe their specific interactions with their membrane lipid environment. This brief review focuses on close interactions between MS channels and the lipid bilayer and emphasizes the central role that the transbilayer pressure profile plays in mechanosensitivity and gating of these fascinating membrane proteins.

Evolutionary specialization of MscCG, an MscS-like mechanosensitive channel, in amino acid transport in Corynebacterium glutamicum.

MscCG, a mechanosensitive channel of Corynebacterium glutamicum provides a major export mechanism for glutamate in this Gram-positive bacterium, which has for many years been used for industrial production of glutamate and other amino acids. The functional characterization of MscCG is therefore, of great significance to understand its conductive properties for different amino acids. Here we report the first successful giant spheroplast preparation of C. glutamicum amenable to the patch clamp technique, which enabled us to investigate mechanosensitive channel activities of MscCG in the native membrane of this bacterium. Single channel recordings from these spheroplasts revealed the presence of three types of mechanosensitive channels, MscCG, MscCG2, and CgMscL, which differ largely from each other in their conductance and mechanosensitivity. MscCG has a relatively small conductance of ~340 pS followed by an intermediate MscCG2 conductance of ~1.0 nS and comparably very large conductance of 3.7 nS exhibited by CgMscL. By applying Laplace's law, we determined that very moderate membrane tension of ~5.5 mN/m was required for half activation of MscCG compared to ~12 mN/m required for half activation of both MscCG2 and CgMscL. Furthermore, by combining the micropipette aspiration technique with molecular dynamics simulations we measured mechanical properties of the C. glutamicum membrane, whose area elasticity module of KA ≈ 15 mN/m is characteristic of a very soft membrane compared to the three times larger area expansion modulus of KA ≈ 44 mN/m of the more elastic E. coli membrane. Moreover, we demonstrate that the "soft" properties of the C. glutamicum membrane have a significant impact on the MscCG gating characterized by a strong voltage-dependent hysteresis in the membrane of C. glutamicum compared to a complete absence of the hysteresis in the E. coli cell membrane. We thus propose that MscCG has evolved and adapted as an MscS-like channel to the mechanical properties of the C. glutamicum membrane enabling the channel to specialize in transport of amino acids such as glutamate, which are major osmolytes helping the bacterial cells survive extreme osmotic stress.

Genetic analysis of the regulation of the voltage-gated calcium channel homolog Cch1 by the γ subunit homolog Ecm7 and cortical ER protein Scs2 in yeast.

The yeast Cch1/Mid1 Ca2+ channel is equivalent to animal voltage-gated Ca2+ channels and activated in cells incubated in low Ca2+ medium. We herein investigated the third subunit, Ecm7, under the same cell culture conditions. The deletion of ECM7 slightly lowered Ca2+ influx activity in the CNB1+ background, in which calcineurin potentially dephosphorylates Cch1, but markedly lowered this activity in the cnb1Δ background. The deletion of the C-terminal cytoplasmic region of Ecm7 also reduced Ca2+ influx activity. We identified a novel Cch1-interacting protein, Scs2, which is known as a cortical endoplasmic reticulum membrane protein. The deletion of SCS2 did not affect Ca2+ influx activity when calcineurin was inhibited by FK506, but enhanced this activity by 35% when the enzyme was not inhibited. However, this enhancement was canceled by the deletion of ECM7. These results suggest that Cch1/Mid1 is regulated differentially by Ecm7 and Scs2 in a manner that is dependent on the phosphorylation status of Cch1.

Activation of the mechanosensitive ion channel MscL by mechanical stimulation of supported Droplet-Hydrogel bilayers.

The droplet on hydrogel bilayer (DHB) is a novel platform for investigating the function of ion channels. Advantages of this setup include tight control of all bilayer components, which is compelling for the investigation of mechanosensitive (MS) ion channels, since they are highly sensitive to their lipid environment. However, the activation of MS ion channels in planar supported lipid bilayers, such as the DHB, has not yet been established. Here we present the activation of the large conductance MS channel of E. coli, (MscL), in DHBs. By selectively stretching the droplet monolayer with nanolitre injections of buffer, we induced quantifiable DHB tension, which could be related to channel activity. The MscL activity response revealed that the droplet monolayer tension equilibrated over time, likely by insertion of lipid from solution. Our study thus establishes a method to controllably activate MS channels in DHBs and thereby advances studies of MS channels in this novel platform.

Coupling of a voltage-gated Ca2+ channel homologue with a plasma membrane H+ -ATPase in yeast.

Yeast has a homologue of mammalian voltage-gated Ca2+ channels (VGCCs), enabling the efficient uptake of Ca2+ . It comprises two indispensable subunits, Cch1 and Mid1, equivalent to the mammalian pore-forming α1 and auxiliary α2 /δ subunits, respectively. Unlike the physiological roles of Cch1/Mid1 channels, the regulatory mechanisms of the yeast VGCC homologue remain unclear. Therefore, we screened candidate proteins that interact with Mid1 by an unbiased proteomic approach and identified a plasma membrane H+ -ATPase, Pma1, as a candidate. Mid1 coimmunoprecipitated with Pma1, and Mid1-EGFP colocalized with Pma1-mCherry at the plasma membrane. The physiological relevance of their interaction was determined using the temperature-sensitive mutant, pma1-10. At the nonpermissive temperature, the membrane potential was less negative and Ca2+ uptake was lower in pma1-10 than in wild-type cells. Increased extracellular H+ increased the rate of Ca2+ uptake. Therefore, H+ extrusion by Pma1 may be important for Ca2+ influx through Cch1/Mid1. These results suggest that Pma1 interacts physically with Cch1/Mid1 Ca2+ channels to enhance their activity via its H+ -pumping activity.

Energy of Liposome Patch Adhesion to the Pipet Glass Determined by Confocal Fluorescence Microscopy.

The formation of the gigaseal in the patch clamp technique is dependent on the adhesion between the cell or liposome membrane and the glass pipet. The adhesion results in a capillary force causing creep of the patch membrane up the pipet. The membrane can be immobilized by counteracting the capillary force by positive pressure applied to the patch pipet. We use this phenomenon to develop a method for static measurement of the adhesion free energy of the lipid bilayer to the glass. Confocal fluorescent microscopy is used to track the bilayer creep inside the pipet and measure the immobilization pressure at various salt concentrations and pH. The adhesion energy is simply related to this pressure. For the studied phospholipid bilayers, its values were in the 0.3-0.7 mJ/m2 range, increased with salt concentration, and had a maximum as a function of pH. This method offers a way to measure bilayer-glass adhesion energy in patch clamp experiments that is more precise than dynamic methods.

Bioelectromagnetics Research within an Australian Context: The Australian Centre for Electromagnetic Bioeffects Research (ACEBR).

Mobile phone subscriptions continue to increase across the world, with the electromagnetic fields (EMF) emitted by these devices, as well as by related technologies such as Wi-Fi and smart meters, now ubiquitous. This increase in use and consequent exposure to mobile communication (MC)-related EMF has led to concern about possible health effects that could arise from this exposure. Although much research has been conducted since the introduction of these technologies, uncertainty about the impact on health remains. The Australian Centre for Electromagnetic Bioeffects Research (ACEBR) is a National Health and Medical Research Council Centre of Research Excellence that is undertaking research addressing the most important aspects of the MC-EMF health debate, with a strong focus on mechanisms, neurodegenerative diseases, cancer, and exposure dosimetry. This research takes as its starting point the current scientific status quo, but also addresses the adequacy of the evidence for the status quo. Risk communication research complements the above, and aims to ensure that whatever is found, it is communicated effectively and appropriately. This paper provides a summary of this ACEBR research (both completed and ongoing), and discusses the rationale for conducting it in light of the prevailing science.

The impact of the C-terminal domain on the gating properties of MscCG from Corynebacterium glutamicum.

The mechanosensitive (MS) channel MscCG from the soil bacterium Corynebacterium glutamicum functions as a major glutamate exporter. MscCG belongs to a subfamily of the bacterial MscS-like channels, which play an important role in osmoregulation. To understand the structural and functional features of MscCG, we investigated the role of the carboxyl-terminal domain, whose relevance for the channel gating has been unknown. The chimeric channel MscS-(C-MscCG), which is a fusion protein between the carboxyl terminal domain of MscCG and the MscS channel, was examined by the patch clamp technique. We found that the chimeric channel exhibited MS channel activity in Escherichia coli spheroplasts characterized by a lower activation threshold and slow closing compared to MscS. The chimeric channel MscS-(C-MscCG) was successfully reconstituted into azolectin liposomes and exhibited gating hysteresis in a voltage-dependent manner, especially at high pipette voltages. Moreover, the channel remained open after releasing pipette pressure at membrane potentials physiologically relevant for C. glutamicum. This contribution to the gating hysteresis of the C-terminal domain of MscCG confers to the channel gating properties highly suitable for release of intracellular solutes.

Transmembrane Topologies of Ca2+-permeable Mechanosensitive Channels MCA1 and MCA2 in Arabidopsis thaliana.

Sensing mechanical stresses, including touch, stretch, compression, and gravity, is crucial for growth and development in plants. A good mechanosensor candidate is the Ca(2+)-permeable mechanosensitive (MS) channel, the pore of which opens to permeate Ca(2+) in response to mechanical stresses. However, the structure-function relationships of plant MS channels are poorly understood. Arabidopsis MCA1 and MCA2 form a homotetramer and exhibit Ca(2+)-permeable MS channel activity; however, their structures have only been partially elucidated. The transmembrane topologies of these ion channels need to be determined in more detail to elucidate the underlying regulatory mechanisms. We herein determined the topologies of MCA1 and MCA2 using two independent methods, the Suc2C reporter and split-ubiquitin yeast two-hybrid methods, and found that both proteins are single-pass type I integral membrane proteins with extracellular N termini and intracellular C termini. These results imply that an EF hand-like motif, coiled-coil motif, and plac8 motif are all present in the cytoplasm. Thus, the activities of both channels can be regulated by intracellular Ca(2+) and protein interactions.

Magnetic nanoparticles for "smart liposomes".

Liposomal drug delivery systems (LDDSs) are promising tools used for the treatment of diseases where highly toxic pharmacological agents are administered. Currently, destabilising LDDSs by a specific stimulus at a target site remains a major challenge. The bacterial mechanosensitive channel of large conductance (MscL) presents an excellent candidate biomolecule that could be employed as a remotely controlled pore-forming nanovalve for triggered drug release from LDDSs. In this study, we developed superparamagnetic nanoparticles for activation of the MscL nanovalves by magnetic field. Synthesised CoFe2O4 nanoparticles with the radius less than 10 nm were labelled by SH groups for attachment to MscL. Activation of MscL by magnetic field with the nanoparticles attached was examined by the patch clamp technique showing that the number of activated channels under ramp pressure increased upon application of the magnetic field. In addition, we have not observed any cytotoxicity of the nanoparticles in human cultured cells. Our study suggests the possibility of using magnetic nanoparticles as a specific trigger for activation of MscL nanovalves for drug release in LDDSs.

Patch clamp characterization of the effect of cardiolipin on MscS of E. coli.

The bacterial mechanosensitive channels MscS and MscL are gated by an increase in membrane tension when the bacterium experiences hypoosmotic shock. It has been well established that membrane lipids modulate the mechanosensitivity and gating behavior of these channels. The focus of this study is a negatively charged phospholipid, cardiolipin, which has been shown to localize at curved regions of the bacterial cell, including the poles and the septum, and to have a strong preference for binding to membrane proteins. Here we characterize the effect of cardiolipin on MscS, the mechanosensitive channel of small conductance, using patch-clamp electrophysiology. We compare the gating kinetics and mechanosensitivity of the channel in both azolectin and mixtures of pure lipids DOPE/DOPC liposomes with and without cardiolipin. In azolectin liposomes, the addition of 10 % cardiolipin abolishes hysteresis of MscS, but MscL remains largely unaffected, indicating that cardiolipin may stabilize the closed state of MscS. On the other hand, mixtures of DOPE/DOPC abolish the hysteresis gating of MscS even in the absence of cardiolipin, and the addition of cardiolipin increases the opening and closing thresholds of both MscS and MscL. In addition, we show that MscS gates more frequently when cardiolipin is present in both the azolectin and pure lipid systems; this dose-dependent effect ultimately destabilizes the open state of MscS and we consider the functional implications of this cardiolipin effect in the bacterial osmotic response. Our results show that cardiolipin modulates the mechanosensitivity and gating characteristics of MscS, indicating its important role in the physiology of bacterial cells.

Organellar mechanosensitive channels involved in hypo-osmoregulation in fission yeast.

MscS and MscL, bacterial mechanosensitive channels, play crucial roles in the hypo-osmotic shock response. However, only MscS has homologs in eukaryotes. These homologs are called MscS-like proteins or MSL proteins. MSL proteins have changed both structurally and functionally during evolution and are now localized not only to the membrane of the chloroplast, which is thought to be a descendant of an ancient, free-living bacterium, but also the cell membrane and the endoplasmic reticulum (ER) membrane, suggesting that the role of MSL proteins has diverged. In this brief review, we mainly focus on two MSL proteins in the fission yeast Schizosaccharomyces pombe that are localized in the ER membrane and protect cells from hypo-osmotic shock-induced death by regulating intracellular Ca(2+) concentrations. We also discuss Arabidopsis thaliana MSL proteins and other yeast ion channels in terms of osmoregulation in eukaryotes.