Biology subjects

Sukharev, S. I.

Publications and source records attributed to Sukharev, S. I..

6 recordsLinked to original sources

bioRxiv2026

Desensitization, inactivation, and the tension-proof safety mechanism of inactivated MscS

MscS is the main low-threshold tension-activated osmolyte release valve in bacteria. Working alone or together with high-threshold MscL, it regulates turgor and protects cells from mechanical rupture during osmotic down-shock. The channel exhibits complex adaptive behavior, including desensitization and full inactivation, both of which occur at relatively low sub-lytic tension. There is debate over whether the commonly observed non-conductive state of MscS with the tension-sensing helices splayed away from the gate corresponds to the closed or inactivated state. In this work, using specialized pressure protocols in patch-clamp electrophysiology, we highlight the difference between reversible adaptation (desensitization) and inactivation. We show that inactivated channels cannot be reactivated with high tension, up to the limit of patch stability. This aligns with cryo-EM studies by Zhang et al. (Nature, 2021, 590:509-5018), who applied extreme tension to the splayed nanodisc-reconstituted MscS (PDB 6VYK) by depleting lipids with cyclodextrin, and observed a new flattened but apparently non-conductive structure (PDB 6VYM). To characterize these two states, we performed a steered Molecular Dynamics simulation from the initial splayed structure to the flattened conformation, confirming that they are connected through a smooth conformational pathway and remain largely dehydrated and entirely non-conductive throughout the transition. The data show that the initial splayed conformation meets all the criteria of the inactivated state, distorting but not opening under extreme tension. By combining patch-clamp experiments with simulations based on cryo-EM data, we demonstrate that inactivated MscS resists activation, thereby maintaining the membrane barrier when tension exceeds the activation threshold. SIGNIFICANCEBacterial energetics, which relies on the electrochemical proton gradient across the inner membrane as an intermediary, conflicts with the presence of a dense population of highly conductive mechano-activated channels in the same cytoplasmic membrane, which must be proton-tight. Moreover, the tension activation threshold for the common MscS channel is low and can be easily exceeded by fluctuations in the concentrations of internal or external osmolytes. In this paper, we describe the important adaptive inactivation of MscS at tensions near its activation threshold, and specifically, the complete resistance of inactivated MscS to opening at any tension. The data show another layer of tight regulation of MscS residing in the energy-coupling membranes.

biophysics↗

bioRxiv2024

Turgor and the conformational pathway for MscS recovery

The bacterial mechanosensitive channel MscS is an adaptive osmolyte release valve that cycles between closed, open, and inactivated states. Since some of these conformations are stable only in the lipid environment under specific conditions, the structures that are currently available cannot explain the entire functional cycle. Previous patch-clamp characterization has provided insights into the missing functional state by estimating protein expansion areas associated with the closed-to-open and closed-to-inactivated transitions and indicating that the closed state must be the most compact. In this paper, we model the conformational transition of MscS from the splayed conformation with the uncoupled gate to the putative compact closed state. The compaction pathway revealed in preliminary extrapolated motion simulations (ExMoS) involved an upward sliding motion of the internal TM3 barrel inside the outer sheath formed by TM1-TM2 helical pairs. This move leads to several structural changes: (1) the relocation of the characteristic kink at G113 to a new position at G121, (2) the establishment of the hydrophobic TM2-TM3 contact, (3) a new pattern of interactions with membrane lipids, and (4) the formation of stabilizing salt bridges between TM1-TM2 loops and the cytoplasmic cage domain. In the intact bacterial cell, the driving force for this upward motion is likely to be turgor pressure normal to the plane of the membrane acting on the upper hemisphere of the cage domain from the inside. Under continuing lipid synthesis in the inner leaflet of the plasma membrane, turgor pressure is also predicted to maximize the lateral pressure of lipids in the membrane, thus driving MscS compaction. Steered simulations were performed on the splayed state to mimic these effects by applying normal forces to the upper part of the cage domain and by applying lateral compression to the TM1-TM2 pairs, emulating the pressure of lipids. The structure arrived at the predicted compact state of the channel. This state was critically stabilized by displacing non-bilayer lipids from the TM2-TM3 crevices into the bilayer. We propose that the energized metabolic state of the cell generating high turgor and promoting lipid synthesis should strongly favor the compact closed state of MscS. The normal forces pressing the dome of the cage domain against the membrane may provide a common recovery mechanism for the entire family of MscS-like channels found exclusively in organisms with walled cells, which evolved to function under turgor pressure. A conversion of turgor into membrane tension under hypoosmotic cytoplasm swelling and peptidoglycan expansion will drive opposite processes of opening followed by adaptive MscS closure and inactivation.

biophysics↗

bioRxiv2024

On the lipid dependence of bacterial mechanosensitive channel gating in situ

For bacterial mechanosensitive channels acting as turgor-adjusting osmolyte release valves, membrane tension is the primary stimulus driving opening transitions. Because tension is transmitted through the surrounding lipid bilayer, it is possible that the presence or absence of different lipid species may influence the function of these channels. In this work, we characterize the lipid dependence of chromosome-encoded MscS and MscL in E. coli strains with genetically altered lipid composition. We use two previously generated strains that lack one or two major lipid species (PE, PG, or CL) and engineer a third strain that is highly enriched in CL due to the presence of hyperactive cardiolipin synthase ClsA. We characterize the functional behavior of these channels using patch-clamp and quantify the relative tension midpoints, closing rates, inactivation depth, and the rate of recovery back to the closed state. We also measure the osmotic survival of lipid-deficient strains, which characterizes the functional consequences of lipid-mediated channel function at the cell level. We find that the opening and closing behavior of MscS and MscL tolerate the absence of specific lipid species remarkably well. The lack of cardiolipin (CL), however, reduces the active MscS population relative to MscL and decreases the closing rate, slightly increasing the propensity of MscS toward inactivation and slowing the recovery process. The data points to the robustness of the osmolyte release system and the importance of cardiolipin for the adaptive behavior of MscS.

biophysics↗

bioRxiv2024

Polymer-solubilized structure of the mechanosensitive channel MscS suggests the role of protein-lipid interactions in the functional gating cycle

Interpretations of experimental conformations of mechanosensitive channels gated by force from lipids become more reliable when native lipids are preserved in the structures. Escherichia coli MscS is an adaptive osmolyte release valve that regulates turgor in osmotically stressed cells. MscS promptly opens under abrupt super-threshold tensions in the membrane, but at lower and more gradually applied tensions, it silently inactivates from the closed state. A central question has been whether to assign the commonly observed non-conductive conformation with splayed peripheral helices to a closed or inactivated state. We present a 3-[A] MscS cryo-EM structure obtained in Glyco-DIBMA polymers, which avoid complete lipid removal. Within the complex, we observe densities for endogenous phospholipids intercalating between the peripheral and pore-lining helices. The lipidomic analysis shows a 2-3 fold enrichment of phosphatidylglycerol in Glyco-DIBMA-extracted MscS samples. The computed pressure of these lipids on the protein surface enforces the characteristic kinks in the pore-lining helices, sterically stabilizing the separation of the peripheral helices. Mutations of residues coordinating lipids in the crevices eliminate inactivation, allowing us to classify this group of structures as the inactivated state. Our study reveals a novel inactivation mechanism in which intercalated lipids physically decouple the tension-sensing helices from the gate.

biophysics↗

bioRxiv2023

Mechanosensitive channel MscS is critical for termination of the bacterial hypoosmotic permeability response

Free-living microorganisms are subjected to drastic changes in osmolarity. To avoid lysis under sudden osmotic down-shock, bacteria quickly expel small metabolites through the tension-activated channels MscL, MscS, and MscK. We examined five chromosomal knockout strains, {Delta}mscL, {Delta}mscS, a double knockout {Delta}mscS {Delta}mscK, and a triple knockout {Delta}mscL {Delta}mscS {Delta}mscK in comparison to the wild-type parental strain. Stopped-flow experiments confirmed that both MscS and MscL mediate fast osmolyte release and curb cell swelling, but osmotic viability assays indicated that they are not equivalent. MscS alone was capable of rescuing the cell population, but in some strains MscL did not rescue and additionally became toxic in the absence of both MscS and MscK. Furthermore, MscS was upregulated in the {Delta}mscL strain, suggesting either a cross-talk between the two genes/proteins or the influence of cell mechanics on mscS expression. The data shows that for the proper termination of the permeability response, the high-threshold (MscL) and the low-threshold (MscS/MscK) channels must act sequentially. In the absence of low-threshold channels, at the end of the release phase, MscL should stabilize membrane tension at around 10 mN/m. Patch-clamp protocols emulating the tension changes during the release phase indicated that the non-inactivating MscL, residing at its own tension threshold, flickers and produces a protracted leakage. The MscS/MscK population, when present, stays open at this stage to reduce tension below the MscL threshold and silence the large channel. When MscS reaches its own threshold, it inactivates and thus ensures proper termination of the hypoosmotic permeability response. This functional interplay between the high- and low-threshold channels is further supported by the compromised osmotic survival of bacteria expressing non-inactivating MscS mutants. Summary (for the table of contents)The kinetics of hypotonic osmolyte release from E. coli is analyzed in conjunction with bacterial survival. It is shown that MscL, the high-threshold emergency release valve, rescues bacteria from down-shocks only in the presence of MscS, MscK or other low-threshold channels that are necessary to pacify MscL at the end of the release phase.

biophysics↗

bioRxiv2020

Mechanical Activation of MscL Revealed by a Locally Distributed Tension Molecular Dynamics Approach

Membrane tension perceived by mechanosensitive (MS) proteins mediates cellular responses to mechanical stimuli and osmotic stresses, and it also guides multiple biological functions including cardiovascular control and development. In bacteria, MS channels function as tension-activated pores limiting excessive turgor pressure, with MscL (MS channel of large conductance) acting as an emergency release valve preventing cell lysis. Previous attempts to simulate gating transitions in MscL by either directly applying steering forces to the protein or by increasing the whole system tension were not fully successful and often disrupted the integrity of the system. We present a novel locally distributed tension molecular dynamics (LDT-MD) simulation method that allows application of forces continuously distributed among lipids surrounding the channel using a specially constructed collective variable. We report reproducible and reversible transitions of MscL to the open state with measured parameters of lateral expansion and conductivity that exactly satisfy experimental values. The LDT-MD method enables exploration of the MscL gating process with different pulling velocities and variable tension asymmetry between the inner and outer membrane leaflets. We use LDT-MD in combination with well-tempered metadynamics to reconstruct the tension-dependent free energy landscape for the opening transition in MscL. SIGNIFICANCEMembrane-embedded mechanosensitive (MS) proteins are essential for numerous biological functions including cardiovascular control and development, osmotic regulation, touch and pain sensing. In this work, we present a novel molecular dynamics simulation method that allows rapid and systematic exploration of structure, dynamics, and energetics of the mechanical transduction process in MS proteins under tightly controlled local tension distributed in the lipid rim around the protein. We provide a detailed description of the gating transition for the tension-activated bacterial mechanosensitive channel of large conductance, MscL, which is the best characterized channel of this type. MscL functions as a tension-activated emergency osmolyte release valve that limits excessive turgor pressure, prevents cell lysis and thus imparts environmental stability to most free-living bacteria.

biophysics↗