A Structural Mechanism for MscS Gating in Lipid Bilayers

Abstract

The mechanosensitive channel of small conductance (MscS) is a key determinant in the prokaryotic response to osmotic challenges. Here, we have determined the structural rearrangements associated with MscS activation in membranes using patch-clamp, EPR spectroscopy, and computational analyses. MscS was trapped in its open conformation after modifying the transbilayer pressure profile through the asymmetric incorporation of lysophospholipids. The transition from the closed to the open state is accompanied by the downward tilting of the TM1-TM2 hairpin, and by the expansion, tilt, and rotation of the TM3 helices. These movements expand the permeation pathway, leading to an increase in water accessibility around TM3. Our open MscS model is compatible with single channel conductance measurements and supports the notion that helix tilting is associated with efficient pore widening in mechanosensitive channels.Mechanosensation is involved in many physiological roles, including osmotic balance, touch, and hearing (1,2). At the molecular level, mechanosensitivity relies on the activity of ion channels that transduce a variety of mechanical stimuli to open a conductive pore. Mechanosensitive (MS) channels are grouped by function rather than sequence similarity (3,4). In prokaryotic systems, MS channels respond directly to bilayer deformations, with a transduction mechanism defined at the protein-lipid interface (5,6). Although this is also true for some eukaryotic MS channels (7), many also respond to mechanical deformations through their association with the cytoskeletal network (8).While the molecular identification of eukaryotic MS channels remains challenging (2,9,10), the biophysical and structural properties of prokaryotic MS channels have proved far more tractable at the molecular level. The crystal structures for the MS channels of large (MscL) and small (MscS) conductance (11-13), have provided a molecular framework to interpret functional and biophysical data and have helped establish the basic mechanistic principles by which these two distinct channels sense the physical state of the bilayer (14-17). Nevertheless, given the critical role that lipid-protein interactions play in prokaryotic Correspondence should be addressed to E.P at eperozo@uchicago.edu. Supporting Online Material Material and Methods Fig. S1 Functional, spectroscopic, and computational studies have shown that in the pentameric MscL activation gating proceeds as a result of a large tilt of both transmembrane (TM) segments (14,17,18). Concerted helical rotation and tilting generates a large aqueous pore, much as in the iris of a camera lens. However, an equivalent gating mechanism is not as obvious in the case of MscS. With three TM segments arranged as a homoheptamer (12), the structural design of MscS is very different to that of MscL. Furthermore, while the MscL crystal structure appears to be a good representation of the closed conformation in its native environment (19,20), the functional state represented by the MscS crystal struc...

Structure and molecular mechanism of an anion-selective mechanosensitive channel of small conductance

“…As indicated by biochemical characterization (13) and confirmed by structural studies, the functional MscS channel is a homoheptamer (10)(11)(12). The structural study of the EcMscS A106V mutant showed that substantial rotational rearrangement of the TM region results in an increase in the pore size, presumably reflecting EcMscS channels in an open state, which is also consistent with electron paramagnetic resonance spectroscopy studies (11,14). A structural comparison of EcMscS in the closed state with the EcMscS A106V mutant revealed striking conformational changes in the TM domain but not in the cytoplasmic region (11).…”

State-stabilizing Interactions in Bacterial Mechanosensitive Channel Gating and Adaptation

“…The general character of transition in our open-state model (tilting and retraction of the helices) was consistent with both the A106V partially open crystal structure and the independently proposed open-state model based on constraints from scanning cysteine mutagenesis, spin labeling, and EPR (43). The authors of the EPR model reported that MscS could be stabilized in a conductive state by adding large amounts of lysophosphatidylcholine to the reconstituted proteoliposomes.…”

MscS-Like10 is a stretch-activated ion channel from Arabidopsis thaliana with a preference for anions

“…40). For example, the characteristic alteration between small and large hydrophobic residues appear to be responsible for proper TM3 packing in the MscS heptamer (41), and the hydrophobic seal residues L105 and L109 are essential for complete channel closure (39,(41)(42)(43)(44). Although these structural motifs are for the most part preserved in MscM and MSC1, MSL10 shares very little homology in the predicted pore-lining region (Fig.…”