Abstract
Prokaryotic mechanosensitive (MS) channels open by sensing the physical state of the membrane. As such, lipid-protein interactions represent the defining molecular process underlying mechanotransduction. Here, we describe cryo-electron microscopy (cryo-EM) structures of the E. coli small-conductance mechanosensitive channel (MscS) in nanodiscs (ND). They reveal a novel membrane-anchoring fold that plays a significant role in channel activation and establish a new location for the lipid bilayer, shifted ~14 Å from previous consensus placements. Two types of lipid densities are explicitly observed. A phospholipid that 'hooks' the top of each TM2-TM3 hairpin and likely plays a role in force sensing, and a bundle of acyl chains occluding the permeation path above the L105 cuff. These observations reshape our understanding of force-from-lipids gating in MscS and highlight the key role of allosteric interactions between TM segments and phospholipids bound to key dynamic components of the channel.
Highlights
The substantial effect of the pore lipid on water dynamics further suggests that under physiological conditions the pore lipid might be able to act as a low dielectric blocker, suggesting that the transition to the open conformation in MscS could be accompanied by a reduction in the occupancy of the pore lipid along the permeation pathway
The proposed gating mechanisms where lipids act as ligands are based on the assumption that the TM2/TM3a cavity is open and accessible to the lipid bilayer (Pliotas et al, 2015) are unlikely due to the location of the TM2/TM3a cavity in relation to the membrane annulus around MscS (Figure 2B) seems incompatible with a proposed phospholipid exchange between bilayer and TM pockets
The suggestion that lateral tension would ’pull’ on lipid acyl chains located in TM pockets appears unrealistic
Summary
A unique mechanism has been proposed based on the idea that bilayer lipids can drive MscS conformation by acting as ligands while they freely exchange between the membrane and hydrophobic pockets between TM2 and TM3a (Pliotas et al, 2015) At rest, these pockets are saturated with phospholipids, preventing structural rearrangements in TM3a. We have solved the nanodisc-reconstituted MscS (MscS-ND) by single particle cryo-electron microscopy (cryoEM) under a variety of lipid compositions and protein constructs (Figure 1—figure supplement 1) as well as a DDM detergent structure These structures, together with electrophysological and computational data highlight new membrane-interacting regions at MscS N-terminal end and define structurally bound lipids with potentially important roles in mechanotransduction, gating, and permeation
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