Mechanosensitive channels (MSCs) are critical for cells to sense and respond to mechanical stimuli. MSCs are widely expressed, and defects in mechano-transduction are associated with numerous diseases. MSCs is a ubiquitous turgor-regulating bacterial MSC that is used as a model system for the entire family of channels in both prokaryotes and eukaryotes. The existing structures of E. coli MSCs fall into two categories: (1) splayed non-conductive structures with a decoupled gate, typically observed in the presence of regular-chain lipids, and (2) expanded, semi-conductive structures, observed in either delipidated samples or in the presence of short-chain lipids. The G168D mutant of MSCs does not exhibit tension-dependent inactivation in patch-clamp electrophysiology, has dramatically decreased osmotic viability, and does not adopt the splayed, non-conductive conformation associated with the inactivated state under the conditions used in our cryo-EM studies. Glycine 168 is situated at the tightly packed interface between the TM3b helix, which runs parallel to the membrane, and the β domain forming the upper part of the cytoplasmic cage. The destabilization of the inactivated state caused by a bulkier, charged aspartate at this site suggests that these contacts may not be present in the closed state. Our structure of G168D in mixed micelles shows a semi-open state with partially straightened TM3 helices and a loose TM3b-β contact. Interestingly, the particles were divided between regular heptamers and a smaller subset of octamers. The inter-subunit contacts in both stoichiometries appeared similar, and patch-clamp shows no deviation from the standard 1.1 nS unitary channel conductance. Based on the low fidelity in complex assembly of this non-inactivating mutant, we hypothesize that the inactivated state may be integral to the initial assembly stage of newly synthesized MSCs channels from protomers.
Moller et al. (Sun,) studied this question.
Synapse has enriched 5 closely related papers on similar clinical questions. Consider them for comparative context: