Mechanosensitive channels in bacteria: signs of closure?

Bacterial mechanosensitive (MS) channels are gated by the perturbation of membrane tension, forming non-selective pores of 16–40 Å through which hydrated ions and solutes can flow.MS channels have a key role in the survival of hypoosmotic shock, but might also have other roles during cell-wall remodelling.Using electrophysiology, three principal structural classes of MS channels have been defined in Escherichia coli. Species can have multiple homologues of each class, but not all have demonstrated MS channel activity. The homologues differ in their degree of conservation of the pore-lining helix residues, and their threshold sensitivity to tension might reflect this and be selected to enable them to have specific cellular functions.MscS and MscL homologues are also found in plants, oomycetes, algae and in some fungi. MscS homologues are associated with chloroplast shape and development in Arabidopsis and Chlamydomonas.Crystal structures of the Mycobacterium tuberculosis MscL (MscL-Mt) and the E. coli MscS (MscS-Ec) channels have revealed a homopentamer and a homoheptamer respectively. Structurally the channels are unrelated, as MscS has more complex packing and an extensive cytoplasmic domain that is required for assembly. Both MscL and MscS use a hydrophobic seal to maintain the channel pore in the closed state.The emerging consensus is that the crystal structures represent either closed states or intermediates in the transition from closed to open states. MscS-Ec characteristically exhibits a desensitized, inactivated state and it is possible that the crystal structure is in this conformation rather than in the 'natural' closed state. MscL-Mt is generally accepted to have been crystallized in the closed state.The structural transitions in MscL and MscS gating have been studied using biophysical, genetic and biochemical approaches. Both channels gate by tilting and rotating the helices surrounding the pore, which involves specific conserved residues.Recent molecular analysis of MS channels has focused on the interaction of the channel-protein residues with surrounding membrane lipids. In this Review we define the absence of specific amino acids at the protein–lipid interface that might block mechanogating as central to MS channel function. We term this 'negative space'.