SummaryHow ion channels are gated to regulate ion flux in and out of cells is the subject of intense interest. The E. coli mechanosensitive channel, MscS, opens to allow rapid ion efflux, relieving the turgor pressure that would otherwise destroy the cell. We present a 3.45 Å resolution structure for the MscS channel in an open conformation. This structure has a pore diameter of ~13 Å created by substantial rotational re-arrangement of the three transmembrane helices. The structure suggests a molecular mechanism that underlies MscS gating and its decay of conductivity during prolonged activation. Support for this mechanism is provided by single channel analysis of mutants with altered gating characteristics.

The ability of proteins to sense membrane tension is pervasive in biology. A higher resolution structure of E. coli MscS, the channel of small conductance, identifies alkyl chains inside pockets formed by the transmembrane helices (TMs). Purified MscS contains E. coli lipids and fluorescence quenching demonstrates that phospholipid acyl chains exchange between bilayer and TM pockets. Molecular dynamics and biophysical analyses show that the volume of the pockets and thus the number of lipid acyl chain within them decreases upon channel opening. Phospholipids with one acyl chain per head group (lysolipids) displace normal phospholipids (two acyl chains) from MscS pockets and trigger channel opening. We propose the extent of acyl chain interdigitation in these pockets determines the conformation of MscS. Where interdigitation is perturbed by increased membrane tension or by lysolipids, the closed state becomes unstable and the channel gates.

Mechanosensitive (MS) channels were first demonstrated in bacterial cells by using patch clamp analysis of giant bacterial protoplasts and by fusion of membranes with liposomes. Both approaches indicated the presence of high-conductance channels in the membranes of gram-positive and gram-negative bacteria (15,29,53,60). Initially the data were greeted with scepticism, based on the similarity of the conductances of MS channels to those of porins and the recognized need of the cytoplasmic membrane to exhibit tight control over H ϩ permeability in order to effect energy transduction. Activation of MS channels by membrane-intercalating amphipathic compounds suggested that these channels are sensitive to mechanical perturbations in the lipid bilayer (22,28). Support for the presence of channels was provided by the discovery and reconstitution of two distinct channel activities from Escherichia coli, each with unique properties (52). Further support came from the discovery that the efflux of solutes from E. coli cells in response to a lowering of the external osmolarity could be prevented by gadolinium ions, which are classical inhibitors of MS channels in higher organisms (6).A landmark event was the purification and cloning of the first MS channel protein, MscL, from E. coli. This heroic piece of biochemistry required that each fraction derived from the solubilized and fractionated membrane be reconstituted into liposomes and the MS channel activity be measured (51). Availability of the amino-terminal sequence of the protein led to identification of the gene. Following this breakthrough, a new age of MS channel protein structure-function analysis dawned (7,(9)(10)(11)42), culminating in the crystal structure of a mycobacterial MscL channel (13) (Fig. 1). Extensive genetic and biophysical analyses of MscL protein movement in real time, coupled with model building, electron paramagnetic resonance spectroscopy, and site-directed spin labeling studies, provided an explanation of how the protein can exist in at least two states-one tightly closed and the other creating a large pore in the membrane (23,42,48,49) (Fig. 2). MS channels are now thought to be important to many bacteria (8) and archaea (20,21,24).The genetic advances with MscL posed a further problemwhy does an mscL null mutant lack an apparent physiological phenotype? Patch clamp analysis had revealed the presence of at least two MS channels in E. coli membranes, and subsequent studies led to the possibility that five or more genetically distinct channels exist (5). Such apparent biochemical redundancy implied that observation of a phenotype might require the construction of a mutant lacking more than one channel protein. Preliminary support for the protective role of MscL was discovered by expressing the channel in Vibrio and observing protection from hypoosmotic shock (38). The discovery of the structural gene for MscS, the second major MS channel in E. coli, allowed this functional hypothesis to be tested (25). Through the genetic analysis of a missense mu...