Molecular dynamics studies of the *gating mechanism of a mechanosensitive channel

Mechanosensitive channels are integral membrane proteins that gate in response to tension in the membrane. One such channel, MscL, poses an especially interesting challenge to modelers both because the channel is activated by membrane tension alone, and because the transition to the open state necessarily involves an unusually large conformational change from a leakproof closed channel to a large, non-specific pore 3 nm across. With the availability of an x-ray crystal structure of MscL, molecular dynamics simulations were carried out which have contributed to our understanding of the sequence of motions involved in MscL gating, suggested the likely form of the open state, and identified residues crucial in setting MscL's tension threshold. The gating process was investigated through simulations of the bare protein under conditions of constant surface tension. Under a range of conditions the transmembrane helices flattened as the pore widened. Simulations taking into account the nonhomogeneous distribution of stresses in the membrane have shown what parts of the channel are involved in sensing membrane tension. Steered Molecular Dynamics simulations were used to investigate how forces arising from membrane tension induce gating of the channel. A fully expanded state was obtained that revealed the mechanism for transducing membrane forces into channel opening. The expanded state agrees well with proposed models of MscL gating, in that it entails an iris-like expansion of the pore accompanied by tilting of the transmembrane helices. The channel was most easily opened when force was applied predominantly on the cytoplasmic side of MscL. Comparison of simulations in which gating progressed to varying degrees identified residues that pose steric hindrance to channel opening. MscL can also be gated in the absence of applied membrane tension through the introduction of molecules that change the membrane's intrinsic curvature. To test whether the gating can be explained by changes in the lateral pressure distribution within the membrane, molecular dynamics simulations of membranes mimicking the experimental conditions were performed. The lateral pressure profiles calculated from these simulations show how changes in membrane composition alter the membrane stress distribution, providing a physical mechanism for MscL gating.