mechanotransduction.org

http://www.pnas.org – Analytic estimates for the forces and free energy generated by bilayer deformation reveal a compelling and intuitive model for MscL channel gating analogous to the nucleation of a second phase. We argue that the competition between hydrophobic mismatch and tension results in a surprisingly rich story that can provide both a quantitative comparison with measurements of opening tension for MscL when reconstituted in bilayers of different thickness, and qualitative insights into the function of the MscL channel and other transmembrane proteins. The mechanosensitive channel (MscL) is a compelling example of the interaction between a protein and the surrounding bilayer membrane. The channel is gated mechanically by membrane tension and is thought to function as an emergency relief valve in bacteria (1). MscL is a member of a growing class of proteins that have been determined to be mechanosensitive (2, 3). The dependence of the conductance on membrane tension has been studied extensively in patch-clamp experiments (4–6). In terms of the observed conductance, these studies have revealed that the channel is very nearly a two-state system. MscL spends the vast majority of its life in either a closed state (C) or an open state (O) characterized by a discrete conductance. When the bilayer tension is small, the protein is exclusively in the closed configuration. As the tension grows, the open state becomes ever more prevalent, until it dominates at high tension. The simplest structural interpretation of this conductance data is to assume that each discrete conductance corresponds to a well defined channel conformation. This assumption seems to be compatible with the conductance data. Patch-clamp experiments have also revealed that at least three additional discrete, intermediate conductance levels exist (4), suggesting three additional short-lived substates (S1–S3). Rees and coworkers (7) have solved the structure for one conformation that appears to be the open state (6, 7) by using x-ray crystallography. MscL has also been trapped in the open state (6, 8). Betanzos et al. (8) have probed the open-state structure by using disulfide crosslinking, while Perozo et al. (6) have used electron paramagnetic resonance spectroscopy (EPRS) and site-directed spin labeling (SDSL) to deduce its geometry. Sukharev et al. (9) have also proposed an open-state conformation based on structural considerations. Read »