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* Department of Physics, University of Illinois at Urbana-Champaign, and Beckman Institute for Advanced Science and Technology, Urbana, Illinois; Institute for Molecular Pediatrics Science and Department of Biochemistry and Molecular Biology, University of Chicago, Chicago, Illinois; and Department of Molecular Physiology and Biological Physics, University of Virginia, Charlottesville, Virginia
Correspondence: Address reprint requests to K. Schulten, E-mail: kschulte{at}ks.uiuc.edu.
The mechanosensitive channel of small conductance (MscS) is a membrane protein thought to act as a safety valve in bacteria, regulating the release of ions and small solutes when gated by membrane tension under challenging osmotic conditions. The influence of voltage on channel activation and the functional state depicted by the available crystal structure of MscS remain debated. Therefore, in an effort to relate electrophysiological measurements on MscS and properties of the MscS crystal conformation, we report here MscS's response to voltage and pressure as determined by patch-clamp experiments, as well as MscS electrostatics and transport properties as determined through all-atom molecular dynamics simulations of the protein embedded in a lipid bilayer, a 224,000-atom system. The experiments reveal that MscS is a slightly anion-selective channel with a conductance of 
1 ns, activated by pressure and inactivated in a voltage-dependent manner. On the other hand, the simulations, covering over 200 ns and including biasing electrostatic potentials, show that MscS restrained to the crystal conformation exhibits low conductance; unrestrained it increases the channel radius upon application of a large electrostatic bias and exhibits then ion conduction that matches experimentally determined conductances. The simulated conductance stems mainly from Cl– ions.
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