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Osmolyte passage times
through MscS Cytoplasmic domain.

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Bacterial cells enclose themselves with a cell membrane to maintain an optimal interior ion concentration and interior electrical potential. The potential is about -0.1 V inside versus 0 V outside, the potential difference amounting to an important energy reservoir for the cell. Closing itself off from the outside can be dangerous, though, for a cell, namely when sudden changes outside the cell can bring a cell to burst. This can happen when ions outside the cell are washed away suddenly: outside water pushes itself then into the cell as the water prefers to be near ions found then only inside the cell, a behavior called osmosis. The osmotic push can be so hard that the cell can burst. To prevent such burst, cells evolved safety valves, one being called mechanosensitive channel of small conductance or MscS. Under pressure, the valve, i.e., MscS, opens and enough ions leave the cell to keep water from pushing in too hard (see the Mar 2008 highlight, "Observation and Simulation depict Cell's Safety Valve", Feb 2007 highlight, "Observing and Modeling a crucial Membrane Channel", May 2006 highlight, "Electrical Safety Valve", and the Nov 2004 highlight, "Japanese Lantern Protein"). But since the electrical potential inside is negative, mainly negative ions would leave the cell, discharging its potential and leaving the cell without energy. A theoretical and computational study, the latter carried out using NAMD, reports now that MscS developed apparently an ingenious solution: ions going through the MscS valve must pass a balloon-like filter that manages to mix positive and negative ions so that only a 1:1 mixture leaves a cell under osmotic shock, thereby providing protection against the shock without compromising the cell's electrical potential. More information can be found on our MscS website.