Mechanosensitive Channel MscS Research Articles

Water Dynamics and Dewetting Transitions in the Small Mechanosensitive Channel MscS

The dynamics of confined water in capillaries and nanotubes suggests that gating of ion channels may involve not only changes of the pore geometry, but also transitions between water-filled and empty states in certain locations. The recently solved heptameric structure of the small mechanosensitive channel of Escherichia coli, MscS, has revealed a relatively wide (7–15 Å) yet highly hydrophobic transmembrane pore. Continuum estimations based on the properties of pore surface suggest low conductance and a thermodynamic possibility of dewetting. To test the predictions we performed molecular dynamics simulations of MscS filled with flexible TIP3P water. Irrespective to the initial conditions, several independent 6-ns simulations converged to the same stable state with the pore water-filled in the wider part, but predominantly empty in the narrow hydrophobic part, displaying intermittent vapor-liquid transitions. The polar gain-of-function substitution L109S in the constriction resulted in a stable hydration of the entire pore. Steered passages of Cl − ions through the narrow part of the pore consistently produced partial ion dehydration and required a force of 200–400 pN to overcome an estimated barrier of 10–20 kcal/mole, implying negligibly low conductance. We conclude that the crystal structure of MscS does not represent an open state. We infer that MscS gate, which is similar to that of the nicotinic ACh receptor, involves a vapor-lock mechanism where limited changes of geometry or surface polarity can locally switch the regime between water-filled (conducting) and empty (nonconducting) states.

A role for mechanosensitive channels in survival of stationary phase: regulation of channel expression by RpoS.

The mechanosensitive (MS) channels MscS and MscL are essential for the survival of hypoosmotic shock by Escherichia coli cells. We demonstrate that MscS and MscL are induced by osmotic stress and by entry into stationary phase. Reduced levels of MS proteins and reduced expression of mscL- and mscS-LacZ fusions in an rpoS mutant strain suggested that the RNA polymerase holoenzyme containing sigmaS is responsible, at least in part, for regulating production of MS channel proteins. Consistent with the model that the effect of sigmaS is direct, the MscS and MscL promoters both use RNA polymerase containing sigmaS in vitro. Conversely, clpP or rssB mutations, which cause enhanced levels of sigmaS, show increased MS channel protein synthesis. RpoS null mutants are sensitive to hypoosmotic shock upon entry into stationary phase. These data suggest that MscS and MscL are components of the RpoS regulon and play an important role in ensuring structural integrity in stationary phase bacteria.

Pulled Sideways

Bacteria lack rigid cell walls and must respond rapidly to salt-induced gradients in osmotic pressure. In Escherichia coli , the mechanosensitive channel proteins MscL and MscS open in response to pressure applied perpendicularly to the membrane, which has the effect of pulling laterally on the membrane-embedded proteins. Bass et al. (see the Perspective by Bezanilla and Perozo) provide the crystal structure of MscS and describe how the transmembrane helices might reorient themselves as a result of being pulled sideways. The authors also propose how changes in transmembrane electrical potential might trigger movement of positively charged arginines and voltage-gate the osmolyte flux. R. B. Bass, P. Strop, M. Barclay, D. C. Rees, Crystal structure of Escherichia coli MscS, a voltage-modulated and mechanosensitive channel. Science 298 , 1582-1587 (2002). [Abstract] [Full Text] F. Bezanilla, E. Perozo, Force and voltage sensors in one structure. Science 298 , 1562-1563 (2002). [Summary] [Full Text]