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 A) 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.

[1]  Boris Martinac,et al.  Open channel structure of MscL and the gating mechanism of mechanosensitive channels , 2002, Nature.

[2]  N. Unwin Acetylcholine receptor channel imaged in the open state , 1995, Nature.

[3]  W. L. Jorgensen,et al.  Comparison of simple potential functions for simulating liquid water , 1983 .

[4]  B. Hille,et al.  Ionic channels of excitable membranes , 2001 .

[5]  W. Im,et al.  Ion permeation and selectivity of OmpF porin: a theoretical study based on molecular dynamics, Brownian dynamics, and continuum electrodiffusion theory. , 2002, Journal of molecular biology.

[6]  D. Eisenberg,et al.  Atomic solvation parameters applied to molecular dynamics of proteins in solution , 1992, Protein science : a publication of the Protein Society.

[7]  K Schulten,et al.  VMD: visual molecular dynamics. , 1996, Journal of molecular graphics.

[8]  C Kung,et al.  Hydrophilicity of a single residue within MscL correlates with increased channel mechanosensitivity. , 1999, Biophysical journal.

[9]  G. R. Smith,et al.  Dynamic properties of Na+ ions in models of ion channels: a molecular dynamics study. , 1998, Biophysical journal.

[10]  G. Hummer,et al.  Water conduction through the hydrophobic channel of a carbon nanotube , 2001, Nature.

[11]  Youxing Jiang,et al.  Crystal structure and mechanism of a calcium-gated potassium channel , 2002, Nature.

[12]  Pavel Strop,et al.  Crystal Structure of Escherichia coli MscS, a Voltage-Modulated and Mechanosensitive Channel , 2002, Science.

[13]  Kazuhito Hashimoto,et al.  Recent Studies on Super-Hydrophobic Films , 2001 .

[14]  K. Schulten,et al.  Control of the Selectivity of the Aquaporin Water Channel Family by Global Orientational Tuning , 2002, Science.

[15]  I. Booth,et al.  Domain organization of the MscS mechanosensitive channel of Escherichia coli , 2003, The EMBO journal.

[16]  Ronald M. Welch,et al.  Climatic Impact of Tropical Lowland Deforestation on Nearby Montane Cloud Forests , 2001, Science.

[17]  Laxmikant V. Kale,et al.  NAMD2: Greater Scalability for Parallel Molecular Dynamics , 1999 .

[18]  A. Parsegian,et al.  Energy of an Ion crossing a Low Dielectric Membrane: Solutions to Four Relevant Electrostatic Problems , 1969, Nature.

[19]  B. Eisenberg,et al.  Progress and Prospects in Permeation , 1999, The Journal of general physiology.

[20]  D C Rees,et al.  Structure of the MscL homolog from Mycobacterium tuberculosis: a gated mechanosensitive ion channel. , 1998, Science.

[21]  D. Paschek,et al.  Gibbs ensemble simulation of water in spherical cavities , 2000 .

[22]  I. Booth,et al.  Protection of Escherichia coli cells against extreme turgor by activation of MscS and MscL mechanosensitive channels: identification of genes required for MscS activity , 1999, The EMBO journal.

[23]  Deri Morgan,et al.  The voltage dependence of NADPH oxidase reveals why phagocytes need proton channels , 2003, Nature.

[24]  D. Hackos,et al.  Constitutive Activation of the Shaker Kv Channel , 2003, The Journal of general physiology.

[25]  Werner Braun,et al.  Exact and efficient analytical calculation of the accessible surface areas and their gradients for macromolecules , 1998 .

[26]  R. Evans REVIEW ARTICLE: Fluids adsorbed in narrow pores: phase equilibria and structure , 1990 .

[27]  K. Schulten,et al.  Gating of MscL studied by steered molecular dynamics. , 2003, Biophysical journal.

[28]  H. Robert Guy,et al.  The gating mechanism of the large mechanosensitive channel MscL , 2001, Nature.

[29]  N. Aluru,et al.  Anomalously Immobilized Water: A New Water Phase Induced by Confinement in Nanotubes , 2003 .

[30]  S. Bezrukov,et al.  Polymeric nonelectrolytes to probe pore geometry: application to the alpha-toxin transmembrane channel. , 1999, Biophysical journal.

[31]  A. Auerbach,et al.  Desensitization of Mouse Nicotinic Acetylcholine Receptor Channels , 1998, The Journal of general physiology.

[32]  J. Gouaux,et al.  Structure of Staphylococcal α-Hemolysin, a Heptameric Transmembrane Pore , 1996, Science.

[33]  B. L. de Groot,et al.  The mechanism of proton exclusion in the aquaporin-1 water channel. , 2003, Journal of molecular biology.

[34]  G. Grest,et al.  Wetting and domain-growth kinetics in confined geometries. , 1992, Physical review. A, Atomic, molecular, and optical physics.

[35]  K. Schulten,et al.  Reconstructing Potentials of Mean Force through Time Series Analysis of Steered Molecular Dynamics Simulations , 1999 .

[36]  Lixin Tang,et al.  Channel gating governed symmetrically by conserved leucine residues in the M2 domain of nicotinic receptors , 1995, Nature.

[37]  Oliver Beckstein,et al.  Liquid–vapor oscillations of water in hydrophobic nanopores , 2003, Proceedings of the National Academy of Sciences of the United States of America.

[38]  C. Kung,et al.  One face of a transmembrane helix is crucial in mechanosensitive channel gating. , 1998, Proceedings of the National Academy of Sciences of the United States of America.

[39]  Intermittent permeation of cylindrical nanopores by water. , 2002, Physical review letters.

[40]  Mark S.P. Sansom,et al.  A Hydrophobic Gating Mechanism for Nanopores , 2001 .

[41]  Youxing Jiang,et al.  The open pore conformation of potassium channels , 2002, Nature.

[42]  C Kung,et al.  Pressure-sensitive ion channel in Escherichia coli. , 1987, Proceedings of the National Academy of Sciences of the United States of America.

[43]  A. Gross,et al.  Potassium channel gating observed with site-directed mass tagging , 2003, Nature Structural Biology.

[44]  H. Robert Guy,et al.  A large iris-like expansion of a mechanosensitive channel protein induced by membrane tension , 2002, Nature Structural Biology.

[45]  Y. Fujiyoshi,et al.  Structure and gating mechanism of the acetylcholine receptor pore , 2003, Nature.

[46]  B. Roux,et al.  Molecular dynamics of the KcsA K(+) channel in a bilayer membrane. , 2000, Biophysical journal.

[47]  M. Sansom,et al.  Potassium and sodium ions in a potassium channel studied by molecular dynamics simulations. , 2001, Biochimica et biophysica acta.

[48]  Sergei Sukharev,et al.  Purification of the small mechanosensitive channel of Escherichia coli (MscS): the subunit structure, conduction, and gating characteristics in liposomes. , 2002, Biophysical journal.

[49]  H. Guy,et al.  On the Conformation of the COOH-terminal Domain of the Large Mechanosensitive Channel MscL , 2003, The Journal of general physiology.

[50]  J. Mccammon,et al.  Time-correlation analysis of simulated water motion in flexible and rigid gramicidin channels. , 1991, Biophysical journal.

[51]  B. Roux,et al.  Energetics of ion conduction through the K + channel , 2022 .

[52]  I. Booth,et al.  The Closed Structure of the MscS Mechanosensitive Channel , 2003, Journal of Biological Chemistry.

[53]  D. Chandler,et al.  Hydrophobicity at Small and Large Length Scales , 1999 .

[54]  K. Schulten,et al.  Energetics of glycerol conduction through aquaglyceroporin GlpF , 2002, Proceedings of the National Academy of Sciences of the United States of America.

[55]  Youxing Jiang,et al.  The principle of gating charge movement in a voltage-dependent K+ channel , 2003, Nature.

[56]  Wolfgang Busch,et al.  Two Families of Mechanosensitive Channel Proteins , 2003, Microbiology and Molecular Biology Reviews.