Ion Conduction through MscS as Determined by Electrophysiology and Simulation

Abstract

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 approximately 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.

Molecular and electrophysiological characterization of a mechanosensitive channel expressed in the chloroplasts of Chlamydomonas

“…On the other hand, the activation threshold did not change with the membrane potential. Recent studies on MscS have also demonstrated that the activation threshold does not change with voltage (15,27). Thus, voltage-independent activation of ⌬N-MSC1 resembles that of MscS, although MscS and ⌬N-MSC1 may exhibit voltage-sensitive gating under certain experimental conditions.…”

“…Evaluation of these data with the Goldman-HodgkinKatz equation indicated that the ratio of the permeability is g K :g Cl :g Br :g I ϭ 1:7:9:9. This anion preference is stronger than that of MscS, which has a g Cl /g K of 1.2-1.5 (14,15). Decreasing the bath CaCl 2 concentration to 1 mM did not shift the reversal potential, showing that the permeability to Ca 2ϩ was insignificant (Fig.…”

Bacterial Mechanosensitive Channels: Models for Studying Mechanosensory Transduction

“…Another way to determine the likely conductive state of the crystal structure is to simulate the flow of water or ions through the pore. The electrostatic properties and the ion conduction of MscS were also investigated using a new mesoscopic approach (135) for simulations of passive ion transport over long simulation times along with atomistic simulations using biasing electrostatic potentials (136). Both types of simulations showed low conductance for the state of the crystal structure but conductance levels in agreement with experimental data for an open state of the channel.…”

“…Anishkin et al The simulations of the wild type produced a hydrated pore, while simulations of the mutant showed a different hydration pattern and structure of the TM helices that is consistent with the mutants' tendency toward inactivation observed experimentally. Different simulation techniques at the atomistic and mesoscopic level (47,135,136) have also been employed to investigate the role of the cytoplasmic domain in the conductance of anions and cations through the channel.…”

Explicit and implicit modeling of nanobubbles in hydrophobic confinement

“…Once a nanobubble is formed, it leads to strong and long-ranged hydrophobic forces among the "dry" regions as the system further tries to minimize unfavorable interface area (see for instance the simple plate model above) and can induce quick protein folding and collapse (ten Wolde and Chandler 2002) or stable protein assemblies (Liu et al 2005). Importantly, the evaporation of liquid in the narrow hydrophobic pore region of ion channel proteins (Anishkin and Sukharev 2004, Beckstein et al 2001, Sotomayor et al 2007) seems to be important for the control of physiological ion conductance. The presence of a vapor bubble blocks ion permeation as the ion would have to desolvate to travel through the vapor.…”