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Gating prokaryotic mechanosensitive channels

Nature Reviews Molecular Cell Biology volume 7, pages 109–119 (2006)Cite this article

Key Points

  • When cells are exposed to a low external osmolarity, water influx generates large turgor pressures that can ultimately affect the integrity of the plasma membrane. In prokaryotes, this type of osmotic challenge is normally relieved by the opening of non-selective mechanosensitive channels.

  • Mechanosensitive channels can be studied in prokaryotes using patch-clamp methods that can be applied to giant Escherichia coli spheroplasts or using giant liposome preparations that contain reconstituted protein. Numerous activities of mechanosensitive channels have been reported under various conditions in the E. coli inner membrane.

  • According to their single-channel properties, mechanosensitive channels are classified as MscL, MscS and MscM, for mechanosensitive channels of large, small and mini conductances, respectively.

  • The structures of MscL and MscS are known at atomic resolution, and they are markedly different. For example, MscL is a homopentamer and its transmembrane (TM)-1 helices line the aqueous channel, whereas MscS is a homoheptamer and its aqueous channel is lined mostly by TM3 helices.

  • The fact that both MscL and MscS can be activated by pressure gradients after their reconstitution into defined lipid systems indicates that gating is triggered by tension that is transmitted directly through the bilayer.

  • Gating in MscL is associated with a massive conformational change, which leads to the generation of a >25-Ă… aqueous pore. This change involves the movement of TM1 helices away from the five-fold symmetry axis through a large tilting movement relative to the plane of the bilayer. The central tenets of this 'helix-tilt' hypothesis have been confirmed using functional, biochemical, computational and spectroscopic approaches.

  • Gating in MscS is significantly more complex than in MscL, because the channel is not only activated by tension, but can also be modulated by voltage. In addition, it has a complex inactivation mechanism that significantly reduces the probability of it being open in the presence of a sustained stimulus. Current proposals for gating are based on the tilting of the TM1–TM2 hairpins, which would promote the movement of TM3 away from the seven-fold symmetry axis and would change the overall conformation of the cytoplasmic domain.

Abstract

Prokaryotic mechanosensitive channels function as molecular switches that transduce bilayer deformations into protein motion. These protein structural rearrangements generate large non-selective pores that function as a prokaryotic 'last line of defence' to sudden osmotic challenges. Once considered an electrophysiological artefact, recent structural, spectroscopic and functional data have placed this class of protein at the centre of efforts to understand the molecular basis of lipid–protein interactions and their influence on protein function.

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Figure 1: The role of mechanosensitive channels in the prokaryotic response to hypo-osmotic challenges.
Figure 2: Functional and structural properties of MscL.
Figure 3: Functional and structural properties of MscS.
Figure 4: Hydrophobic gates and permeation pathways in mechanosensitive channels.
Figure 5: Structural models for the gating of MscL.
Figure 6: The gating mechanism of MscS.

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Acknowledgements

I am grateful to B. Martinac, D. M. Cortes, V. Vasquez and J. Wu for illuminating discussions regarding mechanosensitive channels and mechanosensitivity. I am also grateful for the support of the United States Public Health Service.

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  1. Department of Biochemistry and Molecular Biology, and Institute of Molecular Pediatric Science, University of Chicago Pritzker School of Medicine, 929 East 57th Street, Chicago, 60637, Illinois, USA

    Eduardo Perozo

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Glossary

Spheroplast

A Gram-negative bacterium that has had its cell wall almost completely removed.

Membrane–liposome fusion

A strategy to electrophysiologically characterize inaccessible membranes that involves fusing them with liposomes. The large proteoliposomes are then amenable to standard patch-clamp techniques.

Sm proteins

A class of proteins that form the core of small nuclear ribonucleoprotein particles (snRNPs), and are therefore key components of several mRNA-processing assemblies, including the spliceosome.

Hydrophobic mismatch

Defined as the difference between the hydrophobic length of the transmembrane structures (α-helices or β-sheets) of integral membrane proteins and the hydrophobic thickness of the membranes they span.

Amphipath

A molecule that contains both hydrophobic and hydrophilic regions, and tends to become incorporated into membrane interfaces.

Lysolipid

A phospholipid molecule that lacks one of the two acyl chains.

NiEdda

(nickel ethylenediaminediacetic acid). A chelated Ni(II) complex that is used to measure solvent accessibility in electron-paramagnetic-resonance spectroscopy.

Vapour lock

A mechanism by which solvent flow in a narrow space is interrupted by the formation of a 'de-wetted' region.

S3–S4 'paddle'

A charged structural motif that is found in voltage-dependentchannels and is proposed to be responsible for transmembrane voltage sensing.

Crystallographic B-factor

(also known as temperature or Debye–Waller factor). It describes the degree to which the electron density is spread out owing to local disorder or lattice disorder, or both.

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Perozo, E. Gating prokaryotic mechanosensitive channels. Nat Rev Mol Cell Biol 7, 109–119 (2006). https://doi.org/10.1038/nrm1833

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