PDBsum entry 1mxm

Membrane protein

PDB id: 1mxm

Contents

Protein chains

(+ 1 more) 254 a.a. *

* Residue conservation analysis

References listed in PDB file

Key reference

Title

Crystal structure of escherichia coli mscs, A voltage-Modulated and mechanosensitive channel.

Authors

R.B.Bass, P.Strop, M.Barclay, D.C.Rees.

Ref.

Science, 2002, 298, 1582-1587. [DOI no: 10.1126/science.1077945]

PubMed id

12446901

Abstract

The mechanosensitive channel of small conductance (MscS) responds both to stretching of the cell membrane and to membrane depolarization. The crystal structure at 3.9 angstroms resolution demonstrates that Escherichia coli MscS folds as a membrane-spanning heptamer with a large cytoplasmic region. Each subunit contains three transmembrane helices (TM1, -2, and -3), with the TM3 helices lining the pore, while TM1 and TM2, with membrane-embedded arginines, are likely candidates for the tension and voltage sensors. The transmembrane pore, apparently captured in an open state, connects to a large chamber, formed within the cytoplasmic region, that connects to the cytoplasm through openings that may function as molecular filters. Although MscS is likely to be structurally distinct from other ion channels, similarities in gating mechanisms suggest common structural elements.

Figure 2.
Fig. 2. Ribbon diagrams of MscS. (A) The polypeptide fold of a MscS subunit, viewed perpendicular to the sevenfold axis, with a rainbow gradient coloring scheme from the NH[2]- terminus (blue) to the COOH-terminus (red). The membrane-spanning helices in the transmembrane domain are labeled TM1, TM2, and TM3. The two remaining domains of the monomer, located in the cytoplasm, as well as location of the strands, are also indicated. Brackets denote approximate dimensions of the molecule in both (A) and (B). (B) The MscS heptamer viewed down the sevenfold axis, looking into the permeation pathway from outside the cell. The coloring of the subunits is as in (C). (C) Stereo side view of the MscS heptamer, viewed from the same direction as in (A), with each subunit represented in a separate color. The orientation of this view is such that the periplasm would be at the top, and the cytoplasm would be at the bottom of the figure. This figure was prepared with MOLSCRIPT (52) and RASTER-3D (53).

Figure 4.
Fig. 4. Model for gating the MscS channel. The transmembrane region of the channel, in an open state, is shown in an expanded view, with one subunit highlighted in dark blue and the remaining six subunits represented in transparent light blue. The side chains of arginine residues at positions 46, 74, and 88 are depicted as larger, colored ball and sticks in the highlighted subunit and as smaller, blue ball and sticks in the remaining subunits. The inferred position of the membrane bilayer is schematically indicated as a gray box. Arrows drawn normal and parallel to the plane of the membrane denote the likely directions of force when depolarization and mechanical stress are applied to the membrane, respectively. Curved arrow schematically illustrates the motion the TM1 and TM2 helices could exhibit in response to either or both of these forces.

The above figures are reprinted by permission from the AAAs: Science (2002, 298, 1582-1587) copyright 2002.