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Chapter 13 : MscL, a Bacterial Mechanosensitive Channel

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Abstract:

This chapter discusses properties, structure, and the mechanism of gating of the large mechanosensitive (MS) channel MscL, which is probably the best understood tension-gated channel to date. The progress has been rapid, and within 10 years of MscL cloning we have a reasonably supported structural model of gating. The membrane topology determined with the PhoA fusion approach indicated that the short N-terminal (S1, ~15 residues) and the larger C-terminal (S3, ~40 residues) segments are cytoplasmic, whereas the loop connecting M1 and M2 segments (S2, ~25 residues) resides on the extracellular side (periplasm). MscL is activated directly by tension in the lipid bilayer in which the protein is embedded. Upon a strong osmotic downshift, hydrostatic pressure building up inside the cell causes a distension of the elastic cell wall and eventually stresses the inner membrane. Analysis of occupancies of substates and rates of subtransitions as functions of tension provided valuable information about the positions of intermediate states and major barriers on the reaction coordinate. MscL remains stable and functional in liposomes made of exogenous lipids. Initial characterization of MscL using scanning cysteine mutagenesis, site-specific spin labeling, and electron paramagnetic resonance (EPR) spectroscopy demonstrated that the transmembrane region of EcoMscL has essentially the same organization as TbMscL, validating the correctness of the homology-based alignment of the EcoMscL model. The hypothetical S1 bundle was proposed to act as the second gate because a poreoccluding element was needed to explain the postulated expanded low-conducting substate.

Citation: Sukharev S, Anishkin A, Chiang C, Betanzos M, Guy H. 2005. MscL, a Bacterial Mechanosensitive Channel, p 259-290. In Kubalski A, Martinac B (ed), Bacterial Ion Channels and Their Eukaryotic Homologs. ASM Press, Washington, DC. doi: 10.1128/9781555816452.ch13

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Figures

Image of Figure 1.
Figure 1.

The (TbMscL) and (EcoMscL) MscL homologs. (A) Alignment of the two sequences indicates 37% identity (asterisks). Boxes denote the short N-terminal domain (S1), the first (M1) and second (M2) transmembrane domains, and the cytoplasmic helical domain (S3). The region of M1 forming the main gate is shaded in gray. The alignment is truncated at the end of EcoMscL; thus, 23 extra residues at the C-terminal segment of TbMscL are not shown. (B) The TbMscL crystal structure and the model of EcoMscL built by homology. Upper panels represent the views from the top (periplasm) and the bottom panels are the side views. One subunit in each structure is represented as striped rods to show the topology of the polypeptide comprising S1, periplasmic loop (S2), cytoplasmic domains (S3), and M2-S3 connecting linkers. As seen from the alignment, the loops as well as S2- M3 linkers are divergent in the two species, and the correspondence between the model and the crystal structure is best in the more conserved transmembrane part. The N-terminal domain and the C-terminal end of TbMscL were not fully resolved; thus the S1 of EcoMscL was modeled anew as a short bundle of amphipathic helices.

Citation: Sukharev S, Anishkin A, Chiang C, Betanzos M, Guy H. 2005. MscL, a Bacterial Mechanosensitive Channel, p 259-290. In Kubalski A, Martinac B (ed), Bacterial Ion Channels and Their Eukaryotic Homologs. ASM Press, Washington, DC. doi: 10.1128/9781555816452.ch13
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Image of Figure 2.
Figure 2.

MscL activities and conduction states. (A) A portion of a typical current trace recorded at −20 mV with openings shown as upward transitions. Downward deflections from the fully open state represent short and long subconducting states. (B) Amplitude histogram based on a 5-min recording containing about 10 opening events. The histogram was fit with 11 gaussian peaks defining the amplitudes of the closed, fully open states, and nine intermediate levels. (C) The linear kinetic scheme aligned with the protein area scale chosen as a reaction coordinate. The positions of each conducting state and the main energy barrier were concluded from analyses of state occupancies and subtransition rates as a function of tension.

Citation: Sukharev S, Anishkin A, Chiang C, Betanzos M, Guy H. 2005. MscL, a Bacterial Mechanosensitive Channel, p 259-290. In Kubalski A, Martinac B (ed), Bacterial Ion Channels and Their Eukaryotic Homologs. ASM Press, Washington, DC. doi: 10.1128/9781555816452.ch13
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Image of Figure 3.
Figure 3.

Transmembrane helices of MscL in the crystal-like closed conformation (center) and two possible pathways for channel expansion. The 10-helix barrel-stave model with 10 almost parallel helices all participating in pore lining is shown on the left. The alternative five-helix tilted model is shown on the right. The highly tilted arrangement permits a larger pore lined primarily by M1 helices.

Citation: Sukharev S, Anishkin A, Chiang C, Betanzos M, Guy H. 2005. MscL, a Bacterial Mechanosensitive Channel, p 259-290. In Kubalski A, Martinac B (ed), Bacterial Ion Channels and Their Eukaryotic Homologs. ASM Press, Washington, DC. doi: 10.1128/9781555816452.ch13
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Image of Figure 4.
Figure 4.

The AFM image of the expanded conformation of MscL (A) next to the spacefilled “tilted” model of the open MscL (B). (C) Arrangement of helices with S3 domains separated. The AFM image is taken from , where the MscL homolog from serovar Typhimurium was studied in supported alkylsilane monolayers. Courtesy of Vladimir Tsukruk.

Citation: Sukharev S, Anishkin A, Chiang C, Betanzos M, Guy H. 2005. MscL, a Bacterial Mechanosensitive Channel, p 259-290. In Kubalski A, Martinac B (ed), Bacterial Ion Channels and Their Eukaryotic Homologs. ASM Press, Washington, DC. doi: 10.1128/9781555816452.ch13
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Image of Figure 5.
Figure 5.

Schematic representations of the closed-(left) and open-state models (right) of the transmembrane barrel of EcoMscL. The helices are depicted as cylinders and positions of alpha-carbons of the pore-lining residues are shown as spheres. The closed-to-open transition in the SG model is accompanied by a relatively small (20 to 30°) counterclockwise rotation of M1, which does not change substantially the pore-exposed face of the helix.

Citation: Sukharev S, Anishkin A, Chiang C, Betanzos M, Guy H. 2005. MscL, a Bacterial Mechanosensitive Channel, p 259-290. In Kubalski A, Martinac B (ed), Bacterial Ion Channels and Their Eukaryotic Homologs. ASM Press, Washington, DC. doi: 10.1128/9781555816452.ch13
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