Mechanosensitive channels: what can they do and how do they do it?

Abstract

While mechanobiological processes employ diverse mechanisms, at their heart are force-induced perturbations in the structure and dynamics of molecules capable of triggering subsequent events. Among the best characterized force-sensing systems are bacterial mechanosensitive channels. These channels reflect an intimate coupling of protein conformation with the mechanics of the surrounding membrane; the membrane serves as an adaptable sensor that responds to an input of applied force and converts it into an output signal, interpreted for the cell by mechanosensitive channels. The cell can exploit this information in a number of ways: ensuring cellular viability in the presence of osmotic stress and perhaps also serving as a signal transducer for membrane tension or other functions. This review focuses on the bacterial mechanosensitive channels of large (MscL) and small (MscS) conductance and their eukaryotic homologs, with an emphasis on the outstanding issues surrounding the function and mechanism of this fascinating class of molecules.

Figure 1

Functional and structural characteristics of MscL and MscS channels from bacteria and eukaryotes. (Top panel) Expression of MscS and MscL protects a bacterial strain lacking endogenous MscS and MscL from a 0.5 M osmotic downshock while expression of MscK and Cv-bCNG does not (Levina et al., 1999; Caldwell et al., 2010). Asterisks, over-expression of YbdG and MSL3 provides protection (Haswell et al., 2008; Schumann et al., 2010). (Top middle panel) Conductance of endogenous channels in giant E. coli spheroplasts (MscL, MscS, MscK, MscM, (reviewed in (Kung et al., 2010))), channels heterologously expressed in giant E. coli spheroplasts (MSC1, (Nakayama et al., 2007)) or endogenous channels in Arabidopsis thaliana root cells (MSL10, (Haswell et al., 2008)). (Bottom middle panel) Tension to gate expressed relative to MscL channels in the same patch (Berrier et al., 1996; Edwards et al., 2005; Li et al., 2007). (Bottom panel) Channel monomer topologies as predicted by TOPCONS (http://topcons.net/). Mature versions (after processing of chloroplast targeting sequences) of MSC1 and MSL3 are shown, and sequence loops connecting transmembrane helices were omitted for clarity.

Figure 2

Schematic representations of two models of gating for mechanosensitive channels. (A) The membrane mediated mechanism and (B) the trapdoor mechanism. (A) The closed state of a channel embedded in a bilayer will be at equilibrium in the absence of applied tension (left). The application of tension to a membrane (right) will serve to stabilize conformational states with greater cross-sectional areas of an embedded channel, with the larger areas favored by increasing tension. However, the change in protein conformation will perturb the membrane - protein interactions (schematically indicated by the compressed bilayer dimensions adjacent to the protein), which will contribute an unfavorable free energy term proportional to the interaction area between protein and membrane. As a consequence of these two competing effects, the interplay between tension and protein conformation can give rise to a rich variety of outcomes for the effect of tension on channel function, without the membrane pulling directly on the channel. (B) In the trapdoor mechanism, tension is coupled to the channel through an extramembrane component, such as the cytoskeleton or peptidoglycan (depicted by the bar with embedded ovals above the membrane), connected to a gate (trapdoor) covering the channel in the closed state (left). Movement of the extramembrane component relative to the membrane stretches the connecting spring, resulting in opening of the trapdoor (right). In this simplified mechanism, the membrane and channel are more passive participants with the applied tension doing work outside the membrane. It should be emphasized that these representations depict idealized mechanisms that are not mutually exclusive; activation barriers could be present that require direct interaction between components even when the overall transition is energetically favorable.

Figure 3

Structurally characterized forms of MscL as observed in the crystal structures of (A) M. tuberculosis MscL (Chang et al., 1998) and (B) S. aureus MscL (Liu et al., 2009), and (C) the model of the open state of EcMscL developed by Sukharev and Guy (“SG-model”; (Sukharev et al., 2001b)). For each structure, three representations are provided: (left) Chain traces of the subunits in each oligomeric channel with subunits depicted in different colors and α-helices and β-sheets shown as cylinders and arrows, respectively. The symmetry axis of each channel, assumed to be parallel to the membrane normal, is oriented vertically, with the cytoplasmic region positioned at the bottom. (middle) Chain traces of the transmembrane region of each channel. The same coloring scheme is used as in the left panel, and the view (down the membrane normal) is rotated 90° about the horizontal axis. The cytoplasmic helical bundle of MtMscL has been omitted. (right) Space filling representation of the structures depicted in the middle panel. Scale bars of 3.4 nm and 5.0 nm are indicated. Although admittedly a crude estimate, the cross-sections of the membrane spanning region of each channel may be approximated as regular polygons (pentagon or square), giving values for the corresponding areas of MtMscL, SaMscL and the SG-model as 20, 25 and 43 nm², respectively. Figure prepared with Molscript (Kraulis, 1991) and Raster-3D (Merritt and Bacon, 1997).

Figure 4

Structurally characterized forms of E. coli MscS as observed in the crystal structures of (A) non-conducting/inactivated (Bass et al., 2002; Steinbacher et al., 2007) and (B) open (Wang et al., 2008) states. The organization of this figure parallels that represented in Figure 3 for MscL. The scale bar between the space-filling models equals 3.7 nm; assuming that the cross-sections of each structure are depicted as regular heptagons with this side length, the corresponding areas are 50 nm². If lipids can intercalate between the splayed TM1-TM2 of adjacent subunits in the non-conducting/inactivated structure (Figure 4A), the cross-sectional area will be reduced from this value (see text); as a rough guide to the magnitude of this effect, a square of area 1 nm² is depicted, which corresponds approximately to the gap between adjacent subunits