1.A.22 The Large Conductance Mechanosensitive Ion Channel (MscL) Family
MscL of E. coli has been extensively characterized, and limited functional studies have been performed on some of its homologues (Häse et al., 1995; Sukharev et al., 1996, 1999; Sukharev et al., 1999, 2001). The MscL protein of E. coli is 136 amino acyl residues in length and spans the membrane twice as α-helices (Blount et al., 1996a,b). It forms a homopentameric channel with ten TMSs (Blount et al., 1996a,b; Sukharev et al., 1999). The channel transports ions fairly nonspecifically with slight selectivity for cations over anions (Sukharev et al., 1994). Mechanosensitivity has been demonstrated for several MscL homologues using patch-clamp methodology (Blount et al., 1996a,b; Blount et al., 1997; Sukharev et al., 1996). It has been shown to release proteins such as thioredoxin during osmotic downshift (Ajouz et al., 1998). Expression of the E. coli mscL gene has been shown to protect Vibrio alginolyticus and Bacillus subtilis from cell lysis during osmotic downshift (Nakamaru et al., 1999; Hoffmann et al., 2008).
The levels of both MscL and MscS channels in Bacillus subtilis are high during exponential phase growth, very low in stationary phase and non-detectable in spores (Wahome et al., 2009). Bacterial mechanosensitive channels, MscL and MscS, 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. 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 (Haswell et al., 2011).
MscL is gated by changes in bilayer deformation and by the membrane potential (Andersson et al. 2008). The structure of the MscL channel in membranes of varying thickness and curvature has been studied (Wang et al. 2018). Temperature-sensitive mutants have revealed aspects of the thermodynamic stability of the MscL structure (Owada et al. 2019). Membrane tension is not a mediator of long-range intracellular signaling, but local variations in tension mediate distinct processes in sub-cellular domains (Shi et al. 2018). Allosteric activation of MscL channels is triggered by lipid-mediated modification of mechanosensitive nano-pockets. Single-channel recordings have revealed a significant decrease in the pressure activation threshold of the modified channel and a sub-conducting state in the absence of applied tension (Kapsalis et al. 2019).
The three-dimensional structure of the M. tuberculosis MscL has been solved to 3.5 Å resolution, and the crystal structure has been shown to reflect that in the intact cell membrane (Chang et al., 1998; Perozo et al., 2001). This structure provided the basis for a model that explains gate opening and closing in response to membrane tension. Tension is proposed to expand the 10 TMS/5 subunit transmembrane barrel via the linker between the two TMSs [S1 (N-terminal) and M1 (C-terminal)]. S1 segments form a bundle when the channel is closed, and cross-linking between S1 segments prevents opening. S1 and M1 interact in the open channel, and cross-linking S1 to M1 impedes channel closing. The opening of MscL is accompanied by the disassociation of a carboxl-terminal protrusion and pore formation (Yoshimura et al., 2008). Phylogenetic, structural and functional analysis have been presented by Pivetti et al. (2003). How these channels may respond to change in the mechanical environment the lipid bilayer provides is discussed by Kung et al. (2010). Channel opening uses a helix-tilt mechanism and opens to a 2.8 nm diameter pore (Wang et al. 2014). Water may act as a 'lubricant' (softener) during TM1 helix elongation that may play a role in gating (Bavi et al. 2016).
Price et al. (2011) have demonstrated in vitro synthesis and oligomerization of the mechanosensitive channel, MscL, into functional ion channels. They showed that insertion requires YidC (2.A.9.3.1) but subsequent oligomerization to the functional pentamer occurs spontaneously. MscL acts as an 'emergency relief valve', protecting bacteria from lysis upon acute osmotic down-shock. MscL is reversibly and directly gated by changes in membrane tension. In the open state, MscL forms a non- selective 3 nS conductance channel which gates at tensions close to the lytic limit of the bacterial membrane. An earlier crystal structure at 3.5 A resolution of a pentameric MscL from Mycobacterium tuberculosis represented a closed-state or non-conducting conformation. MscL has a complex gating behaviour; it exhibits several intermediates between the closed and open states, including one putative non-conductive expanded state and at least three sub-conducting states. Liu et al. 2009 presented the crystal structure of a carboxy-terminal truncation mutant (Delta95-120) of MscL from Staphylococcus aureus (SaMscL(CDelta26)) at 3.8 A resolution. SaMscL(CDelta26) forms a tetrameric channel with both transmembrane helices tilted away from the membrane normal at angles close to that inferred for the open state, probably corresponding to a non-conductive but partially expanded intermediate state (Liu et al. 2009).
The MscL channel functions as a last-ditch emergency release valve, discharging cytoplasmic solutes upon decreases in the osmotic pressure. Opening this large gated pore allows passage of molecules up to 30 Å in diameter. MscL undergoes large conformational changes and contains structural/functional themes that recur in higher organisms and help elucidate how other, structurally more complex, channels function. These features of MscL include (i) the ability to directly sense and respond to biophysical changes in the membrane, (ii) an alpha helix ('slide helix') or series of charges ('knot in a rope') at the cytoplasmic membrane boundary to guide transmembrane movements, and (iii) important subunit interfaces that, when disrupted, appear to cause the channel to gate inappropriately. MscL may have medical applications: the modality of the MscL channel can be changed, suggesting its use as a triggered nanovalve in nanodevices, including those for drug targeting. The antibiotic streptomycin opens MscL and uses it as one of the primary paths to the cytoplasm.
The generalized transport reactions are:
(a) proteins (in) → proteins (out)
(b) ions (out) ions (in)
(c) osmolytes (in) osmolytes (out).