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). Mutational loss of the hydrophobic interaction between membrane lipids and the periplasmic rim of the channel's funnel impairs the function of MscL, presumably by blocking channel opening (Yoshimura et al. 2004). Cyclodextrins can be used for structural and functional studies of mechanosensitive channels (Zhang et al. 2021).
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). Synergistic modes of regulation by lipid molecules in membrane tension-activated mechanosensitive MscL channels have been decribed and discussed (Wang et al. 2021). Hybrid-supported lipid bilayers (HSLBs) contain phospholipids and diblock copolymers. Manzer et al. 2021 used cell-free expression of MscL to assemble it with HSLB by either cotranslational integration of the protein into hybrid vesicles, followed by fusion of these proteoliposomes, or by preformation of a HSLB followed by the cell-free synthesis of the protein directly into the HSLB.
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.
Each subunit of an MscL consiss of two transmembrane segments TMS1 and TMS2 connected by a periplasmic loop. The closed pore is lined by five TMS1 helices. The two halves of the protein was cloned, and the first half conferred channel activity by itself while the second half conferred mechanosensitivity when exposed to the first half (Park et al. 2004).
The generalized transport reactions are:
(a) proteins (in) → proteins (out)
(b) ions (out) ions (in)
(c) osmolytes (in) osmolytes (out).
Large mechanosensitive ion channel: MscL, with a subunit size of 136 aas with 2 TMSs; it catalyzes efflux of ions (slightly cation selective), osmolytes and small proteins. Residues in the putative primary gate are present in the first TMS (Levin and Blount 2004). Protein-lipid interactions are important for gating, dependent on TMS tilting (Iscla et al., 2011b). The carboxyl-terminal cytoplasmic helices assemble into a pentameric bundle that resembles cartilage oligomeric matrix protein, and these are required for the selective formation of the pentamer (Ando et al. 2015). Lysophospholipids can increase the size of particles that can be transported (Foo et al. 2015). 500 - 700 channels are needed for 80% survival follwing a large changes in osmotic pressure, a number of channels similar to that found in wild type E. coli cells (Chure et al. 2018). its activation threshold decreases with membrane thickness; the membrane-thickness-dependent MscL opening mainly arises from structural changes in MscL to match the altered membrane thickness by stretching (Katsuta et al. 2018). MscL can provide a route for antibiotic entry into the E. coli cell, and agonists are available to facilitate their entry (Wray et al. 2020; Zhao et al. 2020). MscL has been used to design a synthetic mechanosensitive signaling pathway in compartmentalized artificial cells (Hindley et al. 2019). Available information at the ultrastructural level on lipids tightly bound to transport proteins and the impact of altered bulk membrane lipid composition has been reviewed (Stieger et al. 2021). Competition between hydrophobic mismatch and tension may result in opening tension for MscL (Wiggins and Phillips 2004).
MscL of E. coli (P0A742)
Osmotic adaptation channel that influences sporulation and secondary metabolite production, Sco3190 (MscL) (Wang et al. 2007).
Sco3190 of Streptomyces coelicolor.
Large conductance mechanosensitive channel protein, MscL, of 101 aas and 2 TMSs. When the membrane is stretched, MscL responds to the increase of membrane tension and opens a nonselective pore to about 30 A wide, exhibiting a large unitary conductance of approximately 3 nS. The structures of this archaeal MscL, trapped in the closed and expanded intermediate states, has been solved (Li et al. 2015). The comparative analysis of these two new structures reveals significant conformational rearrangements in the different domains of MscL. The large changes observed in the tilt angles of the two transmembrane helices (TMS1 and TMS2) fit well with the helix-pivoting model. Meanwhile, the periplasmic loop region transforms from a folded structure, containing an omega-shaped loop and a short beta-hairpin, to an extended and partly disordered conformation during channel expansion. Moreover, a significant rotating and sliding of the N-terminal helix (N-helix) is coupled to the tilting movements of TMS1 and TMS2. The dynamic relationships between the N-helix and TMS1/TMS2 suggest that the N-helix serves as a membrane-anchored stopper that limits the tilts of TM1 and TM2 in the gating process (Li et al. 2015). Residues I21-T30 in TMS 1 constitute the hydrophobic gate, and the packing of aromatic rings of F23 in each subunit of Ma-MscL is critical to the hydrophobic gate (Zhang et al. 2021). Hydrophilic substitutions of the other functionally important residues, A22 and G26, modulate channel gating by attenuating the hydrophobicity of the F23 constriction.
MscL of Methanosarcina acetivorans
MscL protein of 171 aas and 2 or 3 TMSs.
MscL of Chondrus crispus (Carrageen Irish moss) (Polymorpha crispa)
Putative large-conductance mechanosensitive channel of 101 aas and 2 TMSs.
MscL channel of Tetraselmis virus 1
MscL homologue of 101 aas and 2 TMSs.
MscL of Cafeteria roenbergensis virus
Large mechanosensitive ion channel of 151 aas and 2 TMSs. The 3-D structure is known, and it may reflect a nearly closed rather than fully closed state. Modeling support a clockwise rotation of the pore-forming first TMS promotes gating (Bartlett et al. 2004).
MscL of Mycobacterium tuberculosis (P0A5K8)
MscL (activated by arachidonate (Balleza et al., 2010), 45% identical to MscL of Bacillus subtilis (1.A.22.1.3)).
MscL of Rhizobium etli (Q2KCQ1)
The pentameric MscL channel (Iscla et al., 2011). The high resolution structure of a proposed closed but expanded tetrameric intermediate state has been determined (Liu et al. 2009). Adhesive forces to surfaces play an important role, next to other established driving forces, in staphylococcal MscL channel gating (Carniello et al. 2020). Thus, transmembrane antibiotic uptake and solute efflux in infectious staphylococcal biofilms is greatly stimulated when bacteria experience adhesion forces from surfaces as in biofilms.
MscL of Staphylococcus aureus (P68805)
MscL; rescues cells form osmotic downshift (Bucarey et al., 2012).
MscL of Micromonospora aurantica (D9T6D3)
Large-conductance mechanosensitive channel, MscL
MscL of Synechococcus sp.
Large-conductance mechanosensitive channel, MscL
MscL of Leuconostoc citreum
Large-conductance mechanosensitive channel, MscL
MscL of Renibacterium salmoninarum