A large iris-like expansion of a mechanosensitive channel protein induced by membrane tension

MscL, a bacterial mechanosensitive channel of large conductance, is the first structurally characterized mechanosensor protein. Molecular models of its gating mechanisms are tested here. Disulfide crosslinking shows that M1 transmembrane alpha-helices in MscL of resting Escherichia coli are arranged similarly to those in the crystal structure of MscL from Mycobacterium tuberculosis. An expanded conformation was trapped in osmotically shocked cells by the specific bridging between Cys 20 and Cys 36 of adjacent M1 helices. These bridges stabilized the open channel. Disulfide bonds engineered between the M1 and M2 helices of adjacent subunits (Cys 32-Cys 81) do not prevent channel gating. These findings support gating models in which interactions between M1 and M2 of adjacent subunits remain unaltered while their tilts simultaneously increase. The MscL barrel, therefore, undergoes a large concerted iris-like expansion and flattening when perturbed by membrane tension.     

Membrane topology and multimeric structure of a mechanosensitive channel protein of Escherichia coli.

We have studied the membrane topology and multimeric structure of a mechanosensitive channel, MscL, which we previously isolated and cloned from Escherichia coli. We have localized this 15-kDa protein to the inner membrane and, by PhoA fusion, have shown that it contains two transmembrane domains with both the amino and carboxyl termini on the cytoplasmic side. Mutation of the glutamate at position 56 to histidine led to changes in channel kinetics which were dependent upon the pH on the periplasmic, but not cytoplasmic side of the membrane, providing additional evidence for the periplasmic positioning of this part of the molecule. Tandems of two MscL subunits expressed as a single polypeptide formed functional channels, suggesting an even number of transmembrane domains per subunit (amino and carboxyl termini on the same side of the membrane), and an even number of subunits per functional complex. Finally, cross-linking studies suggest that the functional MscL complex is a homohexamer. In summary, these data are all consistent with a protein domain assignment and topological model which we propose and discuss.     

Engineering covalent oligomers of the mechanosensitive channel of large conductance from Escherichia coli with native conductance and gating characteristics

To obtain a gene construct for making single substitutions per channel and to determine the quaternary structure of the mechanosensitive channel MscL from Escherichia coli, covalent oligomers (monomer to hexamer) were engineered by gene fusion; up to six copies of the mscL gene were fused in tandem. All the multimeric tandem constructs yielded functional channels with wild-type conductance and dwell times. Importantly, only the covalent pentamer opened at the same relative pressure (compared to the pressure required to open MscS) as the wild-type MscL channel. The in vivo data strongly suggest that pentameric MscL represents the functional state of the channel.     

Functional design of bacterial mechanosensitive channels. Comparisons and contrasts illuminated by random mutagenesis

MscS and MscL are mechanosensitive channels found in bacterial plasma membranes that open large pores in response to membrane tension. These channels function to alleviate excess cell turgor invoked by rapid osmotic downshock. Although much is known of the structure and molecular mechanisms underlying MscL, genes correlating with MscS activity have only recently been identified. Previously, it was shown that eliminating the expression of Escherichia coli yggB removed a major portion of MscS activity. YggB is distinct from MscL by having no obvious structural similarity. Here we have reconstituted purified YggB in proteoliposomes and have successfully detected MscS channel activity, confirming that purified YggB protein encodes MscS activity. Additionally, to define functional regions of the channel protein, we have randomly mutagenized the structural gene and isolated a mutant that evokes a gain-of-function phenotype. Physiological experiments demonstrate that the mutated channel allows leakage of solutes from the cell, suggesting inappropriate channel opening. Interestingly, this mutation is analogous in position and character to mutations yielding a similar phenotype in MscL. Hence, although MscS and MscL mechanosensitive channels are structurally quite distinct, there may be analogies in their gating mechanisms.     

The dynamics of protein-protein interactions between domains of MscL at the cytoplasmic-lipid interface

The bacterial mechanosensitive channel of large conductance, MscL, is one of the best characterized mechanosensitive channels serving as a paradigm for how proteins can sense and transduce mechanical forces. The physiological role of MscL is that of an emergency release valve that opens a large pore upon a sudden drop in the osmolarity of the environment. A crystal structure of a closed state of MscL shows it as a homopentamer, with each subunit consisting of two transmembrane domains (TM). There is consensus that the TM helices move in an iris like manner tilting in the plane of the membrane while gating. An N-terminal amphipathic helix that lies along the cytoplasmic membrane (S1), and the portion of TM2 near the cytoplasmic interface (TM2_ci), are relatively close in the crystal structure, yet predicted to be dynamic upon gating. Here we determine how these two regions interact in the channel complex, and study how these interactions change as the channel opens. We have screened 143 double-cysteine mutants of E. coli MscL for their efficiency in disulfide bridging and generated a map of protein-protein interactions between these two regions. Interesting candidates have been further studied by patch clamp and show differences in channel activity under different redox potentials; the results suggest a model for the dynamics of these two domains during MscL gating.     

The dynamics of protein-protein interactions between domains of MscL at the cytoplasmic-lipid interface

The bacterial mechanosensitive channel of large conductance, MscL, is one of the best characterized mechanosensitive channels serving as a paradigm for how proteins can sense and transduce mechanical forces. The physiological role of MscL is that of an emergency release valve that opens a large pore upon a sudden drop in the osmolarity of the environment. A crystal structure of a closed state of MscL shows it as a homopentamer, with each subunit consisting of two transmembrane domains (TM). There is consensus that the TM helices move in an iris like manner tilting in the plane of the membrane while gating. An N-terminal amphipathic helix that lies along the cytoplasmic membrane (S1), and the portion of TM2 near the cytoplasmic interface (TM2_ci), are relatively close in the crystal structure, yet predicted to be dynamic upon gating. Here we determine how these two regions interact in the channel complex, and study how these interactions change as the channel opens. We have screened 143 double-cysteine mutants of E. coli MscL for their efficiency in disulfide bridging and generated a map of protein-protein interactions between these two regions. Interesting candidates have been further studied by patch clamp and show differences in channel activity under different redox potentials; the results suggest a model for the dynamics of these two domains during MscL gating.     

Molecular dissection of the large mechanosensitive ion channel (MscL) of E. coli: Mutants with altered channel gating and pressure sensitivity

In the search for the essential functional domains of the large mechanosensitive ion channel (MscL) of E. coli, we have cloned several mutants of the mscL gene into a glutathione S-transferase fusion protein expression system. The resulting mutated MscL proteins had either amino acid additions, substitutions or deletions in the amphipathic N-terminal region, and/or deletions in the amphipathic central or hydrophilic C-terminal regions. Proteolytic digestion of the isolated fusion proteins by thrombin yielded virtually pure recombinant MscL proteins that were reconstituted into artificial liposomes and examined for function by the patch-clamp technique. The addition of amino acid residues to the N-terminus of the MscL did not affect channel activity, whereas N-terminal deletions or changes to the N-terminal amino acid sequence were poorly tolerated and resulted in channels exhibiting altered pressure sensitivity and gating. Deletion of 27 amino acids from the C-terminus resulted in MscL protein that formed channels similar to the wild-type, while deletion of 33 C-terminal amino acids extinguished channel activity. Similarly, deletion of the internal amphipathic region of the MscL abolished activity. In accordance with a recently proposed spatial model of the MscL, our results suggest that (i) the N-terminal portion participates in the channel activation by pressure, and (ii) the essential channel functions are associated with both, the putative central amphipathic α-helical portion of the protein and the six C-terminal residues RKKEEP forming a charge cluster following the putative M2 membrane spanning α-helix. Received: 25 September 1996/Revised: 21 November 1996     

Structural models of the MscL gating mechanism.

Three-dimensional structural models of the mechanosensitive channel of large conductance, MscL, from the bacteria Mycobacterium tuberculosis and Escherichia coli were developed for closed, intermediate, and open conformations. The modeling began with the crystal structure of M. tuberculosis MscL, a homopentamer with two transmembrane alpha-helices, M1 and M2, per subunit. The first 12 N-terminal residues, not resolved in the crystal structure, were modeled as an amphipathic alpha-helix, called S1. A bundle of five parallel S1 helices are postulated to form a cytoplasmic gate. As membrane tension induces expansion, the tilts of M1 and M2 are postulated to increase as they move away from the axis of the pore. Substantial expansion is postulated to occur before the increased stress in the S1 to M1 linkers pulls the S1 bundle apart. During the opening transition, the S1 helices and C-terminus amphipathic alpha-helices, S3, are postulated to dock parallel to the membrane surface on the perimeter of the complex. The proposed gating mechanism reveals critical spatial relationships between the expandable transmembrane barrel formed by M1 and M2, the gate formed by S1 helices, and "strings" that link S1s to M1s. These models are consistent with numerous experimental results and modeling criteria.     

Structure and function of the bacterial mechanosensitive channel of large conductance.

Mechanosensation in bacteria involves transducing membrane stress into an electrochemical response. In Escherichia coli and other bacteria, this function is carried out by a number of proteins including MscL, the mechanosensitive channel of large conductance. MscL is the best characterized of all mechanosensitive channels. It has been the subject of numerous structural and functional investigations. The explosion in experimental data on MscL recently culminated in the solution of the three-dimensional structure of the MscL homologue from Mycobacterium tuberculosis. In this review, much of these data are united and interpreted in terms of the newly published M. tuberculosis MscL crystal structure.     

The effect of local bending on gating of MscL using a representative volume element and finite element simulation

Many physiological processes such as cell division, endocytosis and exocytosis cause severe local curvature of the cell membrane. Local curvature has been shown experimentally to modulate numerous mechanosensitive (MS) ion channels. In order to quantify the effects of local curvature we introduced a coarse grain representative volume element for the bacterial mechanosensitive ion channel of large conductance (MscL) using continuum elasticity. Our model is designed to be consistent with the channel conformation in the closed and open states to capture its major continuum rheological behavior in response to the local membrane curvature. Herein we show that change in the local curvature of the lipid bilayer can modulate MscL activity considerably by changing both bilayer thickness and lateral pressure profile. Intriguingly, although bending in any direction results in almost the same free-energy cost, inward (cytoplasmic) bending favors channel opening, whereas outward (periplasmic) bending facilitates closing of the narrowest part of the MscL pore. This quantitative study using MscL as a model channel may have wide reaching consequences for the effect of local curvature on the physiological function of other types of prokaryotic and eukaryotic membrane proteins.     

The effect of local bending on gating of MscL using a representative volume element and finite element simulation

Many physiological processes such as cell division, endocytosis and exocytosis cause severe local curvature of the cell membrane. Local curvature has been shown experimentally to modulate numerous mechanosensitive (MS) ion channels. In order to quantify the effects of local curvature we introduced a coarse grain representative volume element for the bacterial mechanosensitive ion channel of large conductance (MscL) using continuum elasticity. Our model is designed to be consistent with the channel conformation in the closed and open states to capture its major continuum rheological behavior in response to the local membrane curvature. Herein we show that change in the local curvature of the lipid bilayer can modulate MscL activity considerably by changing both bilayer thickness and lateral pressure profile. Intriguingly, although bending in any direction results in almost the same free-energy cost, inward (cytoplasmic) bending favors channel opening, whereas outward (periplasmic) bending facilitates closing of the narrowest part of the MscL pore. This quantitative study using MscL as a model channel may have wide reaching consequences for the effect of local curvature on the physiological function of other types of prokaryotic and eukaryotic membrane proteins.     

Dynamics of Protein-Protein Interactions at the MscL Periplasmic-Lipid Interface

MscL, the highly conserved bacterial mechanosensitive channel of large conductance, is one of the best studied mechanosensors. It is a homopentameric channel that serves as a biological emergency release valve that prevents cell lysis from acute osmotic stress. We previously showed that the periplasmic region of the protein, particularly a single residue located at the TM1/periplasmic loop interface, F47 of Staphylococcus aureus and I49 of Escherichia coli MscL, plays a major role in both the open dwell time and mechanosensitivity of the channel. Here, we introduced cysteine mutations at these sites and found they formed disulfide bridges that decreased the channel open dwell time. By scanning a likely interacting domain, we also found that these sites could be disulfide trapped by addition of cysteine mutations in other locations within the periplasmic loop of MscL, and this also led to rapid channel kinetics. Together, the data suggest structural rearrangements and protein-protein interactions that occur within this region upon normal gating, and further suggest that locking portions of the channel into a transition state decreases the stability of the open state.     

Dynamics of Protein-Protein Interactions at the MscL Periplasmic-Lipid Interface

MscL, the highly conserved bacterial mechanosensitive channel of large conductance, is one of the best studied mechanosensors. It is a homopentameric channel that serves as a biological emergency release valve that prevents cell lysis from acute osmotic stress. We previously showed that the periplasmic region of the protein, particularly a single residue located at the TM1/periplasmic loop interface, F47 of Staphylococcus aureus and I49 of Escherichia coli MscL, plays a major role in both the open dwell time and mechanosensitivity of the channel. Here, we introduced cysteine mutations at these sites and found they formed disulfide bridges that decreased the channel open dwell time. By scanning a likely interacting domain, we also found that these sites could be disulfide trapped by addition of cysteine mutations in other locations within the periplasmic loop of MscL, and this also led to rapid channel kinetics. Together, the data suggest structural rearrangements and protein-protein interactions that occur within this region upon normal gating, and further suggest that locking portions of the channel into a transition state decreases the stability of the open state.     

Contributions of the different extramembranous domains of the mechanosensitive ion channel MscL to its response to membrane tension

MscL is a mechanosensitive channel that is gated by tension in the membrane bilayer alone. It is a homo-oligomer of a protein comprising two transmembrane segments connected by an external loop, with the NH(2) and COOH termini located in the cytoplasm. The contributions of the extramembranous domains of the channel to its activity were investigated by specific proteolysis during patch-clamp experiments. Limited proteolysis of the COOH terminus or the NH(2) terminus increased the mechanosensitivity of the channel without changing its conductance. Strikingly, after cleavage of the external loop of each monomer, the channel was still functional, and its mechanosensitivity was increased dramatically, indicating that the loop acts as a spring that resists the opening of the channel and promotes its closure when it is open. These results indicate that the integrity of most of the extramembranous domains is not essential for mechanosensitivity. They suggest that these domains counteract the movement of the transmembrane helices to which they are connected, thus setting the level of sensitivity of the channel to tension.     

Estimation of the pore size of the large-conductance mechanosensitive ion channel of Escherichia coli.

The open channel diameter of Escherichia coli recombinant large-conductance mechanosensitive ion channels (MscL) was estimated using the model of Hille (Hille, B. 1968. Pharmacological modifications of the sodium channels of frog nerve. J. Gen. Physiol. 51:199-219) that relates the pore size to conductance. Based on the MscL conductance of 3.8 nS, and assumed pore lengths, a channel diameter of 34 to 46 A was calculated. To estimate the pore size experimentally, the effect of large organic ions on the conductance of MscL was examined. Poly-L-lysines (PLLs) with a diameter of 37 A or larger significantly reduced channel conductance, whereas spermine (approximately 15 A), PLL19 (approximately 25 A) and 1,1'-bis-(3-(1'-methyl-(4,4'-bipyridinium)-1-yl)-propyl)-4,4'-b ipyridinium (approximately 30 A) had no effect. The smaller organic ions putrescine, cadaverine, spermine, and succinate all permeated the channel. We conclude that the open pore diameter of the MscL is approximately 40 A, indicating that the MscL has one of the largest channel pores yet described. This channel diameter is consistent with the proposed homohexameric model of the MscL.     

Cysteine scanning of MscL transmembrane domains reveals residues critical for mechanosensitive channel gating

The mechanosensitive channel of large conductance (MscL), a bacterial channel, is perhaps the best characterized mechanosensitive protein. A structure of the Mycobacterium tuberculosis ortholog has been solved by x-ray crystallography, but details of how the channel gates remain obscure. Here, cysteine scanning was used to identify residues within the transmembrane domains of Escherichia coli MscL that are crucial for normal function. Utilizing genetic screens, we identified several mutations that induced gain-of-function or loss-of-function phenotypes in vivo. Mutants that exhibited the most severe phenotypes were further characterized using electrophysiological techniques and chemical modifications of the substituted cysteines. Our results verify the importance of residues in the putative primary gate in the first transmembrane domain, corroborate other residues previously noted as critical for normal function, and identify new ones. In addition, evaluation of disulfide bridging in native membranes suggests alterations of existing structural models for the “fully closed” state of the channel.     

Domain organization of the MscS mechanosensitive channel of Escherichia coli

The major structural features of the Escherichia coli MscS mechanosensitive channel protein have been explored using alkaline phosphatase (PhoA) fusions, precise deletions and site-directed mutations. PhoA protein fusion data, combined with the positive-inside rule, strongly support a model in which MscS crosses the membrane three times, adopting an N(out)-C(in) configuration. Deletion data suggest that the C-terminal domain of the protein is essential for the stability of the MscS channel, whereas the protein will tolerate small deletions at the N-terminus. Four mutants that exhibit either gain-of-function (GOF) or loss-of-function have been identified: a double mutation I48D/S49P inactivates MscS, whereas the MscS mutants T93R, A102P and L109S cause a strong GOF phenotype. The similarity of MscS to the last two domains of MscK (formerly KefA) is reinforced by the demonstration that expression of a truncated MscK protein can substitute for MscL and MscS in downshock survival assays. The data derived from studies of the organization, conservation and the influence of mutations provide significant insights into the structure of the MscS channel.     

On the structure of the N-terminal domain of the MscL channel: helical bundle or membrane interface

The mechanosensitive channel of large conductance, MscL, serves as a biological emergency release valve protecting bacteria from acute osmotic downshock and is to date the best characterized mechanosensitive channel. A well-recognized and supported model for Escherichia coli MscL gating proposes that the N-terminal 11 amino acids of this protein form a bundle of amphipathic helices in the closed state that functionally serves as a cytoplasmic second gate. However, a recently reexamined crystal structure of a closed state of the Mycobacterium tuberculosis MscL shows these helices running along the cytoplasmic surface of the membrane. Thus, it is unclear if one structural model is correct or if they both reflect valid closed states. Here, we have systematically reevaluated this region utilizing cysteine-scanning, in vivo functional characterization, in vivo SCAM, electrophysiological studies, and disulfide-trapping experiments. The disulfide-trapping pattern and functional studies do not support the helical bundle and second-gate hypothesis but correlate well with the proposed structure for M. tuberculosis MscL. We propose a functional model that is consistent with the collective data.     

Novel splice variants of amphiphysin I are expressed in retina

The MscL channel is a mechanosensitive channel which is gated by membrane stress or tension. Here, we describe a series of simulations which apply simulated mechanical stress to a molecular model of the MscL channel using two methods - direct force application to the transmembrane segments, and anisotropic pressure coupling. In the latter simulations, pressures less than that equivalent to a bilayer tension of 12 dyn/cm did not cause the channel to open, while pressures in excess of this value resulted in the channel opening. These results are in approximate agreement with experimental findings.     

Loss-of-function mutations at the rim of the funnel of mechanosensitive channel MscL

MscL is a bacterial mechanosensitive channel that is activated directly by membrane stretch. Although the gene has been cloned and the crystal structure of the closed channel has been defined, how membrane tension causes conformational changes in MscL remains largely unknown. To identify the site where MscL senses membrane tension, we examined the function of the mutants generated by random and scanning mutagenesis. In vitro (patch-clamp) and in vivo (hypoosmotic-shock) experiments showed that when a hydrophilic amino acid replaces one of the hydrophobic residues that are thought to make contact with the membrane lipid near the periplasmic end of the M1 or M2 transmembrane domain, MscL loses the ability to open in response to membrane tension. Hydrophilic (asparagine) substitution of the other residues in the lipid-protein interface did not impair the channel's mechanosensitivity. These observations suggest that the disturbance of the hydrophobic interaction between the membrane lipid and the periplasmic rim of the channel's funnel impairs the function of MscL.