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.

The large mechanosensitive ion channel (MscL) of Escherichia coli was expressed on a plasmid encoding MscL as a fusion protein with glutathione S-transferase in an Escherichia coli strain containing a disruption in the chromosomal mscL gene. After purification of the fusion protein using glutathione-coated beads, thrombin cleavage allowed recovery of the MscL protein. The purified protein was reconstituted into artificial liposomes and found to be fully functional when examined with the patch-clamp technique. The reconstituted recombinant MscL protein formed ion channels that exhibited characteristic conductance and pressure sensitivity and were blocked by the mechanosensitive ion channel inhibitor gadolinium. The recombinant MscL protein was also used to raise specific anti-MscL polyclonal antibodies which abolished channel activity when preincubated with the MscL protein.

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 alpha-helical portion of the protein and the six C-terminal residues RKKEEP forming a charge cluster following the putative M2 membrane spanning alpha-helix.

Mechanosensitive (MS) ion channels have been documented in a variety of cells belonging to Eukarya and Eubacteria. We report the novel finding of two types of MS ion channels in the cell membrane of the halophilic archaeon Haloferax volcanii, a member of the Archaea that comprise the third phylogenetic domain. The two channels, MscA1 and MscA2, differed in their kinetic properties with MscA1 exhibiting more frequent openclosed transitions than MscA2. Both channels have large conductances that rectify between ؊40 mV and ؉40 mV where the conductance of MscA1 ranged from 380 to 680 picosiemens, whereas MscA2 ranged from 850 to 490 picosiemens. Both channels were blocked by submillimolar gadolinium. In addition, the channels of either membrane vesicles or detergent-solubilized membrane proteins remained functional upon reconstitution into artificial liposomes, a result that indicates that these channels are activated by mechanical force transmitted via the lipid bilayer alone. Subsequently a 37-kDa protein corresponding to the MscA1 channel activity was purified. With the possible functional similarity to bacterial MS channels, our finding of MS channels in Archaea emphasizes the ubiquity and importance of these channels in all domains of the evolutionary tree.According to the recent revision the universal phylogenetic tree is composed of three domains: Eukarya, Eubacteria, and Archaea (formerly archaebacteria) (1-6). From this scheme archaebacteria, which are prokaryotes like eubacteria, constitute an intermediary domain between eubacteria and eukaryotes, and although prokaryotes, archaebacteria are neither phylogenetically closer to eubacteria or to eukaryotes (7). As a distinct group of microorganisms Archaea comprise several different families of cells adapted to environments of certain habitats characterized by extreme temperatures such as in ocean hydrothermal vents, or high salt concentrations as occur in the Dead Sea (3,8).The existence of ion channels in cell membranes of different organisms belonging to the eubacterial and eukaryotic phylogenetic domains has been well documented. In contrast, the existence of ion channels in cell membranes of Archaea was only recently documented with the discovery of porin-like channels in the archaebacterium Haloferax volcanii (formerly Halobacterium volcanii) (9). In the present study we report the finding of two types of MS ion channels in the plasma membrane of the same microorganism that seem to share many properties of the described bacterial MS 1 ion channels (10 -18). The finding of this class of channels in the cell membrane of an archaeon demonstrates that MS channels, as well as porins, are present in organisms belonging to all domains of the evolutionary tree and indicates the importance of these types of membrane pores in the phylogeny of ion channels. EXPERIMENTAL PROCEDURESIsolation of the Cell Envelope-Cells of H. volcanii were grown and membranes prepared as described previously (9). Cells were cultured in nutrient rich media (in mM: 3350 NaCl, 170 MgCl 2 ...

OmpC and PhoE porins of Escherichia coli were examined by the patch‐clamp technique following reconstitution in liposomes, and were observed primarily in the open (conducting) state. With application of negative voltage and positive hydrostatic pressure, OmpC exhibited marked gating towards a more closed state whereas PhoE remained largely unaffected by pressure application. Hybrid chimeric OmpC‐PhoE proteins showed an increased tendency for pressure‐dependent gating as the OmpC proportion in the chimeric molecule increased. In addition, several PhoE mutants with amino acid substitutions and insertions in either the L3 or L4 loop of the monomer exhibited pressure sensitivity comparable with the wild‐type OmpC porin. Our data support the structural plasticity model of porins and are consistent with the ‘charge‐screening‐unscreening’ hypothesis that describes how these proteins may exist in distinct conformations.