Gram-negative bacteria characteristically are surrounded by an additional membrane layer, the outer membrane. Although outer membrane components often play important roles in the interaction of symbiotic or pathogenic bacteria with their host organisms, the major role of this membrane must usually be to serve as a permeability barrier to prevent the entry of noxious compounds and at the same time to allow the influx of nutrient molecules. This review summarizes the development in the field since our previous review (H. Nikaido and M. Vaara, Microbiol. Rev. 49:1-32, 1985) was published. With the discovery of protein channels, structural knowledge enables us to understand in molecular detail how porins, specific channels, TonB-linked receptors, and other proteins function. We are now beginning to see how the export of large proteins occurs across the outer membrane. With our knowledge of the lipopolysaccharide-phospholipid asymmetric bilayer of the outer membrane, we are finally beginning to understand how this bilayer can retard the entry of lipophilic compounds, owing to our increasing knowledge about the chemistry of lipopolysaccharide from diverse organisms and the way in which lipopolysaccharide structure is modified by environmental conditions.

Molecular Basis of Bacterial Outer Membrane Permeability Revisited

The organization of behavior: A neuropsychological theory

P2X receptors are membrane ion channels that open in response to the binding of extracellular ATP. Seven genes in vertebrates encode P2X receptor subunits, which are 40–50% identical in amino acid ...

Molecular Physiology of P2X Receptors

The shapes of eukaryotic cells and ultimately the organisms that they form are defined by cycles of mechanosensing, mechanotransduction and mechanoresponse Local sensing of force or geometry is transduced into biochemical signals that result in cell responses even for complex mechanical parameters such as substrate rigidity and cell-level form These responses regulate cell growth, differentiation, shape changes and cell death Recent tissue scaffolds that have been engineered at the micro- and nanoscale level now enable better dissection of the mechanosensing, transduction and response mechanisms

https://faculty.washington.edu/nsniadec/ME599/S09/private/25Vogel.pdf

Local force and geometry sensing regulate cell functions.

Integrin-mediated cell adhesions provide dynamic, bidirectional links between the extracellular matrix and the cytoskeleton Besides having central roles in cell migration and morphogenesis, focal adhesions and related structures convey information across the cell membrane, to regulate extracellular-matrix assembly, cell proliferation, differentiation, and death This review describes integrin functions, mechanosensors, molecular switches and signal-transduction pathways activated and integrated by adhesion, with a unifying theme being the importance of local physical forces

Transmembrane crosstalk between the extracellular matrix--cytoskeleton crosstalk.

The simplest cell-like structure, the lipid bilayer vesicle, can respond to mechanical deformation by elastic membrane dilation/thinning and curvature changes. When a protein is inserted in the lip...

Molecular basis of mechanotransduction in living cells

All cellular organisms respond to vibration, touch, gravity or changes in osmolarity, although the molecules on which such mechanosensations depend are unknown. Candidates include certain channels that gate in response to membrane stretch. Patch-clamp experiments with Escherichia coli envelope have revealed a mechanosensitive channel with very large conductance (MscL) and one with a smaller conductance (MscS) which may be important in osmoregulation. Here we have solubilized and fractionated the envelope, reconstituted the MscL activity in vitro, and traced it to a small protein, whose gene, mscL, we then cloned. Insertional disruption of mscL removes the channel activity, whereas re-expression of mscL borne on an expression plasmid restores it. MscL-channel activities were observed in material from a cell-free expression system with mscL as the only template. The mscL nucleotide sequence predicts a unique protein of only 136 amino acids, with a highly hydrophobic core and very different from porins or other known proteins.

A large-conductance mechanosensitive channel in E. coli encoded by mscL alone

We have used the patch-clamp electrical recording technique on giant spheroplasts of Escherichia coli and have discovered pressure-activated ion channels. The channels have the following properties: activation by slight positive or negative pressure; voltage dependence; large conductance; selectivity for anions over cations; dependence of activity on the species of permeant ions. We believe that these channels may be involved in bacterial osmoregulation and osmotaxis.

https://www.pnas.org/content/pnas/84/8/2297.full.pdf

Pressure-sensitive ion channel in Escherichia coli.

In mechanosensitive (MS) channels, gating is initiated by changes in intra-bilayer pressure profiles originating from bilayer deformation. Here we evaluated two physical mechanisms as triggers of MS channel gating: the energetic cost of protein-bilayer hydrophobic mismatches and the geometric consequences of bilayer intrinsic curvature. Structural changes in the Escherichia coli large MS channel (MscL) were studied under nominally zero transbilayer pressures using both patch clamp and EPR spectroscopic approaches. Changes in membrane intrinsic curvature induced by the external addition of lysophosphatidylcholine (LPC) generated massive spectroscopic changes in the narrow constriction that forms the channel 'gate', trapping the channel in the fully open state. Hydrophobic mismatch alone was unable to open the channel, but decreasing bilayer thickness lowered MscL activation energy, stabilizing a structurally distinct closed channel intermediate. We propose that the mechanism of mechanotransduction in MS channels is defined by both local and global asymmetries in the transbilayer pressure profile at the lipid-protein interface.

Physical principles underlying the transduction of bilayer deformation forces during mechanosensitive channel gating

The mechanosensitive cation channel (MscCa) transduces membrane stretch into cation (Na+, K+, Ca2+ and Mg2+) flux across the cell membrane, and is implicated in cell-volume regulation1, cell locomotion2, muscle dystrophy3 and cardiac arrhythmias4. However, the membrane protein(s) that form the MscCa in vertebrates remain unknown. Here, we use an identification strategy that is based on detergent solubilization of frog oocyte membrane proteins, followed by liposome reconstitution and evaluation by patch-clamp5. The oocyte was chosen because it expresses the prototypical MscCa (≥107MscCa/oocyte)6 that is preserved in cytoskeleton-deficient membrane vesicles7. We identified a membrane-protein fraction that reconstituted high MscCa activity and showed an abundance of a protein that had a relative molecular mass of 80,000 (Mr 80K). This protein was identified, by immunological techniques, as the canonical transient receptor potential channel 1 (TRPC1)8,9,10. Heterologous expression of the human TRPC1 resulted in a >1,000% increase in MscCa patch density, whereas injection of a TRPC1-specific antisense RNA abolished endogenous MscCa activity. Transfection of human TRPC1 into CHO-K1 cells also significantly increased MscCa expression. These observations indicate that TRPC1 is a component of the vertebrate MscCa, which is gated by tension developed in the lipid bilayer, as is the case in various prokaryotic mechanosensitive (Ms) channels11.

Boris Martinac

Papers

Molecular basis of mechanotransduction in living cells

A large-conductance mechanosensitive channel in E. coli encoded by mscL alone

Pressure-sensitive ion channel in Escherichia coli.

Physical principles underlying the transduction of bilayer deformation forces during mechanosensitive channel gating

TRPC1 forms the stretch-activated cation channel in vertebrate cells