A mechanosensitive channel protein and its gene in E. coli

Receptor molecules that respond directly to gravity, touch, vibration, or osmotic pressure are inferred from their functions but not yet characterized as isolated proteins or products of cloned genes. These receptors are often in low abundance and in animal and plant tissues that are inaccessible, thus making biochemical analysis difficult. Yet, the application of the sensitive patch-clamp technique to measure transmembrane currents has demonstrated the ubiquity of ion channels whose opening is favored by membrane stretch forces. We have discovered in E. coli the activity of a mechanosensitive ion channel of large conductance (MscL), and have successfully isolated the corresponding protein and gene (Sukharev et al. 1994a). MscL channel appears to respond directly to stretch force in the lipid bilayer since it is active in artificial patches having only highly enriched MscL protein and lipids. Structurally, MscL is an integral membrane protein of only 136 amino-acid residues. Each channel pore is likely to be enclosed by six assembled MscL subunits. Hydropathy analysis suggests that the protein is largely hydrophobic with a more hydrophilic carboxyl tail. Targeted deletions and substitutions show that not all regions of the molecule contribute to channel function; however, strategic single amino-acid changes can alter channel kinetics and mechanosensitivity. MscL and its gene now form the first tangible system to study mechanosensing using a combination of genetic, biochemical, and biophysical techniques.