Activities of a mechanosensitive ion channel in an E. coli mutant lacking the major lipoprotein.

The activity of the mechanosensitive (MS) ion channels in membrane patches, excised from E. coli spheroplasts, was analyzed using the patch-clamp technique. Outer membranes from a mutant lacking the major lipoprotein (Lpp) and its wild-type parent were examined. The MS-channel activities in the wild-type membrane rarely revealed substates at the time resolution used. These channels showed a stretch sensitivity indicated by the 1/Sp (the suction for an e-fold increase in channel open probability) of 4.9 mm Hg suction. The MS-channel activities of lpp included a prominent substrate and showed a weaker mechanosensitivity with an 1/Sp of 10.0 mm Hg. Whereas small amphipaths (chlorpromazine, trinitrophenol) or a larger amphipath (lysolecithin) all activated the MS channel in the wild-type membrane under minimal suction, only the larger lysolecithin could activate the MS channel in the lpp membranes. After lysolecithin addition, the lpp membrane became more effective in transmitting the stretch force to the MS channel, as indicated by a steepening of the Boltzmann curve. We discuss one interpretation of these results, in which the major lipoprotein services as a natural amphipath inserted in the inner monolayer and the loss of this natural amphipath makes the bilayer less able to transmit the gating force.