Alamethicin-induced single channel conductance fluctuations in biological membranes

Abstract

Alamethicin, a polypeptide of molecular weight ∼2,000, induces voltage-dependent conductance phenomena in artificial lipid bilayer membranes that are in some ways similar to those found in excitable membranes¹. Alamethicin-indueed conductance is probably a consequence of the formation of predominantly cation-selective channels which span the bilayer, and single open channels fluctuate at random between several well defined conductance states^2–5. Statistical analysis of single channel conductance fluctuations in planar lipid bilayers of various compositions has revealed that, whereas the conductance values are relatively constant in a variety of membranes, the average lifetime of the individual conductance states depends strongly on the lipid used to form the bilayer. This variability has tentatively been attributed to differences in lipid fluidity^6,7. Alamethicin is therefore a probe of some aspects of membrane composition. The study of single alamethicin channels in various natural membranes would allow comparison with the well defined bilayer systems and might yield information on lipid properties of natural membranes that could be important for the functioning of natural membrane channels and receptors. The experiments reported here show that alamethicin channels have similar conductances in the sarcolemmal (surface) membrane of frog and rat muscle and in lipid bilayers formed from synthetic lecithin. The average lifetime of the conductance states is, however, one order of magnitude larger in the biological membranes.