Direct evidence for Ca++-induced lateral phase separation in black membranes of lipid mixtures by the analysis of gramicidin A single-channels.

Single-channel conductance fluctuations are analysed for gramicidin A incorporated into binary-mixed black lipid membranes of charged phosphatidic acid and neutral lecithin in different molar ratios. At very low Ca++ concentrations in the electrolyte (i.e. in the presence of EDTA) homogeneous lipid mixtures are identified through their conductance and life time probability distributions for integral gramicidin pores. As for the pure lipid components, the conductance histograms each show a single maximum with regular width and for all channels a single mean lifetime is found. For Ca++-levels (10(-6) - 10(-5) M) that are close to the critical demixing concentration (approximately 10(-4) M) unusually broad conductance distributions and reduced lifetimes are found provided the PC content, chi, of the membrane is close to the critical mixture (chi crit approximately equal to 0.5). We interpret this as a first example of the coupling of a membrane function (the transport of ions) to a lipid matrix with locally fluctuating composition close to a critical demixing point. For cCA++ = 10(-2) M the conductance histogram of gramicidin A in an equimolar mixture of PA and PC shows two well-separated maxima. A correlation analysis between conductance and lifetime of the single pores shows that the two channel populations also differ significantly in their mean channel lifetime, tau *. This finding is interpreted as being direct evidence for Ca++-induced lateral phase separation in black lipid membranes, as has been postulated recently.