Intermembrane contact affects calcium binding to phospholipid vesicles.

Binding of Ca2+ to liposomes composed of phosphatidylserine (PtdSer) was analyzed by potentiometric titrations. Ca2+ binding to large unilamellar PtdSer vesicles was saturable at a stoichiometry of 1:2 (Ca2+/PtdSer). At approximately 6 X 10(-4) M [Ca2+]free, the binding curve exhibited a discontinuity that can be attributed to the formation of a Ca2+/PtdSer complex with a higher affinity. When both Ca2+ and Mg2+ are present, depending on the relative concentrations, Mg2+ can either complete or can enhance Ca2+ binding. Concomitant to the enhanced binding, the vesicle suspension was found to aggregate, suggesting that close contact of membranes is a prerequisite for the abrupt change in affinity. This concept was tested by binding studies with liposomes of mixed composition. It was found that the incorporation of 50 mol% phosphatidylethanolamine (PtdEtn) into PtdSer liposomes produced a similar binding pattern to that of pure PtdSer with a saturable stoichiometry of 1:2 (Ca2+/PtdSer). However, incorporation of 50 mol% phosphatidylcholine (PtdCho) completely abolished the discontinuous shift in affinity and apparent saturation was reached at a stoichiometry of 1:4 (Ca2+/PtdSer). In addition, Ca2+ binding to PtdSer liposomes with 10 mol% galactosylcerebroside was not altered when compared to pure PtdSer, whereas 10 mol% of the glycolipid GL-4 abolished the increased binding. The results are closely correlated with recent findings on the role of the membrane composition in Ca2+-induced fusion of liposomes and argue in favor of a specific Ca2+/PtdSer complex (with 1:2 stoichiometry) forming only at points of close contact between membranes and serving as the trigger for membrane fusion.