The dissimilar effect of diacylglycerols on Ca(2+)-induced phosphatidylserine vesicle fusion.

We have studied the effect of physiological concentrations of different diacylglycerols on Ca(2+)-induced fusion between phosphatidylserine vesicles. We monitored vesicle fusion as mixing of membrane lipids under conditions where the limiting factor was the aggregation and also in conditions where this aggregation was not the limiting factor. We found that diacylglycerols have a different modulating effect on the Ca(2+)-induced fusion: i) depending on their interfacial conformation, so that 1,2-isomers of diacylglycerols containing unsaturated or short saturated acyl chains stimulated fusion and their 1,3-isomers did not, and ii) depending on their specific type of bilayer interior perturbation, so that diacylglycerols containing unsaturated or short chain saturated acyl chains stimulated fusion but those containing long-chain saturated acyl chains did not. These requirements resembled those required for the diacylglycerol activation of protein kinase C, suggesting that diacylglycerol acts in both the specific activation of this enzyme and the induction of membrane fusion through the same perturbation of lipid structure. We found that polylysine affected the stimulatory role of 1,2-dioleoylglycerol differently, depending on whether aggregation was the limiting factor of fusion. When we studied the effect of very low concentrations of diacylglycerols on the bulk structural properties of phosphatidylserine, we found that they neither significantly perturbed the thermotropic transitions of phosphatidylserine nor affected the interaction of Ca2+ with the phosphate group of phosphatidylserine. The underlying mechanism of fusion between phosphatidylserine vesicles is discussed.