Mass action kinetics of vesicle aggregation and fusion

The kinetics of Ca²⁺-induced fusion of both large and small phosphatidylserine vesicles has been examined theoretically and experimentally. With the Tb/DPA assay for mixing of vesicle contents, the kinetics of vesicle aggregation can be separated from the kinetics of vesicle fusion by varying the initial concentration of vesicles. Both the rate constant for vesicle dimerization and the rate constant for the actual membrane fusion can be determined by this analysis. The kinetics of release of vesicle contents during and following the fusion event have also been examined. The theoretical analysis can be easily adapted to other fusion assays and, as such, provides a unique opportunity for a quantitative evaluation of the factors which affect the fusion of vesicles such as temperature, cations bound to the lipids, and the membrane composition. The effect of aggregation reversibility on the kinetics of the overall fusion process is considered in detail. Analysis of the initial kinetics of the Ca²⁺-induced fusion of phosphatidylserine vesicles shows that the fusion step itself can be very rapid (of the order of ms-s) and the initial leakage associated with the fusion event, per se, is essentially zero for the large vesicles and quite small for the small vesicles.