title: Ca2+-induced fusion of phosphatidylserine vesicles: mass action kinetic analysis of membrane lipid mixing and aqueous contents mixing.
abstract: We have investigated the initial kinetics of Ca2+-induced aggregation and fusion of phosphatidylserine large unilamellar vesicles at 3, 5 and 10 mM Ca2+ and 15, 25 and 35 degrees C, utilizing the Tb/dipicolinate (Tb/DPA) assay for mixing of aqueous vesicle contents and a resonance energy transfer (RET) assay for mixing of bilayer lipids. Separate rate constants for vesicle aggregation as well as deaggregation and for the fusion reaction itself were determined by analysis of the data in terms of a mass action kinetic model. At 15 degrees C the aggregation rate constants for either assay are the same, indicating that at this temperature all vesicle aggregation events that result in lipid mixing lead to mixing of aqueous contents as well. By contrast, at 35 degrees C the RET aggregation rate constants are higher than the Tb/DPA aggregation rate constants, indicating a significant frequency of reversible vesicle aggregation events that do result in mixing of bilayer lipids, but not in mixing of aqueous vesicle contents. In any conditions, the RET fusion rate constants are considerably higher than the Tb/DPA fusion rate constants, demonstrating the higher tendency of the vesicles, once aggregated, to mix lipids than to mix aqueous contents. This possibly reflects the formation of an intermediate fusion structure. With increasing Ca2+ concentrations the RET and the Tb/DPA fusion rate constants increase in parallel with the respective aggregation rate constants. This suggests that fusion susceptibility is conferred on the vesicles during the process of vesicle aggregation and not solely as a result of the interaction of Ca2+ with isolated vesicles. Aggregation of the vesicles in the presence of Mg2+ produces neither mixing of aqueous vesicle contents nor mixing of bilayer lipids.
mesh_terms: D000818:Animals; D001921:Brain; D002118:Calcium; D002417:Cattle; D002449:Cell Aggregation; D007700:Kinetics; D008081:Liposomes; D008433:Mathematics; D008563:Membrane Lipids / Q000378:metabolism*; D008954:Models, Biological; D010718:Phosphatidylserines / Q000378:metabolism; D014867:Water
chemical_list: D008081:Liposomes; D008563:Membrane Lipids; D010718:Phosphatidylserines; D014867:Water; D002118:Calcium