Kinetics of intermixing of lipids and mixing of aqueous contents during vesicle fusion

Abstract

The kinetics of Ca 2+-induced fusion of small unilamellar phosphatidylserine (PS) vesicles was investigated by continuous monitoring of the mixing of vesicle lipids and, in parallel, the mixing of aqueous contents. The release of vesicle contents during fusion to the external medium was also determined by monitoring the release of carboxyfluorescein and the release of the aqueous compounds which were employed to report the occurrence of fusion upon mixing of the intravesicular volumes. The results demonstrate that the initial rates of lipid mixing and the mixing of aqueous contents are very similar, and appear to be much faster than the initial rate of vesicle leakage. This suggests that the majority of the initial events during PS vesicle fusion represents the fusion of vesicles prior to leakage of vesicle contents. It is concluded that, under conditions where leakage of vesicle contents during fusion is limited, both fusion assays accurately reflect the occurrence of this process, thus indicating the usefulness of these assays as reliable, complementary tools in the study of fusion of (model) membranes.