LIPID MIXING ASSAYS TO DETERMINE FUSION IN LIPOSOME SYSTEMS

Lipid vesicles or liposomes constitute an important and valuable model to study fundamental elements involved in the mechanism of membrane fusion. Their amenability to variations in composition, including the possibility of reconstituting proteins in their membranes, their access to investigating at the molecular level interactions with fusogens, such as certain ions, polypeptides, proteins, and polymers, and their ability to recognize intra- and intermembrane forces in and between liposomal membranes during close approach and aggregation have made liposomes a popular experimental system to unravel basic features of membrane fusion. Fusion of liposomes or vesicles prepared from synthetic amphiphiles is an uncontrolled event—that is, in contrast to biological fusion it is not a transient phenomenon. Rather, once initiated, fusion of liposomes proceeds until equilibrium is reached and under these conditions, the original vesicle structure may be entirely lost. This has been observed by electron microscopic techniques in the case of Ca2+-induced fusion of vesicles consisting of phosphatidylserine (PS), which revert to cochleate cylinders. It is even more extreme in the case of divalent cation-induced fusion of didodecyl phosphate vesicles, where long tubular hexagonal structures are formed. © 1993, Elsevier Inc. All rights reserved.