Transbilayer redistribution of phosphatidylethanolamine during fusion of phospholipid vesicles. Dependence on fusion rate, lipid phase separation, and formation of nonbilayer structures

The effect of membrane fusion on the transbilayer distribution of dioleoyl- and dipalmitoylphosphatidylethanolamine (DOPE and DPPE, respectively) in phosphatidylserine (PS) vesicles was investigated. A 7-fold increase in the external pool of DOPE, as determined by labeling of the vesicle surface with 2,4,6-trinitrobenzenesulfonic acid, was observed when multilamellar vesicles (MLV) consisting of PS and DOPE were incubated with small unilamellar vesicles (SUV) of PS in the presence of Ca2+. In contrast, no significant redistribution of DPPE was seen when similar experiments were performed by using PS bilayers that contained DPPE instead of DOPE. Redistribution of neither DOPE nor DPPE could be detected during SUV-SUV fusion. By using the resonance energy transfer fusion assay for mixing of membrane lipids [Struck, D.K., Hoekstra, D., & Pagano, R. E. (1981) Biochemistry 20, 4093-4099; Hoekstra, D. (1982) Biochemistry 21, 2833-2840], it was demonstrated that fusion between SUV and MLV had actually occurred. The results also suggested that (partial) fusion of internal bilayers within the multilamellar system must have occurred. Although Mg2+-induced fusion between SUV and MLV was also observed, no redistribution of DOPE was seen in this case. It is concluded that the observed translocation of DOPE during fusion was probably mediated via inverted micellar structures, which were formed when the lipid was converted to the hexagonal (HII) phase resulting from lipid phase separation between PS and DOPE. However, induction of the hexagonal phase per se, i.e., in the absence of fusion, did not cause substantial transbilayer redistribution of DOPE, suggesting that fusion was intimately involved in this phenomenon. It is suggested that fusion represents the "driving force" for transbilayer DOPE redistribution, requiring a (partial) overlap between the kinetics of phase separation and fusion