Effects of hemagglutinin fusion peptide on poly(ethylene glycol)-mediated fusion of phosphatidylcholine vesicles

The effects of hemagglutinin (HA) fusion peptide (X-31) on poly(ethylene gl ycol)- (PEG-) mediated vesicle fusion in three different vesicle systems ha ve been compared: dioleoylphosphatidylcholine (DOPC) small unilamellar vesi cles (SUV) and large unilamellar vesicles (LU-V) and palmitoyloleoylphospha tidylcholine (POPC) large unilamellar perturbed vesicles (pert. LIN). POPC LUVs were asymmetrically perturbed by hydrolyzing 2.5% of the outer leaflet lipid with phospholipase A(2) and removing hydrolysis products with BSA. T he mixing of vesicle contents showed that these perturbed vesicles fused in the presence of PEG as did DOPC SUV, but unperturbed LUV did not. Fusion p eptide had different effects on the fusion of these different types of vesi cles: fusion was not induced in the absence of PEG of in unperturbed DOPC L UV even in the presence of PEG. Fusion was enhanced in DOPC SUV at low pept ide surface occupancy but hindered at high surface occupancy. Finally, fusi on was hindered in proportion to peptide concentration in perturbed POPC LU V. Contents leakage assays demonstrated that the peptide enhanced leakage i n all vesicles. The peptide enhanced lipid transfer between both fusogenic and nonfusogenic vesicles. Peptide binding was detected in terms of enhance d tryptophan fluorescence or through transfer of tryptophan excited-state e nergy to membrane-bound diphenylhexatriene (DPH). The peptide had a higher affinity for vesicles with packing defects (SUV and perturbed LUV). Quasi-e lastic light scattering (QELS) indicated that the peptide caused vesicles t o aggregate. We conclude that binding of the fusion peptide to vesicle memb ranes has a significant effect on membrane properties but does not induce f usion. Indeed, the fusion peptide inhibited fusion of perturbed LUV. It can , however, enhance fusion between highly curved membranes that normally fus e when brought into close contact by PEG.