Influence of lipid composition on physical properties and PEG-mediated fusion of curved and uncurved model membrane vesicles: Nature's own fusogenic lipid bilayer

Poly(ethylene glycol) (PEG)-mediated fusion of phosphatidylcholine model membranes has been shown to mimic the protein-mediated biomembrane process [Lee, J., and Lentz, B. R. (1998) Proc. Natl. Acad. Sci. U.S.A. 95, 9274-9279]. Unlike the simple model membranes used in this earlier study, the lipid composition of fusogenic biomembranes is quite complex. The purpose of this paper was to examine PEG-mediated fusion of highly curved (SW) and largely uncurved (LUV) membrane vesicles composed of different lipids in order to identify lipid compositions that produce highly fusogenic membranes. Starting with liposomes composed of five lipids with different physical properties, dioleoylphosphatidylcholine (DOPC), dioleoylphosphatidylethanolamine (DOPE), dioleoylphosphatidylserine (DOPS), bovine brain sphingomyelin (SM), and cholesterol (CH), we systematically varied the composition and tested for the extent of PEG-mediated fusion after 5 min of treatment. We found that a vesicle system composed of four lipids, DOPC/DOPE/SM/CH, fused optimally at a 35/30/15/20 molar ratio. Each lipid seemed to play a part in optimizing the membrane for fusion. PE disrupted outer leaflet packing as demonstrated with TMA-DPH lifetime, C-6-NBD-PC partitioning, and DPH anisotropy measurements, and thus significantly enhanced fusion and rupture, without significantly altering interbilayer approach (X-ray diffraction). An optimal ratio of PC/PE (35/30) produced a balance between fusion and rupture. CH and SM, when present at an optimal. ratio of 3/4 in vesicles containing the optimal PC/PE ratio, reduced rupture without significantly reducing fusion. This optimal CH/SM ratio also enhanced outer leaflet packing, suggesting that fusion is dependent not only on outer leaflet packing but also on the properties of the inner leaflet. Addition of CH without SM enhanced rupture relative to fusion, while SM alone reduced both rupture and fusion. The optimal lipid composition is very close to the natural synaptic vesicle composition, suggesting that the synaptic vesicle composition is optimized with respect to fusogenicity.