Modulation of membrane fusion by asymmetric transbilayer distributions of amino lipids.

The fusion of model lipid bilayers containing synthetic amino lipids and the regulation of this fusion by inducing transbilayer asymmetry of these amino lipids via imposed pH gradients are demonstrated. Liposomes of 100 nm diameter consisting of 5 mol% 1,2-dioleoyl-3-(N,N-dimethylamino)propane (AL1) in a mixture of egg phosphatidylcholine (EPC), dioleoylphosphatidylethanolamine (DOPE), and cholesterol in a ratio of 35:20:45 do not fuse at pH 4.0. Fusion also is not observed upon increasing the external pH of these vesicles to 7.5, which results in the rapid transport of AL1 to the inner monolayer, as measured by a fluorescent probe sensitive to surface charge. However, dissipation of the imposed pH gradient leads to redistribution of AL1 to the outer monolayer at pH 7.5 and causes liposomal fusion, as detected by fluorescent lipid-mixing assay and freeze-fracture electron microscopy. The effect of varying the hydrocarbon structure of AL1 on the rate of fusion is demonstrated with five synthetic analogues, AL2-AL6. Higher rates of fusion occur with lipids containing longer unsaturated acyl chains and with lower values of pKa for the membrane-bound amino lipids. Fusion is also associated with destabilization of the bilayer at pH 7.5, as indicated by the formation of the hexagonal HII phase.