Fusion of Sendai virus with liposomes: dependence on the viral fusion protein (F) and the lipid composition of liposomes.

The characteristics of fusion of the membrane of Sendai virus with that of liposomes has been investigated using two different methods to monitor the fusion reaction. The first method, which permits quantitation of lipid fused with virus, depends on separation by centrifugation of unfused liposomes from those fused with virus. The second involves the digestion after fusion of internal viral proteins by trypsin contained in liposomes; this assay is completely independent of exchange of lipid between liposomal and viral membranes in the absence of fusion. A fusion-inactive mutant virus, pa-cl, with an uncleaved F protein served as the appropriate control in these experiments. It was found that fusion of the virus with liposomes that contained no protein required cleavage of the F protein; such cleavage was previously shown to be required for fusion of the virus with cell membranes. This indicates the relevance of this model system for studies of fusion. Kinetic studies indicated that at neutral pH fusion was 88% complete in 10 min at 37 degrees. Investigation of the effects of liposomal lipid composition indicated that the presence of cholesterol in the liposomal membrane was required for fusion; a 0.3-0.4-mole fraction of cholesterol was optimal. The presence of neuraminic acid in the membrane was not essential for fusion. The results obtained are compatible with previous evidence suggesting a hydrophobic interaction between the cleaved F protein and the target membrane during fusion.