pH-dependent fusion between the flavivirus West Nile and liposomal model membranes

Fusion between purified [3H]uridine-labelled West Nile virus (WNV) particles and liposomes containing RNase, was assayed by degradation of the viral RNA to trichloroacetic acid-soluble material. Fusion of virus with liposomes containing phosphatidylcholine, phosphatidylethanolamine, sphingomyelin and cholesterol (at a molar ratio of 1:1:1:1.5) was found to be dependent on pH with maximum fusion occurring at pH 6.7 and below. At pH 6.6 fusion was rapid and was essentially complete within 2 min at 37 degrees C. At this time, approximately 50% of the viral RNA had been degraded and increasing the concentration of liposomes or time allowed for fusion increased this percentage only slightly. Fusion was dependent on temperature, was almost totally non-leaky and was not dependent on the presence of divalent cations. The lipid composition of liposomes was found to influence both the pH optimum for fusion and the maximum degree of fusion observed. Electron microscopy was used to visualize the fusion reaction between liposomes and virus particles.