Fusion of influenza virus membranes with liposomes at pH 7.5.

Influenza virus X-31 (H3N2) membranes fuse with liposomes containing ganglioside GD1a at pH 7.5. Fusion was demonstrated by electron microscopy and also can be measured by counting the labeled virus proteins incorporated into liposomes after bound virus has been removed. Liposomes composed of lipids that have no net charge behave as reported by other investigators and do not fuse with influenza X-31 membranes at neutral pH, but they do fuse at low pH. Therefore, the liposomal composition is a factor in whether liposomes fuse with influenza virus membranes at neutral pH, probably by determining whether binding occurs. The liposomal composition necessary for fusion at neutral pH needs to be individualized for each influenza subtype. To establish that a virus requires low pH for membrane fusion, it is first necessary to establish that fusion does not occur at neutral pH under conditions where adequate binding occurs.