Visualization of individual fusion events between VSV and liposomes at low pH, after negative staining. (A) At pH 7.5, some viral particles interact with liposomes, but no fusion is detected. (B) At pH 6.0, viral particles and liposomes aggregate and numerous fusion events are visible. Fusion events are characterized by the presence of several nucleocapsids within a liposome, the membrane of which contains glycoproteins in the post-fusion conformation, often clustered into locally ordered arrays (arrow; magnified inset). (C) At pH 5.5 individual fusion events can be seen, demonstrating that fusion occurs at the base of the virion and that spikes located on the cylindrical part of the virus form ordered helical arrays.