Movie S1.

TIRF microscopy movie showing hemifusion of individual VSV particles. Representative TIRF microscopy movie showing how labeled virus particles bind to a lipid bilayer and undergo hemifusion after a pH drop. The initial pH at the beginning of the movie is 8.0; at about 2.5 s into the movie (25 s in the real experiment) the pH drops to 6.0. Virons that hemifuse are identified by bright peaks in fluorescence intensity, caused by dilution and dequenching of the lipophilic R18 dye initially incorporated into the virion membrane. The speed of the movie is 10×.