Gradual pH decrease minimizes virion aggregation, making it possible to observe individual VSV particles. (A) After incubation for 15 min at pH 6.6 and 37°C, virions are not aggregated and most of the spikes are in their post-fusion conformation (ectodomain length of 12 nm). (B) The viral preparation shown in A was subsequently incubated at pH 6.0 and 20°C for 20 min. All the spikes are in their post-fusion conformation and the virions have retained their initial shape. The arrow indicates a virion with its surface completely covered by spikes in their post-fusion conformation. (C–G) The viral preparation shown in A was subsequently incubated at pH 5.5 and 20°C for 20 min. Most of the particles are disrupted. Extensive lattices of G are visible at the surface of the virion (arrows in C, disrupted virion in D). In many cases (E–G) the particles are highly damaged, precluding the observation of large regular networks of G. Nevertheless, local regular arrays are still observed (arrows). In all frames, the bars indicate 100 nm.