Sindbis virus-mediated cell fusion from without is a two-step event.

The process by which Sindbis virus induces the fusion of BHK-21 cells in monolayer cultures after exposure to low pH was examined. The fusion process was found to require an exposure to low pH followed by a return to conditions closer to neutral pH. The rate at which cells fused was affected by the pH to which the monolayers were exposed, subsequent to the initial exposure to acid conditions. Virus-mediated cell fusion was not observed if the virus-cell complexes were maintained at the acid pH required for membrane fusion.