Low-pH conformational changes of rabies virus glycoprotein and their role in membrane fusion

Fusion of rabies virus with membranes occurs at acidic pH and is mediated by the viral spike glycoprotein (G). In this paper, we provide the basis for a description of structural transitions associated with exposure to low pH and of their role in membrane fusion. First, we have extended previous studies of fusion kinetics and we have shown that low-pH inhibition of fusion is detectable at 0.5 pH units higher than fusion. Second, low-pH-induced conformational changes were analyzed by using electron microscopy and monoclonal antibody binding assays. The existence of a pH-dependent equilibrium between the native and a low-pH inactive conformation was demonstrated. Third, besides these two conformations, we, using the fluorescent probe ANS (8-anilino-1-naphthalenesulfonic acid), provide evidence for the existence of a transient third state which appears to be more hydrophobic than the native state. Our results suggest that this transient state is responsible for viral aggregation at low pH and could play a role in the first steps of the fusion mechanism.