Flaviviruses are a group of positive-stranded RNA viruses. West Nile virus (WNV) is a mosquito-borne neurotropic member of this genus that is an emerging pathogen. The envelope (E) protein is an approximately 53-kDa protein that mediates virus attachment to cells and drives the pH-dependent fusion of viral and cellular membranes in the endosomes of infected cells. On immature virions, the 20-kDa premembrane (prM) protein interacts with the E protein to form 60 heterotrimeric spikes and functions to prevent adventitious fusion between viral and cellular membranes during egress through acidic compartments of the secretory pathway. Transit through these compartments catalyzes a pH-dependent rearrangement of prM and E proteins on the virion that exposes a recognition motif on prM for cellular furin-like serine proteases.