Class III viral fusion protein. Envelope phosphoglycoprotein which mediates the fusion of viral and host endosomal membranes leading to virus entry into the host cell. After receptor-mediated internalization of the virus, gp64 undergoes conformational change into a fusion-competent state at low pH, and the nucleocapsid is released into the cytoplasm after cell fusion. May also play role in budding. Homotrimer; disulfide-linked. Palmitoylated. Belongs to the baculoviridae gp64 family.