Vesicular stomatitis virus G protein acquires pH-independent fusion activity during transport in a polarized endometrial cell line

Entry of vesicular stomatitis virus (VSV), the prototype member of the rhab dovirus family, occurs by receptor-mediated endocytosis. Subsequently, duri ng traversal through the endosomal compartments, the VSV G protein acquires a low-pH-induced fusion-competent form, allowing for fusion of the viral m embrane with endosomal and lysosomal membranes. This fusion event releases genomic RNA into the cytoplasm of the cell. Here we provide evidence that t he VSV G protein acquires a fusion-competent form during exocytosis in a po larized endometrial cell line, HEC-1A, VSV infection of HEC-1A cells result s in high viral yields and giant cell formation. Syncytium formation is blo cked in a concentration-dependent manner by treatment with the lysosomotrop ic weak base ammonium chloride, which raises intravesicular pH, Virus relea se is somewhat delayed by treatment with ammonium chloride, but virus yield s gradually reach those of control cells. In addition, inhibition of vacuol ar H+-ATPases by treatment with bafilomycin A1 also inhibited cell to cell fusion without altering virus yields. Virions released from infected HEC ce lls were themselves not fusion competent, since viral entry required an act ive H+-ATPase and a low-pH-induced conformational change in the viral G pro tein. Thus, the conformation change leading to fusion competence during exo cytotic transport is reversible and reverts during or after release of the virion from the infected cell.