Role of the mutation Q252R in activating membrane fusion in the murine leukemia virus surface envelope protein

Entry of retroviruses into host cells requires the fusion between the viral and cellular membranes. It is unclear how receptor binding induces conformational changes within the surface envelope protein (SU) that activate the fusion machinery residing in the transmembrane envelope protein (TM). In this report, we have isolated a point mutation, Q252R, within the proline-rich region of the 4070A murine leukemia virus SU that altered the virus-cell binding characteristics and induced cell-cell fusion. Q252R displays a SU shedding-sensitive phenotype. Cell-cell fusion is receptor dependent and is observed only in the presence of MuLV Gag-Pol. Both cellular binding and fusion by Q252R are greatly enhanced in conjunction of G100R, a mutation within the SU variable region A which increases viral binding through an independent mechanism. Deletion of a conserved histidine (His36) at the SU N terminus abolished cell-cell fusion by G100R/Q252R Env without compromising virus-cell binding. Although G100R/Q252R virus has no detectable titer, replacement of the N-terminal nine 4070A SU amino acids with the equivalent ecotropic MuLV sequence restored viral infectivity. These studies provide insights into the functional cooperation between multiple elements of SU required to signal receptor binding and activate the fusion machinery.