Characterization of early events during retroviral entry

The retrovirus, avian sarcoma and leukosis virus (ASLV), enters cells through a pH-independent fusion reaction that is catalyzed by the viral envelope protein, EnvA. The membrane distal SU subunit of EnvA is responsible for attaching the virus to the host receptor (Tva), while membrane-associated TM subunit is believed to mediate fusion of the viral and cellular membranes. Current models of retroviral entry hypothesize that interactions between the viral envelope protein and the host receptor(s) induce the conformational changes necessary for activating the of fusogenic potential of the envelope protein. To test this hypothesis, we have evaluated the effects of purified, soluble receptor (sTva) on the function of EnvA in vitro and in culture. We have demonstrated that sTva-binding induces conformational changes throughout EnvA, enabling it to interact directly with target membranes. Furthermore, sTva efficiently triggered viral infection of receptor-deficient cells. Thus, receptor binding triggered the conversion of EnvA to a membrane-binding conformation and activated the fusogenic potential of the protein on virions. The acquisition of membrane-binding activity coincided with receptor-disengagement, suggesting that during viral entry activated-EnvA dissociates from the receptor possibly to facilitate membrane fusion. Further, this suggested that lipids might act as a co-factor for additional conformational changes in EnvA. Mutations within the receptor-binding site uncoupled receptor-triggered conformational changes in the receptor-binding subunit from membrane binding. Thus, communication between the receptor-binding subunit, SU, and the fusogenic subunit, TM, appeared crucial for efficient activation of the fusogenic-state. Furthermore, both the conversion of EnvA to a membrane-binding state and sTva-induced infection demonstrated non-linear dose-response relationships suggesting that receptor-triggered changes involved concerted and cooperative changes in the EnvA trimer. Thus, we have demonstrated that receptor binding can convert a pH-independent envelope protein to a fusion active conformation. Moreover, receptor-triggers activation results in structural rearrangements throughout the envelope protein and likely involves interactions between the subunits of EnvA and within the EnvA oligomer