The mature avian leukosis virus subgroup A envelope glycoprotein is metastable, and refolding induced by the synergistic effects of receptor binding and low pH is coupled to infection.

The spring-loaded model stipulates that influenza virus infection is coupled to the transition of the virus hemagglutinin (HA) from a metastable conformation to a highly stable conformation at low pH. The properties of retrovirus envelope glycoproteins indicate that infection is coupled to an analogous conformational change. As a test of this hypothesis, the requirements for avian leukosis virus A (ALV-A) infection were examined. These studies indicate that, like HA, the conformation of the mature ALV-A envelope glycoprotein is metastable and that infection is linked to refolding at low pH. However, unlike HA, low-pH activation is only observed after priming by receptor. Therefore, ALV-A infection is dependent on the synergistic effects of receptor binding and low pH, suggesting that receptor binding superimposes an additional constraint on activation of ALV-A fusion that proceeds by a mechanism comparable to that of influenza virus.