Stella M. Hurtley

In studying enveloped animal virus entry into cells the influenza virus has been used to define a general model. Influenza viruses bind to receptors at the cell membrane, are internalized by endocytosis, and inside the endosome are exposed to a low pH, which activates a fusion protein in the viral envelope so that the viral capsid is released into the cytosol for viral replication. The entry of Avian Leukosis Virus (ALV) has been thought to be independent of the low pH of endosomes, and its envelope protein has been considered pH independent in its ability to promote viral membrane fusion.  Mothes et al. now challenge this idea and find that the viral envelope protein when bound to its receptor does require an acid bath to promote fusion after all. It is the receptor binding itself that converts the viral envelope fusion protein into a pH-sensitive conformation. In the light of these findings the entry mechanism for a whole variety of animal viruses, including other retroviruses like HIV, may need to be reexamined. -- SMH