Many viruses use a pH-dependent pathway for fusion with host cell membrane, the
mechanism of which is still poorly understood. Here we report that a subtle leucine (Leu)-
valine (Val) change at position 501 in the envelope glycoproteins (Envs) of two related
retroviruses, jaagsiekte sheep retrovirus (JSRV) and enzootic nasal tumor virus (ENTV), is
responsible for their distinct low pH requirements for membrane fusion and infection. The
Leu and Val residues are predicted to reside within the C-terminal heptad repeat (HR2) …