| Retrovirus entry is mediated by specific interactions between the retroviral envelope glycoprotein (Env) and the host cell surface receptor. Replacements of aspartate 84 with lysine (D84K), tryptophan 142 with methionine (W142M), or histidine 8 with arginine (H8R) on SU abolished ecotropic MLV infection. D84 and W142 are essential for receptor binding, while H8 is essential for virus-cell membrane fusion. Here we present genetic evidence that a single receptor binding site consisting of a D84 on one monomer of SU and a W142 on the adjacent monomer is sufficient for triggering the completion of fusion events, as well as evidence that the signal from initial binding is transferred from D84 to H8 within the same monomer rather than across monomers or through both monomers. In addition, we demonstrated that steric hindrance and charge-charge repulsion are the major reasons for loss of infection and binding of D84K or R virus. Using a reverse genetic approach, we further determined that D84 in ecotropic SU might interact with rK234 in the receptor. Finally, we also demonstrated that an aromatic ring side chain is essential to the function of H8, two second-site suppressor mutations (glutamine 227-to-arginine and aspartate 243-to-tyrosine) are universal suppressors of H8 down mutations, all H8 down mutants block infection by arresting membrane fusion at the hemifusion state, and the N-terminal fusion domain of SU containing this conserved histidine is functionally conserved among at least two genera of mammalian retrovirus. |
