A hydrophobic patch in ecotropic murine leukemia virus envelope protein isthe putative binding site for a critical tyrosine residue on the cellular receptor

In the receptor for ecotropic murine leukemia viruses, tyrosine 235 contrib utes a critical hydrophobic side chain to the virus-receptor interaction (1 4). Here we report that tryptophan 142 in ecotropic Moloney murine leukemia virus envelope protein is essential to virus binding and infection. Replac ement of tryptophan 142 by alanine or serine resulted in misfolding. Howeve r, replacement by methionine (W142M) allowed correct folding of the majorit y of glycoprotein molecules. W142M virus showed a marked reduction in virus binding and was almost noninfectious, suggesting that tryptophan 142 is in volved in receptor binding. In contrast, W142Y virus containing a replaceme nt of tryptophan 142 with an aromatic residue (tyrosine) was as efficient a s wild-type virus in infection and binding of cells expressing the wild-typ e receptor. However, W142Y virus was 100-fold less efficient than wild-type virus in infection of cells expressing a mutant receptor containing trypto phan instead of the critical tyrosine. These results strongly support trypt ophan 142 being an essential residue on the virus envelope protein that int eracts directly with the critical hydrophobic residue at position 235 of th e ecotropic receptor. Tryptophan 142 forms one side of a shallow hydrophobi c pocket on the surface of the envelope protein, suggesting that it might c omprise the complete putative binding site for tyrosine 235. We discuss the implications of our findings with respect to two models of the envelope pr otein trimer. Interestingly, both models place tryptophan 142 at the interf ace between adjacent subunits of the trimer.