Identification of a receptor-binding pocket on the envelope protein of friend murine leukemia virus

Based on previous structural and functional studies, a potential receptor-b inding site composed of residues that form a pocket at one end of the two l ong antiparallel helices in the receptor-binding domain of Friend 57 murine leukemia virus envelope protein (RBD) has been proposed. To test this hypo thesis, directed substitutions for residues in the pocket were introduced a nd consequences for infection and for receptor binding were measured. Recep tor binding was measured initially by a sensitive assay based on coexpressi on of receptor and RED in Xenopus oocytes, and the findings were confirmed by using purified proteins. Three residues that are critical for both bindi ng and infection (S84, D86, and W102), with side chains that extend into th e pocket, were identified. Moreover, when mCAT-1 was overexpressed, the inf ectivity of Fr57-MLV carrying pocket substitutions was partially restored. Substitutions for 18 adjacent residues and 11 other previously unexamined s urface-exposed residues outside of the RED pocket had no detectable effect on function, Taken together, these findings support a model in which the RE D pocket interacts directly with mCAT-1 (likely residues, Y235 and E237) an d multiple receptor-envelope complexes are required to form the fusion pore .