Characterization of the prototype foamy virus envelope glycoprotein receptor-binding domain.

The foamy virus (FV) glycoprotein precursor gp130(Env) undergoes a highly unusual biosynthesis, resulting in the generation of three particle-associated, mature subunits, leader peptide (LP), surface (SU), and transmembrane (TM). Little structural and functional information on the extracellular domains of FV Env is available. In this study, we characterized the prototype FV (PFV) Env receptor-binding domain (RBD) by flow cytometric analysis of recombinant PFV Env immunoadhesin binding to target cells. The extracellular domains of the C-terminal TM subunit as well as targeting of the recombinant immunoadhesins by the cognate LP to the secretory pathway were dispensable for target cell binding, suggesting that the PFV Env RBD is contained within the SU subunit. N- and C-terminal deletion analysis of the SU domain revealed a minimal continuous RBD spanning amino acids (aa) 225 to 555; however, internal deletions covering the region from aa 397 to 483, but not aa 262 to 300 or aa 342 to 396, were tolerated without significant influence on host cell binding. Analysis of individual cysteine point mutants in PFV SU revealed that only most of those located in the nonessential region from aa 397 to 483 retained residual binding activity. Interestingly, analysis of various N-glycosylation site mutants suggests an important role of carbohydrate chain attachment to N391, either for direct interaction with the receptor or for correct folding of the PFV Env RBD. Taken together, these results suggest that a bipartite sequence motif spanning aa 225 to 396 and aa 484 to 555 is essential for formation of the PFV Env RBD, with N-glycosylation site at position 391 playing a crucial role for host cell binding.