Single N-glycosylation site of bovine leukemia virus SU is involved in conformation and viral escape

The bovine leukaemia virus (BLV) envelope protein (Env) is synthesized as a polyprotein precursor (gp72) proteolytically cleaved into the mature surface (SU) and transmembrane (TM) glycoproteins. The amino-terminal region of SU contains conformational epitopes F, G and H, which require a glycosylated SU to be recognized by monoclonal antibodies (MAbs) and antibodies from BLV-infected cattle. The SU contains eight asparagine (N) residues that are putative N-glycosylation sites. The N129, N203, N230 and N251 appear involved in carbohydrate binding, play an essential role in the in vitro infection. To determine which sites were actually glycosylated, we generated mutated SU forms, where each N-glycosylation site was changed to alanine (A). Subsequently, these N to A mutations were inserted into the env gene to generate Env mutants. The increase of electrophoretic mobility of EnvA256 and EnvA271 derived SU showed that the asparagine residues N256 and N271 were also glycosylated. ELISA revealed that only the N129 oligosaccharide determined the antigenic conformation of SU. The syncytium formation induced by EnvA129 showed that fusogenic capacity was independent of amino-terminal SU glycan conformational structure. Finally, anti-BLV serum inhibited syncytia formation even with the EnvA129 mutant. The latter inhibition was higher than Env, suggesting that the oligosaccharides could be also involved in the glycan shield for viral escape.