Hyper-replicative bovine leukemia virus by mutation of an envelope N-linked glycosylation site

[en] Reverse genetics can be used in the bovine leukemia virus\BLV) system to characterize mechanisms of viral persistence\and pathogenesis. The question addressed here pertains\to the role of glycans bound to the BLV envelope\glycoprotein (SU). A commonly accepted hypothesis is that\addition of carbohydrates to the SU protein potentially\eates a structure called guillemotleft glycan shield guillemotright that confers\resistance to the virus against the host immune response.\On the other hand, glycosylation can also modulate attachment\of the virus to the cell membrane. To unravel the role\of SU glycosylation, three complementary strategies were\ pharmacological inhibition of different glycosylation\ interference with glycan attachment and\directed mutagenesis of N-glycosylation sites in an\infectious BLV provirus. The different approaches show\that glycosylation is required for cell fusion, as expected.\Simultaneous mutation of all 8 potential N-glycosylation\sites destroys infectivity. Surprisingly, mutation of the\asparagine residue at position 230 creates a virus having an\increased capacity to form syncytia in vitro. Compared to\type BLV, mutant N230 also replicates at accelerated\rates in vivo. Collectively, this data thus illustrates an example\of a N-glycosylation site that restricts viral replication,\ntrasting with the hypothesis supported by glycan shield\