The role of gp55 N-glycosylation in pathogenesis of Friend spleen focus-forming virus

The product of the envelope gene (gp55) of Friend spleen focus-forming virus is responsible for the acute form of erythroleukaemia caused by this virus. In order to investigate the role that the four known N-linked carbohydrate side-chains of gp55 play in pathogenesis, we have inactivated the four N-glycosylation signals by mutating the asparagine residues of these four sites into serine. When glycosylation sites 1 and/or 2 were altered, the viruses remained fully pathogenic. However, mutation at either of glycosylation sites 3 or 4 rendered the virus apathogenic, independent of mutations at other sites. Furthermore, when site 3 was changed, a new product appeared which seemed to have acquired a carbohydrate chain at a position normally not glycosylated, presumably at position Asn378.