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 Asn~Ts.

The protein encoded by the envelope gene of Friend spleen focus-forming virus is responsible for the acute leukaemogenicity of this virus. In order to correlate glycosylation and intracellular processing of this protein with viral pathogenicity, envelope gene products of pathogenic and apathogenic glycosylation mutants were expressed in Rat-1 cells and metabolically labelled with [6-3H]glucosamine. Following immunoprecipitation, primary and secondary gene products (gp55, gp65) were separated by preparative polyacrylamide gel electrophoresis. Oligosaccharides were released from tryptic glycopeptides by treatment with endo-beta-N-acetylglucosaminidase H (gp55), peptide-N4-(N-acetyl-beta-glucosaminyl)asparagine amidase F (gp65) or by reductive beta-elimination. Resulting glycans were characterized by co-chromatography with authentic oligosaccharide standards using different HPLC systems and digestion with exoglycosidases. The results revealed that the primary envelope gene products of pathogenic glycosylation mutants were, in part, further processed in Rat-1 cells similar to wild-type glycoprotein, resulting in polypeptides carrying complex-type N-glycans as well as partially sialylated O-linked oligosaccharides. In contrast, corresponding glycoproteins encoded by apathogenic mutants were found to remain at the level of the primary translation product exclusively comprising high-mannose-type N-glycans. Hence, intracellular maturation of the envelope gene products in this model cell line seems to correlate with the in vivo pathogenicity of the glycosylation mutants studied.