Genetic and sialylation sources of heterogeneity of the murine leukemia virus membrane envelope glycoproteins gp69/71

The authors analyzed the causes for heterogeneity of the membrane envelope glycoprotein of murine leukemia viruses (MuLVs). When analyzed by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate, this glycoprotein frequently separates into two components which have been termed gp69/71 because they migrate in the gels with apparent molecular weights of 69,000 and 71,000, respectively. Cloning experiments with MuLV produced by cultured Eveline cells indicates that gp69 and gp71 are encoded by separate closely related viral genomes which may frequently become incorporated into heterozygous virus particles. This and other evidence demonstrate that the size difference between gp69 and gp71 is caused by a difference in the amino acid sequences of their polypeptide chains. The gp69 and gp71 glycoprotein also have extensive isoelectric point microheterogeneity which can be reduced by removal of sialic acids with neuraminidase. Analyses of D-[³H]glucosamine-labeled glycopeptides by anion exchange chromatography, by gel filtration, and by high voltage paper elctrophoresis indicate that the envelope glycoproteins are glycosylated at several sites. It appears that the glycoproteins contain several complex oligosaccharides which are heterogeneously sialylated and at least two other relatively small neutral oligosaccharides. It is concluded that heterogeneity of the envelope glycoprotein of MuLVs can be caused by at least two factors, a genetic heterogeneity caused by multiplicity of viral genomes and a variability in the numbers of sialic acid residues attached to oligosaccharides.