Carbohydrate heterogeneity of vesicular stomatitis virus G glycoprotein allows localization of the defect in a glycosylation mutant of CHO cells

The carbohydrate portion of the G glycoprotein of vesicular stomatitis virus (VSV) grown in CHO cells (CHO/VSV) has been fractionated on BioGelP6, concanavalin A-Sepharose, and pea lectin-agarose. The results suggest that, in addition to sialic acid and fucose heterogeneity, the asparagine-linked complex carbohydrate moieties of CHO/VSV also display branching heterogeneity. Although the majority of the glycopeptides bind to concanavalin A-Sepharose in a manner typical of certain biantennary carbohydrate structures, a significant proportion do not bind to the lectin. The latter behavior is typical of tri- or tetraantennary (branched) carbohydrate structures. The CHO/VSV glycopeptides which do not bind to concanavalin A-Sepharose separate into bound and unbound fractions on pea lectin-agarose suggesting that they include at least two different types of (branched) carbohydrate structures. Glycopeptides from the G glycoprotein of VSV grown in two, independently derived CHO glycosylation mutants which belong to complementation group 4 (Lec4 mutants) were examined in the same manner. In contrast to glycopeptides from CHO/VSV, glycopeptides from Lec4/VSV which passed through concanavalin A-Sepharose did not contain a component which subsequently bound to pea lectin-agarose. A glycopeptide fraction with these lectin-binding properties was also missing from cell surface glycopeptides derived from Lec4 cells. The combined results are consistent with the hypothesis that Lec4 CHO glycosylation mutants lack a glycosyltransferase activity responsible for the addition of a (branch) N-acetylglucosamine residue linked β1,6 to mannose.