Preferential transfer to truncated oligosaccharides to the first sequon of yeast exoglucanase in Saccharomyces cerevisiae alg3 cells.

In addition to the exoglucanases (Exg) secreted into the culture medium by wild type cells, ExgIa and ExgIb, which have oligosaccharides attached to both potential N-glycosylation sites, Saccharomyces cerevisiae alg3 mutant secreted substantial amounts (35--44%) of underglycosylated and unglycosylated forms. Quantification of these forms indicated that no more than 78% of the available N-sites were occupied. About 50% of the transferred oligosaccharides were endo H sensitive, indicating that the lipid-linked precursor had completed its synthesis to Glc3-Man9-GlcNAc2. The other 50% remained endo H-resistant and, accordingly, it should be derived from the precursor oligosaccharide Man5-GlcNAc2 synthesized by this mutant. A closer analysis of forms that have received two oligosaccharides (ExgIb) showed that the first sequon was enriched in truncated residues, whereas the second one was enriched in regular counterparts. Similarly, analysis of the individual underglycosylated glycoforms indicated that 38% of the oligosaccharides attached to the second site were regular. This percentage dropped to 20% for glycoforms carrying the oligosaccharide in the first sequon. The preferential transfer of truncated oligosaccharides to the first glycosylation site seems to be a consequence of (1) the low percentage of truncated lipid linked oligosaccharides that receives the glucotriose unit, and (2) the effect of the glucotriose unit on the selection of N-sites to be glycosylated.