Glycosylation of the overlapping sequons in yeast external invertase: effect of amino acid variation on site selectivity in vivo and in vitro

Yeast invertase contains 14 sequons, all of which are glycosylated to varying degrees except for sequon 5 which is marginally glycosylated, if at all. This sequon overlaps with sequon 4 in a sequence consisting of Asn92-Asn93-Thr94-Ser95 (Reddy et al., 1988, J. Biol. Chem., 263, 6978-6985). To determine whether glycosylation at Asn93 is sterically hindered by the oligosaccharide on Asn92, the latter amino acid was converted to a glutamine residue by site-directed mutagenesis of the SUC2 gene in a plasmid vector which was expressed in Saccharomyces cerevisiae. A glycopeptide encompassing sequons 3 through 6 was purified from a tryptic digest of the mutagenized invertase and sequenced by Edman degradation, which revealed that Asn93 of sequon 5 contained very little, if any, carbohydrate, despite the elimination of sequon 4. When Ser and Thr were inverted to yield Asn-Asn-Ser-Thr carbohydrate was associated primarily with the second sequon, in agreement with numerous studies indicating that Asn-X-Thr is preferred to Asn-X-Ser as an oligosaccharide acceptor. However, when the invertase overlapping sequons were converted to Asn-Asn-Ser-Ser, both sequons were clearly glycosylated, with the latter sequon predominating. These findings rule out steric hindrance as a factor involved in preventing the glycosylation of sequon 5 in invertase. Comparable results were obtained using an in vitro system with sequon-containing tri- and tetrapeptides acceptors, in addition to larger oligosaccharide acceptors.