Analysis of glycosylation site occupancy reveals a role for Ost3p and Ost6p in site-specific N-glycosylation efficiency.

Asparagine-linked glycosylation is the most common post-translational modification of proteins catalyzed in eukaryotes by the multiprotein complex oligosaccharyltransferase. Apart from the catalytic Stt3p, the roles of the subunits are ill defined. Here we describe functional investigations of the Ost3/6p components of the yeast enzyme. We developed novel analytical tools to quantify glycosylation site occupancy by enriching glycoproteins bound to the yeast polysaccharide cell wall, tagging glycosylated asparagines using endoglycosidase H glycan release, and detecting peptides and glycopeptides with LC-ESI-MS/MS. We found that the paralogues Ost3p and Ost6p were required for efficient glycosylation of distinct defined glycosylation sites. Our results describe a novel method for relative quantification of glycosylation occupancy in the genetically tractable yeast system and show that eukaryotic oligosaccharyltransferase isoforms have different activities toward protein substrates at the level of individual glycosylation sites.