Controlling N-linked glycan site occupancy

N-linked glycosylation, a common co-translational modification in eukaryotic cells, involves the transfer of a lipid-linked oligosaccharide onto asparagine residues in a tripeptide sequon on a nascent protein in the lumen of the endoplasmic reticulum. The attachment of an oligosaccharide unit to the polypeptide at the site of occupancy can enhance solubility, improve folding, facilitate secretion, modulate antigenicity, and increase in vivo half-life of the glycoprotein. A number of proteins exhibit variable site occupancy. The efficiency of protein N-glycosylation is dependent on the kinetics of the individual steps in the biosynthesis of the dolichol-linked oligosaccharide and the transfer of the oligosaccharide from the lipid donor substrate to the nascent polypeptide. In this review, we will discuss the role of N-linked glycan site occupancy and give an overview of the possible limitations associated with variable site occupancy. The characterization of the dolichol pyrophosphate biosynthetic pathway and the recent identification of potential rate limiting enzymes in yeast and mammalian cells has made it possible to investigate their role in site occupancy. Genetic and biochemical characterization of oligosaccharide transferase (OST) complex in yeast and mammalian cells have demonstrated the importance of specific OST subunits in protein N-glycosylation. In addition, insights into the location and residues in and around the acceptor tripeptide sequon suggest an influence on N-glycan site occupancy. Insights from these characterizations are being used to elucidate methodologies to control N-glycosylation site heterogeneity.