N-Linked Protein Glycosylation in the Endoplasmic Reticulum

The attachment of glycans to asparagine residues of proteins is an abundant and highly conserved essential modification in eukaryotes. The N-glycosylation process includes two principal phases: the assembly of a lipid-linked oligosaccharide (LLO) and the transfer of the oligosaccharide to selected asparagine residues of polypeptide chains. Biosynthesis of the LLO takes place at both sides of the endoplasmic reticulum (ER) membrane and it involves a series of specific glycosyltransferases that catalyze the assembly of the branched oligosaccharide in a highly defined way. Oligosaccharyltransferase (OST) selects the Asn-X-Ser/Thr consensus sequence on polypeptide chains and generates the N-glycosidic linkage between the side-chain amide of asparagine and the oligosaccharide. This ER-localized pathway results in a systemic modification of the proteome, the basis for the Golgi-catalyzed modification of the N-linked glycans, generating the large diversity of N-glycoproteome in eukaryotic cells. This article focuses on the processes in the ER. Based on the highly conserved nature of this pathway we concentrate on the mechanisms in the eukaryotic model organism Saccharomyces cerevisiae.The presence of glycans on proteins is known to influence their stability and solubility and the glycan core can contribute to folding processes (Shental-Bechor and Levy 2008; Hanson et al. 2009; Culyba et al. 2011). N-glycans also influence the function and activity of proteins (Skropeta 2009). The terminal residues of N-glycans play a key role in the quality control of protein folding in the ER. Ultimately the glycan signals whether a protein is correctly folded and can leave the ER to continue its maturation in the Golgi or whether the protein is not correctly folded and is degraded (Helenius and Aebi 2004; Aebi et al. 2010). It is therefore of great importance that the oligosaccharide to be transferred to proteins is complete. This “quality control” of the oligosaccharide is mediated by the substrate specificity of oligosaccharyltransferase.