The making of N-glycoproteins
N-linked protein glycosylation is the most frequent post-translation modification in eukaryotic cells. The essential process initiates in the Endoplasmic Reticulum (ER), where an oligosaccharide is assembled on the lipid carrier, dolichylpyrophosphate. Oligosaccharyltransferase transfers the oligosaccharide to selected AsN-X-Ser/Thr sequons (X = any amino acid residue except proline) of polypeptides that enter the lumen of the ER. N-linked glycans then serve as signaling molecules that display the folding status of the attached protein. This signal directs the folding and quality control machinery to the client glycoproteins. Properly folded secretory glycoproteins are directed towards the Golgi compartment where trimming and remodeling of N-linked glycans occurs in a species-, protein- and site specific manner.
Based on experimental approaches in pro- and eukaryotic model systems, the underlying molecular concepts of the N-glycosylation process and the quality control process in the ER as well as the remodeling of the N-linked glycan in the Golgi will be discussed.