N-linked glycosylation

Many proteins are modified in the lumen of the endoplasmic reticulum and then transported via vesicles from the rough and smooth endoplasmic reticulum to the Golgi complex, where further modification may take place. In the ER, signal sequences are cleaved away, disulfide bridges are formed, and glycosylation occurs. Sugars, lipids, phosphate or sulfate groups may be added or removed from the proteins passing from one Golgi sac to another.

The majority of proteins synthesized in the rough endoplasmic reticulum undergo glycosylation. Glycosylation may be N-linked or O-linked. In N-linked glycosylation, the glycan is attached to a nitrogen of asparagine (and sometimes arginine). O-linked glycosylation involves attachment to an oxygen of serine or threonine. In N-linked glycosylation, the donor is dolichol phosphate. Dolichol is a long-chain isoprenoid lipid. (Isoprenes are a major family of lipids. Examples of isoprenoid compounds include carotene, retinol (vitamin A), tocopherol (vitamin E), and squalene (a precursor of cholesterol and the steroids).

The initial steps of N-linked glycosylation occur in the endoplasmic reticulum. After transfer to the Golgi apparatus, the process continues with the loss of a variable number of residues and acquisition a more complex structure during a process called 'terminal glycosylation'. O-linked glycosylation takes place in the cis-Golgi compartment after N-glycosylation.