Membrane proteins are often covalently linked to oligosaccharides, which are branched glycoside-linked sugars (averaging around 15 sugar residues). As glycans, they are the sugars linked to glycoproteins. Glycoproteins are rare in the cytosol, but common on secreted and membrane proteins. Oligosaccharides are typically linked to proteins via the hydroxyl group on serine or threonine. Occasional linkages are to modified amino acids like hydroxylysine or hydroxyproline (O-glycosylation), and to the amide nitrogen on asparagine (N-glycosylation). The oligosaccharide domains of glycoproteins often play a major role in membrane protein function. For example, the glycoproteins, along with the polar domains of integral and peripheral proteins and glycolipids, are a major feature of the glycocalyx. A cell membrane and its glycocalyx are illustrated below.
Oligosaccharides begin their synthesis in the rough endoplasmic reticulum (RER), with the creation of a core glycoside. Partial glycans are enzymatically linked to compatible amino acids of a membrane protein. As these proteins travel through the Golgi vesicles of the endomembrane system, terminal glycosylation attaches more sugars to the core glycoside to complete glycoprotein synthesis. When vesicles budding from the transGolgi vesicles fuse with the plasma membrane, the sugars on the glycoproteins end up on the exterior cell surface. This is illustrated in the link below.
291 The Path to Sugar Coated Cells