Liposomes and Phospholipid Binding Proteins in Glycoprotein Biosynthesis

The metabolic process of glycosylation of proteins on the amide nitrogen of specific asparagine residues in proteins, referred to as N -linked glycosylation, has common features found in all eucaryotic cells. The distinguishing general features are (1) the preassembly in the endoplasmic reticulum (ER) of the precursor core oligosaccharide on a lipid carrier, dolichol phosphate (dol-P); (2) the transfer of the oligosaccharide from dolichol pyrophosphate to the specific asparagine residue of the protein as a cotranslational event; and (3) the modification of the protein-linked oligosaccharide by removal and addition of sugar residues (processing) as the glycoprotein proceeds through the ER and Golgi complex (see ref. 1 for a comprehensive review of glycoprotein biosynthesis). Preassembly in the ER of the precursor oligosaccharide on dol-P involves the addition of two GlcNAc residues (one of these as GlcNAc-1-P), nine mannose residues, and, in most cases, three glucose residues to generate the dol-P-P-GlcNAc2Man9Glc3 molecule. Fourteen glycosyltransfer steps are involved, presumably catalyzed by 14 specific transferases. Addition of the two GlcNAc residues and five of the mannose residues occurs on the cytosolic side of the ER membrane with sugar nucleotide precursors, whereas the remaining four mannose residues and the three glucose residues are added in the lumen of the ER with dol-P-linked sugar precursors. The numerous enzymes catalyzing glycosyl transfer reactions are thus functionally and topologically located either on the cytosolic side or on the luminal side of the ER membrane; and other proteins involved, such as a translocase or flippase for moving the dol-P-P-linked GlcNAc2Man5 from the cytosolic side to the lumen of the ER, have topological features.