POLYISOPRENOID GLYCOLIPIDS INVOLVED IN GLYCOPROTEIN-BIOSYNTHESIS

Polyisoprenol-linked mono- and oligosaccarides may function as activated glycosyl carriers in the biosynthesis of some glycoproteins in animal tissues. The lipophilic glycosyl carrier of monosaccharides is the phosphomonoester of dolichol, the C80-100-polyisoprenol, containing a saturated terminal isoprene unit. In this biosynthetic process, sugars are initially transferred to dolichol monophosphate from their nucleotide derivatives by membrane-associated glycosyltransferases. These dolichol-linked monosaccharides serve as glycosyl donors in the glycosylation of oligosaccharide phospholipids. It appears likely that dolichol is also the lipid moiety of the oligosaccharide intermediates. Detailed enzymatic studies with oligosaccharide phospholipids formed by rat liver, a mouse myeloma and hen oviduct have revealed that these intermediates function as oligosaccharide donors in the assembly of at least one class of glycoproteins. The exact nature of the glycoproteins glycosylated by lipid intermediates and the sub-cellular site(s) of this assembly process remain to be established. The possibility, that the mannose and GlcNAc-containing core found in many glycoproteins, is assembled at the lipid-level is now being investigated.