The Assembly and Transfer of Oligosaccharide Chains to Proteins

Abstract

Starting about two decades ago a series of developments provided us with a basic understanding of the mechanism of synthesis of membrane and secretory glycoproteins. These were the distinction between membrane-bound and free polysomes (Redman, 1969), the discovery of the signal peptide (Blobel & Sabatini, 1971; Milstein et al., 1972), the demonstration of the cotranslational glycosylation of ovalbumin (Kiely et al., 1976), the findings that, in accord with the signal hypothesis, newly translated and glycosylated proteins were sequestered within the lumen of the endoplasmic reticulum (Lingappa et al., 1978) and that viral coat proteins were inserted with their carbohydrate chains facing the lumen (Katz et al., 1977). All of these findings set the scene for in-depth biochemical studies on the mechanisms of assembly of the oligosaccharide chains of the glycoproteins. These studies (summarized in Parodi & Leloir, 1979; Struck & Lennarz, 1980; Hubbard & Ivatt, 1981; and Kornfeld & Kornfeld, 1985) have led to the following general conclusions: 1. All eukaryotic cells are capable of the synthesis of glycoproteins that are destined to become components of the plasma membrane. In addition, many cell types commit a significant portion of their protein biosynthetic activity to the synthesis of secreted and/or lysosome-packaged glycoproteins. 2. The synthesis of membrane, secretory, or lysosomal glycoproteins is a highly segregated process that occurs within an intracellular membrane system composed of the endoplasmic reticulum, transfer vesicles, Golgi apparatus, and secretory vesicles. During their translation, glycosylation and processing the glycoproteins are completely isolated from the cytoplasm and travel to the cell surface (in the case of membrane glycoproteins), to the extracellular environment (in the case of secretory glycoproteins), or to the lysosomes (in the case of lysosomal enzymes) as part of, or within, these membrane compartments. 3. The assembly of N-linked oligosaccharide chains occurs in the endoplasmic reticulum and involves the stepwise preassembly of the oligosaccharide chain on dolichyl phosphate followed by en bloc transfer of the oligosaccharyl unit to the growing polypeptide chain. 4. Subsequent modifications to the oligosaccharide chains of N-linked glycoproteins are initiated in the rough endoplasmic reticulum and completed in the Golgi apparatus.