FIGURE 14-14 Processing of N-linked oligosaccharide chains on glycoproteins within cis-, medial-, and trans-Golgi cisternae in vertebrate cells. The enzymes catalyzing each step are localized to the indicated compartments. After removal of three mannose residues in the cis-Golgi (step 1), the protein moves by cisternal maturation to the medial-Golgi. Here three N-acetylglucosamine (GlcNAc) residues are added (steps 2 and 4), two more mannose residues are removed (step 3), and a single fucose is added (step 5). Processing is completed in the trans-Golgi by addition of three galactose residues (step 6) and finally by linkage of an N-acetylneuraminic acid residue to each of the galactose residues (step 7). Specific transferase enzymes add sugars to the oligosaccharide, one at a time, from sugar nucleotide precursors imported from the cytosol. This pathway represents the Golgi processing events for a typical mammalian glycoprotein. Variations in the structure of N-linked oligosaccharides can result from differences in processing steps in the Golgi. See R. Kornfeld and S. Kornfeld, 1985, Annu. Rev. Biochem. 45:631.