A great deal has been learned about the synthesis of N-and O-linked oligosaccharides, proteoglycans, and glycolipids (see reviews 1-5). Many of the reactions have been studied in detail, and a number of the glycosyltransн ferases involved have been purified. In addition, the subcellular locations of these reactions have been intensely studied. It is now known that nearly all of these steps occur in the Golgi apparatus (GA). Among the exceptions are the early stages of N-linked oligosaccharide synthesis, which occur in the rough endoplasmic reticulum (RER).

However, comparable understanding has not been available for the topoн graphical aspects of glycosylation. The topographical problem is based on the fact that sugar residues must move across membranes as part of these reacн tions. While the nucleotide sugar substrates are made either in the cytoplasm or, in the case of CMP-NeuAc, in the nucleus (6-8), the macromolecular products are found in the lumen of the RER and GA. Moreover, because virtually none of these macromolecules are found in the cytoplasm, the glycosylation reactions must be organized to ensure the transmembrane moveн ment of sugar residues.