The Involvement of the Golgi Apparatus in the Biosynthesis and Secretion of Glycoproteins and Polysaccharides

The transfer of glycosyl radicals either onto oligosaccharides or proteins frequently occurs by a series of transglycosylation reactions to different acceptors and donors before the radicals are incorporated into the final compounds (Northcote, 1969a). In all cases the initial donor of the glycosyl radical is a nucleoside diphosphate sugar compound. The transglycosylases occur on the endomembrane system of plant and animal cells, and this membrane system consists of the nuclear membranes, rough and smooth endoplasmic reticulum, Golgi bodies (Powell and Brew, 1974), and plasma membrane (Hassid, 1967; Northcote, 1974a; Jentoft et al., 1976; Molnar, 1976). (Some storage polysaccharides, for example, starch, are formed in discrete membrane-bounded organelles such as amyloplasts.) It is generally thought that, for glycoproteins, the initial glycosylation occurs within the endoplasmic reticulum and that the terminal units of the oligosaccharide are added onto the molecule within the Golgi apparatus (Schachter et al., 1970; Bennett et al., 1974; Autuori et al., 1975b; Molnar, 1976). A similar sequence of synthetic sites might also be used for polysaccharides, and some of these polymers may even be built up on carrier proteins within the membranes (Bowles and North-cote, 1976; Green and Northcote, 1978).