Organization of the Glycoprotein and Polysaccharide Synthetic Pathways in the Plant Golgi Apparatus

Abstract

The Golgi apparatus serves as a processing and sorting station for secretory proteins as they pass from their site of origin, the endoplasmic reticulum, to their final destination, the cell surface or the lysosomal/vacuolar systems. There is general consensus that in animal cells the Golgi apparatus consists of at least four distinct functional compartments known as cis, medial and trans Golgi cisternae, and the trans Golgi network (TGN) (Farquhar, 1985; Griffiths and Simons, 1986). Proteins enter the stack at its cis face and depart from the opposite trans face (Dunphy and Rothman, 1985), and transport between the cisternal compartments is mediated by transport vesicles (Duden et al., 1991). The definition of these compartments is based on biochemical fractionation studies, as well as on the histochemical and immuno-cytochemical localization of specific glycosyltransferases and their products (Kornfeld and Kornfeld, 1985; Roth, 1987). Cis Golgi cisternae contain the enzyme N-acetylglucosamine (GlcNAc)-1-phosphodiester α-N-acetylglucosaminidase, which is involved in the addition of mannose 6-phosphate residues to the oligosaccharide side chains of lysosomal enzymes. The enzyme GlcNAc transferase I is found in the medial cisternae, whereas most of the galactosyltransferase activity is located in the trans cisternae. The addition of terminal sialic acid residues, finally, is largely confined to the TGN. Thus the presence of a specific type of Golgi compartment in a biochemical fraction can be defined by the presence of specific marker enzyme activities, and the types of cisternae through which a given N-linked glycoprotein has passed can be determined by the types of modifications that are present on its oligosaccharide side chain(s). This knowledge of the functional organization of the Golgi apparatus has proven immensely valuable for designing experiments to explore mechanisms of trafficking through the Golgi apparatus of animal cells, and for deciphering the sites of action of drugs such as brefeldin A that interfere with the secretory pathway.