location of the Golgi apparatus relative to its secretory state were described even before it was viewed with the electron microscope and before its functional relationship to the rER was established. It is active both in cells that secrete protein by exocytosis and in cells that synthesize large amounts of membrane and membrane-associated proteins such as nerve cells. In the light microscope, secretory cells that have a large Golgi apparatus (e.g., plasma cells, osteoblasts, and cells of the epididymis) typically exhibit a clear area partially surrounded by ergastoplasm (Fig. 2.32). In EM, the Golgi apparatus appears as a series of stacked, ﬂattened, membrane-limited sacs or cisternae and tubular extensions embedded in a network of microtubules near the microtubule-organizing center (see page 65). Small vesicles involved in vesicular transport are seen in association with the cisternae. The Golgi apparatus is polarized both morphologically and functionally. The ﬂattened cisternae located closest to the rER represent the forming face, or the cis-Golgi network (CGN); the cisternae located away from the rER represent the maturing face, or the trans-Golgi network (TGN); (Figs. 2.33 and 2.34). The cisternae located between the TGN and CGN are commonly referred as the medial-Golgi network.
The Golgi apparatus functions in the post-translational modification, sorting, and packaging of proteins.
Small COP-II coated transport vesicle s carry newly synthesized proteins (both secretory and membrane) from the rER to the CGN. From there, they travel within the transport vesicles from one cisterna to the next. The vesicles bud from one cisterna and fuse with the adjacent cisternae (Fig. 2.35). As proteins and lipids travel through the Golgi stacks, they undergo a series of post-translational modifcations that involve remodeling of N-linked oligosaccharides previously added in the rER.
FIGURE 2.33 • Electron micrograph of the Golgi apparatus. This electron micrograph shows the extensive Golgi apparatus in an islet cell of the pancreas. The fattened membrane sacs of the Golgi apparatus are arranged in layers. The CGN is represented by the fattened vesicles on the outer convex surface, whereas the fattened vesicles of the inner convex region constitute the TGN. Budding off the TGN are several vesicles (1). These vesicles are released (2) and eventually become secretory vesicles (3). 55,000.
In general, glycoproteins and glycolipids have their oligosaccharides trimmed and translocated. Glycosylation of proteins and lipids uses several carbohydrate-processing enzymes that add, remove, and modify sugar moieties of oligosaccharide chains. M-6-P is added to those proteins destined to travel to late endosomes and lysosomes (see page 37). In addition, glycoproteins are phosphorylated and sulfated. The proteolytic cleavage of certain proteins is also initiated within the cisternae.
Four major pathways of protein secretion from the Golgi apparatus disperse proteins to various cell destinations.
As noted, proteins exit the Golgi apparatus from the TGN. This network and the associated tubulovesicular array serve as
FIGURE 2.34 • Electron micrograph of Golgi cisternae. a. This transmission electron micrograph shows a quick-frozen isolated Golgi apparatus replica from a cultured Chinese hamster ovary (CHO) cell line. The trans-Golgi cisternae are in the process of coated vesicle formation. b. Incubation of the trans-Golgi cisternae with the coatomer-depleted cytosol shows a decrease in vesicle formation activity. Note the lack of vesicles and the fenestrated shape of the trans-Golgi cisternae. 85,000. (Courtesy of Dr. John E. Heuser, Washington University School of Medicine.)