FIGURE 2.35 • The Golgi apparatus and vesicular traffcking. The Golgi apparatus contains several stacks of fattened cisternae with dilated edges. The Golgi cisternae form separate functional compartments. The closest compartment to the rER represents the CGN, to which COP-II–coated transport vesicles originating from the rER fuse and deliver newly synthesized pro teins. Retrograde transport from CGN to rER, as well retrograde transport between Golgi cisternae, is mediated by COP-I–coated vesicles. Once proteins have been modifed within the CGN, the transport vesicles bud off dilated ends of this compartment, and proteins are trans ferred into medial-Golgi cisternae. The process continues; in the same fashion, proteins are translocated into the trans-Golgi cisternae and the sorting station for shuttling vesicles that deliver proteins to the following locations (see Fig. 2.36).
 Apical plasma membrane. Many extracellular and membrane proteins are delivered to this site. This constitutive pathway most likely uses non–clathrin-coated vesicles. In most cells, secretory proteins destined for the apical plasma membrane have specific sorting signals that guide their sorting process in the TGN. Proteins are then delivered to the apical cell surface.
 Basolateral plasma membrane. Proteins targeted to the basolateral domain also have a specific sorting signal attached to them by the TGN. This constitutive pathway uses vesicles coated with an as yet unidentified protein associated with an epithelium-specific adaptor protein. The transported membrane proteins are continuously incorporated into the basolateral cell surface. This type of targeting is present in most polarized epithelial cells. In liver hepatocytes, however, the process of protein sorting into the basolateral and apical domains is quite different. All integral plasma-membrane proteins that are destined for both apical and basolateral domains are first transported from the TGN to the basolateral plasma membrane. From here, both proteins are endocytosed and sorted into early endo somal compartments. Basolateral proteins are recycled back into the basolateral membrane, whereas apical pro teins are transported across the cytoplasm to the apical cell membrane via transcytosis.
 Endosomes or lysosomes. Most proteins destined for organelles bear specific signal sequences. They are sorted in the TGN and delivered to specific organelles. However, TGN sorting mechanisms are never completely accurate. For instance, about 10% of lysosomal integral membrane proteins (limps) instead travel directly into early or late endosomes, take an extended route traveling via the apical plasma membrane (see Fig. 2.20), and from there move back into the endosomal pathways. Enzymes destined for lysosomes using M-6-P markers (see page 37) are delivered into early or late endosomes as they develop into mature lysosomes.
 Apical cytoplasm. Proteins that were aggregated or crystallized in the TGN as a result of changes in pH and Ca2 concentration are stored in large secretory vesicles. These vesicles undergo a maturation process in which secretory proteins are retained within the vesicle. All other nonsecretory proteins are recycled into the endosomal compartment or TGN in clathrin-coated vesicles