During their passage through the Golgi apparatus, transported molecules undergo an ordered series of covalent modifications. Each Golgi stack has two distinct faces: a cis face (or entry face) and a trans face (or exit face). Both cis and trans faces are closely associated with special compartments, each composed of a network of interconnected tubular and cisternal structures: the cis Golgi network (CGN) and the trans Golgi network (TGN), respectively. The CGN is a collection of fused vesicular tubular clusters arriving from the ER. Proteins and lipids enter the cis Golgi network and exit from the trans Golgi network, bound for the cell surface or another compartment. Both networks are important for protein sorting: proteins entering the CGN can either move onward in the Golgi apparatus or be returned to the ER. Similarly, proteins exiting from the TGN move onward and are sorted according to their next destination: endosomes, secretory vesicles, or the cell surface. They also can be returned to an earlier compartment. Some membrane proteins are retained in the part of the Golgi apparatus where they function.
As described in Chapter 12, a single species of N-linked oligosaccharide is attached en bloc to many proteins in the ER and then trimmed while the protein is still in the ER. The oligosaccharide intermediates created by the trimming reactions serve to help proteins fold and to help transport misfolded proteins to the cytosol for degradation in proteasomes. Thus, they play an important role in controlling the quality of proteins exiting from the ER. Once these ER functions have been fulfilled, the cell reutilizes the oligosaccharides for new functions. This begins in the Golgi apparatus, which generates the heterogeneous oligosaccharide structures seen in mature proteins. After arrival in the CGN, proteins enter the first of the Golgi processing compartments (the cis Golgi cisternae). They then move to the next compartment (the medial cisternae) and finally to the trans cisternae, where glycosylation is completed. The lumen of the trans cisternae is thought to be continuous with the TGN, the place where proteins are segregated into different transport packages and dispatched to their final destinations.
The oligosaccharide processing steps occur in an organized sequence in the Golgi stack, with each cisterna containing a characteristic mixture of processing enzymes. Proteins are modified in successive stages as they move from cisterna to cisterna across the stack, so that the stack forms a multistage processing unit.
Investigators discovered the functional differences between the cis, medial, and trans subdivisions of the Golgi apparatus by localizing the enzymes involved in processing N-linked oligosaccharides in distinct regions of the organelle, both by physical fractionation of the organelle and by labeling the enzymes in electron microscope sections with antibodies (Figure 13–28). The removal of mannose and the addition of N-acetylglucosamine, for example, occur in the cis and medial cisternae, while the addition of galactose and sialic acid occurs in the trans cisterna and trans Golgi network. Figure 13–29 summarizes the functional compartmentalization of the Golgi apparatus.
oligosaccharide chains are processed in the golgi apparatus
Whereas the ER lumen is full of soluble lumenal resident proteins and enzymes, the resident proteins in the Golgi apparatus are all membrane bound, as the enzymatic reactions apparently occur entirely on membrane surfaces. All of the Golgi glycosidases and glycosyl transferases, for example, are single-pass transmembrane proteins, many of which are organized in multienzyme complexes.
Figure 13–28 Molecular compartmentalization of the Golgi apparatus.