Protein sorting Golgi apparatus

Figure 46-1. Diagrammatic representation of the two branches of protein sorting occurring by synthesis on (1) cytosolic and (2) membrane-bound polyribosomes. The mitochondrial proteins listed are encoded by nuclear genes. Some of the signals used in further sorting of these proteins are listed in Table 46-4. (ER, endoplasmic reticulum GA, Golgi apparatus.)...

The Golgi Apparatus Is Involved in Glycosylation Sorting of Proteins... 

Teasdale, R. D. and lackson, M. R. Signal-mediated sorting of membrane proteins between the endoplasmic reticulum and the golgi apparatus. Ann. Rev Cell Dev. Biol. 12 27-54,1996. 

In spite of the variety of appearances of eukaryotic cells, their intracellular structures are essentially the same. Because of their extensive internal membrane structure, however, the problem of precise protein sorting for eukaryotic cells becomes much more difficult than that for bacteria. Figure 4 schematically illustrates this situation. There are various membrane-bound compartments within the cell. Such compartments are called organelles. Besides the plasma membrane, a typical animal cell has the nucleus, the mitochondrion (which has two membranes see Fig. 6), the peroxisome, the ER, the Golgi apparatus, the lysosome, and the endosome, among others. As for the Golgi apparatus, there are more precise distinctions between the cis, medial, and trans cisternae, and the TGN trans Golgi network) (see Fig. 8). In typical plant cells, the chloroplast (which has three membranes see Fig. 7) and the cell wall are added, and the lysosome is replaced with the vacuole. 

In a simplified view, the total flow is as follows (Fig. 8). Both soluble and membrane proteins that are translated at the membrane-bound ribosome are first localized at the ER. Some of them are transported to the Golgi apparatus, whereas others remain at the ER. At the Golgi apparatus, including the trans Golgi network (TGN), the next selection occurs some are transported to the plasma membrane, others to the endosome and to the lysosome/vacuole finally, and still others remain there. The lysosome is also an important organelle for the other transport system, the endocytic pathway. In this pathway, proteins at the plasma membrane are internalized by endocytosis. The sorting to lysosomes is treated in the next section. 

A variety of protein import pathways into the vacuole are known (Burd et al., 1998 Bryant and Stevens, 1998). It includes the sorting from the Golgi apparatus, endocytosis, autophagy (where a part of the cytoplasm such as a mitochondrion is engulfed into a newly formed vacuole and is degraded), direct import from the cytosol, and the vacuolar inheritance from the mother cell. Of these, the pathways from the Golgi... 

The Golgi apparatus (3) is a complex network, also enclosed, consisting of flattened membrane saccules ( cisterns ), which are stacked on top of each other in layers. Proteins mature here and are sorted and packed. A distinction is made between the ds, medial, and trans Golgi regions, as well as a trans Golgi network (tGN). The post-translational modification of proteins, which starts in the ER, continues in these sections. 

Endoproteases cleave these proteins to activate them in the ER during sorting in the Golgi apparatus, during storage in secretory vesicles, or at the time of use when they arrive at their final destinations. 

The Golgi apparatus, a system of flattened membrane-bound sacs, is the sorting and packaging center of the cell. It receives membrane vesicles from the RER, further modifies the proteins within them, and then packages the modified proteins in other vesicles which eventually fuse with the plasma membrane or other subcellular organelles. 

Exocytosis is the secretion of proteins out of the cell across the plasma membrane into the extracellular space. Proteins destined to be secreted are synthesized on ribosomes bound to the RER membrane and are then transported in membrane-bound vesicles to the Golgi apparatus where they are sorted and packaged up into secretory vesicles. All cells continuously secrete proteins via the constitutive pathway, whereas only specialized cells (e.g. of the pancreas, nerve cells) secrete proteins via the regulated secretory pathway in response to certain stimuli. 

Golgi apparatus is a membrane-enclosed organelle in the endoplasmic reticulum of eukaryotic cells where proteins are modified post-transcriptionally, mainly by glyco-sylation, and where proteins and lipids are sorted for transport to their destinations. 

The biogenesis of both acetylcholine receptor and chromaffin granules share several common properties. The specific polypeptides are synthesized and transported into the membrane by a vectorial translation process. The specific proteins are sorted out by the Golgi apparatus and eventually fuse with the plasma membrane via the secretory pathway. Yet the acetylcholine receptor functions on the plasma membrane, and therefore it should stay on this membrane for a long time (2-7 days). On the other hand, the function of chromaffin granules is to store neurotransmitters. Therefore they stay most of their lifetime inside the cell and their fusion with the plasma membrane is temporary. Soon after the secretion process, the constituents of the chromaffin granule membrane must be removed from the plasma membrane by endocytosis. 

How is the final location of the acetylcholine receptor and secretory vesicles determined Where is the information stored to determine this localization There are two key locations in cells where sorting out of membrane proteins might occur. One of these is the Golgi apparatus and the second one involves the plasma membrane. After chromaffin granules have fused with the plasma membrane, their specific lipids and proteins are immediately and specifically removed from the latter membrane. The mechanism for this selective removal is unknown. In the Golgi apparatus, newly-synthesized membrane proteins are sorted out to various membrane vesicles. We propose a model for the sorting out of secretory vesicles from vesicles destined to deliver plasma membrane proteins. 

---