Golgi apparatus, protein sorting pathways

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... 

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. 

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. 

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