At its trans face the Golgi apparatus breaks up into a complex system of tubes and sheets called the trans-Golgi network (Fig. 10.8). Although there is some final processing of proteins in the trans-Golgi network, most of the proteins reaching this point have received all the modifications necessary to make them fully functional and to specify their final destination. Rather, the trans-Golgi network is the place where proteins are sorted into the appropriate vesicles and sent down one of three major pathways: constitutive secretion, regulated secretion, or transport to lysosomes.
Vesicles for constitutive or regulated secretion, though functionally different, look very much alike and are directed to the cell surface (Fig. 10.1). When the vesicle membrane comes into contact with the plasma membrane, it fuses with it. At the point of fusion the membrane is broken through, the contents of the vesicle are expelled to the extracellular space, and the vesicle membrane becomes a part of the plasma membrane. This process, by which the contents of a vesicle are delivered to the plasma membrane and following membrane fusion and breakthrough are released to the outside, is called exocytosis.
The difference between the constitutive and regulated pathways of secretion by exocy-tosis is that the former is always "on" (vesicles containing secretory proteins are presented for exocytosis continuously) while the regulated pathway is an intermittent one in which the vesicles containing the substance to be secreted accumulate in the cytoplasm until they receive a specific signal, usually an increase in the concentration of calcium ions in the surrounding cytosol, whereupon exocytosis proceeds rapidly (page 343). After secretion, vesicular membrane proteins are retrieved from the plasma membrane by endocytosis and transported to the endosomes (Fig. 10.1). From the endosomes vesicles are targeted to the lysosomes, back to the Golgi, or into the pool of regulated secretory vesicles. In a cell that is not growing in size, the amount of membrane area added to the plasma membrane by exocytosis is balanced over a period of minutes by endocytosis of the same area of plasma membrane.
Notice that in exocytosis the membrane of the vesicle becomes incorporated into the plasma membrane; consequently the integral proteins and lipids of the vesicle membrane become the integral proteins and lipids of the plasma membrane. This is the principal, if not the only way, that integral proteins made on the rough endoplasmic reticulum are added to the plasma membrane.
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