shows that, although the proteins that an individual cell expresses and packages in secretory vesicles differ, they contain common sorting signals, which function properly even when the proteins are expressed in cells that do not normally make them.
It is unclear how the aggregates of secretory proteins are segregated into secretory vesicles. Secretory vesicles have unique proteins in their membrane, some of which might serve as receptors for aggregated protein in the TGN. The aggregates are much too big, however, for each molecule of the secreted protein to be bound by its own cargo receptor, as occurs for transport of the lysosomal enzymes. The uptake of the aggregates into secretory vesicles may therefore more closely resemble the uptake of particles by phagocytosis at the cell surface, where the plasma membrane zippers up around large structures.
Initially, most of the membrane of the secretory vesicles that leave the TGN is only loosely wrapped around the clusters of aggregated secretory proteins. Morphologically, these immature secretory vesicles resemble dilated trans Golgi cisternae that have pinched off from the Golgi stack. As the vesicles mature, they fuse with one another and their contents become concentrated (Figure 13–64A), probably as the result of both the continuous retrieval of membrane that is recycled to the TGN, and the progressive acidification of the vesicle lumen that results from the increasing concentration of V-type ATPases in the vesicle membrane that acidify all endocytic and exocytic organelles (see Figure 13–37). The degree of concentration of proteins during the formation and maturation of secretory vesicles is only a small part of the total 200–400-fold concentration of these proteins that occurs after they leave the ER. Secretory and membrane proteins become concentrated as they move from the ER through the Golgi apparatus because of an extensive retrograde retrieval process mediated by COPI-coated transport vesicles that carry soluble ER resident proteins back to the ER, while excluding the secretory and membrane proteins (see Figure 13–25).
Membrane recycling is important for returning Golgi components to the Golgi apparatus, as well as for concentrating the contents of secretory vesicles. The vesicles that mediate this retrieval originate as clathrin-coated buds on the surface of immature secretory vesicles, often being seen even on budding secretory vesicles that have not yet pinched off from the Golgi stack (Figure 13–64B).
Because the final mature secretory vesicles are so densely filled with contents, the secretory cell can disgorge large amounts of material promptly by exocytosis when triggered to do so (Figure 13–65).
precursors of secretory proteins are proteolytically processed during the formation of secretory vesicles
Concentration is not the only process to which secretory proteins are subjected as the secretory vesicles mature. Many protein hormones and small neuropeptides,