As noted in the chapter introduction, all eukaryotic cells continuously secrete certain proteins (constitutive secretion). Specialized secretory cells store other proteins in vesicles and secrete them only when triggered by a specific stimulus. One example of such regulated secretion occurs in pancreatic β cells, which store newly made insulin in specialized secretory vesicles and secrete it in response to an elevation in blood glucose (see Figure 16-39). These and other secretory cells simultaneously use two different types of secretory vesicles to move proteins from the trans-Golgi network to the cell surface: unregulated transport vesicles (also called constitutive secretory vesicles) and regulated transport vesicles.

A common mechanism appears to sort regulated proteins as diverse as ACTH (adrenocorticotropic hormone), insulin, and trypsinogen into regulated secretory vesicles. Evidence for such a common mechanism comes from experiments in which recombinant DNA techniques were used to induce the synthesis of insulin and trypsinogen in pituitary tumor cells already synthesizing ACTH. In these cells, which do not normally express insulin or trypsinogen, all three proteins segregate into the same regulated secretory vesicles and are secreted together when a hormone binds to a receptor on the pituitary cells and causes a rise in cytosolic Ca²⁺. Although these three proteins share no identical amino acid sequences that might serve as a sorting sequence, they must have some common feature that signals their incorporation into regulated secretory vesicles.

Morphologic evidence suggests that sorting into the regulated secretory pathway is controlled by selective protein aggregation. For instance, immature vesicles in this pathway—those that have just budded from the trans-Golgi network—contain diffuse aggregates of secretory protein that are visible in the electron microscope. These aggregates are also found in vesicles that are in the process of budding, indicating that proteins destined for regulated secretory vesicles selectively aggregate together before their incorporation into the vesicles.

Other studies have shown that regulated secretory vesicles from mammalian secretory cells contain three acidic proteins, chromogranin A, chromogranin B, and secretogranin II, that together form aggregates when incubated at the ionic conditions (pH 6.5 and 1 mM Ca²⁺) thought to occur in the trans-Golgi network; such aggregates do not form at the neutral pH of the ER. The selective aggregation of regulated secretory proteins together with chromogranin A, chromogranin B, or secretogranin II could be the basis for the sorting of these proteins into regulated secretory vesicles. Secretory proteins that do not associate with these proteins, and thus do not form aggregates, would be sorted into unregulated transport vesicles by default.