SORTING AND SECRETORY PATHWAYS IN EXOCRINE CELLS

Exocrine secretory cells contain multiple post-Golgi pathways from protein secretion. The major pathway in pancreatic and parotid acinar cells involves protein sorting into storage granules that undergo exocytosis with or without stimulation by secretagogues. This route of release is paralleled by a minor nongranular (but vesicular) pathway that originates by budding from maturing secretory granules. The nongranular pathway carries the same polypeptides that undergo storage in the granules but in different relative amounts. These features indicate that sorting into the stimulus-regulated pathway reflects not only the deposition of secretory proteins into immature granules but may also involve selective aggregation of proteins along with exclusion and vesicle-mediated secretion of other polypeptides that are inefficiently retained. Storage granules represent a distinct compartment of the secretory pathway, as indicated by the specific composition of their limiting membranes. Little is known about processes that maintain the low content and limited diversity of integral proteins of the granule membrane as compared to the membranes with which it fuses during exocytosis and formation. Future studies will examine the role of the nongranular secretory pathway in acinar cells, the branchpoint of pathways that are directed to the apical or basolateral cell surfaces, the structural determinants of secretory sorting, and the distribution and function of specific granule membrane polypeptides.