Endocrine, neuroendocrine and exocrine cells store regulated secretory proteins in secretory granules, while constitutive and constitutive-like secretory proteins are secreted directly without storage. Sorting of secretory proteins takes place in the trans-Golgi network (sorting for entry) or immature secretory granules (sorting by retention). The relative contribution of these sorting steps and the sorting signals and mechanisms involved in each step has been the subject of intense studies and debate in recent years. New evidence now suggests that: (1) two proteins with structurally similar sorting signals can use different sorting mechanisms; (2) one protein with multiple sorting signals can be sorted differently in different cell types; and (3) one cell type can recognize different sorting signals and use different sorting mechanisms. The latter finding suggests that sorting must be a regulated event. While the current image of sorting is complex, recent findings are pointing to common features that form a mosaic of related sorting mechanisms.
Bibliographical noteFunding Information:
Drs B.H. Fasciotto and S.G. Venkatesh are thanked for critical reading of the manuscript. Work in the authors' laboratory was supported by PHS grants R01 DK53367 and R01 DE12205, Jewish Hospital Foundation, Louisville, Kentucky and Grants-in-aid from the Kentucky Affiliate of the American Heart Association.
- Disulfide loop
- Secretory granules