Sorting of an exocrine secretory protein to the regulated secretory pathway in endocrine cells.

Regulated secretory proteins are stored in secretory granules. While the sorting and storage process appears similar in endocrine and exocrine cells, the extent of overlap of sorting between endocrine and exocrine cell types is not clear. It is predicted that exocrine regulated secretory proteins that are stored with high efficiency in exocrine granules would also be stored efficiently in endocrine granules. To test this hypothesis, parotid secretory protein (PSP), which is stored efficiently in parotid acinar cells, was expressed in the endocrine cell lines GH4C1 and PC12. PSP undergoes stimulated secretion in both cell types. Secretion is similar to that of the endocrine regulated secretory protein chromogranin A but distinct from secreted alkaline phosphatase, a marker for the constitutive secretory pathway in endocrine cells. Subcellular fractionation of GH4C1 cells revealed that PSP co-fractionates with chromogranin A but not with secreted alkaline phosphatase.

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