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Increased storage of basic proline-rich secretory proteins induced in rat parotid acinar cells by isoproterenol is accompanied by increased storage of a chondroitin sulfate-containing proteoglycan. Amino acid analysis of the purified proteoglycan and the chondroitinase digestion products reveals that the polypeptide backbone is a proline-rich protein. Most sulfation occurs in Golgi elements; however, a small fraction of the proteoglycan can be labeled by incubating isolated secretion granules with [35S]phosphoadenosine phosphosulfate ([35S]PAPS), and the amount of sulfate incorporation decreases with increased granule maturity. In vitro incorporation is sensitive to inhibitors of PAPS transport and occurs in intact granules as shown by radioautography. Both the increased production of a chondroitin sulfate proteoglycan following isoproterenol treatment and its sulfation at sites of secretory condensation and storage suggest that sulfation may aid secretory packaging by reducing the known fixed positive charge that stems from the large concentration of basic secretory proteins.

An antiserum raised against purified rat parotid secretory granule membrane proteins has been used to identify organelle-specific protein phosphorylation events following stimulation of intact cells from the rat parotid gland. After lobules were prelabeled with [32P]orthophosphate and exposed to secretagogues, phosphoproteins were immunoprecipitated with the granule membrane protein antiserum, separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and visualized by autoradiography. Parallel studies of stimulated amylase release were performed. Isoproterenol treatment of parotid lobules resulted in an increase in the phosphate content of immunoprecipitable 60- and 72-kDa proteins that correlated with amylase release in a time-dependent manner. Forskolin addition mimicked these effects, but only the isoproterenol effects were reversed by propranolol treatment. To confirm the specificity of the antiserum to the secretory granule membrane fraction, subcellular isolation techniques were employed following in situ phosphorylation. The 60- and 72-kDa phosphoproteins were immunoprecipitated from both a particulate fraction and a purified secretory granule fraction. Furthermore, the extraction properties of both species suggest that they are integral membrane proteins. These findings support the possibility that stimulus-regulated secretion may involve phosphorylation of integral membrane proteins of the exocrine secretory granule.