Proteoglycan sulfation and storage parallels storage of basic secretory proteins in exocrine cells

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Abstract
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.