A novel serine-specific kinase activity associated with exocrine secretory granules

Zymogen secretion from exocrine cells involves an exocytotic process that is highly regulated by the modification of cytoplasmic components at different cellular levels. In the present studies, purified secretory granules were prepared from rabbit gastric chief cells, rat pancreatic acinar cells, and parotid glands to characterize a Mg(2+)-dependent protein kinase activity. In chief cell granules, endogenous pepsinogen, a fortuitous substrate, was phosphorylated at optimal Mg2+ and K+ concentrations of 40 and 50 mM, respectively. Adenosine 3',5'-cyclic monophosphate, Ca2+, and calmodulin had no significant effects on the kinase activity. In contrast, Mn2+ or Zn2+ inhibited the kinase activity. In addition to pepsinogen, the exogenous substrates casein, myelin basic protein, and lysine-rich histone were also phosphorylated by the granule-associated kinase. All substrates were exclusively phosphorylated on serine residues. ATP, but not GTP, served as the donor in the phosphorate transfer reaction. Casein kinase (CK) inhibitors CKI-7 and dibromoribofuransylbenzimidazole at concentrations (10 microM) that significantly inhibited CK activities in the tissue homogenate failed to inhibit the granule-associated kinase activity. The kinase activity was localized to the granule membrane and could be removed from the membrane with either 5 mM EDTA or alkaline carbonate extraction. Furthermore, protease digestion sensitivity revealed that the kinase was localized on the cytoplasmic face of the granules. Our results therefore indicate that the secretory granules of exocrine gastric chief cells, pancreatic acini, and parotid acini possess a unique serine-specific protein kinase activity. The cytoplasmic orientation of the kinase activity suggests a possible role in vesicle processing or the exocytotic process.