A novel serine specific kinase activity associated with exocrine secretory granules. Tang, Laura H., Irvin M. Modlin, Todd A. Caulfield, and James R. Goldenring*. Gastrointestinal Surgical Research Unit, Department of Surgery, Yale University School of Medicine, and the West Haven Veterans Administration Medical Center, New Haven CT 06510, Institute for Molecular Medicine and Genetics, Medical College of Georgia, Augusta, GA 30912
APStracts 2:0089G, 1995.
Zymogen secretion from exocrine cells involves an exocytotic process which 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 magnesium-dependent protein kinase activity. In chief cell granules, endogenous pepsinogen, a fortuitous substrate, was phosphorylated at optimal magnesium and potassium concentrations of 40 mM and 50 mM, respectively. Cyclic AMP, calcium 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 dibromo -ribofuransylbenzimidazole (DRB) at the concentrations (10 NM) 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. 

Received 3 August 1994; accepted in final form 15 April 1995.
APS Manuscript Number G289-4.
Article publication pending Am. J. Physiol. (Gastrointest. Liver
Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on  9 May 1995.