Phosphorylation of low molecular mass cytosolic proteins by protein kinase C and protein kinase A in the rabbit exocrine pancreas

Subcellular fractionation of rabbit pancreatic acini was performed to study the distribution of endogenous substrates for protein kinase C. Substrates for protein kinase C were found to be predominantly low molecular mass proteins of cytosolic origin. At least three of these soluble substrates, with molecular masses of 17‐19 kDa, were relatively heavily phosphorylated by endogenous as well as purified pancreatic protein kinase C. In the same molecular mass range, 16‐18 kDa, soluble proteins were also phosphorylated by protein kinase A. Moreover, addition of cyclic AMP under conditions that activated protein kinase C gave a more than additive labelling of these low molecular mass proteins. The latter observation may be of interest in view of the potentiating effect cyclic‐AMP‐activated protein kinase A has on amylase secretion stimulated by secretagogues which increase free cytosolic Ca2+ and activate protein kinase C.