Calmodulin-Stimulated Protein Kinase Activity from Rat Pancreas

Previous work from our laboratory has demonstrated that neurohumoral stimulation of the exocrine pancreas is associated with the phosphorylation of the Mr29,000 ribosomal protein S6. In a cell-free system using pancreatic postmicrosomal supernatant as the kinase donor, we found that the following co-factors stimulate the phosphorylation of the Mr29,000 ribosomal protein: calcium with calmodulin, calcium with phosphatidyl serine, and cAMP. These findings suggest that the pancreas contains a calcium-calmodulin-dependent protein kinase (CaM-PK) that can phosphorylate the Mr29,000 ribosomal protein. A CaM-PK activity was partially purified sequentially by ion exchange, gel filtration, and calmodulin-affinity chromatography. Phosphorylation of the Mr29,000 ribosomal protein by the partially purified CaM-PK was dependent on the presence of both calcium and calmodulin and not on the other co-factors. The CaM-PK fraction contained a phosphoprotein of Mr51,000 whose phosphorylation was also dependent on calcium and calmodulin. When125I-calmodulin-binding proteins from the CaM-PK fraction were identified using electrophoretic transfers of SDS-polyacrylamide gels, a single Mr51,000 protein was labeled. The preparation enriched in CaM-PK activity contained an Mr51,000 protein that underwent phosphorylation in a calcium-calmodulin-dependent manner and an Mr51,000 calmodulin-binding protein. It is therefore possible that the CaM-PK may comprise a calmodulin-binding phosphoprotein component of Mr51,000.