Evidence for Protein Kinase C in Bovine Adrenocortical Membrane Preparations Using [35S] Gamma-thio-ATP As a Phosphate Donor

Thio-substituted ATP is a sensitive probe for detecting protein kinase C activity as demonstrated in bovine adrenocortical cell membrane preparations. A single endogenous protein substrate with a molecular weight of approximately 47 Kd was rapidly phosphorylated with [3 5S] gamma-thio-ATP as phosphate donor. Phosphorylation was significantly increased in 30 seconds and reached a plateau by 3 minutes. The activity of the endogenous membrane kinase was unaffected by ACTH, cAMP, calmodulin or trifluoperazine but was responsive to combinations of calcium (Ca), diolein and phosphatidyl serine (PS). In addition, the kinase was activated by the tumor promoting phorbol ester, 12-0-tetradecanoylphorbol-13-acetate, indicating that the membrane contains a protein kinase C and a single 47 Kd phosphorylatable protein substrate. The same substrate is phosphorylated by Ca/diolein/PS activated kinase in membrane preparations from a broad range of rat tissues. Attempts to identify the substrate indicate that it is neither the type I regulatory subunit of cAMP dependent protein kinase nor mitochondrial cytochrome P450.