Heat-resistant inhibitors of protein kinase C from bovine brain

Bovine brain cytosol is shown to contain two heat-resistant inhibitors of protein kinase C, with the following characteristics: 1. One protein kinase C inhibitor can be easily purified to homogeneity. Evidence is presented that this polypeptide of Mr 19,000 is calmodulin. It inhibits protein kinase C with an EC50 of about 2.5 microM and the inhibition is Ca2+-independent. It inhibits only intact protein kinase C. Removal of the regulatory domain of protein kinase C, by limited proteolysis with trypsin, abolishes the inhibition. 2. Another protein kinase C inhibitory activity has been partially purified. Its Mr is low (Mr 600-700, as estimated by gel chromatography). It is not digested by proteases, is hydrophilic, acid- and alkali-resistant, acts Ca2+-independently, and, in contrast to calmodulin, inhibits even the catalytic fragment of protein kinase C after removal of the regulatory domain by limited proteolysis. This inhibition is, at least partially, due to a competition with ATP. Besides protein kinase C, calcium/calmodulin-dependent protein kinase II is inhibited to a similar extent. cAMP-dependent protein kinase is not affected.