Endogenous inhibitor of a cyclic AMP-independent (A type) protein kinase

The cytosol fraction of the cells of Saccharomyces cerevisiae contains a low-molecular-mass, heat-stable inhibitor for endogenous cAMP-independent protein kinase. The inhibitor (Mr 15 000) is specific toward the protein kinase of A type, while the protein kinase of G type and the catalytic subunit of cAMP-dependent protein kinase are not affected. The following results suggest a protein structure of the inhibitor: 1. preincubation of the inhibitor with trypsin totally abolished its activity; 2. the inhibitor can be labelled by reductive alkylation of the amino acids with [14C]formaldehyde and sodium cyanoborohydride. The kinetic experiments have shown that the inhibitor is a competitive effector of the protein kinase of A type with respect to the protein substrate.