Purification and characterization of a dimer form of the cAMP-dependent protein kinase from mouse liver cytosol

A protein kinase that phosphorylates histones and polysomal proteins was partially purified from mouse liver cytosol. The active enzyme has a molecular mass of 100 kDa and a phosphorylatable subunit of 54 kDa. Biochemical as well as immunological data suggest that the enzyme is a heterodimer composed of the catalytic subunit of cyclic AMP-dependent protein kinase and the RII regulatory subunit. This RC form does not seem to dissociate upon activation with 3', 5' cyclic AMP and exhibits identical specificity as the classical cAMP-dependent protein kinase (2.7.1.37). The enzyme is affected by the 3', 5' cyclic phosphates of adenosine mainly, but also of guanosine, uridine and cytidine in a substrate-dependent manner. Cyclic nucleotides slightly stimulate phosphate incorporation into histones, while phosphorylation of polysomal proteins in intact polysomes is dramatically increased. The substrate- specific stimulatory effects of 3', 5' cyclic nucleotides are due to repression of the inhibition exerted upon the reaction, by negatively charged macromolecules such as RNA, DNA and to a lesser extent heparin.