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 pa rtially purified from mouse liver cytosol. The active enzyme has a molecula r mass of 100 kDa and a phosphorylatable subunit of 54 kDa. Biochemical as well as immunological data suggest that the enzyme is a heterodimer compose d of the catalytic subunit of cyclic AMP-dependent protein kinase and the R II regulatory subunit. This RC form does not seem to dissociate upon activa tion with 3', 5' cyclic AMP and exhibits identical specificity as the class ical cAMP-dependent protein kinase (2.7.1.37). The enzyme is affected by th e 3', 5' cyclic phosphates of adenosine mainly, but also of guanosine, urid ine and cytidine in a substrate-dependent manner. Cyclic nucleotides slight ly stimulate phosphate incorporation into histones, while phosphorylation o f polysomal proteins in intact polysomes is dramatically increased. The sub strate- specific stimulatory effects of 3', 5' cyclic nucleotides are due t o repression of the inhibition exerted upon the reaction, by negatively cha rged macromolecules such as RNA, DNA and to a lesser extent heparin.