Molecular Forms and Subunit Composition of a Cyclic Adenosine 3′,5′-Monophosphate-dependent Protein Kinase Purified from Bovine Heart Muscle

A cyclic adenosine 3',5'-monophosphate (cyclic AMP)dependent protein kinase has been puritied from bovine heart muscle.Its molecular weight was estimated to be 280,000 by gel filtration chromatography, and it was composed of cyclic AMP-independent protein kinase and cyclic AMPbinding subunits with molecular weights of 42,000 and 55,000, respectively.When the purified protein kinase was subjected to polyacrylamide gel electrophoresis, ultracentrifugation, or storage there appeared smaller forms of cyclic AMPdependent kinase with molecular weights of approximately 140,000 and 90,000.A close structural relationship between all of these forms of protein kinase was suggested by the observation that each was composed of the same two kinds of subunits.During the past few years, investigations designed to elucidate the mechanisms by which cyclic adenosine 3', 5'-monophosphate exerts its diverse biological effects have concentrated on the ability of this nucleotide to stimulate protein kinase activity (l-lo).Cyclic AMPl-dependent protein kinases appear to serve as proximate receptors of cyclic AMP linking the membrane-bound, hormone-sensitive adenylyl cyclase system and the physiological responses of the cell.Protein kinases alter the activity of specific cellular proteins by catalyzing the transfer of the y-phosphate of ATP to specific serine and threonine residues (1, 8, 11, 12).Optimal concentrations of cyclic AMP have been shown to stimulate protein kinase activit(y 2-to lo-fold in vitro (3) and in vivo (1).Proteins which serve as substrates for the protein kinases derived from mammalian tissues include histones (1, 3), protamine (I2),