The cyclic adenosine 3'5'-monophosphate (cAMP)-dependent protein kinase in mammalian tissue appears at present to be a specific enzyme (receptor) through which cAMP carries out its function as the ubiquitous "second messenger" in response to hormonal stimulus. The objective of this research project is to study the interactions, in molecular terms, between protein kinase and its effector cAMP and between protein kinase and its substrate ATP. The study will utilize homogeneous protein kinase isolated from rabbit skeletal muscle. The detail of the protein kinase-nucleotide interactions will be established by systematic studies involving analytical ultracentrifuge, gel electrophoresis, molecular sieve chromatography, spectrofluorimetry, and chemical cross-linking. Particular emphasis is placed on the elucidation of the mechanism for the action of cAMP on the protein kinase in terms of subunit-subunit interactions. Since protein kinase catalyzes the conversion of a larger number of metabolic enzymes and physiologically active proteins to their functional states by phosphorylation reaction, and the rate of the phosphorylation is greatly stimulated by cAMP, the interactions under proposed study are of fundamental importance for understanding hormonal regulations of cellular activities in molecular terms.