Interaction whereas the catalytic subunit (C) of both types of kinase appears essentially identical (1-3). The physiological relevance of cAMP-dependent protein phosphorylation and dephosphorylation in regulation of enzymes in key metabolic pathways has been clearly demonstrated (1, 4, 5), and additional advances have been made in understanding the effects of phosphorylation on many nonenzyme substrates (6, 7). The major recognized function of the regulatory subunit is inhibition of the kinase activity of the C present in the holoenzyme (1). The physiological significance of the existence of two types of cAMP-dependent protein kinase differing only in their regulatory subunit has not been fully elucidated. An increasing number of indications, however, suggest thatThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact. the regulatory subunit may have other roles in addition to inhibition of C. (i) The concentration of regulatory subunits can be selectively increased several fold without any change in the concentration of C in certain cells in culture treated with dibutyryl cAMP (Bt2cAMP) or hormones that raise cAMP levels (reviewed in ref. 8). Under these conditions some cells appeared to acquire more differentiated characteristics, but the function of the excess regulatory subunit is not known. (ii) The protein kinase regulatory subunit seems capable of interacting with other cellular components in addition to C. The subcellular localization of both type I and type II cAMP-dependent protein kinases appears to be mediated by the attachment of the regulatory subunit to certain structural elements of the cell. A portion of both kinases have been recovered in particulate fractions of cell homogenates, and the ratio of the soluble/particulate kinase varied from one tissue to another (9-11). Kinase association with the particulate fraction was shown to be mediated by the regulatory subunit (either RI or RI,), since cAMP produced release of C but not the regulatory subunit into the soluble phase (10, 12). In addition, a specific association of RI, with microtubule-associated protein 2 (MAP2) (13,14) and with a calmodulin-binding multimeric complex (15) has been demonstrated. Finally, experiments using cDNA probes have allowed a direct demonstration of cAMP-mediated increases in specific mRNAs coding for a number of proteins (reviewed in ref. 8 SPreliminary results of this work were presented at the