Protein Kinase A Type I and Type II Define Distinct Intracellular Signaling Compartments

Abstract

Abstract-Protein kinase A (PKA) is a key regulatory enzyme that, on activation by cAMP, modulates a wide variety of cellular functions. PKA isoforms type I and type II possess different structural features and biochemical characteristics, resulting in nonredundant function. However, how different PKA isoforms expressed in the same cell manage to perform distinct functions on activation by the same soluble intracellular messenger, cAMP, remains to be established. Here, we provide a mechanism for the different function of PKA isoforms subsets in cardiac myocytes and demonstrate that PKA-RI and PKA-RII, by binding to AKAPs (A kinase anchoring proteins), are tethered to different subcellular locales, thus defining distinct intracellular signaling compartments. Within such compartments, PKA-RI and PKA-RII respond to distinct, spatially restricted cAMP signals generated in response to specific G protein-coupled receptor agonists and regulated by unique subsets of the cAMP degrading phosphodiesterases. Key Words: cAMP Ⅲ compartmentalization Ⅲ compartmentation Ⅲ adrenergic stimulation Ⅲ prostaglandin Ⅲ protein kinase A P rotein kinase A (PKA) is a key regulatory enzyme in the heart that is involved in the catecholamine-mediated control of excitation-contraction coupling, as well as in a myriad of other functions including activation of transcription factors and control of metabolic enzymes. The second messenger cAMP activates PKA by binding to the regulatory (R) subunits, causing release of the activated catalytic (C) subunits.The fact that, following cAMP-engagement, PKA mediates a plethora of cellular responses has raised the question of how specificity is maintained. In recent years, features of this pathway that contribute to specificity have been uncovered. 1 A key role is played by AKAPs (A kinase anchoring proteins), a family of proteins that act as molecular scaffolds to anchor PKA in the vicinity of specific substrate molecules, 2 thus focusing PKA activity toward relevant substrates.A second mechanism contributing to specificity revolves around the spatial control of the cAMP signal itself. Restriction of intracellular diffusion of cAMP has been shown by using a variety of approaches, [3][4][5] including direct imaging of gradients of cAMP in response to activation of various G protein-coupled receptors (GPCRs). 6 A key role in shaping cAMP intracellular pools is played by phosphodiesterases (PDEs), the enzymes that hydrolyze cAMP. 7 Indeed, individual PDE isoforms have been shown to be functionally coupled to specific GPCRs to degrade cAMP selectively in response to a given stimulus. 8 Cardiac myocytes express all four types of PKA isozymes, PKA-RI␣, PKA-RII␣, PKA-RI␤, and PKA-RII␤. 9 PKA isoforms show different subcellular localization, with PKA-RII being mainly associated with the particulate fraction of cell lysates whereas PKA-RI has been found preferentially in the cytosol. 10,11 PKA isoforms also show different biochemical properties. PKA-RI is more readily dissociated by cAMP than PKA-RII, 12,13 an...

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