Intracellular Targeting of Protein Kinases and Phosphatases

Abstract

Compartmentalization of kinases and phosphatases is a key determinant in the specificity of second messenger؊ mediated signaling events. Localization of the cAMPdependent protein kinase (PKA) and other signaling enzymes is mediated by interaction with A-kinase anchoring proteins (AKAPs). This study focused on recent advances that further our understanding of AKAPs, with particular emphasis on the bidirectional regulation of signaling events by AKAP signaling complexes and their contribution to the control of actin reorganization events. Diabetes 51 (Suppl. 3):S385-S388, 2002 E xtracellular signals, such as hormones, neurotransmitters, and growth factors, regulate a wide variety of cellular activities, including ion channel modulation, neuronal excitation, cell growth, cell differentiation, and insulin secretion events (1). Intracellular transduction systems receive these signals via receptors and transmit them quickly and precisely, resulting in the amplification of specific biological responses. However, cells often are exposed to several messengers simultaneously, and maintaining the fidelity of these networks is crucial in eliciting the appropriate physiological response. Doing so requires the accurate selection of effector molecules for regulated activation and deactivation, often by phosphorylation and dephosphorylation events. A principal strategy in achieving this selection specificity is compartmentalization of signaling enzymes (2-4). This study highlights the most recent advances in our understanding of compartmentalization of multivalent signaling complexes by A-kinase anchoring proteins (AKAPs) and the functional consequences they mediate. CYCLIC AMP-DEPENDENT PROTEIN KINASEOne of the best-characterized signaling pathways involves the activation of the cAMP-dependent protein kinase (PKA). PKA is a serine/threonine kinase composed of two catalytic (C) subunits that are held in an inactive state by association with a regulatory (R) subunit dimer (5-8). The catalytic subunits are expressed from three different genes-C␣, C␤, and C␥-whereas the R subunits are expressed from four different genes-RI␣, RI␤, RII␣, and RII␤ (9 -12). The R subunit is a modular polypeptide containing an NH 2 -terminal dimerization domain, an autophosphorylation site that serves as a principal contact site for the C subunit, and two cAMP binding sites. Activation of PKA is solely accomplished by the major, diffusible secondary messenger cAMP (13,14). Binding of cAMP to each R subunit relieves the autoinhibitory contact, allowing the C subunits to dissociate (15,16), thereby resulting in phosphorylation of local substrates.Two forms of the heterotetrameric PKA holoenzyme exist: type I (RI␣ and RI␤ dimer) and type II (RII␣ and RII␤ dimer). Type I PKA is predominantly cytoplasmic, whereas type II PKA associates with specific cellular structures and organelles (17). Discrete localization of type II PKA within the cell is chiefly caused by association with nonenzymatic scaffolding AKAPs (3,18,19). This method of regulation ensures tha...

Mechanisms of action of glucagon-like peptide 1 in the pancreas

“…To add to the complexity of this whole area of regulation, there is evidence that PDEs are also tethered in the AKAP/PKA complex (Dodge et al, 2001). Scott and colleagues outlined possible regulatory aspects of PKA observed in other cell types that have not yet been explored in the β cell (Alto et al, 2002).…”

AKAPs (A-kinase anchoring proteins) and molecules that compose their G-protein-coupled receptor signalling complexes

“…In AKAP79, a homologous sequence occurs in residues 392-405. These sequences are amphipathic helices that provide a hydrophobic pocket for the binding of PKA R-subunit dimers [13,53]. The affinity of the R subunits for binding to these regions is in the nanomolar range: 0.5 nM for RI and 4.5 nM for RII [54].This sequence provides a basis for the Ht-31 blocking peptide (residues 493-515) [4], which has been shown to block the interaction of the β 2 AR with AKAP79 [47] and AKAP250 [19].…”

α-Kinase Anchoring Protein αKAP Interacts with SERCA2A to Spatially Position Ca2+/Calmodulin-dependent Protein Kinase II and Modulate Phospholamban Phosphorylation

“…Specificity of the phosphorylation/dephosphorylation event is in part achieved by selective employment of a protein kinase/phosphatase cascade and subcellular targeting (1,2). Both spatial and temporal specificity of signaling events is achieved by the compartmentalization of the signaling complexes through adaptor or anchoring proteins (1,2). Recent studies have highlighted novel aspects of integrating spatially and temporally the cAMP signaling cascades via a diverse family of protein kinase A anchoring proteins (AKAPs) 2 (3).…”

“…Specificity of the phosphorylation/dephosphorylation event is in part achieved by selective employment of a protein kinase/phosphatase cascade and subcellular targeting (1,2). Both spatial and temporal specificity of signaling events is achieved by the compartmentalization of the signaling complexes through adaptor or anchoring proteins (1,2).…”

“…The phosphorylation/dephosphorylation cycle is critical for controlling a diverse series of signaling processes in cell biology (1,2). Specificity of the phosphorylation/dephosphorylation event is in part achieved by selective employment of a protein kinase/phosphatase cascade and subcellular targeting (1,2).…”