The A-Kinase Anchoring Protein (AKAP) Glycogen Synthase Kinase 3β Interaction Protein (GSKIP) Regulates β-Catenin through Its Interactions with Both Protein Kinase A (PKA) and GSK3β

Abstract

The A-kinase anchoring protein (AKAP) GSK3␤ interaction protein (GSKIP) is a cytosolic scaffolding protein binding protein kinase A (PKA) and glycogen synthase kinase 3␤ (GSK3␤). Here we show that both the AKAP function of GSKIP, i.e. its direct interaction with PKA, and its direct interaction with GSK3␤ are required for the regulation of ␤-catenin and thus Wnt signaling. A cytoplasmic destruction complex targets ␤-catenin for degradation and thus prevents Wnt signaling. Wnt signals cause ␤-catenin accumulation and translocation into the nucleus, where it induces Wnt target gene expression. GSKIP facilitates control of the ␤-catenin stabilizing phosphorylation at Ser-675 by PKA. Its interaction with GSK3␤ facilitates control of the destabilizing phosphorylation of ␤-catenin at Ser-33/Ser-37/Thr-41. The influence of GSKIP on ␤-catenin is explained by its scavenger function; it recruits the kinases away from the destruction complex without forming a complex with ␤-catenin. The regulation of ␤-catenin by GSKIP is specific for this AKAP as AKAP220, which also binds PKA and GSK3␤, did not affect Wnt signaling. We find that the binding domain of AKAP220 for GSK3␤ is a conserved GSK3␤ interaction domain (GID), which is also present in GSKIP. Our findings highlight an essential compartmentalization of both PKA and GSK3␤ by GSKIP, and ascribe a function to a cytosolic AKAP-PKA interaction as a regulatory factor in the control of canonical Wnt signaling. Wnt signaling controls different biological processes, including embryonic development, cell cycle progression, glycogen metabolism, and immune regulation; deregulation is associated with diseases such as cancer, type 2 diabetes, inflammatory, and Alzheimer's and Parkinson's diseases.A-kinase anchoring proteins (AKAPs) 3 are a family of about 50 scaffolding proteins. Their conserved function is the compartmentalization of protein kinase A (PKA). PKA holoenzyme consists of a dimer of regulatory (RI␣, RI␤, RII␣, or RII␤) and two catalytic subunits each bound to one R subunit. AKAPs directly interact with R subunits and tether the kinase to defined cellular compartments such as vesicles, the sarcoplasmic reticulum, or the cytoskeleton. This compartmentalization confers a tight spatiotemporal control to PKA signaling, and enables PKA to elicit a specific cellular response to each of the many stimuli that cause cAMP elevation and thereby lead to activation of this ubiquitous kinase. AKAPs directly interact with further signaling proteins, thus mediating crosstalk between signaling systems: phosphatases, dephosphorylating PKA-phosphorylated substrates, adenylyl cyclases, synthesizing cAMP, and phosphodiesterases (PDEs), hydrolyzing cAMP. Several AKAPs bind further kinases such as protein kinase C (PKC), which are activated by signals other than cAMP, e.g. Ca 2ϩ . AKAPs and their interactions play key roles in a variety of physiological processes such as vasopressin-mediated water reabsorption in renal principal cells and cardiac myocyte contractility (1-5, 7, 8).A new example...

GSK3 is a negative regulator of the thermogenic program in brown adipocytes

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