Networking with AKAPs: context-dependent regulation of anchored enzymes

Abstract

A-Kinase Anchoring Proteins (AKAPs) orchestrate and synchronize cellular events by tethering the cAMP-dependent protein kinase (PKA) and other signaling enzymes to organelles and membranes. The control of kinases and phosphatases that are held in proximity to activators, effectors, and substrates favors the rapid dissemination of information from one cellular location to the next. This article charts the inception of the PKA-anchoring hypothesis, the characterization of AKAPs and their nomenclature, and the physiological roles of context-specific AKAP signaling complexes.

Figure 1

Structural aspects of AKAP interactions with the regulatory subunits of PKA. A. Structure of AKAP-IS in complex with the RII X-type 4 helix bundle. The AKAP-IS helix (top) interacts with the dimerization/docking (D/D) domain of RII through interactions within the core hydrophobic interface (yellow), and polar contacts that form hydrogen bonds with RII. B. Detailed top-view of the protein-protein interactions between AKAP-IS and RII. The residues important for AKAP-RII interactions (hydrophobic and H bonding) are numbered, with corresponding interaction sites in RII highlighted. C. Helical wheel rendering of the AKAP-IS amphipathic helix, showing a face of hydrophobic residues (yellow). D. Sequence alignment of the RII binding domain of AKAP peptides. The pdb accession number 2izx was used in A and B.

Figure 2

The coordination of PKA signaling events in a cardiac myocyte by localized scaffolding of AKAPs. A. In a cardiomyocyte, multiple AKAPs (yellow) coordinate physiologic and pathophysiologic signaling events, including excitation-contraction coupling, hypertrophic remodeling, gene transcription, and oxygen homeostasis. B. Adult mouse cardio-myocytes were fixed and incubated with antibodies against RIIα (blue) and AKAP-Lbc (green), and actin was visualized by phalloidin staining (red). Cells were then imaged using immunofluorescence microscopy. RyR, ryanodine receptor; βAR, β-adrenergic receptor; NCX, sodium-calcium exchangr ; CG-NAP, centrosome- and golgi-localized PKN-associated protein; MAP2, mitochondrial-associated protein 2 ; LTCC, L-type calcium channel; AC5/6, adenylyl cyclase 5 or 6; PLB, phospholamban; SERCA2, Sarcoplasmic-endoplasmic calcium pump 2.

Figure 3

AKAP-Lbc integrates multiple signaling pathways. GPCRs such as M1 muscarinic receptor (M1-R) initiate a signaling cascade involving PKC-mediated activation of PKD and phosphorylation of HDAC in the nucleus. In another pathway, Gα12-coupled receptors such as the lysophospholipid receptor (LPA-R) stimulate the intrinsic Rho-GEF activity of AKAP-Lbc. When cAMP concentrations are low (top panel), PKD activity is relatively low and Rho-GEF activity relatively high. Gαs-coupled receptors such as βAR stimulate production of cAMP (orange cloud) by adenylyl cyclases (bottom panel), leading to the activation of PKA. Phosphorylation of AKAP-Lbc by PKA accentuates PKD signaling and curtails Rho-GEF activity. For simplicity, both panels show conditions where Gαq/11 and Gα12 pathways are active. HDAC, histone deacetylase; LPA-R, Lysophosphatidic acid receptor; βAR, beta-adrenergic receptor; LC3, microtubule associated protein light chain 3.

Figure 4

AKAP79/150 forms customized signaling complexes. Different combinations of enzymes and substrates are anchored by AKAP79/150. A. PKA stimulates calcium channel (Ca_v1.2) currents while also negatively regulating the action of adrenergic receptors and adenylyl cyclase, which are responsible for PKA activation. B. Signaling through the M1 muscarinic receptor activates PKC which phosphorylates and terminates M-current through the potassium channel KCNQ2. C. SAP97 recruits AKAP79/150 and its anchored enzymes PP2B and PKA, whose opposing actions regulate amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) current through GluR1.