Dimerization/docking domain of the type Ialpha regulatory subunit of cAMP-dependent protein kinase. Requirements for dimerization and docking are distinct but overlapping

Identification of protein kinase A signalling molecules in renal collecting ducts

Body water homeostasis is maintained by the correct balance between water intake and water loss through urine, faeces, sweat and breath. It is known that elevated circulating levels of the antidiuretic hormone vasopressin decrease urine volume to prevent excessive water loss from the body. Vasopressin/cAMP/protein kinase A (PKA) signalling is the canonical pathway in renal collecting ducts for phosphorylating aquaporin-2 (AQP2) water channels, which leads to the reabsorption of water from urine via AQP2. Although recent omics data have verified various downstream targets of PKA, crucial regulators that mediate PKA-induced AQP2 phosphorylation remain unknown, mainly because vasopressin is usually used to activate PKA as a positive control. Vasopressin is extremely potent and phosphorylates various PKA substrates non-specifically, making it difficult to narrow down the candidate mediators responsible for AQP2 phosphorylation. The intracellular localization of PKA is tightly regulated by its scaffold proteins, also known as A-kinase anchoring proteins (AKAPs). Furthermore, each AKAP has a target domain that determines its intracellular localization, enabling the creation of a local PKA signalling network. Although vasopressin activates most PKAs independently of their intracellular localization, some chemical compounds preferentially act on PKAs localized on AQP2-containing vesicles while simultaneously phosphorylating AQP2 and its surrounding PKA substrates. Immunoprecipitation with antibodies against phosphorylated PKA substrates followed by mass spectrometry analysis revealed that the PKA substrate in proximity to AQP2 was lipopolysaccharide-responsive and beige-like anchor (LRBA). Furthermore, Lrba knockout studies revealed that LRBA was required for vasopressin-induced AQP2 phosphorylation.

Role of A-Kinase Anchoring Protein 1 in Retinal Ganglion Cells: Neurodegeneration and Neuroprotection

A-Kinase anchoring protein 1 (AKAP1) is a multifunctional mitochondrial scaffold protein that regulates mitochondrial dynamics, bioenergetics, and calcium homeostasis by anchoring several proteins, including protein kinase A, to the outer mitochondrial membrane. Glaucoma is a complex, multifactorial disease characterized by a slow and progressive degeneration of the optic nerve and retinal ganglion cells (RGCs), ultimately resulting in vision loss. Impairment of the mitochondrial network and function is linked to glaucomatous neurodegeneration. Loss of AKAP1 induces dynamin-related protein 1 dephosphorylation-mediated mitochondrial fragmentation and loss of RGCs. Elevated intraocular pressure triggers a significant reduction in AKAP1 protein expression in the glaucomatous retina. Amplification of AKAP1 expression protects RGCs from oxidative stress. Hence, modulation of AKAP1 could be considered a potential therapeutic target for neuroprotective intervention in glaucoma and other mitochondria-associated optic neuropathies. This review covers the current research on the role of AKAP1 in the maintenance of mitochondrial dynamics, bioenergetics, and mitophagy in RGCs and provides a scientific basis to identify and develop new therapeutic strategies that could protect RGCs and their axons in glaucoma.

Role of Protein Kinase A Activation in the Immune System with an Emphasis on Lipopolysaccharide-Responsive and Beige-like Anchor Protein in B Cells

Cyclic AMP-dependent protein kinase A (PKA) is a ubiquitous enzymatic complex that is involved in a broad spectrum of intracellular receptor signaling. The activity of PKA depends on A-kinase anchoring proteins (AKAPs) that attach to PKAs close to their substrates to control signaling. Although the relevance of PKA-AKAP signaling in the immune system is evident in T cells, its relevance in B and other immune cells remains relatively unclear. In the last decade, lipopolysaccharide-responsive and beige-like anchor protein (LRBA) has emerged as an AKAP that is ubiquitously expressed in B and T cells, specifically after activation. A deficiency of LRBA leads to immune dysregulation and immunodeficiency. The cellular mechanisms regulated by LRBA have not yet been investigated. Therefore, this review summarizes the functions of PKA in immunity and provides the most recent information regarding LRBA deficiency to deepen our understanding of immune regulation and immunological diseases.

Regulation of Cardiac PKA Signaling by cAMP and Oxidants

Pathologies, such as cancer, inflammatory and cardiac diseases are commonly associated with long-term increased production and release of reactive oxygen species referred to as oxidative stress. Thereby, protein oxidation conveys protein dysfunction and contributes to disease progression. Importantly, trials to scavenge oxidants by systemic antioxidant therapy failed. This observation supports the notion that oxidants are indispensable physiological signaling molecules that induce oxidative post-translational modifications in target proteins. In cardiac myocytes, the main driver of cardiac contractility is the activation of the β-adrenoceptor-signaling cascade leading to increased cellular cAMP production and activation of its main effector, the cAMP-dependent protein kinase (PKA). PKA-mediated phosphorylation of substrate proteins that are involved in excitation-contraction coupling are responsible for the observed positive inotropic and lusitropic effects. PKA-actions are counteracted by cellular protein phosphatases (PP) that dephosphorylate substrate proteins and thus allow the termination of PKA-signaling. Both, kinase and phosphatase are redox-sensitive and susceptible to oxidation on critical cysteine residues. Thereby, oxidation of the regulatory PKA and PP subunits is considered to regulate subcellular kinase and phosphatase localization, while intradisulfide formation of the catalytic subunits negatively impacts on catalytic activity with direct consequences on substrate (de)phosphorylation and cardiac contractile function. This review article attempts to incorporate the current perception of the functionally relevant regulation of cardiac contractility by classical cAMP-dependent signaling with the contribution of oxidant modification.

The crystal structure of yeast regulatory subunit reveals key evolutionary insights into Protein Kinase A oligomerization

Protein Kinase A (PKA) is a widespread enzyme that plays a key role in many signaling pathways from lower eukaryotes to metazoans. In mammals, the regulatory (R) subunits sequester and target the catalytic (C) subunits to proper subcellular locations. This targeting is accomplished by the dimerization and docking (D/D) domain of the R subunits. The activation of the holoenzyme depends on the binding of the second messenger cAMP. The only available structures of the D/D domain proceed from mammalian sources. Unlike dimeric mammalian counterparts, the R subunit from Saccharomyces cerevisiae (Bcy1) forms tetramers in solution. Here we describe the first high-resolution structure of a non-mammalian D/D domain. The tetramer in the crystals of the Bcy1 D/D domain is a dimer of dimers that retain the classical D/D domain fold. By using phylogenetic and structural analyses combined with site-directed mutagenesis, we found that fungal R subunits present an insertion of a single amino acid at the D/D domain that shifts the position of a downstream, conserved arginine. This residue participates in intra-dimer interactions in mammalian D/D domains, while due to this insertion it is involved in inter-dimer contacts in Bcy1, which are crucial for the stability of the tetramer. This surprising finding challenges well-established concepts regarding the oligomeric state within the PKAR protein family and provides important insights into the yet unexplored structural diversity of the D/D domains and the molecular determinants of R subunit oligomerization.

Subcellular Organization of the cAMP Signaling Pathway

The field of cAMP signaling is witnessing exciting developments with the recognition that cAMP is compartmentalized and that spatial regulation of cAMP is critical for faithful signal coding. This realization has changed our understanding of cAMP signaling from a model in which cAMP connects a receptor at the plasma membrane to an intracellular effector in a linear pathway to a model in which cAMP signals propagate within a complex network of alternative branches and the specific functional outcome strictly depends on local regulation of cAMP levels and on selective activation of a limited number of branches within the network. In this review, we cover some of the early studies and summarize more recent evidence supporting the model of compartmentalized cAMP signaling, and we discuss how this knowledge is starting to provide original mechanistic insight into cell physiology and a novel framework for the identification of disease mechanisms that potentially opens new avenues for therapeutic interventions. SIGNIFICANCE STATEMENT: cAMP mediates the intracellular response to multiple hormones and neurotransmitters. Signal fidelity and accurate coordination of a plethora of different cellular functions is achieved via organization of multiprotein signalosomes and cAMP compartmentalization in subcellular nanodomains. Defining the organization and regulation of subcellular cAMP nanocompartments is necessary if we want to understand the complex functional ramifications of pharmacological treatments that target G protein-coupled receptors and for generating a blueprint that can be used to develop precision medicine interventions.

Structural analyses of the PKA RIIβ holoenzyme containing the oncogenic DnaJB1-PKAc fusion protein reveal protomer asymmetry and fusion-induced allosteric perturbations in fibrolamellar hepatocellular carcinoma

When the J-domain of the heat shock protein DnaJB1 is fused to the catalytic (C) subunit of cAMP-dependent protein kinase (PKA), replacing exon 1, this fusion protein, J-C subunit (J-C), becomes the driver of fibrolamellar hepatocellular carcinoma (FL-HCC). Here, we use cryo-electron microscopy (cryo-EM) to characterize J-C bound to RIIβ, the major PKA regulatory (R) subunit in liver, thus reporting the first cryo-EM structure of any PKA holoenzyme. We report several differences in both structure and dynamics that could not be captured by the conventional crystallography approaches used to obtain prior structures. Most striking is the asymmetry caused by the absence of the second cyclic nucleotide binding (CNB) domain and the J-domain in one of the RIIβ:J-C protomers. Using molecular dynamics (MD) simulations, we discovered that this asymmetry is already present in the wild-type (WT) RIIβ2C2 but had been masked in the previous crystal structure. This asymmetry may link to the intrinsic allosteric regulation of all PKA holoenzymes and could also explain why most disease mutations in PKA regulatory subunits are dominant negative. The cryo-EM structure, combined with small-angle X-ray scattering (SAXS), also allowed us to predict the general position of the Dimerization/Docking (D/D) domain, which is essential for localization and interacting with membrane-anchored A-Kinase-Anchoring Proteins (AKAPs). This position provides a multivalent mechanism for interaction of the RIIβ holoenzyme with membranes and would be perturbed in the oncogenic fusion protein. The J-domain also alters several biochemical properties of the RIIβ holoenzyme: It is easier to activate with cAMP, and the cooperativity is reduced. These results provide new insights into how the finely tuned allosteric PKA signaling network is disrupted by the oncogenic J-C subunit, ultimately leading to the development of FL-HCC.

Receptor-independent modulation of cAMP-dependent protein kinase and protein phosphatase signaling in cardiac myocytes by oxidizing agents

The contraction and relaxation of the heart is controlled by stimulation of the β1-adrenoreceptor (AR) signaling cascade, which leads to activation of cAMP-dependent protein kinase (PKA) and subsequent cardiac protein phosphorylation. Phosphorylation is counteracted by the main cardiac protein phosphatases, PP2A and PP1. Both kinase and phosphatases are sensitive to intramolecular disulfide formation in their catalytic subunits that inhibits their activity. Additionally, intermolecular disulfide formation between PKA type I regulatory subunits (PKA-RI) has been described to enhance PKA's affinity for protein kinase A anchoring proteins, which alters its subcellular distribution. Nitroxyl donors have been shown to affect contractility and relaxation, but the mechanistic basis for this effect is unclear. The present study investigates the impact of several nitroxyl donors and the thiol-oxidizing agent diamide on cardiac myocyte protein phosphorylation and oxidation. Although all tested compounds equally induced intermolecular disulfide formation in PKA-RI, only 1-nitrosocyclohexalycetate (NCA) and diamide induced reproducible protein phosphorylation. Phosphorylation occurred independently of β1-AR activation, but was abolished after pharmacological PKA inhibition and thus potentially attributable to increased PKA activity. NCA treatment of cardiac myocytes induced translocation of PKA and phosphatases to the myofilament compartment as shown by fractionation, immunofluorescence, and proximity ligation assays. Assessment of kinase and phosphatase activity within the myofilament fraction of cardiac myocytes after exposure to NCA revealed activation of PKA and inhibition of phosphatase activity thus explaining the increase in phosphorylation. The data suggest that the NCA-mediated effect on cardiac myocyte protein phosphorylation orchestrates alterations in the kinase/phosphatase balance.

Hedgehog and Gpr161: Regulating cAMP Signaling in the Primary Cilium

Compartmentalization of diverse types of signaling molecules contributes to the precise coordination of signal propagation. The primary cilium fulfills this function by acting as a spatiotemporally confined sensory signaling platform. For the integrity of ciliary signaling, it is mandatory that the ciliary signaling pathways are constantly attuned by alterations in both oscillating small molecules and the presence or absence of their sensor/effector proteins. In this context, ciliary G protein-coupled receptor (GPCR) pathways participate in coordinating the mobilization of the diffusible second messenger molecule 3',5'-cyclic adenosine monophosphate (cAMP). cAMP fluxes in the cilium are primarily sensed by protein kinase A (PKA) complexes, which are essential for the basal repression of Hedgehog (Hh) signaling. Here, we describe the dynamic properties of underlying signaling circuits, as well as strategies for second messenger compartmentalization. As an example, we summarize how receptor-guided cAMP-effector pathways control the off state of Hh signaling. We discuss the evidence that a macromolecular, ciliary-localized signaling complex, composed of the orphan GPCR Gpr161 and type I PKA holoenzymes, is involved in antagonizing Hh functions. Finally, we outline how ciliary cAMP-linked receptor pathways and cAMP-sensing signalosomes may become targets for more efficient combinatory therapy approaches to counteract dysregulation of Hh signaling.

Cardiac ischemia-reperfusion injury induces ROS-dependent loss of PKA regulatory subunit RIα

Type I PKA regulatory α-subunit (RIα; encoded by the Prkar1a gene) serves as the predominant inhibitor protein of the catalytic subunit of cAMP-dependent protein kinase (PKAc). However, recent evidence suggests that PKA signaling can be initiated by cAMP-independent events, especially within the context of cellular oxidative stress such as ischemia-reperfusion (I/R) injury. We determined whether RIα is actively involved in the regulation of PKA activity via reactive oxygen species (ROS)-dependent mechanisms during I/R stress in the heart. Induction of ex vivo global I/R injury in mouse hearts selectively downregulated RIα protein expression, whereas RII subunit expression appears to remain unaltered. Cardiac myocyte cell culture models were used to determine that oxidant stimulus (i.e., H2O2) alone is sufficient to induce RIα protein downregulation. Transient increase of RIα expression (via adenoviral overexpression) negatively affects cell survival and function upon oxidative stress as measured by increased induction of apoptosis and decreased mitochondrial respiration. Furthermore, analysis of mitochondrial subcellular fractions in heart tissue showed that PKA-associated proteins are enriched in subsarcolemmal mitochondria (SSM) fractions and that loss of RIα is most pronounced at SSM upon I/R injury. These data were supported via electron microscopy in A-kinase anchoring protein 1 (AKAP1)-knockout mice, where loss of AKAP1 expression leads to aberrant mitochondrial morphology manifested in SSM but not interfibrillar mitochondria. Thus, we conclude that modification of RIα via ROS-dependent mechanisms induced by I/R injury has the potential to sensitize PKA signaling in the cell without the direct use of the canonical cAMP-dependent activation pathway.NEW & NOTEWORTHY We uncovered a previously undescribed phenomenon involving oxidation-induced activation of PKA signaling in the progression of cardiac ischemia-reperfusion injury. Type I PKA regulatory subunit RIα, but not type II PKA regulatory subunits, is dynamically regulated by oxidative stress to trigger the activation of the catalytic subunit of PKA in cardiac myocytes. This effect may play a critical role in the regulation of subsarcolemmal mitochondria function upon the induction of ischemic injury in the heart.

A-kinase anchoring protein 1 (AKAP1) and its role in some cardiovascular diseases

A-kinase anchoring proteins (AKAPs) play crucial roles in regulating compartmentalized multi-protein signaling networks related to PKA-mediated phosphorylation. The mitochondrial AKAP - AKAP1 proteins are enriched in heart and play cardiac protective roles. This review aims to thoroughly summarize AKAP1 variants from their sequence features to the structure-function relationships between AKAP1 and its binding partners, as well as the molecular mechanisms of AKAP1 in cardiac hypertrophy, hypoxia-induced myocardial infarction and endothelial cells dysfunction, suggesting AKAP1 as a candidate for cardiovascular therapy.

Cysteine-Based Redox Sensing and Its Role in Signaling by Cyclic Nucleotide-Dependent Kinases in the Cardiovascular System

Oxidant molecules are produced in biological systems and historically have been considered causal mediators of damage and disease. While oxidants may contribute to the pathogenesis of disease, evidence continues to emerge that shows these species also play important regulatory roles in health. A major mechanism of oxidant sensing and signaling involves their reaction with reactive cysteine thiols within proteins, inducing oxidative posttranslational modifications that can couple to altered function to enable homeostatic regulation. Protein kinase A and protein kinase G are regulated by oxidants in this way, and this review focuses on our molecular-level understanding of these events and their role in regulating cardiovascular physiology during health and disease.

The unusual flagellar-targeting mechanism and functions of the trypanosome ortholog of the ciliary GTPase Arl13b

The small GTPase Arl13b is one of the most conserved and ancient ciliary proteins. In human and animals, Arl13b is primarily associated with the ciliary membrane, where it acts as a guanine-nucleotide-exchange factor (GEF) for Arl3 and is implicated in a variety of ciliary and cellular functions. We have identified and characterized Trypanosoma brucei (Tb)Arl13, the sole Arl13b homolog in this evolutionarily divergent, protozoan parasite. TbArl13 has conserved flagellar functions and exhibits catalytic activity towards two different TbArl3 homologs. However, TbArl13 is distinctly associated with the axoneme through a dimerization/docking (D/D) domain. Replacing the D/D domain with a sequence encoding a flagellar membrane protein created a viable alternative to the wild-type TbArl13 in our RNA interference (RNAi)-based rescue assay. Therefore, flagellar enrichment is crucial for TbArl13, but mechanisms to achieve this could be flexible. Our findings thus extend the understanding of the roles of Arl13b and Arl13b–Arl3 pathway in a divergent flagellate of medical importance.

This article has an associated First Person interview with the first author of the paper.

Highlighted Article: All roads lead to cilia – how the essential flagellar enrichment of Arl13 is achieved in trypanosome cells using a fundamentally different strategy compared with that of animal cells.

Mimicry of Cellular A Kinase-Anchoring Proteins Is a Conserved and Critical Function of E1A across Various Human Adenovirus Species

The E1A proteins of the various human adenovirus (HAdV) species perform the critical task of converting an infected cell into a setting primed for virus replication. While E1A proteins differ in both sequence and mechanism, the evolutionary pressure on viruses with limited coding capacity ensures that these proteins often have significant overlap in critical functions. HAdV-5 E1A is known to use mimicry to rewire cyclic AMP (cAMP) signaling by decoupling protein kinase A (PKA) from cellular A kinase-anchoring proteins (AKAPs) and utilizing PKA to its own advantage. We show here that E1As from other species of HAdV also possess this viral AKAP (vAKAP) function and examine how they manipulate PKA. E1A from most species of HAdV examined contain a small AKAP-like motif in their N terminus which targets the docking-dimerization domain of PKA as the binding interface for a conserved protein-protein interaction. This motif is also responsible for an E1A-mediated relocalization of PKA regulatory subunits from the cytoplasm into the nucleus, with species-specific E1A proteins having preference for one particular isoform of PKA subunit over another. Importantly, we showed that these newly characterized vAKAPs can integrate into cAMP-responsive transcription as well as contribute to viral genome replication and infectious progeny production for several distinct HAdV species.IMPORTANCE These data enhance the mechanistic knowledge on how HAdV E1A manipulates cellular PKA to benefit infection. The work establishes that mimicry of AKAPs and subversion of PKA-mediated cAMP signaling are conserved features for numerous human adenoviruses. This study also highlights the molecular determinants conferring selective protein-protein interactions between distinct PKA regulatory subunits and the different E1A proteins of these viruses. Additionally, it further emphasizes the utility of using viral proteins like E1A as tools for studying the molecular biology of cellular regulatory pathways.

In silico analysis of the EF-hand proteins in the genome of Giardia intestinalis assembly A

Giardia intestinalis is a parasite that inhabits the small intestine of humans and other mammals, causing a disease that can manifest itself with acute diarrhea. This parasite is an early divergent eukaryote with a compact genome and a life cycle composed of two distinct cell types: the trophozoite, the replicative form, and the cyst, the infectious form. Signal transduction pathways implicated in differentiation processes of G. intestinalis are largely unknown. Calcium, considered an essential messenger in cell signaling, has been shown to regulate a myriad of key cell processes including metabolism, motility, and exocytosis, among other important functions, through calcium-binding proteins (CaBPs). The most important and largest family of CaBPs is the EF-hand protein family. To investigate the nature of calcium signaling pathways present in this protozoan, an in silico analysis of the genome to identify genes encoding EF-hand proteins was undertaken. Twenty-eight sequences containing EF-hand domains were found; most of which have only a pair of domains, and half of the sequences were divergent or unique to Giardia. In addition, the transcription pattern for eight genes encoding EF-hand proteins was assessed during encystation. It was found that all the genes were differentially transcribed suggesting a different function in this process. The in silico results suggest that in G. intestinalis, calcium is involved in the regulation of protein phosphorylation through kinases and phosphatases.

Targeting the Architecture of Deregulated Protein Complexes in Cancer

The architectures of central signaling hubs are precisely organized by static and dynamic protein-protein interactions (PPIs). Upon deregulation, these PPI platforms are capable to propagate or initiate pathophysiological signaling events. This causes the acquisition of molecular features contributing to the etiology or progression of many diseases, including cancer, where deregulated molecular interactions of signaling proteins have been best studied. The reasons for PPI-dependent reprogramming of cancer-initiating cells are manifold; in many cases, mutations perturb PPIs, enzyme activities, protein abundance, or protein localization. Consequently, the pharmaceutical targeting of PPIs promises to be of remarkable therapeutic value. For this review we have selected three key players of oncogenic signaling which are differently affected by PPI deregulation: two (the small G proteins of the RAS family and the transcription factor MYC) are considered "undruggable" using classical drug discovery approaches and in the case of the third protein discussed here, PKA, standard kinase inhibitors, may be unsuitable in the clinic. These circumstances require alternative strategies, which may lie in pharmaceutical drug interference of critical PPIs accountable for oncogenic signaling.

The many faces of compartmentalized PKA signalosomes

Cellular signal transmission requires the dynamic formation of spatiotemporally controlled molecular interactions. At the cell surface information is received by receptor complexes and relayed through intracellular signaling platforms which organize the actions of functionally interacting signaling enzymes and substrates. The list of hormone or neurotransmitter pathways that utilize the ubiquitous cAMP-sensing protein kinase A (PKA) system is expansive. This requires that the specificity, duration, and intensity of PKA responses are spatially and temporally restricted. Hereby, scaffolding proteins take the center stage for ensuring proper signal transmission. They unite second messenger sensors, activators, effectors, and kinase substrates within cellular micro-domains to precisely control and route signal propagation. A-kinase anchoring proteins (AKAPs) organize such subcellular signalosomes by tethering the PKA holoenzyme to distinct cell compartments. AKAPs differ in their modular organization showing pathway specific arrangements of interaction motifs or domains. This enables the cell- and compartment- guided assembly of signalosomes with unique enzyme composition and function. The AKAP-mediated clustering of cAMP and other second messenger sensing and interacting signaling components along with functional successive enzymes facilitates the rapid and precise dissemination of incoming signals. This review article delineates examples for different means of PKA regulation and for snapshots of compartmentalized PKA signalosomes.

Structural features of FAP174, a MYCBP-1 orthologue from Chlamydomonas reinhardtii, revealed by computational and experimental analyses

The ciliary MYCBP-1 (FAP174) from Chlamydomonas reinhardtii is an R2D2 protein and harbors a Dimerization and Docking domain.

Functional and Structural Mimicry of Cellular Protein Kinase A Anchoring Proteins by a Viral Oncoprotein

The oncoproteins of the small DNA tumor viruses interact with a plethora of cellular regulators to commandeer control of the infected cell. During infection, adenovirus E1A deregulates cAMP signalling and repurposes it for activation of viral gene expression. We show that E1A structurally and functionally mimics a cellular A-kinase anchoring protein (AKAP). E1A interacts with and relocalizes protein kinase A (PKA) to the nucleus, likely to virus replication centres, via an interaction with the regulatory subunits of PKA. Binding to PKA requires the N-terminus of E1A, which bears striking similarity to the amphipathic α-helical domain present in cellular AKAPs. E1A also targets the same docking-dimerization domain of PKA normally bound by cellular AKAPs. In addition, the AKAP like motif within E1A could restore PKA interaction to a cellular AKAP in which its normal interaction motif was deleted. During infection, E1A successfully competes with endogenous cellular AKAPs for PKA interaction. E1A's role as a viral AKAP contributes to viral transcription, protein expression and progeny production. These data establish HAdV E1A as the first known viral AKAP. This represents a unique example of viral subversion of a crucial cellular regulatory pathway via structural mimicry of the PKA interaction domain of cellular AKAPs.

Emerging roles of A-kinase anchoring proteins in cardiovascular pathophysiology

Heart and blood vessels ensure adequate perfusion of peripheral organs with blood and nutrients. Alteration of the homeostatic functions of the cardiovascular system can cause hypertension, atherosclerosis, and coronary artery disease leading to heart injury and failure. A-kinase anchoring proteins (AKAPs) constitute a family of scaffolding proteins that are crucially involved in modulating the function of the cardiovascular system both under physiological and pathological conditions. AKAPs assemble multifunctional signaling complexes that ensure correct targeting of the cAMP-dependent protein kinase (PKA) as well as other signaling enzymes to precise subcellular compartments. This allows local regulation of specific effector proteins that control the function of vascular and cardiac cells. This review will focus on recent advances illustrating the role of AKAPs in cardiovascular pathophysiology. The accent will be mainly placed on the molecular events linked to the control of vascular integrity and blood pressure as well as on the cardiac remodeling process associated with heart failure. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Integration of Developmental and Environmental Cues in the Heart edited by Marcus Schaub and Hughes Abriel.

Construction of a Near-Infrared-Activatable Enzyme Platform To Remotely Trigger Intracellular Signal Transduction Using an Upconversion Nanoparticle

Photoactivatable (caged) bioeffectors provide a way to remotely trigger or disable biochemical pathways in living organisms at a desired time and location with a pulse of light (uncaging), but the phototoxicity of ultraviolet (UV) often limits its application. In this study, we have demonstrated the near-infrared (NIR) photoactivatable enzyme platform using protein kinase A (PKA), an important enzyme in cell biology. We successfully photoactivated PKA using NIR to phosphorylate its substrate, and this induced a downstream cellular response in living cells with high spatiotemporal resolution. In addition, this system allows NIR to selectively activate the caged enzyme immobilized on the nanoparticle surface without activating other caged proteins in the cytosol. This NIR-responsive enzyme-nanoparticle system provides an innovative approach to remote-control proteins and enzymes, which can be used by researchers who need to avoid direct UV irradiation or use UV as a secondary channel to turn on a bioeffector.

D-AKAP2:PKA RII:PDZK1 ternary complex structure: insights from the nucleation of a polyvalent scaffold

A-kinase anchoring proteins (AKAPs) regulate cAMP-dependent protein kinase (PKA) signaling in space and time. Dual-specific AKAP2 (D-AKAP2/AKAP10) binds with high affinity to both RI and RII regulatory subunits of PKA and is anchored to transporters through PDZ domain proteins. Here, we describe a structure of D-AKAP2 in complex with two interacting partners and the exact mechanism by which a segment that on its own is disordered presents an α-helix to PKA and a β-strand to PDZK1. These two motifs nucleate a polyvalent scaffold and show how PKA signaling is linked to the regulation of transporters. Formation of the D-AKAP2: PKA binary complex is an important first step for high affinity interaction with PDZK1, and the structure reveals important clues toward understanding this phenomenon. In contrast to many other AKAPs, D-AKAP2 does not interact directly with the membrane protein. Instead, the interaction is facilitated by the C-terminus of D-AKAP2, which contains two binding motifs-the D-AKAP2AKB and the PDZ motif-that are joined by a short linker and only become ordered upon binding to their respective partner signaling proteins. The D-AKAP2AKB binds to the D/D domain of the R-subunit and the C-terminal PDZ motif binds to a PDZ domain (from PDZK1) that serves as a bridging protein to the transporter. This structure also provides insights into the fundamental question of why D-AKAP2 would exhibit a differential mode of binding to the two PKA isoforms.

Creating order from chaos: cellular regulation by kinase anchoring

Second messenger responses rely on where and when the enzymes that propagate these signals become active. Spatial and temporal organization of certain signaling enzymes is controlled in part by A-kinase anchoring proteins (AKAPs). This family of regulatory proteins was originally classified on the basis of their ability to compartmentalize the cyclic adenosine monophosphate (cAMP)-dependent protein kinase (also known as protein kinase A, or PKA). However, it is now recognized that AKAPs position G protein-coupled receptors, adenylyl cyclases, G proteins, and their effector proteins in relation to protein kinases and signal termination enzymes such as phosphodiesterases and protein phosphatases. This arrangement offers a simple and efficient means to limit the scope, duration, and directional flow of information to sites deep within the cell. This review focuses on the pros and cons of reagents that define the biological role of kinase anchoring inside cells and discusses recent advances in our understanding of anchored second messenger signaling in the cardiovascular and immune systems.

Isoform-specific targeting of PKA to multivesicular bodies

PKA RIα subunit is localized to MVBs by the A-kinase–anchoring protein AKAP11 when disassociated from the PKA catalytic subunit.

Although RII protein kinase A (PKA) regulatory subunits are constitutively localized to discrete cellular compartments through binding to A-kinase–anchoring proteins (AKAPs), RI subunits are primarily diffuse in the cytoplasm. In this paper, we report a novel AKAP-dependent localization of RIα to distinct organelles, specifically, multivesicular bodies (MVBs). This localization depends on binding to AKAP11, which binds tightly to free RIα or RIα in complex with catalytic subunit (holoenzyme). However, recruitment to MVBs occurs only with the release of PKA catalytic subunit (PKAc). This recruitment is reversed by reassociation with PKAc, and it is disrupted by the presence of AKAP peptides, mutations in the RIα AKAP-binding site, or knockdown of AKAP11. Cyclic adenosine monophosphate binding not only unleashes active PKAc but also leads to the targeting of AKAP11:RIα to MVBs. Therefore, we show that the RIα holoenzyme is part of a signaling complex with AKAP11, in which AKAP11 may direct RIα functionality after disassociation from PKAc. This model defines a new paradigm for PKA signaling.

Ascl1-induced neuronal differentiation of P19 cells requires expression of a specific inhibitor protein of cyclic AMP-dependent protein kinase

cAMP-dependent protein kinase (PKA) plays a critical role in nervous system development by modulating sonic hedgehog and bone morphogenetic protein signaling. In the current studies, P19 embryonic carcinoma cells were neuronally differentiated by expression of the proneural basic helix-loop-helix transcription factor Ascl1. After expression of Ascl1, but prior to expression of neuronal markers such as microtubule associated protein 2 and neuronal β-tubulin, P19 cells demonstrated a large, transient increase in both mRNA and protein for the endogenous protein kinase inhibitor (PKI)β. PKIβ-targeted shRNA constructs both reduced the levels of PKIβ expression and blocked the neuronal differentiation of P19 cells. This inhibition of differentiation was rescued by transfection of a shRNA-resistant expression vector for the PKIβ protein, and this rescue required the PKA-specific inhibitory sequence of the PKIβ protein. PKIβ played a very specific role in the Ascl1-mediated differentiation process as other PKI isoforms were unable to rescue the deficit conferred by shRNA-mediated knockdown of PKIβ. Our results define a novel requirement for PKIβ and its inhibition of PKA during neuronal differentiation of P19 cells.

cAMP-dependent protein kinase from Plasmodium falciparum: an update

One of the most important public health problems in the world today is the emergence and dissemination of drug-resistant malaria parasites. Plasmodium falciparum is the causative agent of the most lethal form of human malaria. New anti-malarial strategies are urgently required, and their design and development require the identification of potential therapeutic targets. However, the molecular mechanisms controlling the life cycle of the malaria parasite are still poorly understood. The published genome sequence of P. falciparum and previous studies have revealed that several homologues of eukaryotic signalling proteins, such as protein kinases, are relatively conserved. Protein kinases are now widely recognized as important drug targets in protozoan parasites. Cyclic AMP-dependent protein kinase (PKA) is implicated in numerous processes in mammalian cells, and the regulatory mechanisms of the cAMP pathway have been characterized. P. falciparum cAMP-dependent protein kinase plays an important role in the parasite's life cycle and thus represents an attractive target for the development of anti-malarial drugs. In this review, we focus on the P. falciparum cAMP/PKA pathway to provide new insights and an improved understanding of this signalling cascade.

Structure of D-AKAP2:PKA RI complex: insights into AKAP specificity and selectivity

A-kinase anchoring proteins (AKAPs) regulate cyclic AMP-dependent protein kinase (PKA) signaling in space and time. Dual-specific AKAP 2 (D-AKAP2) binds to the dimerization/docking (D/D) domain of both RI and RII regulatory subunits of PKA with high affinity. Here we have determined the structures of the RIalpha D/D domain alone and in complex with D-AKAP2. The D/D domain presents an extensive surface for binding through a well-formed N-terminal helix, and this surface restricts the diversity of AKAPs that can interact. The structures also underscore the importance of a redox-sensitive disulfide in affecting AKAP binding. An unexpected shift in the helical register of D-AKAP2 compared to the RIIalpha:D-AKAP2 complex structure makes the mode of binding to RIalpha novel. Finally, the comparison allows us to deduce a molecular explanation for the sequence and spatial determinants of AKAP specificity.

Transnitrosylating nitric oxide species directly activate type I protein kinase A, providing a novel adenylate cyclase-independent cross-talk to beta-adrenergic-like signaling

The transnitrosylating nitric oxide (NO) donor nitrocysteine (CysNO) induced a disulfide bond between the two regulatory RI subunits of protein kinase A (PKA). The conventional NO donor S-nitroso-N-acetylpenicillamine failed to do this, consistent with our observation that it also did not promote protein S-nitrosylation. This disulfide oxidation event activated PKA and induced vasorelaxation independently of the classical beta-adrenergic or NO signaling pathway. Activation of PKA had also been anticipated to exert a positive inotropic effect on the myocardium but did not. The lack of positive inotropy was explained by CysNO concomitantly activating protein kinase G (PKG) Ialpha. PKG was found to exert a partial negative inotropic influence regardless of whether PKA was activated by classical beta-receptor stimulation or by disulfide bond formation. This work demonstrates that NO molecules that can induce S-nitrosylation directly activate type I PKA, providing a novel cross-talk to beta-adrenergic-like signaling without receptor or adenylate cyclase stimulation. However, the expected positive inotropic consequences of PKA activation by this novel mechanism are countermanded by the simultaneous dual activation of PKGIalpha, which is also activated by CysNO.

Dual specificity A-kinase anchoring proteins (AKAPs) contain an additional binding region that enhances targeting of protein kinase A type I

A-kinase anchoring proteins (AKAPs) target protein kinase A (PKA) to a variety of subcellular locations. Conventional AKAPs contain a 14-18-amino acid sequence that forms an amphipathic helix that binds with high affinity to the regulatory (R) subunit of PKA type II. More recently, a group of dual specificity AKAPs has been classified on the basis of their ability to bind the PKA type I and the PKA type II isozymes. In this study we show that dual specificity AKAPs contain an additional PKA binding determinant called the RI Specifier Region (RISR). A variety of protein interaction assays and immunoprecipitation and immunolocalization experiments indicates that the RISR augments RI binding in vitro and inside cells. Cellular delivery of the RISR peptide uncouples RI anchoring to Ezrin leading to release of T cell inhibition by cAMP. Likewise, expression of mutant Ezrin forms where RI binding has been abrogated by substitution of the RISR sequence prevents cAMP-mediated inhibition of T cell function. Thus, we propose that the RISR acts in synergy with the amphipathic helix in dual specificity anchoring proteins to enhance anchoring of PKA type I.

Spatiotemporal control of cAMP signalling processes by anchored signalling complexes

Ligand-induced changes in cAMP concentration vary in duration, amplitude and extension into the cell. cAMP microdomains are shaped by adenylate cyclases that form cAMP as well as PDEs (phosphodiesterases) that degrade cAMP. Various extracellular signals converge on the cAMP/PKA (protein kinase A) pathway through ligand binding to GPCRs (G-protein-coupled receptors) and the cAMP/PKA pathway is therefore tightly regulated on several levels to maintain specificity in the multitude of signal inputs. AKAPs (A-kinase-anchoring proteins) target PKA to specific substrates and distinct subcellular compartments, providing spatial and temporal specificity for mediation of biological effects channelled through the cAMP/PKA pathway. AKAPs also serve as scaffolding proteins that assemble PKA together with signal terminators such as phosphoprotein phosphatases and cAMP-specific PDEs as well as components of other signalling pathways into multiprotein signalling complexes.

Signaling at A-kinase anchoring proteins organizes anesthesia-sensitive memory in Drosophila

The ubiquitous cAMP-protein kinase A (PKA) signaling pathway exhibits complex temporal requirements during the time course of associative memory processing. This directly raises questions about the molecular mechanisms that provide signaling specificity to this pathway. Here, we use Drosophila olfactory conditioning to show that divergent cAMP signaling is mediated by functionally distinct pools of PKA. One particular pool is organized via the PKA regulatory type II subunit at the level of A-kinase anchoring proteins (AKAPs), a family of scaffolding proteins that provides focal points of spatiotemporal signal integration. This AKAP-bound pool of PKA is acting within neurons of the mushroom bodies to support a late phase of aversive memory. The requirement for AKAP-bound PKA signaling is limited to aversive memory, but dispensable during appetitive memory. This finding suggests the existence of additional mechanisms to support divergence within the cAMP-PKA signaling pathway during memory processing. Together, our results show that subcellular organization of signaling components plays a key role in memory processing.

Regulation of brefeldin A-inhibited guanine nucleotide-exchange protein 1 (BIG1) and BIG2 activity via PKA and protein phosphatase 1gamma

Brefeldin A-inhibited guanine nucleotide-exchange proteins (GEPs) BIG1 and BIG2 activate ADP-ribosylation factor (ARF) GTPases, which are required for vesicular trafficking. Both molecules contain one or more sites for binding protein kinase A, i.e., A kinase-anchoring protein (AKAP) sequences. Elevation of cell cAMP caused PKA-catalyzed phosphorylation and nuclear accumulation of BIG1 but not BIG2. We then asked whether BIG1 phosphorylation altered its GEP activity. Incubation of BIG1 or BIG2 with PKA catalytic subunits and ATP resulted in retardation of their electrophoretic migration, consistent with PKA phosphorylation. Okadaic acid inhibits many protein phosphatases, including protein phosphatase 1 (PP1) and PP2A, that can reverse PKA-catalyzed phosphorylation. Incubation of HepG2 cells with okadaic acid caused concentration-dependent accumulation of presumably phosphorylated BIG1 and BIG2 with decreased mobility, which was increased by subsequent incubation in vitro with specific recombinant phosphatases, PP1gamma > PP2A >> PP1alpha. For assays of GEP activity, BIG1 and BIG2 were immunoprecipitated from cells that had been depleted, respectively, of BIG2 and BIG1 by using specific siRNA. GEP activity of each was significantly decreased after incubation with recombinant PKA plus ATP and restored by incubation with PP1gamma. In agreement with a role for PP1gamma in regulation of BIG, endogenous PP1gamma, but not PP1alpha or beta, was immunoprecipitated with BIG1 or BIG2 from microsomal fractions. All observations are consistent with the effects of BIG1 and BIG2 phosphorylation on vesicular trafficking, via alterations in ARF activation and regulatory roles for cAMP, PKA, and PP1gamma in ARF activation by BIG1 and BIG2.

Oxidant-induced activation of type I protein kinase A is mediated by RI subunit interprotein disulfide bond formation

Here we demonstrate that type I protein kinase A is redoxactive, forming an interprotein disulfide bond between its two regulatory RI subunits in response to cellular hydrogen peroxide. This oxidative disulfide formation causes a subcellular translocation and activation of the kinase, resulting in phosphorylation of established substrate proteins. The translocation is mediated at least in part by the oxidized form of the kinase having an enhanced affinity for alpha-myosin heavy chain, which serves as a protein kinase A (PKA) anchor protein and localizes the PKA to its myofilament substrates troponin I and myosin binding protein C. The functional consequence of these events in cardiac myocytes is that hydrogen peroxide increases contractility independently of beta-adrenergic stimulation and elevations of cAMP. The oxidant-induced phosphorylation of substrate proteins and increased contractility is blocked by the kinase inhibitor H89, indicating that these events involve PKA activation. In essence, type I PKA contains protein thiols that operate as redox sensors, and their oxidation by hydrogen peroxide directly activates the kinase.

Distinct interaction modes of an AKAP bound to two regulatory subunit isoforms of protein kinase A revealed by amide hydrogen/deuterium exchange

The structure of an AKAP docked to the dimerization/docking (D/D) domain of the type II (RIIalpha) isoform of protein kinase A (PKA) has been well characterized, but there currently is no detailed structural information of an AKAP docked to the type I (RIalpha) isoform. Dual-specific AKAP2 (D-AKAP2) binds in the nanomolar range to both isoforms and provided us with an opportunity to characterize the isoform-selective nature of AKAP binding using a common docked ligand. Hydrogen/deuterium (H/D) exchange combined with mass spectrometry (DXMS) was used to probe backbone structural changes of an alpha-helical A-kinase binding (AKB) motif from D-AKAP2 docked to both RIalpha and RIIalpha D/D domains. The region of protection upon complex formation and the magnitude of protection from H/D exchange were determined for both interacting partners in each complex. The backbone of the AKB ligand was more protected when bound to RIalpha compared to RIIalpha, suggesting an increased helical stabilization of the docked AKB ligand. This combined with a broader region of backbone protection induced by the AKAP on the docking surface of RIalpha indicated that there were more binding constraints for the AKB ligand when bound to RIalpha. This was in contrast to RIIalpha, which has a preformed, localized binding surface. These distinct modes of AKAP binding may contribute to the more discriminating nature of the RIalpha AKAP-docking surface. DXMS provides valuable structural information for understanding binding specificity in the absence of a high-resolution structure, and can readily be applied to other protein-ligand and protein-protein interactions.

Regional variations in the localization of insoluble kinase A regulatory isoforms during rodent brain development

In eukaryothes, the second messenger cAMP regulates many cellular functions by binding to the regulatory subunits of cAMP-dependent protein kinases, and releasing the catalytic subunits. In the mammalian brain all four regulatory isoforms (RIalpha and beta, RIIalpha and beta) are present. Apparently, they are simple inhibitors of the catalytic subunits. It is still unclear why four isoforms are needed, but possibly they can target kinase activity at precise intracellular locations. Therefore, we examined the distribution of the insoluble regulatory isoforms in rat and mouse brains during prenatal (from embryonic day 8) and postnatal development up to senescence (13 months), via immunohistochemistry. RIIalpha labelling is always restricted to the ventricular ependyma. Punctated RIIbeta labelling is observed in the embryo from early stages of development, and is mainly localized in the cortical plate. After birth, punctate RIIbeta labelling is present throughout almost the whole brain, often observed in proximity of neurofilaments. It shows different characteristics and relationships to the other isoforms: for example in the CA1 hippocampal field, RIIbeta is substituted by RIalpha 2 weeks after birth, while in CA2 it persists for life. In other regions, as in the reticular formation, RIIbeta and RIalpha aggregates are found in the same cell, although clearly segregated. The different regulatory isoforms show distinct patterns of distribution that change consistently during development. A careful characterization of second messenger systems may be as useful as the study of neurotransmitters to understand neuronal properties and their modifications during development, so as to relate biochemical to functional properties.

PKA: a portrait of protein kinase dynamics

Protein kinases play a critical role in the integration of signaling networks in eukaryotic cells. cAMP-dependent protein kinase (PKA) serves as a prototype for this large and highly diverse enzyme family. The catalytic subunit of PKA provides the best example of how a protein kinase recognizes its substrates, as well as inhibitors, and also show how the enzyme moves through the steps of catalysis. Many of the relevant conformational states associated with the catalytic cycle which have been captured in a crystal lattice are summarized here. From these structures, we can begin to appreciate the molecular events of catalysis as well as the intricate orchestration of critical residues in the catalytic subunit that contribute to catalysis. The entire molecule participates. To fully understand signaling by PKA, however, requires an understanding of a large set of related proteins, not just the catalytic subunit. This includes the regulatory subunits that serve as receptors for cAMP and the A kinase anchoring proteins (AKAPs) that serve as scaffolds for PKA. The AKAPs localize PKA to specific sites in the cell by docking to the N-terminus of the regulatory subunits, thus creating microenvironments for PKA signaling. To fully appreciate the diversity and integration of these molecules, one needs not only high-resolution structures but also an appreciation of how these molecules behave in solution. Thus, in addition to obtaining high-resolution structures by X-ray crystallography and NMR, we have used fluorescent tools and also hydrogen/deuterium exchange coupled with mass spectrometry to probe the dynamic properties of these proteins and how they interact with one another. The molecular features of these molecules are described. Finally, we describe a new recombinantly expressed PKA reporter that allows us to monitor PKA activity in living cells.

Localized effects of cAMP mediated by distinct routes of protein kinase A

More than 20% of the human genome encodes proteins involved in transmembrane and intracellular signaling pathways. The cAMP-protein kinase A (PKA) pathway is one of the most common and versatile signal pathways in eukaryotic cells and is involved in regulation of cellular functions in almost all tissues in mammals. Various extracellular signals converge on this signal pathway through ligand binding to G protein-coupled receptors, and the cAMP-PKA pathway is therefore tightly regulated at several levels to maintain specificity in the multitude of signal inputs. Ligand-induced changes in cAMP concentration vary in duration, amplitude, and extension into the cell, and cAMP microdomains are shaped by adenylyl cyclases that form cAMP as well as phosphodiesterases that degrade cAMP. Different PKA isozymes with distinct biochemical properties and cell-specific expression contribute to cell and organ specificity. A kinase anchoring proteins (AKAPs) target PKA to specific substrates and distinct subcellular compartments providing spatial and temporal specificity for mediation of biological effects channeled through the cAMP-PKA pathway. AKAPs also serve as scaffolding proteins that assemble PKA together with signal terminators such as phosphatases and cAMP-specific phosphodiesterases as well as components of other signaling pathways into multiprotein signaling complexes that serve as crossroads for different paths of cell signaling. Targeting of PKA and integration of a wide repertoire of proteins involved in signal transduction into complex signal networks further increase the specificity required for the precise regulation of numerous cellular and physiological processes.

Analysis of CREM-dependent gene expression during mouse spermatogenesis

The transcription factors CREM, CREB, and ATF-1 constitute a subfamily of beta-Zip transcription factors. Several different kinase cascades regulate the activity of these proteins. The activator splice-isoform CREMtau is specifically and highly expressed in post-meiotic germ cells during mouse spermatogenesis. Male mice lacking CREMtau expression are sterile because of stage-specific arrest of sperm maturation as the spermatids undergo apoptosis. In order to characterize the genes that are controlled by CREM during post-meiotic differentiation of round spermatids, we compared the expression levels of mRNA prepared from testes of wild-type and CREM-deficient mice by suppression subtractive hybridization (SSH) and affymetrix oligonucleotide arrays. A set of 956 unique sequences found in the CREM SSH library was further characterized by generating stage-specific expression profiles during spermatogenesis by hybridization with cDNA from pre-pubertal mice at defined stages of spermatogenesis using nylon DNA arrays. The resulting expression profiles were arranged in a linear order according to similarity in their profile shapes to find co-regulation of functionally related genes. Our data shows that a large number of genes are transcriptionally activated in round spermatids when CREM activity is maximal, including functional groups like transcription factors, proteins involved in signal transduction, and metabolic enzymes, therefore providing novel information of post-meiotic expression of many known as well as novel genes that are either directly or indirectly influenced by CREM expression.

Characterization of a type I regulatory subunit of cAMP-dependent protein kinase from the bivalve mollusk Mytilus galloprovincialis

Two isoforms of the regulatory subunit (R) of cAMP-dependent protein kinase (PKA), named R(myt1) and R(myt2), had been purified in our laboratory from two different tissues of the sea mussel Mytilus galloprovincialis. In this paper, we report the sequences of several peptides obtained from tryptic digestion of R(myt1). As a whole, these sequences showed high homology with regions of type I R subunits from invertebrate and also from mammalian sources, but homology with those of fungal and type II R subunits was much lower, which indicates that R(myt1) can be considered as a type I R isoform. This conclusion is also supported by the following biochemical properties: (1) R(myt1) was proved to have interchain disulfide bonds stabilizing its dimeric structure; (2) it failed to be phosphorylated by the catalytic (C) subunit purified from mussel; (3) it has a higher pI value than that of the R(myt2) isoform; and (4) it showed cross-reactivity with mammalian anti-RIbeta antibody.

Related protein-protein interaction modules present drastically different surface topographies despite a conserved helical platform

The subcellular localization of cAMP-dependent protein kinase (PKA) occurs through interaction with A-Kinase Anchoring Proteins (AKAPs). AKAPs bind to the PKA regulatory subunit dimer of both type Ialpha and type IIalpha (RIalpha and RIIalpha). RIalpha and RIIalpha display characteristic localization within different cell types, which is maintained by interaction of AKAPs with the N-terminal dimerization and docking domain (D/D) of the respective regulatory subunit. Previously, we reported the solution structure of RIIa D/D module, both free and bound to AKAPs. We have now solved the solution structure of the dimerization and docking domain of the type Ialpha regulatory dimer subunit (RIalpha D/D). RIalpha D/D is a compact docking module, with unusual interchain disulfide bonds that help maintain the AKAP interaction surface. In contrast to the shallow hydrophobic groove for AKAP binding across the surface of the RIIalpha D/D dimeric interface, the RIalpha D/D module presents a deep cleft for proposed AKAP binding. RIalpha and RIIalpha D/D interaction modules present drastically differing dimeric topographies, despite a conserved X-type four-helix bundle structure.

A-kinase anchoring protein 4 binding proteins in the fibrous sheath of the sperm flagellum

The fibrous sheath is a unique cytoskeletal structure located in the principal piece of the sperm flagellum and is constructed of two longitudinal columns connected by closely spaced circumferential ribs. Cyclic AMP-dependent protein kinases are secured within specific cytoplasmic domains by A-kinase anchoring proteins (AKAPs), and the most abundant protein in the fibrous sheath is AKAP4. Several other fibrous sheath proteins have been identified, but how the fibrous sheath assembles is not understood. Yeast two-hybrid assays and deletion mutagenesis were used to identify AKAP4-binding proteins and to map the binding regions on AKAP4 and on the proteins identified. We found that AKAP4 binds AKAP3 and two novel spermatogenic cell-specific proteins, Fibrous Sheath Interacting Proteins 1 and 2 (FSIP1, FSIP2). Transcription of Akap4, Akap3, and Fsip1 begins in early spermatid development, whereas transcription of Fsip2 begins in late spermatocyte development. AKAP3 is synthesized in round spermatids and incorporated into the fibrous sheath concurrently with formation of the rib precursors. However, AKAP4 is synthesized and incorporated into the nascent fibrous sheath late in spermatid development. The AKAP4 precursor is processed in the flagellum and only the mature form of AKAP4 appears to bind AKAP3. These results suggest that AKAP3 is involved in organizing the basic structure of the fibrous sheath, whereas AKAP4 has a major role in completing fibrous sheath assembly.

Isoform specific differences in binding of a dual-specificity A-kinase anchoring protein to type I and type II regulatory subunits of PKA

Dual-specificity AKAPs bind to type I (RI) and type II (RII) regulatory subunits of cAMP-dependent protein kinase A (PKA), potentially recruiting distinct cAMP responsive holoenzymes to a given intracellular location. To understand the molecular basis for this "dual" functionality, we have examined the pH-dependence, the salt-dependence, and the kinetics of binding of the A-kinase binding (AKB) domain of D-AKAP2 to the regulatory subunit isoforms of PKA. Using fluorescence anisotropy, we have found that a 27-residue peptide corresponding to the AKB domain of D-AKAP2 bound 25-fold more tightly to RIIalpha than to RIalpha. The higher affinity for RIIalpha was the result of a slower off-rate as determined by surface plasmon resonance. The high-affinity interaction for RIalpha and RIIalpha was pH-independent from pH 7.4 to 5.0. At pH 4.0, both isoforms had a reduction in binding affinity. Additionally, binding of the AKB domain to RIalpha was independent of solution ionic strength, whereas RIIalpha had an increased binding affinity at higher ionic strength. This suggests that the relative energetic contribution of the charge stabilization is different for the two isoforms. This prediction was confirmed by mutagenesis in which acidic mutations, primarily of E10 and D23, in the AKB domain affected binding to RIalpha but not to RIIalpha. These isoform-specific differences provide a foundation for developing isoform-specific peptide inhibitors of PKA anchoring by dual-specificity AKAPs, which can be used to evaluate the physiological significance of dual-specificity modes of PKA anchoring.

A kinase anchoring protein (AKAP) interaction and dimerization of the RIalpha and RIbeta regulatory subunits of protein kinase a in vivo by the yeast two hybrid system

Protein kinase A (PKA) regulatory (R) subunits dimerize through an N-terminal motif. Such dimerization is necessary for binding to PKA anchoring proteins (AKAPs) and targeting of PKA to its site of action. In the present study, we used the yeast two-hybrid system as an in vivo bio-reporter assay and analyzed the formation of homo- and heterodimeric complexes of RIalpha and RIbeta as well as AKAP binding of RI dimers. Native polyacrylamide gel electrophoresis (PAGE) of yeast extracts confirmed the two-hybrid data. Both RIalpha- and RIbeta homodimers as well as an RIalpha:RIbeta heterodimer were observed. Single, double and one triple mutation were introduced into the RIalpha and RIbeta subunits and dimerization properties of the mutants were analyzed. Consistent with previous reports, RIalpha(C37H) dimerized, although the disulfide bridges were disrupted, whereas the additional mutation of F47 or F52 abolished the dimerization. Corresponding mutations (C38H, F48A, F53A) in RIbeta were not sufficient to abolish the RIbeta dimerization, indicating that additional or other amino acids are important. RIalpha:RIbeta heterodimers of the mutants were formed at intermediate stringency. Analysis of ternary complexes by the yeast two-hybrid system revealed that RIalpha and RIbeta homodimers as well as an RIalpha:RIbeta heterodimer and several of the mutants were able to bind to the R-binding domain of AKAP149/D-AKAP1. Furthermore, an RIbeta:AKAP149 complex was identified following introduction of RIbeta into HEK293 cells. Importantly, RIbeta revealed AKAP binding properties similar to those of RIalpha, indicating that RIbeta holoenzymes may be anchored.

Role and identification of protein kinase A anchoring proteins in vasopressin-mediated aquaporin-2 translocation

The antidiuretic hormone arginine vasopressin (AVP) regulates water reabsorption in renal principal cells by inducing a cAMP/protein kinase A-dependent translocation of water channels [aquaporin-2 (AQP2)] from intracellular vesicles into the apical cell membranes. Using primary cultured rat inner medullary collecting duct (IMCD) cells, it has been shown that AQP2 translocation in response to AVP stimulation occurs only if protein kinase A (PKA) is anchored to PKA anchoring proteins (AKAPs), which are present in various subcellular compartments. The identity of the AKAPs involved has not yet been elucidated. One potential candidate is a new splice variant of AKAP18, namely AKAP18 delta.

Genetic evidence for a protein kinase A/cubitus interruptus complex that facilitates processing of cubitus interruptus in Drosophila

Hedgehog (Hh) activates a signal transduction pathway regulating Cubitus interruptus (Ci). In the absence of Hh, full-length Ci (Ci-155) is bound in a complex that includes Costal2 (Cos2) and Fused (Fu). Ci-155 is phosphorylated by protein kinase A (PKA), inducing proteolysis to Ci-75, a transcriptional repressor. Hh signaling blocks proteolysis and produces an activated Ci-155 transcriptional activator. The relationship between PKA and the Ci/Cos2/Fu complex is unclear. Here we examine Hh target gene expression caused by mutant forms of PKA regulatory (PKAr) and catalytic (PKAc) subunits and by the PKAc inhibitor PKI(1-31). The mutant PKAr*, defective in binding cAMP, is shown to activate Hh target genes solely through its ability to bind and inhibit endogenous PKAc. Surprisingly, PKAcA75, a catalytically impaired mutant, also activates Hh target genes. To account for this observation, we propose that PKAc phosphorylation targeting Ci-155 for proteolysis is regulated within a complex that includes PKAc and Ci-155 and excludes PKI(1-31). This complex may permit processive phosphorylation of Ci-155 molecules, facilitating their processing to Ci-75.

Muscle-regulated expression and determinants for neuromuscular junctional localization of the mouse RIalpha regulatory subunit of cAMP-dependent protein kinase

In skeletal muscle, transcription of the gene encoding the mouse type Ialpha (RIalpha) subunit of the cAMP-dependent protein kinase is initiated from the alternative noncoding first exons 1a and 1b. Here, we report that activity of the promoter upstream of exon 1a (Pa) depends on two adjacent E boxes (E1 and E2) in NIH 3T3-transfected fibroblasts as well as in intact muscle. Both basal activity and MyoD transactivation of the Pa promoter require binding of the upstream stimulating factors (USF) to E1. E2 binds either an unknown protein in a USF/E1 complex-dependent manner or MyoD. Both E2-bound proteins seem to function as repressors, but with different strengths, of the USF transactivation potential. Previous work has shown localization of the RIalpha protein at the neuromuscular junction. Using DNA injection into muscle of plasmids encoding segments of RIalpha or RIIalpha fused to green fluorescent protein, we demonstrate that anchoring at the neuromuscular junction is specific to RIalpha subunits and requires the amino-terminal residues 1-81. Mutagenesis of Phe-54 to Ala in the full-length RIalpha-green fluorescent protein template abolishes localization, indicating that dimerization of RIalpha is essential for anchoring. Moreover, two other hydrophobic residues, Val-22 and Ile-27, are crucial for localization of RIalpha at the neuromuscular junction. These amino acids are involved in the interaction of the Caenorhabditis elegans type Ialpha homologue R(CE) with AKAP(CE) and for in vitro binding of RIalpha to dual A-kinase anchoring protein 1. We also show enrichment of dual A-kinase anchoring protein 1 at the neuromuscular junction, suggesting that it could be responsible for RIalpha tethering at this site.