Association of the type I regulatory subunit of cAMP-dependent protein kinase with cardiac myocyte sarcolemma

Molecular evolution of A-kinase anchoring protein (AKAP)-7: implications in comparative PKA compartmentalization

Background

A-Kinase Anchoring Proteins (AKAPs) are molecular scaffolding proteins mediating the assembly of multi-protein complexes containing cAMP-dependent protein kinase A (PKA), directing the kinase in discrete subcellular locations. Splice variants from the AKAP7 gene (AKAP15/18) are vital components of neuronal and cardiac phosphatase complexes, ion channels, cardiac Ca²⁺ handling and renal water transport.

Results

Shown in evolutionary analyses, the formation of the AKAP7-RI/RII binding domain (required for AKAP/PKA-R interaction) corresponds to vertebrate-specific gene duplication events in the PKA-RI/RII subunits. Species analyses of AKAP7 splice variants shows the ancestral AKAP7 splice variant is AKAP7α, while the ancestral long form AKAP7 splice variant is AKAP7γ. Multi-species AKAP7 gene alignments, show the recent formation of AKAP7δ occurs with the loss of native AKAP7γ in rats and basal primates. AKAP7 gene alignments and two dimensional Western analyses indicate that AKAP7γ is produced from an internal translation-start site that is present in the AKAP7δ cDNA of mice and humans but absent in rats. Immunofluorescence analysis of AKAP7 protein localization in both rat and mouse heart suggests AKAP7γ replaces AKAP7δ at the cardiac sarcoplasmic reticulum in species other than rat. DNA sequencing identified Human AKAP7δ insertion-deletions (indels) that promote the production of AKAP7γ instead of AKAP7δ.

Conclusions

This AKAP7 molecular evolution study shows that these vital scaffolding proteins developed in ancestral vertebrates and that independent mutations in the AKAP7 genes of rodents and early primates has resulted in the recent formation of AKAP7δ, a splice variant of likely lesser importance in humans than currently described.

A-kinase anchoring proteins: from protein complexes to physiology and disease

Protein scaffold complexes are a key mechanism by which a common signaling pathway can serve many different functions. Sequestering a signaling enzyme to a specific subcellular environment not only ensures that the enzyme is near its relevant targets, but also segregates this activity to prevent indiscriminate phosphorylation of other substrates. One family of diverse, well-studied scaffolding proteins are the A-kinase anchoring proteins (AKAPs). These anchoring proteins form multi-protein complexes that integrate cAMP signaling with other pathways and signaling events. In this review, we focus on recent advances in the elucidation of AKAP function.

Changes in intracellular cAMP reported by a Redistribution assay using a cAMP-dependent protein kinase-green fluorescent protein chimera

We report on a novel method to monitor changes in intracellular cAMP concentration ([cAMP]i) within intact living cells using a chimeric fusion of the catalytic subunit of cAMP-dependent protein kinase to green fluorescent protein (PKAcat-GFP). In stably transfected unstimulated fibroblasts, fusion protein fluorescence is highly concentrated in aggregates throughout the cytoplasm and absent in the nucleus. Elevation of [cAMP]i disperses GFP fluorescence from the cytoplasmic aggregates within minutes. Spot-photobleach measurements show that the rate of exchange of GFP-labeled catalytic subunits at these aggregates increases in proportion to [cAMP]i. For any given stimulus, the response curve for dispersal of GFP fluorescence from aggregates agrees closely with the increase in total [cAMP]i as measured by standard in vitro methods (SPA). The redistribution of fluorescence is completely reversible: reduction of [cAMP]i results in return of fluorescence to the cytoplasmic aggregates. Consistent behaviour of PKAcat-GFP is seen in different cell backgrounds. We demonstrate that PKA Redistribution assays are suitable for measurement of changes in [cAMP]i brought about by both Gs- and Gi-protein-coupled receptor stimulation as well as by inhibition of cAMP phosphodiesterases.

A-kinase anchoring protein targeting of protein kinase A in the heart

There is increasing evidence that subcellular targeting of signaling molecules is an important means of regulating the protein kinase A (PKA) pathway. Subcellular organization of the signaling molecules in the PKA pathway insures that a signal initiated at the receptor level is transferred efficiently to a PKA substrate eliciting some cellular response. This subcellular targeting appears to regulate the function of a highly specialized cell such as the cardiac myocyte. This review focuses on A-kinase anchoring proteins (AKAPs) which are expressed in the heart. It has been determined that, of the approximately 13 different AKAPs expressed in cardiac tissue, several of these are expressed in cardiac myocytes. These AKAPs bind several PKA substrates and some appear to regulate PKA-dependent phosphorylation of these substrates. AKAP tethering of PKA may be essential for efficient regulation of cardiac muscle contraction. The ability of an AKAP to anchor PKA may be altered in the failing heart, thus compromising the ability of the myocyte to respond to stimuli which elicit the PKA pathway.

Conformational differences among solution structures of the type Ialpha, IIalpha and IIbeta protein kinase A regulatory subunit homodimers: role of the linker regions

The regulatory (R) subunits of the cAMP-dependent protein kinase (protein kinase A or PKA) are multi-domain proteins responsible for conferring cAMP-dependence and localizing PKA to specific subcellular locations. There are four isoforms of the R subunit in mammals that are similar in molecular mass and domain organization, but clearly serve different biological functions. Although high-resolution structures are available for the cAMP-binding domains and dimerization/docking domains of two isoforms, there are no high-resolution structures of any of the intact R subunit homodimer isoforms. The results of small-angle X-ray scattering studies presented here indicate that the RIalpha, RIIalpha, and RIIbeta homodimers differ markedly in overall shape, despite extensive sequence homology and similar molecular masses. The RIIalpha and RIIbeta homodimers have very extended, rod-like shapes, whereas the RIalpha homodimer likely has a compact Y-shape. Based on a comparison of the R subunit sequences, we predict that the linker regions are the likely cause of these large differences in shape among the isoforms. In addition, we show that cAMP binding does not cause large conformational changes in type Ialpha or IIalpha R subunit homodimers, suggesting that the activation of PKA by cAMP involves only local conformational changes in the R subunits.

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.

Amino acid variant in the kinase binding domain of dual-specific A kinase-anchoring protein 2: a disease susceptibility polymorphism

The focus of human genetics in recent years has shifted toward identifying genes that are involved in the development of common diseases such as cancer, diabetes, cardiovascular diseases, and Alzheimer's disease. Because many complex diseases are late-onset, the frequencies of disease susceptibility alleles are expected to decrease in the healthy elderly individuals of the population at large because of their contribution to disease morbidity andor mortality. To test this assumption, we compared allele frequencies of 6,500 single-nucleotide polymorphisms (SNPs) located in approximately 5,000 genes between DNA pools of age-stratified healthy, European-American individuals. A SNP that results in an amino acid change from Ile to Val in the dual-specific A kinase-anchoring protein 2 (d-AKAP2) gene, showed the strongest correlation with age. Subsequent analysis of an independent sample indicated that the Val variant was associated with a statistically significant decrease in the length of the electrocardiogram PR interval. The IleVal SNP is located in the A-kinase-binding domain. An in vitro binding assay revealed that the Ile variant bound approximately 3-fold weaker to the protein kinase A (PKA)-RIalpha isoform than the Val variant. This decreased affinity resulted in alterations in the subcellular distribution of the recombinantly expressed PKA-RIalpha isoform. Our study suggests that alterations in PKA-RIalpha subcellular localization caused by variation in d-AKAP2 may have a negative health prognosis in the aging population, which may be related to cardiac dysfunction. Age-stratified samples appear to be useful for screening SNPs to identify functional gene variants that have an impact on health.

Isoform-specific differences between the type Ialpha and IIalpha cyclic AMP-dependent protein kinase anchoring domains revealed by solution NMR

Cyclic AMP dependent protein kinase (PKA) is controlled, in part, by the subcellular localization of the enzyme (). Discovery of dual specificity anchoring proteins (d-AKAPs) indicates that not only is the type II, but also the type I, enzyme localized (). It appears that the type I enzyme is localized in a novel, dynamic fashion as opposed to the apparent static localization of the type II enzyme. Recently, the structure of the dimerization/docking (D/D) domain from the type II enzyme was solved (). This work revealed an X-type four-helix bundle motif with a hydrophobic patch that modulates AKAP interactions. To understand the dynamic versus static localization of PKA, multidimensional NMR techniques were used to investigate the structural features of the type I D/D domain. Our results indicate a conserved helix-turn-helix motif in the type I and type II D/D domains. However, important differences between the two domains are evident in the extreme NH(2) terminus: this region is extended in the type II domain, whereas it is helical in the type I protein. The NH(2)-terminal residues in RIIalpha contain determinants for anchoring, and the orientation and packing of this helical element in the RIalpha structure may have profound consequences in the recognition surface presented to the AKAPs.

Structural characterization of the membrane-associated regulatory subunit of type I cAMP-dependent protein kinase by mass spectrometry: identification of Ser81 as the in vivo phosphorylation site of RIalpha

The mechanism by which the type Ialpha regulatory subunit (RIalpha) of cAMP-dependent protein kinase is localized to cell membranes is unknown. To determine if structural modification of RIalpha is important for membrane association, both beef skeletal muscle cytosolic RI and beef heart membrane-associated RI were characterized by electrospray ionization mass spectrometry. Total sequence coverage was 98% for both the membrane-associated and cytosolic forms of RI after digestion with AspN protease or trypsin. Sequence data indicated that membrane-associated and cytosolic forms of RI were the same RIalpha gene product. A single RIalpha phosphorylation site was identified at Ser81 located near the autoinhibitory domain of both membrane-associated and cytosolic RIalpha. Because both R subunit preparations were 30-40% phosphorylated, this post-translational modification could not be responsible for the membrane compartmentation of the majority of RIalpha. Mass spectrometry also indicated that membrane-associated RIalpha had a higher extent of disulfide bond formation in the amino-terminal dimerization domain. No other structural differences between cytosolic and membrane-associated RIalpha were detected. Consistent with these data, masses of the intact proteins were identical by LCQ mass spectrometry. Lack of detectable structural differences between membrane-associated and cytosolic RIalpha strongly suggests an interaction between RIalpha and anchoring proteins or membrane lipids as more likely mechanisms for explaining RIalpha membrane association in the heart.

Identification of tethering domains for protein kinase A type Ialpha regulatory subunits on sperm fibrous sheath protein FSC1

The fibrous sheath is a unique cytoskeletal structure in the sperm flagellum believed to modulate sperm motility. FSC1 is the major structural protein of the fibrous sheath. The yeast two-hybrid system was used to identify other proteins that contribute to the structure of the fibrous sheath or participate in sperm motility. When FSC1 was used as the bait to screen a mouse testis cDNA library, two clones were isolated encoding the type Ialpha regulatory subunit (RIalpha) of cAMP-dependent protein kinase. Deletion analysis using the yeast two-hybrid system and in vitro binding assays with glutathione S-transferase-FSC1 fusion proteins identified two RIalpha tethering domains on FSC1. A domain located at residues 219-232 (termed domain A) corresponds to the reported tethering domain for a type II regulatory subunit (RII) of cAMP-dependent protein kinase, indicating that this binding domain has dual specificity to RI and RII. Another RIalpha tethering site (termed domain B) at residues 335-344 shows specific binding of RIalpha and had no significant sequence homology with known RII tethering domains. However, helical wheel projection analysis indicates that domain B is likely to form an amphipathic helix, the secondary structure of RII tethering domains of protein kinase A anchoring proteins. This was supported by the finding that site-directed mutagenesis to disrupt the amphipathic helix eliminated RIalpha binding. This is apparently the first report of an RIalpha-specific protein kinase A anchoring protein tethering domain.

The role of NHERF and E3KARP in the cAMP-mediated inhibition of NHE3

NHE3 is the apically located Na+/H+ exchanger in the gut and in the renal proximal tubule. Acute inhibition of this transporter by cAMP requires the presence of either of two NHE3-associated proteins, NHERF or E3KARP. It has been suggested that these proteins either directly regulate NHE3 activity after being phosphorylated by protein kinase A (PKA) or that they may serve as adapters that localize PKA near NHE3. We studied the role of NHERF and E3KARP in opossum kidney cells, which endogenously express NHE3, NHERF, and ezrin and display cAMP-dependent inhibition of NHE3. In vivo phosphorylation studies showed that NHERF is a phosphoprotein under basal conditions, but does not change its phosphorylation state after 8-bromo-cAMP treatment, and that E3KARP is not phosphorylated at all. Co-immunoprecipitation showed that NHERF and E3KARP bind both NHE3 and ezrin. Using cAMP analogs it was demonstrated that NHE3 activity, measured as sodium-dependent recovery of the intracellular pH after intracellular acidification, is inhibited by PKA type II. Because others have shown that ezrin binds PKA type II and that NHE3 is phosphorylated by PKA we suggest that NHERF and E3KARP are adapters that link NHE3 to ezrin, thereby localizing PKA near NHE3 to allow NHE3 phosphorylation.

cAMP-mediated signal transduction and sarcoplasmic reticulum function in heart failure

There is evidence that the effects of beta-adrenergic receptor agonists on myocardial contractility result principally from the phosphorylation of phospholamban by cAMP-dependent protein kinase and the consequent deinhibition of SERCA2 activity and stimulation of sarcoplasmic reticulum Ca2+ transport. An impairment in beta-adrenergic receptor-stimulated cAMP generation, attributable to down-regulation of beta 1-adrenergic receptors and increased activity of G alpha i and G protein-coupled receptor kinase, has long been recognized in failing human myocardium. This impairment is associated with a compartment-specific decrease in sarcoplasmic reticulum cAMP content that may selectively reduce phospholamban phosphorylation. Published and preliminary results indicate that two plausible explanations for this compartment-specific decrease--a reduction in sarcoplasmic reticulum-associated cAMP-dependent protein kinase or an increase in sarcoplasmic reticulum-associated cAMP phosphodiesterase--are unlikely. Instead, there is reason to believe that the selective reduction in beta 1-adrenergic receptor density in failing myocardium is causally related to this compartment-specific decrease in cAMP content through an as-yet-undetermined mechanism. The fact that the modulation of SERCA2 activity by phospholamban is preserved in failing human myocardium offers an opportunity for improvement in the therapy of heart failure.

Compartmentation of the type I regulatory subunit of cAMP-dependent protein kinase in cardiac ventricular muscle

The species-dependent compartmentation of type I cAMP-dependent protein kinase (PKA I) and its dissociated regulatory subunit (RI) was examined in the heart by biochemical and immunohistochemical means. PKA I and RI were resolved from type II cAMP-dependent protein kinase and its regulatory subunit by DEAE-Sephacel chromatography of the supernatant and Triton X-100 soluble particulate fractions of heart homogenates. The relative amounts of holoenzymes and subunits were determined by cAMP-binding, protein kinase, 8-N3-[32P]cAMP photoaffinity labeling, and Western blot assays. Rat, rabbit, and guinea pig hearts all contained PKA I to varying degrees, but only in the supernatant fractions. Significant amounts of dissociated RI were found in the supernatant fractions, and to a lesser extent the particulate fractions, of these species. In contrast, though no PKA I was detected in the supernatant or particulate fractions of pig and beef heart, half of the cAMP-binding activity in the particulate fraction was attributed to RI. The results suggest that RI may associate with membrane fractions when it is not associated with the PKA catalytic subunit. Immunohistochemical studies of tissue sections from pig, beef, and rat cardiac ventricle using antibodies directed against RI also revealed species-dependent localization of RI. Cardiac myocyte intercalated discs were stained in pig and beef sections with additional sarcolemmal staining in beef sections. Rat ventricle, which contained large amounts of supernatant PKA I, showed nuclear staining. The localization of RI to cardiac myocyte intercalated discs and sarcolemma in certain species suggests a role(s) for this subunit in mediating cAMP-regulated events in these regions.