Identification of a 15-kDa cAMP-dependent protein kinase-anchoring protein associated with skeletal muscle L-type calcium channels

Identification of a novel A-kinase anchoring protein 18 isoform and evidence for its role in the vasopressin-induced aquaporin-2 shuttle in renal principal cells

Arginine vasopressin (AVP) increases the water permeability of renal collecting duct principal cells by inducing the fusion of vesicles containing the water channel aquaporin-2 (AQP2) with the plasma membrane (AQP2 shuttle). This event is initiated by activation of vasopressin V2 receptors, followed by an elevation of cAMP and the activation of protein kinase A (PKA). The tethering of PKA to subcellular compartments by protein kinase A anchoring proteins (AKAPs) is a prerequisite for the AQP2 shuttle. During the search for AKAP(s) involved in the shuttle, a new splice variant of AKAP18, AKAP18delta, was identified. AKAP18delta functions as an AKAP in vitro and in vivo. In the kidney, it is mainly expressed in principal cells of the inner medullary collecting duct, closely resembling the distribution of AQP2. It is present in both the soluble and particulate fractions derived from renal inner medullary tissue. Within the particulate fraction, AKAP18delta was identified on the same intracellular vesicles as AQP2 and PKA. AVP not only recruited AQP2, but also AKAP18delta to the plasma membrane. The elevation of cAMP caused the dissociation of AKAP18delta and PKA. The data suggest that AKAP18delta is involved in the AQP2 shuttle.

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.

AKAP79 increases the functional expression of skeletal muscle Ca2+ channels in Xenopus oocytes

The actions of the kinase A anchoring protein, AKAP79, a key element in the regulation of the cardiac L-type Ca2+ channel, were assessed on skeletal muscle Ca2+ channels expressed in Xenopus oocytes. The channels were reconstituted by expressing the pore forming alpha1s subunit and its accessory subunits, alpha2-delta, beta, and gamma. We report, for the first time, that peak Ca2+ channel currents are greatly increased (3.5-fold) by AKAP79 when co-expressed with the truncated form of the alpha1s subunit. Immunoblots revealed that the increase in current amplitude is not accompanied by a corresponding increase in the membrane levels of the alpha1s subunit. This suggests that AKAP79 does not increase the trafficking of the channel. In addition, we show that the transcript of AKAP150, the rat ortholog of the human AKAP79, is expressed in rat skeletal muscle and propose that AKAP79/150 modulates Ca2+ channel function.

Beta-adrenergic regulation requires direct anchoring of PKA to cardiac CaV1.2 channels via a leucine zipper interaction with A kinase-anchoring protein 15

Activation of beta-adrenergic receptors and consequent phosphorylation by cAMP-dependent protein kinase A (PKA) greatly increases the L-type Ca2+ current through CaV1.2 channels in isolated cardiac myocytes. A kinase-anchoring protein 15 (AKAP15) coimmunoprecipitates with CaV1.2 channels isolated from rat heart membrane extracts and transfected cells, and it colocalizes with CaV1.2 channels and PKA in the transverse tubules of isolated ventricular myocytes. Site-directed mutagenesis studies reveal that AKAP15 directly interacts with the distal C terminus of the cardiac CaV1.2 channel via a leucine zipper-like motif. Disruption of PKA anchoring to CaV1.2 channels via AKAP15 using competing peptides markedly inhibits the beta-adrenergic regulation of CaV1.2 channels via the PKA pathway in ventricular myocytes. These results identify a conserved leucine zipper motif in the C terminus of the CaV1 family of Ca2+ channels that directly anchors an AKAP15-PKA signaling complex to ensure rapid and efficient regulation of L-type Ca2+ currents in response to beta-adrenergic stimulation and local increases in cAMP.

Targeting of protein kinase A by muscle A kinase-anchoring protein (mAKAP) regulates phosphorylation and function of the skeletal muscle ryanodine receptor

Protein kinase A anchoring proteins (AKAPs) tether cAMP-dependent protein kinase (PKA) to specific subcellular locations. The muscle AKAP, mAKAP, co-localizes with the sarcoplasmic reticulum Ca2+ release channel or ryanodine receptor (RyR). The purpose of this study was to determine whether anchoring of PKA by mAKAP regulates RyR function. Either mAKAP or mAKAP-P, which is unable to anchor PKA, was expressed in CHO cells stably expressing the skeletal muscle isoform of RyR (CHO-RyR1). Immunoelectron microscopy showed that mAKAP co-localized with RyR1 in disrupted skeletal muscle. Following the addition of 10 microm forskolin to activate adenylyl cyclase, RyR1 phosphorylation in CHO-RyR1 cells expressing mAKAP increased by 42.4 +/- 6.6% (n = 4) compared with cells expressing mAKAP-P. Forskolin treatment alone did not increase the amplitude of the cytosolic Ca2+ transient in CHO-RyR1 cells expressing mAKAP or mAKAP-P; however, forskolin plus 10 mm caffeine elicited a cytosolic Ca2+ transient, the amplitude of which increased by 22% (p < 0.05) in RyR1/mAKAP-expressing cells compared with RyR1/mAKAP-P-expressing cells. Therefore, localization of PKA by mAKAP at RyR1 increases both PKA-dependent RyR phosphorylation as well as efflux of Ca2+ through the RyR. Therefore, RyR1 function is regulated by mAKAP targeting of PKA, implying an important functional role for PKA phosphorylation of RyR in skeletal muscle.

AKAP7gamma is a nuclear RI-binding AKAP

Spatial regulation of protein kinase A (PKA) is accomplished by its sequestration via A-kinase anchor proteins (AKAPs). PKA activity is critical for mammalian oocyte development, suggesting that PKA must be appropriately positioned in these large cells. A screen for AKAPs in oocytes identified AKAP7gamma, an AKAP originally found in pancreas. Yeast two-hybrid analysis and co-immunoprecipitation studies showed that AKAP7gamma bound the type I PKA regulatory subunit (RI) and that the RI-binding domain overlapped the previously identified type II PKA regulatory subunit (RII) binding domain. Overexpressed AKAP7gamma localized to the nuclei of HEK 293 cells via a nuclear localization signal. In addition, endogenous AKAP7gamma protein was found in both the nucleus and cytoplasm of oocytes. This work identifies AKAP7gamma as the first nuclear AKAP to bind RI and suggests that AKAP7gamma may be responsible for positioning PKA via RI and/or RII to regulate PKA-mediated gene transcription in both somatic cells and oocytes.

Characterization of an A-kinase anchoring protein in human ciliary axonemes

Although protein kinase A (PKA) activation is known to increase ciliary beat frequency in humans the molecular mechanisms involved are unknown. We demonstrate that PKA is associated with ciliary axonemes where it specifically phosphorylates a 23-kDa protein. Because PKA is often localized to subcellular compartments in proximity to its substrate(s) via interactions with A-kinase-anchoring proteins (AKAPs), we investigated whether an AKAP was also associated with ciliary axonemes. This study has identified a novel 28 kDa AKAP (AKAP28)that is highly enriched in airway axonemes. The mRNA for AKAP28 is up-regulated as primary airway cells differentiate and is specifically expressed in tissues containing cilia and/or flagella. Additionally, both Western blot and immunostaining data show that AKAP28 is enriched in airway cilia. These data demonstrate that we have identified the first human axonemal AKAP, a protein that likely plays a role in the signaling necessary for efficient modulation of ciliary beat frequency.

Protein phosphorylation activates the guard cell Ca2+ channel and is a prerequisite for gating by abscisic acid

Protein phosphorylation and cytosolic-free [Ca2+] ([Ca2+]i) contribute to signalling cascades evoked by the water-stress hormone abscisic acid (ABA) that lead to stomatal closure in higher-plant leaves. ABA activates an inward-rectifying Ca2+ channel at the plasma membrane of stomatal guard cells, promoting Ca2+ entry by shifting the voltage-sensitivity of the channels. Because many of these effects could be mediated by kinase/phosphatase action at the membrane, we examined a role for protein (de-)phosphorylation in plasma membrane patches from Vicia guard cells. Ca2+ channel activity decayed rapidly in excised patches, and recovered on adding ATP (K1/2, 1.3 +/- 0.7 mm) but not the non-hydrolyzable analog ATPgammaS. ABA activation of the channel required the presence of ATP and like ABA, the 1/2 A-type protein phosphatase antagonists okadaic acid (OA) and calyculin A (CA) enhanced Ca2+ channel activity by increasing the open probability and number of active channels. Neither ATP nor the antagonists affected the mean open lifetime of the channel, suggesting an action through changes in closed lifetime distributions. Like ABA, OA and CA shifted the voltage-sensitivities of the Ca2+ current and [Ca2+]i increases in intact guard cells towards positive voltages. OA and CA also augmented the [Ca2+]i rise evoked by hyperpolarization and delayed its recovery. These results demonstrate a membrane-delimited interaction between 1/2 A-type protein phosphatase(s) and the Ca2+ channel or associated proteins, and they are consistent with a role for protein (de-)phosphorylation in ABA signalling mediated directly through Ca2+ channel gating that leads to [Ca2+]i increases in the guard cells.

Molecular mechanism of convergent regulation of brain Na(+) channels by protein kinase C and protein kinase A anchored to AKAP-15

Activation of D1-like dopamine (DA) receptors reduces peak Na(+) current in hippocampal neurons voltage-dependent in a manner via phosphorylation of the alpha subunit. This modulation is dependent upon activation of cAMP-dependent protein kinase (PKA) and requires phosphorylation of serine 573 (S573) in the intracellular loop connecting homologous domains I and II (L(I-II)) by PKA anchored to A kinase anchoring protein-15 (AKAP-15). Activation of protein kinase C (PKC) also reduces peak Na(+) currents and enhances the strength of the PKA modulatory pathway. Here we probe the molecular mechanism responsible for the convergent effects of PKA and PKC on brain Na(v)1.2a channels. Analysis of the interaction of AKAP-15 with the intracellular loops of the Na(v)1.2a channel shows that it binds to L(I-II), thereby targeting PKA directly to its sites of phosphorylation on the Na(+) channel by specific protein-protein interactions. Mutagenesis and expression experiments indicate that reduction of peak Na(+) current by PKC requires S554 and S573 in L(I-II) in addition to S1506 in the inactivation gate. In addition, PKC-dependent phosphorylation of S576 in L(I-II) is necessary for enhancement of PKA modulation of brain Na(+) channels. When S576 is phosphorylated by PKC, the increase in modulation by PKA activation requires phosphorylation of S687 in L(I-II). Thus, the maximal modulation of these Na(+) channels by concurrent activation of PKA and PKC requires phosphorylation at four distinct sites in L(I-II): S554, S573, S576, and S687. This convergent regulation provides a novel mechanism by which information from multiple signaling pathways may be integrated at the cellular level in the hippocampus and throughout the central nervous system.

Clues to calcineurin function in mammalian fast-twitch muscle

It is believed that brief, high amplitude Ca2+ transients, as found in fast-twitch muscles, are not sufficient to activate the calcineurin (Cn)-dependent signaling pathway involved in regulation of slow myosin and slow sarcoplasmic reticulum Ca2+-ATPase genes (Olson and Williams, Cell 101: 689-692, 2000). The results reported here try to fill the gap between this molecular knowledge, and the still fragmentary pieces of information on a possible different role of calcineurin in the same type of muscles. In the present work calcineurin was determined immunocytochemically by labeling fast- and slow-twitch fibers of representative rabbit muscles with anti-CnB antibodies, and was assessed by western blotting of isolated subcellular fractions. Calcineurin was found to be largely soluble and to be constitutively overexpressed in fast muscle as CnAalpha and CnAbeta isoforms, the latter appearing to be predominant. Particulate calcineurin was not only associated with myofibrils but also with membranes of various origins. Fluorescence microscopy showed that calcineurin was distributed in the same pattern with respect to sarcomeres in both types of fibers, and formed punctate dots spanning the I-Z-I region, rather than being exclusively located at the Z-line, a disposition described for cardiomyocytes (Frey et al., Proc Natl Acad Sci USA 97: 14,632-14,637, 2000). From knowledge that, in mammalian skeletal muscle fibers, junctional triads are located at the A-I band boundary, we explored the distribution of calcineurin between triadic components, after having verified that it was present in very low amounts in dystrophin-enriched sarcolemmal membranes. Our results demonstrate that a small but significant proportion of calcineurin coenriched with transverse tubules (TT), and copurified with the DHPR and with DHPR-associated PKA-AKAP15/18, thus suggesting that it is assembled as a multiprotein complex in the junctional membrane domain of TT. The membrane specificity of this association is further corroborated by the negative evidence for the presence of calcineurin in SR terminal cisternae. Calcineurin was separated from the DHPR and isolated as a AKAP15/18 subcomplex, including beta2 adrenergic receptor, in addition to PKA and calcineurin, following equilibrium centrifugation of detergent extracts on a linear sucrose gradient. We show that the alpha1 subunit skeletal isoform of the DHPR, is a substrate for calcineurin dephosphorylation, after previous phosphorylation by endogenous PKA.

Effects of the PKA inhibitor H-89 on excitation-contraction coupling in skinned and intact skeletal muscle fibres

This study investigated the effects of the protein kinase A (PKA) inhibitor, H-89, in mechanically-skinned muscle fibres and intact muscle fibres, in order to determine whether PKA phosphorylation is essential for normal excitation-contraction (E-C) coupling. In skinned EDL fibres of the rat, force responses to depolarization (by ion substitution) were inhibited only slightly by 10 microM H-89, a concentration more than sufficient to fully inhibit PKA. Staurosporine (1 microM), a potent non-specific kinase inhibitor, also had little if any effect on depolarization-induced responses. At 1-2 microM, H-89 significantly slowed the repriming rate in rat skinned fibres, most likely due to it deleteriously affecting the T-system potential. With 100 microM H-89, the force response to depolarization by ion substitution was completely abolished. This inhibitory effect was reversed by washout of H-89 and was not due to block of the Ca2+ release channel in the sarcoplasmic reticulum (SR). In intact single fibres of the flexor digitorum longus (FDB) muscle of the mouse, 1-3 microM H-89 had no noticeable effect on action-potential-mediated Ca2+ transients. Higher concentrations (4-10 microM) caused Ca2+ transient failure in fibres stimulated at 20 Hz in a manner indicative of action-potential failure. At 10-100 microM, H-89 also inhibited net Ca2+ uptake by the SR and affected the Ca2+-sensitivity of the contractile apparatus in rat skinned fibres. All such effects were proportionately greater in toad muscle fibres. These results do not support the hypothesis that phosphorylation is essential for the Ca2+ release channel to open in response to voltage-sensor activation in skeletal muscle fibres.

A novel leucine zipper targets AKAP15 and cyclic AMP-dependent protein kinase to the C terminus of the skeletal muscle Ca2+ channel and modulates its function

In skeletal muscle, voltage-dependent potentiation of L-type Ca(2+) channel (Ca(V)1.1) activity requires phosphorylation by cyclic AMP-dependent protein kinase (PKA) anchored via an A kinase-anchoring protein (AKAP15). However, the mechanism by which AKAP15 targets PKA to L-type Ca(2+) channels has not been elucidated. Here we report that AKAP15 directly interacts with the C-terminal domain of the alpha(1) subunit of Ca(V)1.1 via a leucine zipper (LZ) motif. Disruption of the LZ interaction effectively inhibits voltage-dependent potentiation of L-type Ca(2+) channels in skeletal muscle cells. Our results reveal a novel mechanism whereby anchoring of PKA to Ca(2+) channels via LZ interactions ensures rapid and efficient phosphorylation of Ca(2+) channels in response to local signals such as cAMP and depolarization.

Electrical muscle activity pattern and transcriptional and posttranscriptional mechanisms regulate PKA subunit expression in rat skeletal muscle

We have examined protein kinase A (PKA) subunit expression in adult rat skeletal muscles. Northern blots identified PKA catalytic alpha and regulatory (R) I alpha and RII alpha subunits as the major subunits expressed in slowly contracting soleus (SOL) and rapidly contracting extensor digitorum longus (EDL) muscles. In addition, the steady-state RNA levels of PKA subunit mRNAs and activities of RI alpha and RII alpha promoters are similar in SOL and EDL. These data indicate that posttranscriptional mechanisms account for the twofold differences in PKA subunit protein levels reported earlier. Electrical stimulation of denervated SOL with an EDL-like activity pattern (fast pattern) transformed SOL into an EDL-like muscle with regard to PKA protein levels. These experiments suggest that the posttranscriptional regulation is activity pattern-dependent. Denervation specifically increased RI alpha promoter activity and RI alpha mRNA levels in SOL and EDL. Further experiments indicated that the RI alpha 1a upstream sequences were activated following denervation. Direct electrical stimulation prevented the rise in RI alpha mRNA levels following denervation, demonstrating that electrical muscle activity regulates transcription.

Characterization of RII(beta) and D-AKAP1 in differentiated adipocytes

A-kinase anchoring proteins (AKAPs) have been proposed to regulate cAMP-dependent signaling in the cell by targeting RII subunits of protein kinase A (PKA) to specific subcellular compartments. RII(beta) is the predominant PKA subtype in adipose tissue. In gel overlay assays of C3H/10T1/2 adipocytes and adipose tissue, RII(beta) bound to several proteins including a prominent 132-kDa band, which was strongly induced upon differentiation of C3H/10T1/2 cells into adipocytes. Immunoblotting and nuclease protection analysis of C3H/10T1/2 cellular extracts identified this band as D-AKAP1/S-AKAP84, a putative AKAP. Immunocytochemical analysis of C3H/10T1/2 adipocytes revealed that most of D-AKAP1/S-AKAP84, but not RII(beta), was colocalized with a mitochondrial-selective dye, MitoTracker red. These findings were further confirmed in studies where D-AKAP1/ S-AKAP84, but not RII(beta), were localized in purified mitochondria made from C3H/10T1/2 adipocytes. Moreover, D-AKAP1, which is upregulated after differentiation, did not recruit RII(beta) to membrane fractions enriched in mitochondria. These results demonstrate that D-AKAP1/S-AKAP84 does not interact with PKA in differentiated C3H/10T1/2 adipocytes under the conditions tested.

Critical role of cAMP-GEFII--Rim2 complex in incretin-potentiated insulin secretion

Incretins such as glucagon-like peptide-1 and gastric inhibitory polypeptide/glucose-dependent insulinotropic peptide are known to potentiate insulin secretion mainly through a cAMP/protein kinase A (PKA) signaling pathway in pancreatic beta-cells, but the mechanism is not clear. We recently found that the cAMP-binding protein cAMP-GEFII (or Epac 2), interacting with Rim2, a target of the small G protein Rab3, mediates cAMP-dependent, PKA-independent exocytosis in a reconstituted system. In the present study, we investigated the role of the cAMP-GEFII--Rim2 pathway in incretin-potentiated insulin secretion in native pancreatic beta-cells. Treatment of pancreatic islets with antisense oligodeoxynucleotides (ODNs) against cAMP-GEFII alone or with the PKA inhibitor H-89 alone inhibited incretin-potentiated insulin secretion approximately 50%, while a combination of antisense ODNs and H-89 inhibited the secretion approximately 80-90%. The effect of cAMP-GEFII on insulin secretion is mediated by Rim2 and depends on intracellular calcium as well as on cAMP. Treatment of the islets with antisense ODNs attenuated both the first and second phases of insulin secretion potentiated by the cAMP analog 8-bromo-cAMP. These results indicate that the PKA-independent mechanism involving the cAMP-GEFII--Rim2 pathway is critical in the potentiation of insulin secretion by incretins.

Regulation of membrane targeting of the G protein-coupled receptor kinase 2 by protein kinase A and its anchoring protein AKAP79

The beta2 adrenergic receptor (beta2AR) undergoes desensitization by a process involving its phosphorylation by both protein kinase A (PKA) and G protein-coupled receptor kinases (GRKs). The protein kinase A-anchoring protein AKAP79 influences beta2AR phosphorylation by complexing PKA with the receptor at the membrane. Here we show that AKAP79 also regulates the ability of GRK2 to phosphorylate agonist-occupied receptors. In human embryonic kidney 293 cells, overexpression of AKAP79 enhances agonist-induced phosphorylation of both the beta2AR and a mutant of the receptor that cannot be phosphorylated by PKA (beta2AR/PKA-). Mutants of AKAP79 that do not bind PKA or target to the beta2AR markedly inhibit phosphorylation of beta2AR/PKA-. We show that PKA directly phosphorylates GRK2 on serine 685. This modification increases Gbetagamma subunit binding to GRK2 and thus enhances the ability of the kinase to translocate to the membrane and phosphorylate the receptor. Abrogation of the phosphorylation of serine 685 on GRK2 by mutagenesis (S685A) or by expression of a dominant negative AKAP79 mutant reduces GRK2-mediated translocation to beta2AR and phosphorylation of agonist-occupied beta2AR, thus reducing subsequent receptor internalization. Agonist-stimulated PKA-mediated phosphorylation of GRK2 may represent a mechanism for enhancing receptor phosphorylation and desensitization.

AKAP proteins anchor cAMP-dependent protein kinase to KvLQT1/IsK channel complex

In cardiac myocytes, the slow component of the delayed rectifier K(+) current (I(Ks)) is regulated by cAMP. Elevated cAMP increases I(Ks) amplitude, slows its deactivation kinetics, and shifts its activation curve. At the molecular level, I(Ks) channels are composed of KvLQT1/IsK complexes. In a variety of mammalian heterologous expression systems maintained at physiological temperature, we explored cAMP regulation of recombinant KvLQT1/IsK complexes. In these systems, KvLQT1/IsK complexes were totally insensitive to cAMP regulation. cAMP regulation was not restored by coexpression with the dominant negative isoform of KvLQT1 or with the cystic fibrosis transmembrane regulator. In contrast, coexpression of the neuronal A kinase anchoring protein (AKAP)79, a fragment of a cardiac AKAP (mAKAP), or cardiac AKAP15/18 restored cAMP regulation of KvLQT1/IsK complexes inasmuch as cAMP stimulation increased the I(Ks) amplitude, increased its deactivation time constant, and negatively shifted its activation curve. However, in cells expressing an AKAP, the effects of cAMP stimulation on the I(Ks) amplitude remained modest compared with those previously reported in cardiac myocytes. The effects of cAMP stimulation were fully prevented by including the Ht31 peptide (a global disruptor of protein kinase A anchoring) in the intracellular medium. We concluded that cAMP regulation of I(Ks) requires protein kinase A anchoring by AKAPs, which therefore participate with the channel protein complex underlying I(Ks).

The biological functions of A-kinase anchor proteins

cAMP-dependent protein kinase is targeted to discrete subcellular locations by a family of specific anchor proteins (A-kinase anchor proteins, AKAPs). Localization recruits protein kinase A (PKA) holoenzyme close to its substrate/effector proteins, directing and amplifying the biological effects of cAMP signaling.AKAPs include two conserved structural modules: (i) a targeting domain that serves as a scaffold and membrane anchor; and (ii) a tethering domain that interacts with PKA regulatory subunits. Alternative splicing can shuffle targeting and tethering domains to generate a variety of AKAPs with different targeting specificity. Although AKAPs have been identified on the basis of their interaction with PKA, they also bind other signaling molecules, mainly phosphatases and kinases, that regulate AKAP targeting and activate other signal transduction pathways. We suggest that AKAP forms a "transduceosome" by acting as an autonomous multivalent scaffold that assembles and integrates signals derived from multiple pathways. The transduceosome amplifies cAMP and other signals locally and, by stabilizing and reducing the basal activity of PKA, it also exerts long-distance effects. The AKAP transduceosome thus optimizes the amplitude and the signal/noise ratio of cAMP-PKA stimuli travelling from the membrane to the nucleus and other subcellular compartments.

AKAP-mediated targeting of protein kinase a regulates contractility in cardiac myocytes

Compartmentalization of cAMP-dependent protein kinase A (PKA) by A-kinase anchoring proteins (AKAPs) targets PKA to distinct subcellular locations in many cell types. However, the question of whether AKAP-mediated PKA anchoring in the heart regulates cardiac contractile function has not been addressed. We disrupted AKAP-mediated PKA anchoring in cardiac myocytes by introducing, via adenovirus-mediated gene transfer, Ht31, a peptide that binds the PKA regulatory subunit type II (RII) with high affinity. This peptide competes with endogenous AKAPs for RII binding. Ht31P (a proline-substituted derivative), which does not bind RII, was used as a negative control. We then investigated the effects of Ht31 expression on RII distribution, Ca(2+) cycling, cell shortening, and PKA-dependent substrate phosphorylation. By confocal microscopy, we showed redistribution of RII from the perinuclear region and from periodic transverse striations in Ht31P-expressing cells to a diffuse cytosolic localization in Ht31-expressing cells. In the presence of 10 nmol/L isoproterenol, Ht31-expressing myocytes displayed an increased rate and amplitude of cell shortening and relaxation compared with control cells (uninfected and Ht31P-expressing myocytes); with isoproterenol stimulation we observed decreased time to 90% decline in Ca(2+) but no significant difference between Ht31-expressing and control cells in the rate of Ca(2+) cycling or amplitude of the Ca(2+) transient. The increase in PKA-dependent phosphorylation of troponin I and myosin binding protein C on isoproterenol stimulation was significantly reduced in Ht31-expressing cells compared with controls. Our results demonstrate that, in response to beta-adrenergic stimulation, cardiomyocyte function and substrate phosphorylation by PKA is regulated by targeting of PKA by AKAPs.

Regulation of cardiac L-type calcium channels by protein kinase A and protein kinase C

Voltage-dependent L-type Ca(2+) channels are multisubunit transmembrane proteins, which allow the influx of Ca(2+) (I:(Ca)) essential for normal excitability and excitation-contraction coupling in cardiac myocytes. A variety of different receptors and signaling pathways provide dynamic regulation of I:(Ca) in the intact heart. The present review focuses on recent evidence describing the molecular details of regulation of L-type Ca(2+) channels by protein kinase A (PKA) and protein kinase C (PKC) pathways. Multiple G protein-coupled receptors act through cAMP/PKA pathways to regulate L-type channels. ss-Adrenergic receptor stimulation results in a marked increase in I:(Ca), which is mediated by a cAMP/PKA pathway. Growing evidence points to an important role of localized signaling complexes involved in the PKA-mediated regulation of I:(Ca), including A-kinase anchor proteins and binding of phosphatase PP2a to the carboxyl terminus of the alpha(1C) (Ca(v)1.2) subunit. Both alpha(1C) and ss(2a) subunits of the channel are substrates for PKA in vivo. The regulation of L-type Ca(2+) channels by Gq-linked receptors and associated PKC activation is complex, with both stimulation and inhibition of I:(Ca) being observed. The amino terminus of the alpha(1C) subunit is critically involved in PKC regulation. Crosstalk between PKA and PKC pathways occurs in the modulation of I:(Ca). Ultimately, precise regulation of I:(Ca) is needed for normal cardiac function, and alterations in these regulatory pathways may prove important in heart disease.

Regulation of Na(+)-K(+)-ATPase by cAMP-dependent protein kinase anchored on membrane via its anchoring protein

Na(+)-K(+)- ATPase alpha-subunits in basolateral membrane vesicles (BLMVs) purified from rat parotid glands were (32)P-labeled within 5 s by incubation with [gamma-(32)P]ATP at 37 degrees C in the presence of cAMP, but no labeling occurred without cAMP. Phosphorylation of Na(+)-K(+)-ATPase was associated with a decrease in its activity. This alpha-subunit phosphorylation disappeared when BLMVs were briefly incubated with cAMP and subsequent washing before the incubation with [gamma-(32)P]ATP, indicating that catalytic subunit of protein kinase A (PKA) associated to BLMVs via binding with its RII regulatory subunit anchored on the membrane. In the absence of cAMP, a PKA catalytic subunit readily reassociated with the membrane-bound RII subunit. HT-31 peptide inhibited the Na(+)-K(+)-ATPase phosphorylation by membrane-bound endogenous PKA, indicating an involvement of A-kinase anchoring protein (AKAP). AKAP-150 protein in BLMVs was shown by immunoblotting and an RII overlay assay and was coimmunoprecipitated by anti-RII antibody. These results show that Na(+)-K(+)-ATPase of rat parotid gland acinar cells is regulated in vivo by membrane-anchored PKA via AKAP rather than by free cytosolic PKA.

SPECs, small binding proteins for Cdc42

The Rho GTPase, Cdc42, regulates a wide variety of cellular activities including actin polymerization, focal complex assembly, and kinase signaling. We have identified a new family of very small Cdc42-binding proteins, designated SPECs (for Small Protein Effector of Cdc42), that modulates these regulatory activities. The two human members, SPEC1 and SPEC2, encode proteins of 79 and 84 amino acids, respectively. Both contain a conserved N-terminal region and a centrally located CRIB (Cdc42/Rac Interactive Binding) domain. Using a yeast two-hybrid system, we found that both SPECs interact strongly with Cdc42, weakly with Rac1, and not at all with RhoA. Transfection analysis revealed that SPEC1 inhibited Cdc42-induced c-Jun N-terminal kinase (JNK) activation in COS1 cells in a manner that required an intact CRIB domain. Immunofluorescence experiments in NIH-3T3 fibroblasts demonstrated that both SPEC1 and SPEC2 showed a cortical localization and induced the formation of cell surface membrane blebs, which was not dependent on Cdc42 activity. Cotransfection experiments demonstrated that SPEC1 altered Cdc42-induced cell shape changes both in COS1 cells and in NIH-3T3 fibroblasts and that this alteration required an intact CRIB domain. These results suggest that SPECs act as novel scaffold molecules to coordinate and/or mediate Cdc42 signaling activities.

Ras mediates the cAMP-dependent activation of extracellular signal-regulated kinases (ERKs) in melanocytes

In melanocytes and melanoma cells, cAMP activates extracellular signal-regulated kinases (ERKs) and MEK-1 by an unknown mechanism. We demonstrate that B-Raf is activated by cAMP in melanocytes. A dominant-negative mutant of B-Raf, but not of Raf-1, blocked the cAMP-induced activation of ERK, indicating that B-Raf is the MEK-1 upstream regulator mediating this cAMP effect. Studies using Clostridium sordelii lethal toxin and Clostridium difficile toxin B have suggested that Rap-1 or Ras might transduce cAMP action. We show that Ras, but not Rap-1, is activated cell-specifically and mediates the cAMP-dependent activation of ERKs, while Rap-1 is not involved in this process in melanocytes. Our results suggest a novel, cell-specific mechanism involving Ras small GTPase and B-Raf kinase as mediators of ERK activation by cAMP. Also, in melanocytes, Ras or ERK activation by cAMP is not mediated through protein kinase A activation. Neither the Ras exchange factor, Son of sevenless (SOS), nor the cAMP-responsive Rap-1 exchange factor, Epac, participate in the cAMP-dependent activation of Ras. These findings suggest the existence of a melanocyte-specific Ras exchange factor directly regulated by cAMP.

A-kinase anchoring proteins: protein kinase A and beyond

Compartmentalization of kinases and phosphatases is a key determinant in the specificity of second messenger mediated signaling events. Localization of the cAMP-dependent protein kinase (PKA) and other signaling enzymes is mediated by interaction with A-kinase anchoring proteins (AKAPs). In the past year there have been many advances in our understanding of AKAPs, particularly in the field of the functional consequences of PKA anchoring.

Effects of protein kinase A inhibition on rat diaphragm force generation

Although protein kinases are known to play a role in modulating a variety of intracellular functions, the direct effect of inhibition of these enzymes on skeletal muscle force production has not been studied. The purpose of the present study was to examine this issue by determining the effects produced on diaphragm force generation by two protein kinase inhibitors: (a) H7, an inhibitor of both cAMP-dependent protein kinase (PKA) and of protein kinase C, and (b) H89, a selective inhibitor of PKA. Experiments (n=15) were performed using isolated, arterially perfused, electrically stimulated rat diaphragms. Perfusate temperature was adjusted to maintain muscle temperature at 27 degrees C and arterial pressure was kept at 150 Torr. Animals were divided into three groups: (a) a control group perfused with Krebs-Henselheit solution equilibrated with 95% O(2)/5% CO(2), (b) a group in which H7 (2 microM) was added to the perfusate, and (c) a group perfused with solution containing H89 (4 microM). In all three groups, we assessed diaphragm twitch kinetics, force-frequency relationships and in vitro fatiguability. We found that both H7 and H89 administration slowed twitch relaxation, augmented force generation in response to low frequency stimulation, and increased the rate of development of fatigue. Specifically, for control, H7 and H89 groups, respectively, we found: (a) 1/2 relaxation time averaged 64+/-2 S.E.M., 87+/-6 and 90+/-2 ms, P<0. 003, (b) force production during 10-Hz stimulation averaged 12.6+/-1. 1, 20.1+/-2.3, and 20.3+/-2.1 N/cm(2), P<0.035, and (c) force fell to 14.3+/-2.0, 9.5+/-0.5 and 8.7+/-0.2% of its initial value after 20 min of fatiguing stimulation, P<0.035. These data show that it is possible to produce large increases in low frequency skeletal muscle force generation by directly inhibiting PKA. We speculate that it may be possible to pharmacologically augment respiratory muscle force and pressure generation in clinical medicine by administration of PKA inhibitors.

PKA holoenzyme is functionally coupled to CFTR by AKAPs

Cystic fibrosis transmembrane regulator (CFTR) is reported to be preferentially regulated by membrane-bound protein kinase A (PKAII). We tested for close physical and functional association of PKA with CFTR in inside-out membrane patches excised from Calu-3 cells. In the presence of MgATP, 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate (CPT-cAMP) increased the product of CFTR channel number and open probability (from 0.36 +/- 0.12 to 1.23 +/- 0.57, n = 20, P < 0.0025), and this stimulation was abolished by PKI. Thus Calu-3 membrane isolated from cells retains PKA holoenzyme that is functionally coupled to CFTR. PKAII is anchored at specific subcellular sites by A kinase anchoring proteins (AKAPs). Exposure of excised patches to HT-31, a peptide that disrupts the association of PKAII and AKAPs, prevented CPT-cAMP stimulation of CFTR. Therefore, PKA holoenzyme in isolated membrane patches is bound to AKAPs. In whole cell voltage-clamp studies, intracellular dialysis of Calu-3 cells with HT-31 blocked the activation of CFTR by extracellular adenosine. These results suggest that AKAPs mediate PKA compartmentalization with CFTR and are required for activation of CFTR by physiological regulators.

Alternative splicing regulates the subcellular localization of A-kinase anchoring protein 18 isoforms

The cAMP-dependent protein kinase (PKA) is localized to specific subcellular compartments by association with A-kinase anchoring proteins (AKAPs). AKAPs are a family of functionally related proteins that bind the regulatory (R) subunit of PKA with high affinity and target the kinase to specific subcellular organelles. Recently, AKAP18, a low molecular weight plasma membrane AKAP that facilitates PKA-mediated phosphorylation of the L-type Ca(2+) channel, was cloned. We now report the cloning of two additional isoforms of AKAP18, which we have designated AKAP18beta and AKAP18gamma, that arise from alternative mRNA splicing. The AKAP18 isoforms share a common R subunit binding site, but have distinct targeting domains. The original AKAP18 (renamed AKAP18alpha) and AKAP18beta target the plasma membrane when expressed in HEK-293 cells, while AKAP18gamma is cytosolic. When expressed in epithelial cells, AKAP18alpha is targeted to lateral membranes, whereas AKAP18beta is accumulated at the apical membrane. A 23-amino acid insert, following the plasma membrane targeting domain, facilitates the association of AKAP18beta with the apical membrane. The data suggest that AKAP18 isoforms are differentially targeted to modulate distinct intracellular signaling events. Furthermore, the data suggest that plasma membrane AKAPs may be targeted to subdomains of the cell surface, adding additional specificity in intracellular signaling.

Protein kinase A anchoring to the myometrial plasma membrane is required for cyclic adenosine 3',5'-monophosphate regulation of phosphatidylinositide turnover

The importance of the localization of protein kinase A (PKA) to the plasma membrane for cAMP-mediated inhibition of phosphatidylinositide turnover was tested in an immortalized pregnant human myometrial (PHM1-41) cell line, and the putative A kinase anchoring protein (AKAP) involved was identified. Preincubation in PHM1-41 cells with chlorophenylthio-cAMP (CPT-cAMP), forskolin, or relaxin inhibited the ability of oxytocin to stimulate phosphatidylinositide turnover. The addition of a peptide that specifically disrupts interactions of PKA RII subunits with AKAPs (S-Ht31) reversed the effects of these agents, whereas a control peptide was ineffective. The pharmacology of S-Ht31 on this particular membrane event was further characterized. A 10-min incubation with S-Ht31 at a concentration of 1 microM completely reversed the inhibitory effect of relaxin on phosphatidylinositide turnover. S-Ht31 inhibited cAMP-stimulated PKA activity in PHM1-41 cell plasma membranes and decreased the concentration of PKA. Overlay analysis detected a single AKAP of approximately 86 kDa associated with the plasma membrane of PHM1-41 cells, suggesting that the association of PKA with this AKAP is important for the cAMP inhibitory mechanism. The mol wt of this AKAP was similar to that of an AKAP associated with the plasma membrane in the human brain, AKAP79. Antibodies against AKAP79 recognized a band at 86 kDa in purified plasma membranes from the PHM1-41 cells, indicating similar determinants in these proteins. These data suggest that PKA is anchored to the myometrial plasma membrane through association with an AKAP similar to AKAP79, and that this anchoring is required for the cAMP-mediated inhibition of phosphatidylinositide turnover in PHM1-41 cells.

The A-kinase anchor protein MAP2B and cAMP-dependent protein kinase are associated with class C L-type calcium channels in neurons

Phosphorylation by cAMP-dependent protein kinase (PKA) increases the activity of class C L-type Ca(2+) channels which are clustered at postsynaptic sites and are important regulators of neuronal functions. We investigated a possible mechanism that could ensure rapid and efficient phosphorylation of these channels by PKA upon stimulation of cAMP-mediated signaling pathways. A kinase anchor proteins (AKAPs) bind to the regulatory R subunits of PKA and target the holoenzyme to defined subcellular compartments and substrates. Class C channels isolated from rat brain extracts by immunoprecipitation contain an endogenous kinase that phosphorylates kemptide, a classic PKA substrate peptide, and also the main phosphorylation site for PKA in the pore-forming alpha(1) subunit of the class C channel complex, serine 1928. The kinase activity is inhibited by the PKA inhibitory peptide PKI(5-24) and stimulated by cAMP. Physical association of the catalytic C subunit of PKA with the immunoisolated class C channel complex was confirmed by immunoblotting. A direct protein overlay binding assay performed with (32)P-labeled RIIbeta revealed a prominent AKAP with an M(r) of 280,000 in class C channel complexes. The protein was identified by immunoblotting as the microtubule-associated protein MAP2B, a well established AKAP. Class C channels did not contain tubulin and MAP2B association was not disrupted by dilution or addition of nocodazole, two treatments that cause dissociation of microtubules. In vitro experiments show that MAP2B can directly bind to the alpha(1) subunit of the class C channel. Our findings indicate that PKA is an integral part of neuronal class C L-type Ca(2+) channels and suggest that the AKAP MAP2B may mediate this interaction. Neither PKA nor MAP2B were detected in immunoprecipitates of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid-type glutamate receptors or class B N-type Ca(2+) channels. Accordingly, MAP2B docked at class C Ca(2+) channels may be important for recruiting PKA to postsynaptic sites.

Insulin increases near-membrane but not global Ca2+ in isolated skeletal muscle

It has long been debated whether changes in Ca2+ are involved in insulin-stimulated glucose uptake in skeletal muscle. We have now investigated the effect of insulin on the global free myoplasmic Ca2+ concentration and the near-membrane free Ca2+ concentration ([Ca2+]mem) in intact, single skeletal muscle fibers from mice by using fluorescent Ca2+ indicators. Insulin has no effect on the global free myoplasmic Ca2+ concentration. However, insulin increases [Ca2+]mem by approximately 70% and the half-maximal increase in [Ca2+]mem occurs at an insulin concentration of 110 microunits per ml. The increase in [Ca2+]mem by insulin persists when sarcoplasmic reticulum Ca2+ release is inhibited but is lost by perfusing the fiber with a low Ca2+ medium or by addition of L-type Ca2+ channel inhibitors. Thus, insulin appears to stimulate Ca2+ entry into muscle cells via L-type Ca2+ channels. Wortmannin, which inhibits insulin-mediated activation of glucose transport in isolated skeletal muscle, also inhibits the insulin-mediated increase in [Ca2+]mem. These data demonstrate a new facet of insulin signaling and indicate that insulin-mediated increases in [Ca2+]mem in skeletal muscle may underlie important actions of the hormone.

Serine/threonine protein kinases and phosphatases in filamentious fungi

Protein phosphorylation and dephosphorylation are one of the central currencies by which living cells perceive and respond to environmental cues. A number of fundamental processes in fungi such as the cell cycle, transcription, and mating have been shown to require protein phosphorylation. The analysis of protein kinases and phosphatases in filamentous fungi is in its infancy; however, it has already become clear that kinases and phosphatases are likely to be important mediators of fungal proliferation and development as well as signal transduction and infection-related morphogenesis. In this review, we describe, summarize, and consider the rapidly expanding field of protein phosphorylation/dephosphorylation in various aspects of filamentous fungal growth and development.

Regulation of cardiac L-type Ca2+ channel by coexpression of G(alpha s) in Xenopus oocytes

Activation of G(alpha s) via beta-adrenergic receptors enhances the activity of cardiac voltage-dependent Ca2+ channels of the L-type, mainly via protein kinase A (PKA)-dependent phosphorylation. Contribution of a PKA-independent effect of G(alpha s) has been proposed but remains controversial. We demonstrate that, in Xenopus oocytes, antisense knockdown of endogenous G(alpha s) reduced, whereas coexpression of G(alpha s) enhanced, currents via expressed cardiac L-type channels, independently of the presence of the auxiliary subunits alpha2/delta or beta2A. Coexpression of G(alpha s) did not increase the amount of alpha1C protein in whole oocytes or in the plasma membrane (measured immunochemically). Activation of coexpressed beta2 adrenergic receptors did not cause a detectable enhancement of channel activity; rather, a small cAMP-dependent decrease was observed. We conclude that coexpression of G(alpha s), but not its acute activation via beta-adrenergic receptors, enhances the activity of the cardiac L-type Ca2+ channel via a PKA-independent effect on the alpha1C subunit.

Dynamic complexes of beta2-adrenergic receptors with protein kinases and phosphatases and the role of gravin

Signals mediated by G-protein-linked receptors display agonist-induced attenuation and recovery involving both protein kinases and phosphatases. The role of protein kinases and phosphatases in agonist-induced attenuation and recovery of beta-adrenergic receptors was explored by two complementary approaches, antisense RNA suppression and co-immunoprecipitation of target elements. Protein phosphatases 2A and 2B are associated with the unstimulated receptor, the latter displaying a transient decrease followed by a 2-fold increase in the levels of association at 30 min following challenge with agonist. Protein kinase A displays a robust, agonist-induced association with beta-adrenergic receptors over the same period. Suppression of phosphatases 2A and 2B with antisense RNA or inhibition of their activity with calyculin A and FK506, respectively, blocks resensitization following agonist removal. Recycling of receptors to the plasma membrane following agonist-promoted sequestration is severely impaired by loss of either phosphatase 2B or protein kinase C. In addition, loss of protein kinase C diminishes association of phosphatase 2B with beta-adrenergic receptors. Overlay assays performed with the RII subunit of protein kinase A and co-immunoprecipitations reveal proteins of the A kinase-anchoring proteins (AKAP) family, including AKAP250 also known as gravin, associated with the beta-adrenergic receptor. Suppression of gravin expression disrupts recovery from agonist-induced desensitization, confirming the role of gravin in organization of G-protein-linked signaling complexes. The Ht31 peptide, which blocks AKAP protein-protein interactions, blocks association of beta-adrenergic receptors with protein kinase A. These data are the first to reveal dynamic complexes of beta-adrenergic receptors with protein kinases and phosphatases acting via an anchoring protein, gravin.

AKAP15 anchors cAMP-dependent protein kinase to brain sodium channels

The voltage-sensitive sodium channel is regulated by cAMP-dependent protein kinase (PKA) phosphorylation. Using purified preparations of rat brain sodium channels, we have shown that the alpha subunit was phosphorylated by a co-purifying protein kinase. The co-purifying kinase was stimulated by cAMP and phosphorylated PKA substrate peptides. Both the regulatory and catalytic subunits of PKA were detected by immunoblotting in purified sodium channel preparations. Bound PKA was immunoprecipitated with anti-SP19 antibodies directed against the sodium channel alpha subunit. PKA bound to sodium channels phosphorylated the sodium channel alpha subunit on the same four serine residues as observed with exogenously added PKA, indicating that association with the sodium channel does not restrict the sites of phosphorylation. Analysis of proteins with high affinity for the type II alpha regulatory subunit of PKA in a gel overlay assay identified a 15-kDa cAMP-dependent protein kinase-anchoring protein (AKAP) in these preparations. Determination of its amino acid sequence by mass spectrometry revealed two peptides identical to AKAP15, a recently described AKAP that targets PKA to skeletal muscle calcium channels. The co-purifying AKAP was also immunoreactive with antibodies generated against AKAP15, and antibodies directed against AKAP15 co-immunoprecipitated the sodium channel. Our results indicate that PKA is bound to brain sodium channels through interaction with AKAP15. Association of AKAP15 with both skeletal muscle calcium channels and brain sodium channels suggests that it may have broad specificity in targeting PKA to ion channels for regulation.

cAMP-dependent phosphorylation sites and macroscopic activity of recombinant cardiac L-type calcium channels

The involvement of cAMP-dependent phosphorylation sites in establishing the basal activity of cardiac L-type Ca2+ channels was studied in HEK 293 cells transiently cotransfected with mutants of the human cardiac alpha1 and accessory subunits. Systematic individual or combined elimination of high consensus protein kinase A (PKA) sites, by serine to alanine substitutions at the amino and carboxyl termini of the alpha1 subunit, resulted in Ca2+ channel currents indistinguishable from those of wild type channels. Dihydropyridine (DHP)-binding characteristics were also unaltered. To explore the possible involvement of nonconsensus sites, deletion mutants were used. Carboxyl-terminal truncations of the alpha1 subunit distal to residue 1597 resulted in increased channel expression and current amplitudes. Modulation of PKA activity in cells transfected with the wild type channel or any of the mutants did not alter Ca2+ channel functions suggesting that cardiac Ca2+ channels expressed in these cells behave, in terms of lack of PKA control, like Ca2+ channels of smooth muscle cells.

A-kinase anchoring protein 100 (AKAP100) is localized in multiple subcellular compartments in the adult rat heart

Stimulation of beta-adrenergic receptors activates type I and II cyclic AMP-dependent protein kinase A, resulting in phosphorylation of various proteins in the heart. It has been proposed that PKA II compartmentalization by A-kinase-anchoring proteins (AKAPs) regulates cyclic AMP-dependent signaling in the cell. We investigated the expression and localization of AKAP100 in adult hearts. By immunoblotting, we identified AKAP100 in adult rat and human hearts, and showed that type I and II regulatory (RI and II) subunits of PKA are present in the rat heart. By immunofluorescence and confocal microscopy of rat cardiac myocytes and cryostat sections of rat left ventricle papillary muscles, we localized AKAP100 to the nucleus, sarcolemma, intercalated disc, and at the level of the Z-line. After double immunostaining of transverse cross-sections of the papillary muscles with AKAP100 plus alpha-actinin-specific antibodies or AKAP100 plus ryanodine receptor-specific antibodies, confocal images showed AKAP100 localization at the region of the transverse tubule/junctional sarcoplasmic reticulum. RI is distributed differently from RII in the myocytes. RII, but not RI, was colocalized with AKAP100 in the rat heart. Our studies suggest that AKAP100 tethers PKA II to multiple subcellular compartments for phosphorylation of different pools of substrate proteins in the heart.

Regulation of ion channels by cAMP-dependent protein kinase and A-kinase anchoring proteins

Subcellular targeting of the cAMP-dependent protein kinase is achieved, in part, through association with A-kinase anchoring proteins (AKAPs). Recent evidence suggests that specific AKAPs direct the kinase to submembrane sites to facilitate phosphorylation and modulation of a variety of ion channels. A new membrane-anchored AKAP targets cAMP-dependent protein kinase to calcium channels and enhances their regulation in multiple cell types.

Primary structure and function of an A kinase anchoring protein associated with calcium channels

Rapid, voltage-dependent potentiation of skeletal muscle L-type calcium channels requires phosphorylation by cAMP-dependent protein kinase (PKA) anchored via an A kinase anchoring protein (AKAP). Here we report the isolation, primary sequence determination, and functional characterization of AKAP15, a lipid-anchored protein of 81 amino acid residues with a single amphipathic helix that binds PKA. AKAP15 colocalizes with L-type calcium channels in transverse tubules and is associated with L-type calcium channels in transfected cells. A peptide fragment of AKAP15 encompassing the RII-binding domain blocks voltage-dependent potentiation. These results indicate that AKAP15 targets PKA to the calcium channel and plays a critical role in voltage-dependent potentiation and regulation of skeletal muscle contraction. The expression of AKAP15 in the brain and heart suggests that it may mediate rapid PKA regulation of L-type calcium channels in neurons and cardiac myocytes.

A novel lipid-anchored A-kinase Anchoring Protein facilitates cAMP-responsive membrane events

Compartmentalization of protein kinases with substrates is a mechanism that may promote specificity of intracellular phosphorylation events. We have cloned a low-molecular weight A-kinase Anchoring Protein, called AKAP18, which targets the cAMP-dependent protein kinase (PKA) to the plasma membrane, and permits functional coupling to the L-type calcium channel. Membrane anchoring is mediated by the first 10 amino acids of AKAP18, and involves residues Gly1, Cys4 and Cys5 which are lipid-modified through myristoylation and dual palmitoylation, respectively. Transient transfection of AKAP18 into HEK-293 cells expressing the cardiac L-type Ca2+ channel promoted a 34 9% increase in cAMP-responsive Ca2+ currents. In contrast, a targeting-deficient mutant of AKAP18 had no effect on Ca2+ currents in response to the application of a cAMP analog. Further studies demonstrate that AKAP18 facilitates GLP-1-mediated insulin secretion in a pancreatic beta cell line (RINm5F), suggesting that membrane anchoring of the kinase participates in physiologically relevant cAMP-responsive events that may involve ion channel activation.

The mitogen-activated protein kinase p38-2 is necessary for the inhibition of N-type calcium current by bradykinin

Calcium influx via voltage-dependent calcium channels (ICa,V) links depolarization of excitable cells to critical cellular processes, such as secretion, contraction, and gene transcription. Fast regulation of ICa,V (<1 sec) by G-protein-coupled receptors is a relatively well-defined mechanism, whereas slow (30-60 sec) actions of transmitters and hormones on the same current remain poorly understood. In NG108-15 cells, the kinetically slow inhibition of N-type ICa,V by bradykinin (BK) requires the sequential activation of two G-proteins, heterotrimeric G13 and monomeric Rac1/Cdc42. We have now defined a role in this pathway for the relatively fast-acting p38 mitogen-activated protein kinase (MAPK). The slow inhibition of ICa,V by BK was suppressed specifically by SB203580, a compound that inhibits the p38 family of MAPKs. BK potently and selectively activated a newly discovered p38 family member, p38-2. These data provide the first evidence that a MAPK is involved in the regulation of ICa,V by a receptor-mediated process.

Anchoring of protein kinase A facilitates hormone-mediated insulin secretion

Impaired insulin secretion is a characteristic of non-insulin-dependent diabetes mellitus (NIDDM). One possible therapeutic agent for NIDDM is the insulinotropic hormone glucagon-like peptide 1 (GLP-1). GLP-1 stimulates insulin secretion through several mechanisms including activation of protein kinase A (PKA). We now demonstrate that the subcellular targeting of PKA through association with A-kinase-anchoring proteins (AKAPs) facilitates GLP-1-mediated insulin secretion. Disruption of PKA anchoring by the introduction of anchoring inhibitor peptides or expression of soluble AKAP fragments blocks GLP-1 action in primary islets and cAMP-responsive insulin secretion in clonal beta cells (RINm5F). Displacement of PKA also prevented cAMP-mediated elevation of intracellular calcium suggesting that localized PKA phosphorylation events augment calcium flux.

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.

Type II regulatory subunits are not required for the anchoring-dependent modulation of Ca2+ channel activity by cAMP-dependent protein kinase

Preferential phosphorylation of specific proteins by cAMP-dependent protein kinase (PKA) may be mediated in part by the anchoring of PKA to a family of A-kinase anchor proteins (AKAPs) positioned in close proximity to target proteins. This interaction is thought to depend on binding of the type II regulatory (RII) subunits to AKAPs and is essential for PKA-dependent modulation of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate receptor, the L-type Ca2+ channel, and the KCa channel. We hypothesized that the targeted disruption of the gene for the ubiquitously expressed RIIalpha subunit would reveal those tissues and signaling events that require anchored PKA. RIIalpha knockout mice appear normal and healthy. In adult skeletal muscle, RIalpha protein levels increased to partially compensate for the loss of RIIalpha. Nonetheless, a reduction in both catalytic (C) subunit protein levels and total kinase activity was observed. Surprisingly, the anchored PKA-dependent potentiation of the L-type Ca2+ channel in RIIalpha knockout skeletal muscle was unchanged compared with wild type although it was more sensitive to inhibitors of PKA-AKAP interactions. The C subunit colocalized with the L-type Ca2+ channel in transverse tubules in wild-type skeletal muscle and retained this localization in knockout muscle. The RIalpha subunit was shown to bind AKAPs, although with a 500-fold lower affinity than the RIIalpha subunit. The potentiation of the L-type Ca2+ channel in RIIalpha knockout mouse skeletal muscle suggests that, despite a lower affinity for AKAP binding, RIalpha is capable of physiologically relevant anchoring interactions.

cAMP-dependent regulation of cardiac L-type Ca2+ channels requires membrane targeting of PKA and phosphorylation of channel subunits

The cardiac L-type Ca2+ channel is a textbook example of an ion channel regulated by protein phosphorylation; however, the molecular events that underlie its regulation remain unknown. Here, we report that in transiently transfected HEK293 cells expressing L-type channels, elevations in cAMP resulted in phosphorylation of the alpha1C and beta2a channel subunits and increases in channel activity. Channel phosphorylation and regulation were facilitated by submembrane targeting of protein kinase A (PKA), through association with an A-kinase anchoring protein called AKAP79. In transfected cells expressing a mutant AKAP79 that is unable to bind PKA, phosphorylation of the alpha1C subunit and regulation of channel activity were not observed. Furthermore, we have demonstrated that the association of an AKAP with PKA was required for beta-adrenergic receptor-mediated regulation of L-type channels in native cardiac myocytes, illustrating that the events observed in the heterologous expression system reflect those occurring in the native system. Mutation of Ser1928 to alanine in the C-terminus of the alpha1C subunit resulted in a complete loss of cAMP-mediated phosphorylation and a loss of channel regulation. Thus, the PKA-mediated regulation of L-type Ca2+ channels is critically dependent on a functional AKAP and phosphorylation of the alpha1C subunit at Ser1928.