A constitutively active holoenzyme form of the cAMP-dependent protein kinase

Edmond Fischer's kinase legacy: History of the protein kinase inhibitor and protein kinase A

Although Fischer's extraordinary career came to focus mostly on the protein phosphatases, after his co-discovery of Phosphorylase Kinase with Ed Krebs he was clearly intrigued not only by cAMP-dependent protein kinase (PKA), but also by the heat-stable, high-affinity protein kinase inhibitor (PKI). PKI is an intrinsically disordered protein that contains at its N-terminus a pseudo-substrate motif that binds synergistically and with high-affinity to the PKA catalytic (C) subunit. The sequencing and characterization of this inhibitor peptide (IP20) were validated by the structure of the PKA C-subunit solved first as a binary complex with IP20 and then as a ternary complex with ATP and two magnesium ions. A second motif, nuclear export signal (NES), was later discovered in PKI. Both motifs correspond to amphipathic helices that convey high-affinity binding. The dynamic features of full-length PKI, recently captured by NMR, confirmed that the IP20 motif becomes dynamically and sequentially ordered only in the presence of the C-subunit. The type I PKA regulatory (R) subunits also contain a pseudo-substrate ATPMg2-dependent high-affinity inhibitor sequence. PKI and PKA, especially the Cβ subunit, are highly expressed in the brain, and PKI expression is also cell cycle-dependent. In addition, PKI is now linked to several cancers. The full biological importance of PKI and PKA signaling in the brain, and their importance in cancer thus remains to be elucidated.

Phosphorylation, compartmentalization, and cardiac function

Protein phosphorylation is a fundamental element of cell signaling. First discovered as a biochemical switch in glycogen metabolism, we now know that this posttranslational modification permeates all aspects of cellular behavior. In humans, over 540 protein kinases attach phosphate to acceptor amino acids, whereas around 160 phosphoprotein phosphatases remove phosphate to terminate signaling. Aberrant phosphorylation underlies disease, and kinase inhibitor drugs are increasingly used clinically as targeted therapies. Specificity in protein phosphorylation is achieved in part because kinases and phosphatases are spatially organized inside cells. A prototypic example is compartmentalization of the cyclic adenosine 3',5'-monophosphate (cAMP)-dependent protein kinase A through association with A-kinase anchoring proteins. This configuration creates autonomous signaling islands where the anchored kinase is constrained in proximity to activators, effectors, and selected substates. This article primarily focuses on A kinase anchoring protein (AKAP) signaling in the heart with an emphasis on anchoring proteins that spatiotemporally coordinate excitation-contraction coupling and hypertrophic responses.

Swimming regulations for protein kinase A catalytic subunit

cAMP-dependent protein kinase (PKA) plays a central role in important biological processes including synaptic plasticity and sympathetic stimulation of the heart. Elevations of cAMP trigger release of PKA catalytic (C) subunits from PKA holoenzymes, thereby coupling cAMP to protein phosphorylation. Uncontrolled C subunit activity, such as occurs in genetic disorders in which regulatory subunits are depleted, is pathological. Anchoring proteins that associate with PKA regulatory subunits are important for localising PKA activity in cells. However, anchoring does not directly explain how unrestrained 'free swimming' of C subunits is avoided following C subunit release. In this review, I discuss new mechanisms that have been posited to account for this old problem. One straightforward explanation is that cAMP does not trigger C subunit dissociation but instead activates intact PKA holoenzymes whose activity is restrained through anchoring. A comprehensive comparison of observations for and against cAMP-activation of intact PKA holoenzymes does not lend credence to this mechanism. Recent measurements have revealed that PKA regulatory subunits are expressed at very high concentrations, and in large molar excess relative to C subunits. I discuss the implications of these skewed PKA subunit concentrations, before considering how phosphorylation of type II regulatory subunits and myristylation of C subunits are likely to contribute to controlling C subunit diffusion and recapture in cells. Finally, I speculate on future research directions that may be pursued on the basis of these emerging mechanisms.

Potential for therapeutic targeting of AKAP signaling complexes in nervous system disorders

A common feature of neurological and neuropsychiatric disorders is a breakdown in the integrity of intracellular signal transduction pathways. Dysregulation of ion channels and receptors in the cell membrane and the enzymatic mediators that link them to intracellular effectors can lead to synaptic dysfunction and neuronal death. However, therapeutic targeting of these ubiquitous signaling elements can lead to off-target side effects due to their widespread expression in multiple systems of the body. A-kinase anchoring proteins (AKAPs) are multivalent scaffolding proteins that compartmentalize a diverse range of receptor and effector proteins to streamline signaling within nanodomain signalosomes. A number of essential neurological processes are known to critically depend on AKAP-directed signaling and an understanding of the role AKAPs play in nervous system disorders has emerged in recent years. Selective targeting of AKAP protein-protein interactions may be a means to uncouple pathologically active signaling pathways in neurological disorders with a greater degree of specificity. In this review we will discuss the role of AKAPs in both regulating normal nervous system function and dysfunction associated with disease, and the potential for therapeutic targeting of AKAP signaling complexes.

Discovery of Allostery in PKA Signaling

cAMP-dependent protein kinase (PKA) was the second protein kinase to be discovered and the PKA catalytic (C) subunit serves as a prototype for the large protein kinase superfamily that contains over 500 gene products. The protein kinases regulate much of biology in eukaryotic cells and they are now also a major therapeutic target. Although PKA was discovered nearly 50 years ago and the subsequent discovery of the regulatory subunits that bind cAMP and release the catalytic activity from the holoenzyme followed quickly. Thus in PKA we see the convergence of two major signaling mechanisms - protein phosphorylation and second messenger signaling through cAMP. Crystallography provides a foundation for understanding function, and the structure of the isolated regulatory (R) and C-subunits have been extremely informative. Yet it is the R₂C₂ holoenzyme that predominates in cells, and one can only appreciate the allosteric features of PKA signaling by seeing the full length protein. The symmetry and the quaternary constraints that one R:C hetero-dimer exerts on the other in the holoenzyme simply are not present in the isolated subunits or even in the R:C hetero-dimer.

The role of phosphodiesterases in hippocampal synaptic plasticity

Phosphodiesterases (PDEs) degrade cyclic nucleotides, signalling molecules that play important roles in synaptic plasticity and memory. Inhibition of PDEs may therefore enhance synaptic plasticity and memory as a result of elevated levels of these signalling molecules, and this has led to interest in PDE inhibitors as cognitive enhancers. The development of new mouse models in which PDE subtypes have been selectively knocked out and increasing selectivity of PDE antagonists means that this field is currently expanding. Roles for PDE2, 4, 5 and 9 in synaptic plasticity have so far been demonstrated and we review these studies here in the context of cyclic nucleotide signalling more generally. The role of other PDE families in synaptic plasticity has not yet been investigated, and this area promises to advance our understanding of cyclic nucleotide signalling in synaptic plasticity in the future. This article is part of the Special Issue entitled 'Glutamate Receptor-Dependent Synaptic Plasticity'.

The role of A-Kinase anchoring proteins in cAMP-mediated signal transduction pathways

Compartmentalization of signal transduction enzymes is an important mechanism of cellular signaling specificity. This occurs through the interaction of enzymes with scaffolding or anchoring proteins. To date, one of the best-studied examples of kinase anchoring is the targeting of protein kinase A to cellular locations through its association with A-kinase anchoring proteins (AKAPs). AKAPs mediate a high-affinity interaction with the type II regulatory subunit of protein kinase A for the purpose of localizing the kinase to pools of cyclic adenosine monophosphate and within proximity of preferred substrates. Furthermore, AKAPs can organize entire signaling complexes made up of kinases, phosphatases, signaling enzymes, and additional regulatory proteins.

PDE7A1, a cAMP-specific Phosphodiesterase, Inhibits cAMP-dependent Protein Kinase by a Direct Interaction with C

The N-terminal regulatory region of the high affinity cAMP-specific phosphodiesterase, PDE7A1, contains two copies of the cAMP-dependent kinase (PKA) pseudosubstrate site RRGAI. In betaTC3 insulinoma cells, PDE7A1 co-localizes with PKA II in the Golgi-centrosome region. The roles PDE7A1 and its regulatory region play in cAMP signaling were examined by studying interactions with PKA subunits. PDE7A1 associates with the dissociated C subunit of PKA (C), but does not bind tetrameric PKA holoenzyme. High affinity binding of C by PDE7A1 inhibits kinase activity in vitro (IC50 = 0.5 nm). The domain containing PKA pseudosubstrate sites at the N terminus of PDE7A1 mediates complex formation with C. The PDE7A1 N-terminal repeat region inhibits C activity in CHO-K1 cells and also suppresses C dependent, cAMP-independent, physiological responses in yeast. Thus, PDE7A1 possesses a non-catalytic activity that can contribute to the termination of cAMP signals via direct inhibition of C. This study identifies a novel inhibitor of PKA and a non-catalytic affect of a cyclic nucleotide phosphodiesterase.

Phosphorylation of Par-4 by protein kinase A is critical for apoptosis

Despite distinct dissimilarities, diverse cancers express several common protumorigenic traits. We present here evidence that the proapoptotic protein Par-4 utilizes one such common tumorigenic trait to become selectively activated and induce apoptosis in cancer cells. Elevated protein kinase A (PKA) activity noted in cancer cells activated the apoptotic function of ectopic Par-4 or its SAC (selective for apoptosis induction in cancer cells) domain, which induces apoptosis selectively in cancer cells and not in normal or immortalized cells. PKA preferentially phosphorylated Par-4 at the T155 residue within the SAC domain in cancer cells. Moreover, pharmacological-, peptide-, or small interfering RNA-mediated inhibition of PKA activity in cancer cells resulted in abrogation of both T155 phosphorylation and apoptosis by Par-4. The mechanism of activation of endogenous Par-4 was similar to that of ectopic Par-4, and in response to exogenous stimuli, endogenous Par-4 induced apoptosis by a PKA- and phosphorylated T155-dependent mechanism. Enforced elevation of PKA activity in normal cells resulted in apoptosis by the SAC domain of Par-4 in a T155-dependent manner. Together, these observations suggest that selective apoptosis of cancer cells by the SAC domain of Par-4 involves phosphorylation of T155 by PKA. These findings uncover a novel mechanism engaging PKA, a procancerous activity commonly elevated in most tumor cells, to activate the cancer selective apoptotic action of Par-4.

Intracellular targeting of protein kinases and phosphatases

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). This study focused on recent advances that further our understanding of AKAPs, with particular emphasis on the bidirectional regulation of signaling events by AKAP signaling complexes and their contribution to the control of actin reorganization events.

Autoinhibition and isoform-specific dominant negative inhibition of the type II cGMP-dependent protein kinase

In the absence of cyclic nucleotides, the cAMP-dependent protein kinase and cGMP-dependent protein kinases (cGKs) suppress phosphotransfer activity at the catalytic cleft by competitive inhibition of substrate binding with a pseudosubstrate sequence within the holoenzyme. The magnitude of inhibition can be diminished by autophosphorylation near this pseudosubstrate sequence. Activation of type I cGK (cGKI) and type II cGK (cGKII) are differentially regulated by their cyclic nucleotide-binding sites. To address the possibility that the distinct activation mechanisms of cGKII and cGKI result from differences in the autophosphorylation of the inhibitory domain, we investigated the effects of autophosphorylation on the kinetics of activation. Unlike the type I cGKs (cGKIalpha and Ibeta), cGKII autophosphorylation did not alter the basal activity, nor the sensitivity of the enzyme to cyclic nucleotide activation. To determine residues responsible for autoinhibition of cGKII, Ala was substituted for basic residues (Lys(122), Arg(118), and Arg(119)) or a hydrophobic residue (Val(125)) within the putative pseudosubstrate domain of cGKII. The integrity of these residues was essential for full cGKII autoinhibition. Furthermore, a cGKII truncation mutant containing this autoinhibitory region demonstrated a nanomolar IC(50) toward a constitutively active form of cGKII. Finally, we present evidence that the dominant negative properties of this truncation mutant are specific to cGKII when compared with cAMP-dependent protein kinase Calpha and cGKIbeta. These findings extend the known differences in the activation mechanisms among cGK isoforms and allow the design of an isoform-specific cGKII inhibitor.

Formation of inactive cAMP-saturated holoenzyme of cAMP-dependent protein kinase under physiological conditions

The complex of the subunits (RIalpha, Calpha) of cAMP-dependent protein kinase I (cA-PKI) was much more stable (K(d) = 0.25 microm) in the presence of excess cAMP than previously thought. The ternary complex of C subunit with cAMP-saturated RIalpha or RIIalpha was devoid of catalytic activity against either peptide or physiological protein substrates. The ternary complex was destabilized by protein kinase substrate. Extrapolation from the in vitro data suggested about one-fourth of the C subunit to be in ternary complex in maximally cAMP-stimulated cells. Cells overexpressing either RIalpha or RIIalpha showed decreased CRE-dependent gene induction in response to maximal cAMP stimulation. This could be explained by enhanced ternary complex formation. Modulation of ternary complex formation by the level of R subunit may represent a novel way of regulating the cAMP kinase activity in maximally cAMP-stimulated cells.

AKAP mediated signal transduction

Compartmentalization of cyclic AMP-dependent protein kinase (PKA) is achieved through association with A-kinase anchoring proteins (AKAPs). AKAPs are a group of structurally diverse proteins with the common function of binding to the regulatory subunit of PKA and confining the holoenzyme to discrete locations within the cell. This mode of regulation ensures that PKA is exposed to isolated cAMP gradients, which allows for efficient catalytic activation and accurate substrate selection. Several AKAPs coordinate multiple members of signaling cascades, effectively assembling upstream activators and downstream effectors within the same macromolecular complex. Consequently, AKAPs may serve as points of integration for numerous signaling pathways. This review details the most recent advances in our understanding of the various biological functions dependent upon AKAP-anchored signaling complexes.

Analysis of micronuclear, macronuclear and cDNA sequences encoding the regulatory subunit of cAMP-dependent protein kinase of Euplotes octocarinatus: evidence for a ribosomal frameshift

We have isolated and characterized the micronuclear gene encoding the regulatory subunit of cAMP-dependent protein kinase of the ciliated protozoan Euplotes octocarinatus, as well as its macronuclear version and the corresponding cDNA. Analyses of the sequences revealed that the micronuclear gene contains one small 69-bp internal eliminated sequence (IES) that is removed during macronuclear development. The IES is located in the 5'-noncoding region of the micronuclear gene and is flanked by a pair of tetranucleotide 5'-TACA-3' direct repeats. The macronuclear DNA molecule carrying this gene is approximately 1400 bp long and is amplified to about 2000 copies per macronucleus. Sequence analysis suggests that the expression of this gene requires a +1 ribosomal frameshift. The deduced protein shares 31% identity with the cAMP-dependent protein kinase type I regulatory subunit of Homo sapiens, and 53% identity with the regulatory subunit R44 of one of the two cAMP-dependent protein kinases of Paramecium. In addition, it contains two highly conserved cAMP binding sites in the C-terminal domain. The putative autophosphorylation site ARTSV of the regulatory subunit of E. octocarinatus is similar to that of the regulatory subunit R44 of Paramecium but distinct from the consensus motif RRXSZ of other eukaryotic regulatory subunits of cAMP-dependent protein kinases.

Mechanism of activation of cAMP-dependent protein kinase: in Mucor rouxii the apparent specific activity of the cAMP-activated holoenzyme is different than that of its free catalytic subunit

Kinetic constants for peptide phosphorylation by the catalytic subunit of the dimorphic fungus Mucor rouxii protein kinase A were determined using 13 peptides derived from the peptide containing the basic consensus sequence RRASVA, plus kemptide, S6 peptide, and protamine. As a whole, although with a greater Km, the order of preference of the peptides by the M. rouxii catalytic subunit was similar to the one displayed by mammalian protein kinase A. Particularly significant is the replacement of serine by threonine in the basic peptide RRATVA, which impaired its role as a substrate of M. rouxii catalytic subunit. Mucor rouxii protein kinase A is a good model in which to study the mechanism of activation since cAMP alone is not enough to promote activation and dissociation. Four peptides were selected for the study of holoenzyme activation under conditions in which the enzymatic activity was not proportional to the holoenzyme concentration: RRASVA, RRRRASVA, KRRRLSSRA (S6 peptide), and LRRASLG (kemptide); protamine was used as reference. Differential activation degree was observed depending on the peptide used and on cAMP concentration. Ratios of activity between different substrates displayed by the holoenzyme under the above conditions did not reflect the one expected for the free catalytic subunit. The degree of inhibition of the holoenzyme activity by an active peptide derived from the thermostable protein kinase inhibitor was dependent on the substrate used and on the holoenzyme concentration, while it was found to be independent of these two parameters for free catalytic subunit. Polycation modulation of holoenzyme activation by cAMP was also dependent on the polycation itself and on the peptide used as substrate. The observed kinetic differences between holoenzyme and free catalytic subunit were decreased or almost abolished when working at low enzyme or at high cAMP concentrations. Two hypotheses compatible with the results are discussed: substrate participation in the dissociation process and/or holoenzyme activation without dissociation.

Cyclic nucleotide-dependent protein kinases: intracellular receptors for cAMP and cGMP action

Intracellular cAMP and cGMP levels are increased in response to a variety of hormonal and chemical stimuli; these nucleotides play key roles as second messenger signals in modulating myriad physiological processes. The cAMP-dependent protein kinase and cGMP-dependent protein kinase are major intracellular receptors for these nucleotides, and the actions of these enzymes account for much of the cellular responses to increased levels of cAMP or cGMP. This review summarizes many studies that have contributed significantly to an improved understanding of the catalytic, regulatory, and structural properties of these protein kinases. These accumulated findings provide insights into the mechanisms by which these enzymes produce their specific physiological effects and are helpful in considering the actions of other protein kinases as well.

Identification of electrostatic interaction sites between the regulatory and catalytic subunits of cyclic AMP-dependent protein kinase

Two classes of molecules inhibit the catalytic subunit (C) of the cyclic AMP-dependent protein kinase (cAPK), the heat-stable protein kinase inhibitors (PKIs) and the regulatory (R) subunits. Basic sites on C, previously identified as important for R/C interaction in yeast TPK1 and corresponding to Lys213, Lys217, and Lys189 in murine C alpha, were replaced with either Ala or Thr and characterized for their kinetic properties and ability to interact with RI and PKI. rC(K213A) and rC(K217A) were both defective in forming holoenzyme with RI but were inhibited readily with PKI. This contrasts with rC(R133A), which is defective in binding PKI but not RI (Wen & Taylor, 1994). Thus, the C-subunit employs two distinct electrostatic surfaces to achieve high-affinity binding with these two types of inhibitory molecules even though all inhibitors share a common consensus site that occupies the active site cleft. Unlike TPK1, mutation of Lys189 had no effect. The mutant C subunits that were defective in binding RI, rC(K213A) and rC(K217A), were then paired with three RI mutants, rRI(D140A), rRI(E143A), and rRI(D258A), shown previously to be defective in recognition of C. Although the mutations at Asp140 and Asp258 in RI were additive with respect to the C mutations. rC(K213A) and rRI(E143A) were compensatory, thus identifying a specific electrostatic interaction site between RI and C. The results are discussed in terms of the RI and C crystal structures and the sequence homology between the yeast and mammalian enzymes.

Interaction of the regulatory and catalytic subunits of cAMP-dependent protein kinase. Electrostatic sites on the type Ialpha regulatory subunit

Since a basic surface on the catalytic (C) subunit of cAMP-dependent protein kinase is important for binding to the regulatory (R) subunit, acidic residues in R were sought that might contribute to R-C interaction. Using differential labeling by a water-soluble carbodiimide (Buechler, T. A., and Taylor, S. S. (1990) Biochemistry 29, 1937-1943), seven specific carboxylates in RIalpha were identified that were protected from chemical modification in the holoenzyme; each was then replaced with Ala. Of these, rRI(E15A/E106A/D107A)), rRI(E105A), rRI(D140A), rRI(E143A), and rRI(D258A) all were defective in holoenzyme formation and define negative electrostatic surfaces on RIalpha. An additional conserved carboxylate, Glu101 in RIalpha and the equivalent, Glu99 in RIIalpha were mutated to Ala. Replacement of Glu101 had no effect while rRII(E99A) was very defective. RIalpha and RIIalpha thus differ in the molecular details of how they recognize C. Unlike wild-type RI, two additional mutants, rRI(D170A) and rRI(K242A), inhibited C-subunit stoichiometrically in the presence of cAMP and show increases in both on- and off-rates. Asp170, which contributes directly to the hydrogen bonding network in cAMP-binding site A, thus contributes also to holoenzyme stability.

Identification of critical determinants for autoinhibition in the pseudosubstrate region of type I alpha cAMP-dependent protein kinase

The consensus substrate site for cAMP-dependent protein kinase (PKA) is Arg-Arg-Xaa-Ser(P)-Xaa and the autoinhibitory domain of the PKA type I alpha regulatory subunit (RI subunit) contains a similar sequence, Arg92-Arg-Arg-Arg-Gly-Ala-Ile-Ser-Ala-Glu. The italicized amino acids form a putative pseudosubstrate site (Ser is replaced with Ala), which together with adjacent residues could competitively inhibit substrate phosphorylation by the PKA catalytic subunit (C subunit). The present studies determine the contributions of Arg92-95, Ile98, and Glu101 to inhibitory potency. Amino-terminal truncation of RI subunit through Arg92 (delta1-92) or Arg93 (delta1-93) had no detectable effect on inhibition of C subunit. Truncation through Arg94 (delta1-94), or point mutation of Arg95 within truncated mutants (delta1-93.R95A or delta1-92.R95A), caused a dramatic reduction in inhibitory potency. Truncation through Arg95 (delta1-95) had a greater effect than did replacement or deletion of Arg94 or Arg95 alone. Using full-length RI subunit, the inhibitory potency was reduced by replacing Ile98 with Ala, Gly, or Gln, but not by replacing it with Val. The inhibitory potency of RI subunit was unchanged when Glu101 was replaced with Ala or Gln. It is concluded that Arg94, Arg95 and, to a lesser extent, Ile98 are vital constituents of PKA autoinhibition by type I alpha R subunit.

The 44-kDa regulatory subunit of the Paramecium cAMP-dependent protein kinase lacks a dimerization domain and may have a unique autophosphorylation site sequence

The 44-kDa regulatory subunit (R44) of one form of cAMP-dependent protein kinase of Paramecium was purified, and two partial internal amino acid sequences from it were used to clone the corresponding cDNA. This R44 cDNA clone was 1022-bp long, including 978 bp of coding sequence and 7 bp and 37 bp of 5' and 3' untranslated sequences, respectively. A 1.1-kb mRNA was labeled on a Northern blot. The deduced R44 amino acid sequence had 31%-38% positional identity to the sequences of other cloned cAMP-dependent protein kinase regulatory subunits. R44 sequence showed equal sequence similarity to mammalian types I and II regulatory subunits. The N-terminal sequence encoding the regulatory subunit dimerization domain found in most regulatory subunits is not present in the R44 clone, confirming the lack of regulatory subunit dimer formation previously reported for the Paramecium cAMP-dependent protein kinase. The putative autophosphorylation site of R44 contains the amino acid sequence TRTS, distinct from the consensus sequence RRXS, where X is any residue, found in other autophosphorylated cAMP-dependent protein kinase regulatory subunits and many cAMP-dependent protein kinase substrates.

Paramecium has two regulatory subunits of cyclic AMP-dependent protein kinase, one unique to cilia

The subunit composition and intracellular location of the two forms of cAMP-dependent protein kinase of Paramecium cilia were determined using antibodies against the 40-kDa catalytic (C) and 44-kDa regulatory (R44) subunits of the 70-kDa cAMP-dependent protein kinase purified from deciliated cell bodies. Both C and R44 were present in soluble and particulate fractions of cilia and deciliated cells. Crude cilia and a soluble ciliary extract contained a 48-kDa protein (R48) weakly recognized by one of several monoclonal antibodies against R44, but not recognized by an anti-R44 polyclonal serum. Gel-filtration chromatography of a soluble ciliary extract resolved a 220-kDa form containing C and R48 and a 70-kDa form containing C and R44. In the large enzyme, R48 was the only protein to be autophosphorylated under conditions that allow autophosphorylation of R44. The subunits of the large enzyme subsequently were purified to homogeneity by cAMP-agarose chromatography. Both C and R48 were retained by the column and eluted with I M NaCl; no other proteins were purified in this step. These results confirm that the ciliary cAMP-dependent protein kinases have indistinguishable C subunits, but different R subunits. The small ciliary enzyme, like the cell-body enzyme, contains R44, whereas R48 is the R subunit of the large enzyme.

Expression of a chimeric, cGMP-sensitive regulatory subunit of the cAMP-dependent protein kinase type I alpha

To study the fluctuations of cGMP in living cells through changes of energy transfer of dissociable fluorescence labeled subunits, we constructed a cGMP-sensitive probe by combining the N-terminus of the type I regulatory subunit of cAMP-dependent protein kinase (PKA) with the cGMP binding sites of cGMP-dependent protein kinase I alpha (PKG). This chimeric regulatory subunit retained PKA-like dimerization and PKG-compatible cGMP binding constants (Kd = 53 nM) for both binding sites. High affinity interaction with the PKA catalytic subunit was verified by Surface Plasmon Resonance (Kd = 3.15 nM). Additionally, the chimera inhibits the formation of wild-type holoenzyme with an apparent Ki of 1.05 nM. Furthermore, cGMP dissociated the mutant holoenzyme with an apparent activation constant of 146 nM. Thus, our construct provides all the requirements needed to investigate changes in intracellular cGMP concentrations.

Protein kinase A modulates an endogenous calcium channel, but not the calcium-activated chloride channel, in Xenopus oocytes

In Xenopus oocytes, Ca2+ influx through an endogenous voltage-gated Ca2+ channel activates a transient outward Cl- current (ICl(Ca)), which is potentiated by cAMP increase. The site of cAMP effect appears to be the Ca2+ channel instead of the Ca(2+)-activated Cl- channel, because cAMP potentiates the Ba2+ current through the Ca2+ channel in a similar way to the ICl(Ca), and cAMP does not potentiate the Ca(2+)-dependent Cl- current in cells treated with Ca2+ ionophore. Using the catalytic subunit of protein kinase A (PKA) and PKA inhibitors, it was shown that PKA is both necessary and sufficient for the cAMP effect on ICl(Ca). Furthermore, the cAMP/PKA-mediated potentiation of ICl(Ca) was inhibited by both type 1 and type 2A protein phosphatases.

Fluorescence resonance energy transfer within a heterochromatic cAMP-dependent protein kinase holoenzyme under equilibrium conditions: new insights into the conformational changes that result in cAMP-dependent activation

Previous studies of the ligand regulation of the cAMP-dependent protein kinase have demonstrated the cAMP-mediated dissociation of the holoenzyme by using nonequilibrium techniques; i.e., gel filtration, ion-exchange chromatography, and differential centrifugation. While physically mild, these could have caused weakly associated species to dissociate, thereby providing a potentially flawed interpretation of the mechanism of activation of the protein kinase. To assess this, the activation of the cAMP-dependent protein kinase has been monitored under equilibrium conditions using dipolar fluorescence energy transfer to measure changes in the proximity relations between the catalytic (C) and regulatory (R) subunits that compose the holoenzyme. Specifically, we prepared a heterochromatically labeled protein kinase type II holoenzyme, with the regulatory and catalytic subunits labeled with sulforhodamine and carboxyfluorescein, respectively, and monitored the exchange of electronic excitation energy between the C and R subunits by both donor lifetime and steady-state fluorescence. Biochemically, the heterochromatic holoenzyme was closely identical to the native protein with regard to cAMP-induced increase in catalytic activity, reassociation of C and R subunits, inhibition of catalytic activity by the specific protein kinase inhibitor (PKI), and observed dissociation examined by gel filtration upon cAMP addition. However, under equilibrium conditions, the energy-transfer measurements revealed that the addition of cAMP to this heterochromatic reporter complex promoted an estimated 10-A increase in the distance between the derivatization sites on C and R but not a dissociation of these subunits. Addition of PKI plus cAMP promoted full dissociation of the two subunits.(ABSTRACT TRUNCATED AT 250 WORDS)

Protein kinase C mutants in the auto-inhibitory region exhibit two distinct properties

To define the role of the auto-inhibitory region of protein kinase C (PKC), Arg22-Lys23-Gly24-Ala25-Leu26-Arg27, site-directed mutations were introduced into the basic residues. Three mutants, PKCAla22,23, PKCAla27, and PKCAla22,23,27, apparently fell into two distinct types with regard to their biochemical properties and biological activities, as judged by the enhancement of a c-fos promoter in Jurkat cells and by the initiation of germinal vesicle breakdown (GVBD) in Xenopus laevis oocytes. (i) PKCAla22,23 and PKCAla27 had activators independent in vitro kinase activity, high phosphorylation levels in vivo, and localized in both cytosolic and particulate fractions. These mutants were not fully biologically active. (ii) PKCAla22,23,27 had a low phosphorylation level in vivo, was found predominantly in the particulate fraction and was the most biologically active. These results suggest that basic residues in the auto-inhibitory domain account for the regulation of kinase activity and the cytosolic retention of PKC. The particulate association or the cytosolic clearance of PKC may facilitate signal transduction in the cell.

Mutations in the catalytic subunit of cAMP-dependent protein kinase result in unregulated biological activity

Mutations were identified in the catalytic subunit (C) of the cAMP-dependent protein kinase (EC 2.7.1.37) that block inactivation by regulatory subunit (R) without compromising catalytic activity. Randomly mutagenized mouse C expression vectors were screened functionally for clones that stimulated gene induction in the presence of excess R. Point mutations in the C coding sequence were identified that result in a His----Gln substitution at amino acid 87 (His87Gln) and a Trp----Arg change at amino acid 196 (Trp196Arg). In contrast to wild-type C, both mutants retained partial activity in the presence of excess R isoform RI alpha, although only Trp196Arg retained partial activity in the presence of excess R isoform RII alpha. A C expression vector that included both mutations was fully active in promoting gene induction and was virtually unaffected by an 80-fold excess of either RI alpha or RII alpha. These results demonstrate that mutations at His-87 and Trp-196 alter R interactions with C at a site that is not involved in substrate recognition or enzymatic activity. In contrast to these randomly generated mutations, a site-specific alteration of the autophosphorylated Thr-197 to an Ala resulted in an 80% loss of biological activity and partial resistance to R inhibition. The location and proximity of His-87 and Trp-196 in the crystal structure of C suggest a surface domain that may interact with a region of R that is outside of the substrate/pseudosubstrate site.