Protein kinase A-mediated phosphorylation of the Broad-Complex transcription factor in silkworm suppresses its transcriptional activity

The insect-specific transcription factor Broad-Complex (BR-C) is transcriptionally activated by the steroid 20-hydroxyecdysone (20E) and regulates the expression of many target genes involved in insect growth and development. However, although the transcriptional regulation of BR-C proteins has been well studied, how BR-C is regulated at post-transcription and -translation levels is poorly understood. To this end, using liquid chromatography-tandem mass spectrometry analysis, we identified residue Ser-186 as a phosphorylation site of BR-C in silkworm. Site-directed mutagenesis and treatment with specific kinase activators and inhibitors indicated that the Ser-186 residue in silkworm BR-C is phosphorylated by protein kinase A (PKA). Immunostaining assays disclosed that PKA-mediated phosphorylation of silkworm BR-C has no effect on its nuclear import. However, luciferase reporter analysis, electrophoretic mobility shift assays, and chromatin immunoprecipitation revealed that the PKA phosphorylation event suppresses the transcriptional activation of silkworm BR-C target genes and that this inhibition was caused by repression of BR-C binding to its DNA targets. Of note, both in vitro and ex vivo experiments disclosed that a continuous 20E signal inhibits the PKA-mediated BR-C phosphorylation and also the cAMP/PKA pathway, indicating that 20E's inhibitory effect on PKA-mediated phosphorylation of silkworm BR-C contributes to maintaining BR-C transcriptional activity. In conclusion, our findings indicate that PKA-mediated phosphorylation inhibits silkworm BR-C activity by interfering with its binding to DNA and that 20E signaling relieves PKA-mediated phosphorylation of BR-C, thereby maintaining its transcriptional activity.

Protein kinase catalytic subunit (PKAcat) from bovine lens: purification, characterization and phosphorylation of lens crystallins

The purification and functional characterization of protein kinase A catalytic subunit (PKAcat) from bovine lens cytosol has been described. Purification to homogeneity has been achieved by using 100 kDa cut-off membrane filtration followed by Sephacryl S-300 chromatography and finally fractionating on High Q anion exchange column. The purified protein migrates as a single band of molecular mass approximately 41 kDa on 12.5% SDS-PAGE. Proteomic data from ion trap LC-MS when analyzed through NCBI blast program reveals significant homology (52%) with bovine zeta-crystallin and also some homology with pig casein kinase I alpha chain (38%) and SLA-DR1 beta 1 domain (38%). The search does not indicate homology with any known catalytic subunit of PKA. Inspite of the significant homology with the zeta-crystallin, our protein is different from it in terms of molecular mass. pI value of the kinase (5.3) obtained from 2D analysis is also different from zeta-crystallin (8.5). The protein is found to contain 17% alpha-helix, 26.5% beta-sheet, 21.4% turn and 34.7% random coil. The active catalytic subunit of the bovine lens cAMP-dependent kinase belongs to Type I Calpha subtype. The enzyme shows maximum activity at 30 min incubation in presence of 5 mM MgCl(2 )and 50 microM ATP. The kinase shows broad substrate specificity. It prefers Ser over Thr as phosphorylating residue. Phosphorylation of crystallin proteins, major protein fraction of bovine lens and phosphorylation of chaperone protein alpha crystallin by the kinase suggests that the kinase plays some crucial role in regulation of chaperone function within lens.

Mimicking cAMP-dependent allosteric control of protein kinase A through mechanical tension

We report the activation of an enzyme complex by mechanical tension. Protein kinase A, a tetrameric enzyme that, in the cell, is allosterically controlled by cAMP, has been modified by the insertion of a "molecular spring" on the regulatory subunit. The spring is made of DNA, and its stiffness can be varied externally by hybridization to a complementary strand. This allows us to exert a controlled mechanical tension between the two points on the protein's surface where the spring is attached. We show that upon applying the tension, we can activate the enzyme with efficiency comparable to the activation by its natural regulatory molecule, cAMP.

Catalytic subunit of cAMP-dependent protein kinase from a catch muscle of the bivalve mollusk Mytilus galloprovincialis: purification, characterization, and phosphorylation of muscle proteins

cAMP-dependent protein kinase (PKA) plays a crucial role in the release of the catch state of molluskan muscles, but the nature of the enzyme in such tissues is unknown. In this paper, we report the purification of the catalytic (C) subunit of PKA from the posterior adductor muscle (PAM) of the sea mussel Mytilus galloprovincialis. It is a monomeric protein with an apparent molecular mass of 40.0+/-2.0kDa and Stoke's radius 25.1+/-0.3A. The protein kinase activity of the purified enzyme was inhibited by both isoforms of the PKA regulatory (R) subunit that we had previously characterized in the mollusk, and also by the inhibitor peptide PKI(5-24). On the other hand, the main proteins of the contractile apparatus of PAM were partially purified and their ability to be phosphorylated in vitro by purified PKA C subunit was analyzed. The results showed that twitchin, a high molecular mass protein associated with thick filaments, was the better substrate for endogenous PKA. It was rapidly phosphorylated with a stoichiometry of 3.47+/-0.24mol Pmol(-1) protein. Also, catchin, paramyosin, and actin were phosphorylated, although more slowly and to a lesser extent. On the contrary, myosin heavy chain (MHC) and tropomyosin were not phosphorylated under the conditions used.

Efficacy of the phosphorylation of synthetic peptides by purified catalytic subunit of PKA (PKAcat) from bovine lens depends on the amino acid sequence of the peptides

Protein kinase (PK) A catalytic (PKAcat) subunit was purified to homogeneity from bovine lens using a 100-kDa cut-off membrane filtration followed by different chromatographic procedures. The molecular weight of PKAcat was found to be 41 kDa. The kinase phosphorylates histone IIIs and other synthetic modified peptides of VRKRTLRRL with different amino acid environment. The extent of phosphorylation depends not only on the presence of Ser or Thr (phosphorylating residues) but also on other surrounding amino acid residues. Although some peptides compete in phosphorylating histone, they are not very significant. The result suggests that the extent of phosphorylation depends on the amino acid residue(s) surrounding phosphorylable residue(s) on the peptide.

Identification and characterization of novel PKA holoenzymes in human T lymphocytes

Cyclic AMP-dependent protein kinase (PKA) is a holoenzyme that consists of a regulatory (R) subunit dimer and two catalytic (C) subunits that are released upon stimulation by cAMP. Immunoblotting and immunoprecipitation of T-cell protein extracts, immunofluorescence of permeabilized T cells and RT/PCR of T-cell RNA using C subunit-specific primers revealed expression of two catalytically active PKA C subunits C alpha1 (40 kDa) and C beta2 (47 kDa) in these cells. Anti-RI alpha and Anti-RII alpha immunoprecipitations demonstrated that both C alpha1 and C beta2 associate with RI alpha and RII alpha to form PKAI and PKAII holoenzymes. Moreover, Anti-C beta2 immunoprecipitation revealed that C alpha1 coimmunoprecipitates with C beta2. Addition of 8-CPT-cAMP which disrupts the PKA holoenzyme, released C alpha1 but not C beta2 from the Anti-C beta2 precipitate, indicating that C beta2 and C alpha1 form part of the same holoenzyme. Our results demonstrate for the first time that various C subunits may colocate on the same PKA holoenzyme to form novel cAMP-responsive enzymes that may mediate specific effects of cAMP.

Synergistic activation of insect cAMP-dependent protein kinase A (type II) by cyclicAMP and cyclicGMP

The high cGMP sensitivity of cAMP-dependent protein kinase A (type II) (PKAII) from invertebrates led to the hypothesis that cGMP directly activates PKAII under physiological conditions. We tested this idea using PKAII holoenzyme purified from the honeybee brain in an assay with short stimulation times. In the presence of very low cAMP concentrations, we found a synergistic increase in PKAII activation by physiological cGMP concentrations. Cloning honeybee regulatory subunit RII and phylogenetic comparison of the two cyclic nucleotide-binding sites of RII reveal a high relation of domain A of insect RII with cGMP-binding domains of cGMP-dependent protein kinases.

Protein kinase A and signal transduction in T lymphocytes: biochemical and molecular methods

Abnormal T-cell effector functions in systemic lupus erythematosus (SLE) are present and may be associated with disease immunopathogenesis. Our work has led to the characterization of a signaling defect, involving protein kinase A (PKA), leading to abnormal T-cell effector functions in SLE. PKA is a component of the adenylyl cyclase/cyclic adenosine monophosphate/PKA (AC/cAMP/PKA) pathway, a principal signal transduction system in T cells. The aim of this chapter is to provide a comprehensive, technical, step-by-step approach to studying PKA function in T cells. The methods detailed here are (a) chromatographic fractionation of PKA-I and PKA-II isozymes and PKA phosphotransferase activity in purified T cell populations, (b) Western immunoblotting to identify the presence of regulatory (R)-subunit proteins of PKA, and (c) isolation of RNA, and quantification of PKA R subunit-specific transcripts by competitive polymerase chain reaction. Although our emphasis in the chapter is T cells, these methods may be useful for investigation of signaling via PKA in other cell types as well.

Direct comparison of inositol phosphoglycan with prostaglandylinositol cyclic phosphate, two potential mediators of insulin action

Though insulin signalling is thought by many groups to function without second messenger action, others have provided evidence for the existence and action of such regulators. Chemically quite different compounds, however, have been proposed as mediators, such as various inositol phosphoglycans and prostaglandylinositol cyclic phosphate (cyclic PIP). In spite of marked structural differences, these compounds are reported to have the same regulatory properties, i.e. to activate protein ser/thr phosphatases and to inhibit protein kinase A. In order to clarify this discrepancy, the regulatory potency of these different compounds was assayed under identical conditions. It was found that in contrast to cyclic PIP, the synthetic inositol phosphoglycan PIG41 neither directly inhibited protein kinase A nor activated protein ser/thr phosphatases. However, when added to intact cells, such as primary adipocytes, PIG41 inhibited isoproterenol-stimulated lipolysis. This effect most likely results from tyrosine phosphorylation of insulin receptor substrates (IRSs) by PIG41. This tyrosine phosphorylation is not carried out by the insulin receptor tyrosine kinase but by cytosolic tyrosine kinases. This indicates that cyclic PIP, an intracellular regulator, which primarily acts on protein kinase A and on protein ser/thr phosphatases, operates more downstream in the signal transduction cascade as compared to the inositol phosphoglycan PIG41. Thus, cyclic PIP appears to be a suitable candidate to close the gap between IRSs and the protein kinases/phosphatases involved in the signal transduction of insulin.

Characterization of the cAMP-dependent protein kinase catalytic subunit Cγ expressed and purified from sf9 cells

The Cgamma and Calpha subunits of the cAMP-dependent protein kinase (PKA) contain 350 amino acids that are highly homologous (83% amino acid sequence), with 91% homology within the catalytic domain (a.a. 40-300). Unlike Cgamma, the Calpha subunit has been readily purified and characterized as a recombinant protein in vitro, in intact cells, and in vivo. This report describes for the first time the expression, purification, and characterization of Cgamma. The expression of active Cgamma was eukaryote-specific, from mammalian and insect cells, but not bacteria. Active recombinant Cgamma was optimally expressed and purified to homogeneity from Sf9 cells with a 273-fold increase in specific activity and a 21% recovery after sequential CM-Sepharose and Sephacryl S-300 chromatography. The specific activity of pure Cgamma was 0.31 and 0.81 U/mg with kemptide and histone as substrates, respectively. Physical characterization showed Cgamma had a lower apparent molecular weight and Stokes radii than Calpha, suggesting differences in tertiary structures. Steady-state kinetics demonstrated that like Calpha and Cbeta, Cgamma phosphorylates substrates requiring basic amino acids at P-3 and P-2. However, Cgamma generally exhibited a lower Km and Vmax than Calpha for peptide substrates tested. Cgamma also exhibited a distinct pseudosubstrate specificity showing inhibition by homogeneous preparations of RIalpha and RIIalpha-subunits, but not by pure recombinant protein kinase inhibitors PKIalpha and PKIbeta, PKA-specific inhibitors. These studies suggest that Cgamma and Calpha exhibit differences in structure and function in vitro, supporting the hypothesis that functionally different C-subunit isozymes could diversify and/or fine-tune cAMP signal transduction downstream of PKA activation.

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

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

Purification, crystallization and preliminary X-ray diffraction studies of a complex between G protein-coupled receptor kinase 2 and Gbeta1gamma2

G protein-coupled receptor kinase 2 (GRK2) phosphorylates activated G protein-coupled receptors (GPCRs), which ultimately leads to their desensitization and/or downregulation. The enzyme is recruited to the plasma membrane via the interaction of its carboxyl-terminal pleckstrin-homology (PH) domain with the beta and gamma subunits of heterotrimeric G proteins (Gbetagamma). An improved purification scheme for GRK2 has been developed, conditions under which GRK2 forms a complex with Gbeta(1)gamma(2) have been determined and the complex has been crystallized in CHAPS detergent micelles. Crystals of the GRK2-Gbetagamma complex belong to space group C2 and have unit-cell parameters a = 187.0, b = 72.1, c = 122.0 A, beta = 115.2 degrees. A complete data set has been collected to 3.2 A resolution with Cu Kalpha radiation.

Isolation and characterization of the regulatory subunit of cAMP-dependent protein kinase from the filarial parasite Onchocerca volvulus

Protein kinases exert major regulatory effects in eukaryotic signaling events. As these proteins play central regulatory and sensory functions they are interesting targets for antiparasitic drug development and serve as vaccine candidates. A cDNA with an open reading frame of 1122 bp coding for the regulatory subunit of the cAMP-dependent protein kinase (Ov-pka-r) of the pathogenic human nematode Onchocerca volvulus has been isolated. The predicted protein displays 84% homology to the corresponding protein of Caenorhabditis elegans and 71% to the human homologue. The O. volvulus protein has unique features, it includes six cysteine residues, as compared to four residues in mammals. Ov-PKA-r was recombinantly expressed as His-tagged protein and under reducing conditions showed a molecular mass of 52 kDa. In sera from O. volvulus patients IgG antibodies were found that strongly reacted with the recombinant Ov-PKA-r. Using rabbit antisera raised against the recombinant protein for immunohistology allowed the localization of the native Ov-PKA-r within the nervous system and sensory organs of adult O. volvulus worms and of microfilariae. The predominant expression in the nervous system and sensory organs as well as the unique structural features identify this signaling molecule of O. volvulus as a new and interesting target for drug or vaccine development.

Spotting and quantification of phosphoproteins purified by gel electrophoresis and laser ablation-element mass spectrometry with phosphorus-31 detection

Laser ablation inductively coupled plasma-mass spectrometry (ICP-MS) with (31)P detection has been used for spotting of phosphoproteins after one-dimensional polyacrylamide gel electrophoresis (1-D PAGE) and membrane transfer. By analyzing a mixture of myoglobin, alpha-casein and reduced fibrinogen it is demonstrated that phosphoproteins are specifically recognized by this method. A special washing step was found to be necessary to remove phosphate noncovalently bound to proteins. The (31)P signal was found to contain quantitative information both with respect to relative and absolute amounts of phosphorus present in phosphoproteins. Normalizing the (31)P signal from a single laser ablation trace by the total amount of phosphoprotein applied to the gel, a detection limit of 5 pmol of phosphorus is estimated.

High-throughput screening of kinase inhibitors by multiplex capillary electrophoresis with UV absorption detection

Protein kinases play a major role in the transformation of cells and are often used as molecular targets for the new generation of anticancer drugs. We present a novel technique for high-throughput screening of inhibitors of protein kinases. The technique involves the use of multiplexed capillary electrophoresis (CE) for the rapid separation of the peptides, phosphopeptides, and various inhibitors. By means of UV detection, diversified peptides with native amino acid sequences and their phosphorylated counterparts can be directly analyzed without the need for radioactive or fluorescence labeling. The effects of different inhibitors and their IC(50) value were determined using three different situations involving the use of a single purified kinase, two purified kinases, and crude cell extracts, respectively. The results suggest that multiplexed CE/UV may prove to be a straightforward and general approach for high-throughput screening of compound libraries to find potent and selective inhibitors of the various protein kinases.

Evidence for cAMP-dependent protein kinase in the dinoflagellate, Amphidinium operculatum

A cAMP dependent protein kinase (PKA) was identified in the dinoflagellate Amphidinium operculum. In vitro kinase activity towards kemptide, a PKA-specific substrate, was not detectable in crude lysates. However, fractionation of dinoflagellate extracts by gel filtration chromatography showed PKA-like activity toward kemptide at approximately 66 kDa. These findings suggest that possible low molecular mass inhibitors in crude lysates were removed by the gel filtration chromatography. Pre-incubation of extracts with cAMP prior to chromatography resulted in an apparent molecular mass shift in the in vitro kinase assay to 40 kDa. An in-gel kinase assay reflected activity of the free catalytic subunit at approximately 40 kDa. Furthermore, western blotting with an antibody to the human PKA catalytic subunit confirmed a catalytic subunit with a mass of approximately 40 kDa. Results from this study indicate that the PKA in A. operculatum has a catalytic subunit of similar size to that in higher eukaryotes, but with a holoenzyme of a size suggesting a dimeric, rather than tetrameric structure.

Catalytic subunit of protein kinase A caged at the activating phosphothreonine

Caged reagents are photoactivatable molecules with applications in biological research. While a great deal of work has been carried out on small caged molecules, less has been done on caged macromolecules, such as proteins. Caged proteins would be especially useful in signal transduction research. Since most proteins involved in cell signaling are regulated by phosphorylation, a means to cage phosphorylated proteins would be generally applicable. Here we show that the catalytic subunit of protein kinase A can be activated by thiophosphorylation at Thr-197. The modified protein can then be caged with 4-hydroxyphenacyl bromide to yield a derivative with a specific catalytic activity that is reduced by approximately 17-fold. Upon photolysis at near UV wavelengths, an approximately 15-fold increase in activity is observed, representing an approximately 85-90% yield of uncaged product with a quantum yield phi(P) = 0.21. Because protein kinases belong to a superfamily with structurally related catalytic domains, the protein chemistry demonstrated here should be applicable to a wide range of signaling proteins.

Protein kinase A: purification and characterization of the enzyme from two cold-hardy goldenrod gall insects

The catalytic subunit of protein kinase A (PKAc) was purified to apparent homogeneity from two species of cold-hardy goldenrod gall insects, Epiblema scudderiana and Eurosta solidaginis. Final specific activity for both enzymes was approximately 74.5 nmol of phosphate transferred per minute per milligram protein. Molecular weights were 41 and 40 kDa for E. scudderiana and E. solidaginis PKAc, respectively. K(m) values at 24 degrees C for the artificial substrate, Kemptide, were 38.1+/-4.9 and 3.67+/-0.11 microM for E. scudderiana and E. solidaginis PKAc, respectively, whereas K(m) Mg-ATP values were 61.1+/-6.9 and 30.7+/-4.1 microM. Assay at 4 degrees C lowered the K(m) for Kemptide of E. scudderiana PKAc by 55% and addition of 1M glycerol further lowered the K(m). Low assay temperature also enhanced holoenzyme dissociation in both species with the K(a) value for cyclic 3'5'-monophosphate at 4 degrees C lowered to just 13-18% of the value at 24 degrees C. Low temperature did not affect affinity for Mg-ATP or inhibition by PKA inhibitors (PKAi, H7, H89) but increased inhibition by some salts. PKAc from both species showed a break in the Arrhenius relationship at approximately 10 degrees C which suggests a conformational change at low temperature; activation energies (E(a)) were 2.2-3 fold higher for the lower (<10 degrees C) versus higher (>10 degrees C) range. Addition of naturally occurring polyols, 1M glycerol or 0.4M sorbitol, affected E(a) in some cases. Temperature dependent regulation of holoenzyme dissociation and PKAc kinetic properties may have an role in regulating the enzymes involved in polyol synthesis in cold-hardy insects.

Exploring the mechanisms of vascular smooth muscle tone with highly specific, membrane-permeable inhibitors of cyclic GMP-dependent protein kinase Ialpha

The structural similarity of cyclic GMP-dependent protein kinase (cGPK) and cyclic AMP-dependent protein kinase (cAPK) has made it difficult to study cGPK pathways independent of those mediated by cAPK, primarily due to the lack of potent and selective cGPK inhibitors. We recently reported a novel peptide library screen specifically designed to select for tight-binding peptides that identified selective inhibitors of cGPK [Proc Natl Acad Sci USA, 97 (2000) 14772]. Iterative deconvolution of octameric library arrays on paper identified the sequence LRK(5)H (W45). Binding of W45 to cGPK resulted in selective inhibition of the kinase, with K(i) values of 0.8 microM and 560 microM for cGPK and cAPK, respectively. Cellular internalization of highly charged W45 was accomplished by N-terminal fusion of membrane translocation sequences from either the human immunodeficiency virus tyrosine aminotransferase protein (47-59) DT-2 or from the Drosophila Antennapedia homeodomain (43-58) DT-3, respectively. For both fusion peptides, DT-2 and DT-3, we observed a potentiating effect with respect to the inhibitory potency, with K(i) values 40- to 80-fold lower than W45. Fluorescein-labeled DT-2 and DT-3 demonstrated rapid translocation through the cytosol and nuclei in a time-dependent manner using cultured cells and intact tissue samples (cerebral arteries). The physiological effects of DT-2 and DT-3 as selective cGPK inhibitors in smooth muscle were studied in small intact arteries. Nitric oxide, a cyclic GMP/cGPK activator, elicited a concentration-dependent dilation of isolated rat cerebral arteries, which was markedly inhibited by DT-2 and DT-3. Collectively, these results indicate that DT-2 and DT-3 effectively inhibit nitric oxide-induced vasodilation, further emphasizing the central role for cGPK in the modulation of vascular contractility.

Protein kinase and phosphatase responses to anoxia in crayfish, Orconectes virilis: purification and characterization of cAMP-dependent protein kinase

The freshwater crayfish, Orconectes virilis, shows good anoxia tolerance, enduring 20 h in N(2)-bubbled water at 15 degrees C. Metabolic responses to anoxia by tolerant species often include reversible phosphorylation control over selected enzymes. To analyze the role of serine/threonine kinases and phosphatases in signal transduction during anoxia in O. virilis, changes in the activities of cAMP-dependent protein kinase (PKA) and protein phosphatases 1, 2A, and 2C were measured in tail muscle and hepatopancreas over a time course of exposure to N(2)-bubbled water. A strong increase in the percentage of PKA present as the free catalytic subunit (% PKAc) occurred between 1 and 2 h of anoxia exposure whereas phosphatase activities were strongly reduced. This suggests that PKA-mediated events are important in the initial response by tissues to declining oxygen availability. As oxygen deprivation became severe and prolonged (5-20 h) these changes reversed; the % PKAc fell to below control values and activities of phosphatases returned to or rose above control values. Subcellular fractionation also showed a decrease in PKA associated with the plasma membrane after 20 h anoxia whereas cytosolic PKA content increased. PKAc purified from tail muscle showed a molecular weight of 43.8+/-0.4 kDa, a pH optimum of 6.8, a high affinity for Mg ATP (K(m)=131.0+/-14.4 microM) and Kemptide (K(m)=31.6+/-5.2 microM). Crayfish PKAc was sensitive to temperature change; a break in the Arrhenius plot occurred at approximately 15 degrees C with a 2.5-fold rise in activation energy at temperatures <15 degrees C. These studies demonstrate a role for serine/threonine protein kinases and phosphatases in the metabolic adjustments to oxygen depletion by crayfish organs.

Cyclic AMP-dependent protein kinase isoenzymes in human myeloid leukemia (HL60) and breast tumor (MCF-7) cells

Combinations of retinoic acid (RA) and cAMP mediate many biological responses in a large variety of cell types. While the basis for the apparent synergistic effects of RA and cAMP are not clearly defined, it is likely that activation of PKA by cAMP is involved. However, literature reports concerning the identity of PKA isoforms in HL60 and MCF-7 cells are conflicting. The purpose of the present investigation is to identify PKA isoforms in HL60 and MCF-7 cells. Utilization of high-performance anion-exchange liquid chromatography, immunoblotting, and 8-azido-cAMP photoaffinity binding resulted in the finding that HL60 cells contain PKA types I alpha and II alpha, while MCF-7 cells contain PKA types I alpha, II alpha, and II beta. PKA type I alpha in both HL60 and MCF-7 cells eluted from columns as two well-separated peaks. One peak eluted at a low salt concentration in agreement with previous reports. The second HL60 PKA type I alpha peak eluted at a salt concentration intermediate between that eluting the first peak and that eluting PKA type II alpha and contained approximately 62% of the total RI alpha protein. However, the second MCF-7 PKA type I alpha peak contained approximately 66% of the total RI alpha protein and co-eluted with PKA types II alpha and II beta. This "contamination" of PKA type II fractions with PKA type I has led, in some cases, to interpretations that may need reevaluation.

Protein kinase A RI beta subunit deficiency in lupus T lymphocytes: bypassing a block in RI beta translation reconstitutes protein kinase A activity and augments IL-2 production

A profound deficiency of type I protein kinase A (PKA-I or RIalpha/beta2C2) phosphotransferase activity occurs in the T lymphocytes of 80% of subjects with systemic lupus erythematosus (SLE), an autoimmune disorder of unknown etiology. This isozyme deficiency is predominantly the product of reduced or absent beta isoform of the type I regulatory subunit (RIbeta). Transient transfection of RIbeta cDNAs from SLE subjects into autologous T cells that do not synthesize the RIbeta subunit bypassed the block, resulting in RIbeta subunit synthesis and restoration of the PKA-Ibeta (RIbeta2C2) holoenzyme. Transfected T cells activated via the T cell surface receptor complex revealed a significant increase of cAMP-activatable PKA activity that was associated with a significant increase in IL-2 production. These data demonstrate that a disorder of RIbeta translation exists, and that correction of the PKA-I deficiency may enhance T lymphocyte effector functions in SLE.

Type II beta regulatory subunit of cAMP-dependent protein kinase: purification strategies to optimize crystallization

To elucidate the structural basis for important differences between types I and II regulatory subunit isoforms (RI and RII) of adenosine 3',5'-cyclic monophosphate (cAMP)-dependent protein kinase, the full-length RII beta isoform and five RII beta deletion mutants were constructed, expressed, purified, and screened for crystallization. Only one of these six proteins yielded diffraction quality crystals. Crystals were grown of the RII beta deletion mutant (delta 1-111) monomer potentially in complex with two cAMP molecules. X-ray diffraction quality data were obtained only after significant modification to existing purification procedures. Modifications required a Sepharose, not agarose, support for cAMP affinity chromatography followed by rapid, quantitative removal of free cAMP by size-exclusion chromatography under reducing conditions. Data to 2.4 A resolution were collected at 29 degrees C using synchrotron radiation on a single crystal measuring 0.2 x 0.3 x 1.2 mm(3). Data were 99% complete. The hexagonal crystal belonged to space group P6((1)) or P6((5)) with unit cell dimensions a = b = 161.62 A and c = 39.66 A.

The protein kinase A catalytic subunit Cbeta2: molecular characterization and distribution of the splice variant

Cbeta2, a 46 kDa splice variant of the Cbeta isoform, is the largest isoform so far described for catalytic subunits from cAMP-dependent protein kinase in mammals. It differs from Cbeta in the first 15 N-terminal residues which are replaced with a 62-residue domain with no similarity to other known proteins. The Cbeta2 protein was identified in cardiac tissue by MS, microsequencing and C-subunit-isoform-selective antibodies. The Cbeta2 protein has a very low abundance of about 2% of total affinity-purified C subunits from bovine cardiac tissue. This, and the similarity of its biochemical properties to Calpha and Cbeta, are probably some of the reasons why the Cbeta2 protein has escaped detection so far. The abundance of the Cbeta2 protein differs dramatically between tissues, with most protein detected in heart, liver and spleen, and the lowest level in testis. Cbeta2 protein shows kinase activity against synthetic substrates, and is inhibited by the protein kinase inhibitor peptide PKI(5-24). The degree of Cbeta2 removal from tissue extracts by binding to PKI(5-24) depends on the cAMP level, i.e. on the dissociation state of the holoenzyme. Two sites in the protein are phosphorylated: Thr-244 in the activation segment and Ser-385 close to the C-terminus. By affinity purification and immunodetection Cbeta2 was found in cattle, pig, rat, mouse and turkey tissue and in HeLa cells. In the cAMP-insensitive CHO 10260 cell line, which has normal Cbeta but is depleted of Calpha, stable transfection with Cbeta2 restored most of the cAMP-induced morphological changes. Cbeta2 is a ubiquitously expressed protein with characteristic properties of a cAMP-dependent protein kinase catalytic subunit.

Bi-substrate analogue ligands for affinity chromatography of protein kinases

Novel affinity ligands, consisting of ATP-resembling part coupled with specificity determining peptide fragment, were proposed for purification of protein kinases. Following this approach affinity sorbents based on two closely similar ligands AdoC-Aoc-Arg4-Lys and AdoC-Aoc-Arg4-NH(CH2)6NH2, where AdoC stands for adenosine-5'-carboxylic acid and Aoc for amino-octanoic acid, were synthesized and tested for purification of recombinant protein kinase A catalytic subunit directly from crude cell extract. Elution of the enzyme with MgATP as well as L-arginine yielded homogeneous protein kinase A preparation in a single purification step. Also protein kinase A from pig heart homogenate was selectively isolated using MgATP as eluting agent. Protein kinase with acidic specificity determinant (CK2) as well as other proteins possessing nucleotide binding site (L-type pyruvate kinase) or sites for wide variety of different ligands (bovine serum albumin) did not bind to the column, pointing to high selectivity of the bi-functional binding mode of the affinity ligand.

A crystallizable form of RIIbeta regulatory domain obtained by limited proteolysis

The type RIIbeta regulatory subunit of protein kinase A is primarily expressed in adipose tissue and brain. Knockout mice suggest a role for RIIbeta in regulating energy balance and adipose-tissue content, thus making it a potential target for therapeutic intervention in obesity. A truncated version of the RIalpha subunit has been used in a crystallographic study and was used here to design an analogous RIIbeta construct. Despite substantial screening, conditions were not found for the crystallization of the truncated RIIbeta subunit. However, limited proteolysis of the full-length RIIbeta subunit identified boundaries of the 'hinge' region and a fragment containing the two cAMP-binding domains which did crystallize. A recombinant version of the fragment was expressed and crystallized for X-ray diffraction studies. The crystals belong to the orthorhombic space group C222, with unit-cell parameters a = 91.6, b = 105.9, c = 85.8 A, and diffracted to at least 2.3 A.

SDS/PAGE characteristics of protein kinases tightly associated with chick embryo brain ribosomes

Protein kinases tightly associated with chick embryo brain ribosomes washed with Triton X-100 and KCl were characterized by their ability to phosphorylate ribosomes and two exogenous substrates, histone IIA and casein. c-AMP-dependent kinase (PKA) and casein kinases (CK1, CK2) were examined in the presence of specific modulators by SDS/PAGE followed by renaturation in gel assay according to Kameshita & Fujisawa (Anal. Biochem. 1989, 183, 139-143). Basing on these data it can be presumed that PKA activity increases, but the levels of CK2 and CK1 decrease during chick embryo development.

Purification and characterization of protein kinase A from liver of the freeze-tolerant wood frog: role in glycogenolysis during freezing

Freeze tolerance by various amphibians includes cryoprotectant production in the form of glucose. Activation of the catalytic subunit of liver cAMP-dependent protein kinase (PKAc) facilitates activation of glycogenolysis, a critical biochemical process necessary for production of glucose. Here, we purified PKAc from Rana sylvatica liver to determine the extent to which cold temperature, which stimulates cryoprotectant production, affected PKAc activity and function. PKAc was purified to greater than 95% homogeneity, with a final specific activity of 71 nmol phosphate transferred/min/mg protein. The molecular weight of frog liver PKAc was 47.6 +/- 1.1 kDa and K(m) values for the phosphate acceptor kemptide and Mg-ATP were 9.0 +/- 0.1 and 51.8 +/- 1.0 microM at 22 degrees C, respectively. K(m) values for both substrates dropped significantly at 5 degrees C. The enzyme was sensitive to specific inhibitors of mammalian PKAc (PKA(i), H89) but was only moderately inhibited by high salt concentrations. Furthermore, salt inhibition was reduced at low temperature. The effect of temperature on enzyme activity indicated a conformational change in PKAc at 10 +/- 2 degrees C, with calculated activation energies of 51 +/- 4 kJ/mol at temperatures above 10 degrees C and 110 +/- 9 kJ/mol below 10 degrees C. PKAc in wood frog liver plays a crucial role in mediating the freeze-induced glycogenolysis that is responsible for the production of 200-300 mM levels of glucose as a cryoprotectant. Differential effects of low temperature on enzyme function, increased substrate affinity and reduced ion inhibition, appear to be central to this role.

Probing the multidomain structure of the type I regulatory subunit of cAMP-dependent protein kinase using mutational analysis: role and environment of endogenous tryptophans

The regulatory R-subunit of cAMP-dependent protein kinase (cAPK) is a thermostable multidomain protein. It contains a dimerization domain at the N-terminus followed by an inhibitor site that binds the catalytic C-subunit and two tandem cAMP-binding domains (A and B). Two of the three tryptophans in the RIalpha subunit, Trp188 and Trp222, lie in cAMP-binding domain A while Trp260 lies at the junction between domains A and B. The unfolding of wild-type RIalpha (wt-RI), monitored by intrinsic fluorescence, was described previously [Leon, D. A., Dostmann, W. R. G., and Taylor, S. S. (1991) Biochemistry 30, 3035 (1)]. To determine the environment of each tryptophan and the role of the adjacent domain in folding and stabilization of domain A, three point mutations, W188Y, W222Y, and W260Y, were introduced. The secondary structure of wt-RI and the point mutants has been studied by far-UV circular dichroism spectropolarimetry (CD). The CD spectra of wt-RI and the three point mutants are practically identical, and the thermal unfolding behavior is very similar. Intrinsic fluorescence and iodide quenching in the presence of increasing urea established that: (a) Trp222 is the most buried, whereas Trp188 is the most exposed to solvent; (b) Trp260 accounts for the quenching of fluorescence when cAMP is bound; and (c) Trp222 contributes most to the intrinsic fluorescence of the wt-RI-subunit, while Trp188 contributes least. For wt-RI, rR(W188Y), and rR(W260Y), removal of cAMP causes a destabilization, while excess cAMP stabilizes these three proteins. In contrast, rR(W222Y) was not stabilized by excess cAMP.

Purification and characterization of cAMP dependent protein kinase from Microsporum gypseum

A cyclic AMP dependent protein kinase (PKA), its regulatory (R) and catalytic (C) subunits were purified to homogeneity from soluble extract of Microsporum gypseum. Purified enzyme showed a final specific activity of 277.9 nmol phosphate transferred min(-1) mg protein(-1) with kemptide as substrate. The enzyme preparation showed two bands with molecular masses of 76 kDa and 45 kDa on sodium dodecyl polyacrylamide gel electrophoresis. The 76 kDa subunit was found to be the regulatory (R) subunit of PKA holoenzyme as determined by its immunoreactivity and the isoelectric point of this subunit was 3.98. The 45 kDa subunit was found to be the catalytic (C) subunit by its immunoreactivity and phosphotransferase activity. Gel filtration using Sepharose CL-6B revealed the molecular mass of PKA holoenzyme to be 240 kDa, compatible with its tetrameric structure, consisting of two regulatory subunits (76 kDa) and two catalytic subunits (45 kDa). The specificity of enzyme towards protein acceptors in decreasing order of phosphorylation was found to be kemptide, casein, syntide and histone IIs. Purified enzyme had apparent K(m) values of 71 microM and 25 microM for ATP and kemptide, respectively. Phosphorylation was strongly inhibited by mammalian PKA inhibitor (PKI) but not by inhibitors of other protein kinases. The PKA showed maximum activity at pH 7.0 and enzyme activity was inhibited in the presence of N-ethylmaleimide (NEM) which shows the involvement of sulfhydryl groups for the activity of PKA. PKA phosphorylated a number of endogenous proteins suggesting the multifunctional role of cAMP dependent protein kinase in M. gypseum. Further work is under progress to identify the natural substrates of this enzyme through which it may regulate the enzymes involved in phospholipid metabolism.

The localization and activity of cAMP-dependent protein kinase affect cell cycle progression in thyroid cells

cAMP signals are received and transmitted by multiple isoforms of cAMP-dependent protein kinases (PKAs), typically determined by their specific regulatory subunits. We describe changes in the cAMP signal transduction pathway during cell cycle progression in synchronized rat thyroid cells. Both PKA type II (PKAII) localization and nuclear cAMP signaling are significantly modified during G(0) and G(1)-S transitions. G(1) is characterized by PKA activation and amplified cAMP signal transduction. This is associated with a decrease in the concentration of RI and RII regulatory subunits and enhanced anchoring of PKAII to the Golgi-centrosome region. Just prior to S, the cAMP pathway is depressed. Up-regulation of the pathway by exogenous cAMP in G(1) inhibited the subsequent decay of the Cdk inhibitor p27 and delayed the onset of S phase. Forced translocation of endogenous PKAII to the cytosol down-regulated cAMP signaling, advancing the timing of p27 decay and inducing premature exit from G(1). These data indicate that membrane-bound PKA amplifies the transduction of cAMP signals in G(1) and that the length of G(1) is influenced by cAMP-PKA.

Identification of cAMP-dependent protein kinase holoenzymes in preantral- and preovulatory-follicle-enriched ovaries, and their association with A-kinase-anchoring proteins

Undifferentiated cells from preantral (PA) follicles respond to high levels of cAMP in a different manner than do differentiated cells from preovulatory (PO) follicles. We hypothesized that this differential response of PA and PO cells to cAMP could be due, in part, to either a difference in the profile of isoforms that comprise the cAMP-dependent protein kinase (PKA) holoenzymes and/or a difference in the interaction of PKA with A-kinase-anchoring proteins (AKAPs). To test these hypotheses, PKA activity, PKA holoenzymes, PKA subunits and AKAPs from PA and PO ovaries were compared. Soluble PKA holoenzymes and regulatory (R) subunits were separated by DEAE-cellulose chromatography and sucrose-density-gradient centrifugation. PKA R subunits were distinguished by photoaffinity labelling, autophosphorylation, size, isoelectric point and immunoreactivity. AKAPs were identified by RII subunit overlay assays and immunoreactivity. The results showed that extracts from PA and PO ovaries exhibited equivalent PKA holoenzyme profiles and activities, characterized by low levels of PKA type I (PKAI) holoenzyme and two distinct PKAII holoenzyme peaks, one containing only RIIbeta subunits (PKAIIbeta) and one containing both PKAIIbeta and PKAIIalpha holoenzymes. Both PA and PO ovarian extracts also contained PKA catalytic (C)-subunit-free RIalpha, while only PO ovaries exhibited C-subunit-free RIIbeta. Consistent with the elevated levels of C-subunit-free RIIbeta in PO cells, PKA activation in PO cells required higher concentrations of forskolin than that in PA cells. While extracts of PA and PO ovaries exhibited a number of similar AKAPs, including four prominent ones reactive with anti-AKAP-KL antisera (where AKAP-KL is an AKAP especially abundant in kidney and liver), cAMP-agarose affinity chromatography revealed two major differences in AKAP binding to purified R subunits. PO ovaries contained increased levels of AKAP80 (AKAP of 80 kDa) bound selectively to R subunits in DEAE-cellulose peak 2 (comprising PKAIIbeta and RIalpha), but not to R subunits in DEAE-cellulose peak 3 (comprising PKAIIalpha, PKAIIbeta and RIIbeta). PO ovaries also showed increased binding of R subunits to AKAPs reactive with anti-AKAP-KL antisera at 210, 175, 150 and 115 kDa. Thus in PO ovaries, unlike in PA ovaries, the majority of AKAPs are bound to R subunits. These results suggest that altered PKA-AKAP interactions may contribute to the distinct responses of PA and PO follicles to high levels of cAMP, and that higher cAMP levels are required to activate PKA in PO ovaries.

[Identification of PKC isozymes and effect of knockdown of PKC alpha by antisense oligodeoxynucleotide on iNOS expression via interleukin-1 receptor in vascular smooth muscle cells]

Protein kinase C (PKC) family, is now classified into three groups; conventional (cPKC), novel (nPKC) and atypical (aPKC), and to date, 10 members of isozymes have been identified. We have suggested that PKC is essential to interleukin-1 (IL-1)-triggered expression of inducible NO synthase (iNOS), and that by pharmacological analysis, cPKC is not involved in iNOS induction in rat vascular smooth muscle cells (VSMC). In the present study, we identified some PKC isozymes and investigated the effect of PKC alpha knockdown by antisense oligodeoxynucleotide (AS-ODN) strategy on iNOS expression and nuclear translocation of NF-kappa B in RASMC. Western blot analysis revealed the presence of cPKC (alpha), nPKCs (delta and epsilon) and aPKCs (tau and lambda). Short-time (10-20 min) treatment with phorbol 12-myristate 13-acetate (PMA) induced translocation of PKC alpha from cytosolic to particulate fraction. PKC alpha was completely downregulated by treatment with 100 nM PMA for 24 hours. Treatment with AS-ODN against PKC alpha mRNA depleted PKC alpha specifically, and had no detectable effect on the other PKCs. The production of iNOS mRNA, but not nuclear translocation of NF-kappa B, stimulated by IL-1 beta was decreased by PKC alpha knockdown. These results suggest that there are 5 PKC isozymes in RASMC, and that PKC alpha is involved in iNOS expression triggered by IL-1 beta, supporting our previous pharmacological conclusion.

Purification and characterization of a dimer form of the cAMP-dependent protein kinase from mouse liver cytosol

A protein kinase that phosphorylates histones and polysomal proteins was partially purified from mouse liver cytosol. The active enzyme has a molecular mass of 100 kDa and a phosphorylatable subunit of 54 kDa. Biochemical as well as immunological data suggest that the enzyme is a heterodimer composed of the catalytic subunit of cyclic AMP-dependent protein kinase and the RII regulatory subunit. This RC form does not seem to dissociate upon activation with 3', 5' cyclic AMP and exhibits identical specificity as the classical cAMP-dependent protein kinase (2.7.1.37). The enzyme is affected by the 3', 5' cyclic phosphates of adenosine mainly, but also of guanosine, uridine and cytidine in a substrate-dependent manner. Cyclic nucleotides slightly stimulate phosphate incorporation into histones, while phosphorylation of polysomal proteins in intact polysomes is dramatically increased. The substrate- specific stimulatory effects of 3', 5' cyclic nucleotides are due to repression of the inhibition exerted upon the reaction, by negatively charged macromolecules such as RNA, DNA and to a lesser extent heparin.

PKA from Saccharomyces cerevisiae can be activated by cyclic AMP and cyclic GMP

Analysis of Saccharomyces cerevisiae genome revealed no sequence homologous to cyclic GMP (cGMP) dependent protein kinase from other organisms. Here we demonstrate that cyclic AMP (cAMP) dependent protein kinase purified from S. cerevisiae was almost equally activated by cAMP and cGMP in 3 x 10(-6) M concentrations of either nucleotide in the presence of Mg2+ ions. Interestingly, if Mn2+ ions were used instead of Mg2+, cGMP was only 30% as effective as cAMP in the activation of cAMP-dependent protein kinase. Analogs of cAMP such as 8-chloro-cAMP and 3':5'-cyclic monophosphate of ribofuranosylbenzimidazole were as potent as cAMP in the enzyme activation, while N6,2'-O-dibutyryl-cAMP activated the enzyme to a lower extent. It was also found that yeast cAMP-dependent protein kinase can be activated by limited proteolytic digestion. The results presented were obtained with protamine and ribosomal protein S10 used as phosphorylation substrates.

A method for the purification of cAMP-dependent protein kinase using immunoaffinity chromatography

A rapid and efficient method for purifying cAMP-dependent protein kinase (PKA) holoenzyme based on immunoaffinity chromatography was developed. The affinity column was prepared by coupling a polyclonal antibody raised against the PKA regulatory subunit to NHS-activated Sepharose. The holoenzyme purified by this procedure from the bivalve mollusk Mytilus galloprovincialis was shown to be fully active as judged by (1) its cAMP-binding activity, (2) its cAMP-dependent protein kinase activity, and (3) its autophosphorylation ability. Moreover, together with both regulatory and catalytic subunits, which constitute the PKA holoenzyme, a protein with a molecular mass of approximately 200 kDa was copurified, and results from gel-filtration chromatography showed that it was associated with a fraction of PKA. Therefore, this immunoaffinity purification technique could also be useful to isolate such proteins as interact with PKA in vivo.

Crystallization and preliminary X-ray analysis of the unliganded recombinant catalytic subunit of cAMP-dependent protein kinase

X-ray diffraction-quality crystals of the unliganded mouse recombinant catalytic subunit of cAMP-dependent protein kinase were grown by the hanging-drop vapour-diffusion technique using 2-methyl-2,4-pentanediol as precipitant. The crystals belong to the monoclinic space group P21 with unit-cell parameters a = 48.9, b = 147.4, c = 54.2 A, beta = 110.2 degrees. A data set to 3.0 A resolution with 92% completeness has been collected using synchrotron radiation. The unit cell contains four molecules of molecular weight 40 kDa with a corresponding volume solvent content of 45%.

Purification of a novel isoform of the regulatory subunit of cAMP-dependent protein kinase from the bivalve mollusk Mytilus galloprovincialis

Cytosolic extracts from the posterior adductor muscle of the bivalve mollusk Mytilus galloprovincialis contain significant amounts of both cGMP-binding and cGMP-stimulated protein kinase activities. However, photoaffinity labeling with 8-azido-[32P]cGMP revealed only a major cGMP-binding protein with an apparent molecular mass of 54 kDa (p54), lacking protein kinase activity itself. Instead, the purified and cGMP-free p54 protein has the ability to inhibit a mussel protein kinase homologous to the mammalian cAMP-dependent protein kinase (cAPK) catalytic subunit, the inhibition being relieved by cAMP or cGMP, which suggests that it can act as a regulatory subunit of cAPK. However, p54 failed to be recognized by a specific antibody against the regulatory subunit (type RII) previously isolated from mussel. Therefore, p54 must be a novel isoform of cAPK regulatory subunit that seems to have high affinity for both cGMP and cAMP.

Identification of phosphorylation sites on AChR delta-subunit associated with dispersal of AChR clusters on the surface of muscle cells

The innervation of embryonic skeletal muscle cells is marked by the redistribution of nicotinic acetylcholine receptors (AChRs) on muscle surface membranes into high-density patches at nerve-muscle contacts. To investigate the role of protein phosphorylation pathways in the regulation of AChR surface distribution, we have identified the sites on AChR delta-subunits that undergo phosphorylation associated with AChR cluster dispersal in cultured myotubes. We found that PKC-catalyzed AChR phosphorylation is targeted to Ser378, Ser393, and Ser450, all located in the major intracellular domain of the AChR delta-subunit. Adjacent to one of these sites is a PKA consensus target site (Ser377) that was efficiently phosphorylated by purified PKA in vitro. The PKC activator 12-O-tetradecanoylphorbol-13-acetate (TPA) and the phosphoprotein phosphatase inhibitor okadaic acid (OA) produced increased phosphorylation of AChR delta-subunits on the three serine residues that were phosphorylated by purified PKC in vitro. In contrast, treatment of these cells with the PKA activator forskolin, or with the cell-permeable cAMP analogue 8-bromo-cAMP, did not alter the phosphorylation state of surface AChR, suggesting that PKA does not actively phosphorylate the delta-subunit in intact chick myotubes. The effects of TPA and OA included an increase in the proportion of surface AChR that is extracted in Triton X-100, as well as the spreading of AChR from cluster regions to adjacent areas of the muscle cell surface. These findings suggest that PKC-catalyzed phosphorylation on the identified serine residues of AChR delta-subunits may play a role in the surface distribution of these receptors.

Protein kinase A from bat skeletal muscle: a kinetic study of the enzyme from a hibernating mammal

The catalytic subunit of adenosine 3'-5'-cyclic mono-phosphate-dependent protein kinase (PKAc) was purified to homogeneity from skeletal muscle of the little brown bat, Myotis lucifugus. The purification procedure was highly reproducible, resulting in a final activity of 205 nmol phosphate transferred/min/mg protein at 22 degreesC. Identification of the enzyme as a protein kinase A was confirmed through the use of specific PKA inhibitors. The catalytic subunit had a molecular weight of 54.6 +/- 3.5 kDa.Km values for Kemptide and Mg-ATP were 9.1 +/- 0.2 and 94.1 +/- 4.5 microM at 37 degreesC, respectively. Both values decreased significantly at 5 degreesC to 37 and 52% of their values at the higher temperature. Similar temperature effects on Km values were found with the purified commercial pig heart enzyme. Neutral salts had little effect on enzyme activity (I 50 values >400 mM) but NaF had an I 50 of 38 mM; except for fluoride, ions were less inhibitory at 5 degreesC, compared with 37 degreesC. Arrhenius plots showed evidence of a temperature-dependent conformational change; a distinct break in the plot occurred at 10 degreesC giving calculated activation energies of 5.6 +/- 0. 46 kJ/ mol at temperatures above 10 degreesC++and 29.5 +/- 2.0 kJ/mol below 10 degreesC. Porcine PKAc, by contrast, showed a linear Arrhenius plot over the entire temperature range tested and an intermediate activation energy of 15.9 +/- 0.3 kJ/mol. The pH optimum of bat PKAc also changed dramatically with temperature falling from 8.5 at 37 degreesC to 5.5 at 5 degreesC, an effect that could substantially change enzyme activity in vivo at the low body temperature of the hibernating state. Overall, low temperature had both positive (increased the percentage of PKAc, reduced Km values, increased I 50 values for salts) and negative (increased activation energy, acidic shift of pH optimum) effects on PKAc but the substantial positive effects of low temperature on the enzyme suggest an important role for continued PKA action in signal transduction in the hibernating animal.

Regulation of ciliary beat frequency by both PKA and PKG in bovine airway epithelial cells

Ciliary beating is required for the maintenance of lung mucociliary transport. We investigated the role of cyclic nucleotide-dependent protein kinases in stimulating ciliary beat frequency (CBF) in bovine bronchial epithelial cells (BBECs). cAMP-dependent protein kinase (PKA) activity and cGMP-dependent protein kinase (PKG) activity were distinguished after DEAE-Sephacel chromatography of BBEC extracts. cAMP levels and PKA activity are increased in BBECs stimulated with 0.01-1 mM isoproterenol, with a corresponding increase in CBF. cGMP levels and PKG activity are increased in BBECs stimulated with 0.1-10 microM sodium nitroprusside, with a corresponding increase in CBF. Direct protein kinase-activating analogs of cAMP and cGMP (dibutyryl cAMP and 8-bromo-cGMP, respectively) also activate their specific kinases and stimulate CBF. Preincubation of BBECs with inhibitors of PKA or PKG [KT-5720 or Rp-8-(p-chlorophenylthio)-guanosine 3',5'-cyclic monophosphothioate] results in the inhibition of specific kinase activity as well as in the inhibition of CBF. These studies suggest that the activation of either PKA or PKG can lead to the stimulation of CBF in bovine airway epithelium.

Subcellular distribution of the R-subunits of cAMP-dependent protein kinase in LS-174T human colon carcinoma cells

The analysis of the particulate preparations of LS-174T human colon carcinoma cells in confluent stage of growth revealed different distribution for regulatory subunits (R) of cAMP-dependent protein kinase (PKA) in the subcellular compartments. The LS-174T cell lysates were subjected to differential and discontinuous sucrose density gradient centrifugation. The obtained fractions were assayed for marker enzymes and photoaffinity labeled with 8-N3[32P]cAMP. The whole lysates and cytoplasmic fraction exhibited the presence of both RI alpha and RII alpha--subunits of PKA. The fractions exhibiting high activity of the marker enzymes for plasma membranes, Golgi apparatus and mitochondria contained mainly RII alpha. In the fractions of lysosomes and microsomes RI alpha and RII alpha were found in nearly equal amounts.

Activation of myocardial cAMP-dependent protein kinase by lysoplasmenylcholine

Plasmalogen-specific, calcium-independent phospholipase A2 (iPLA2) is activated during myocardial ischemia. Accordingly, we have assessed the activation of myocardial protein kinases by the iPLA2 product, lysoplasmenylcholine. Lysoplasmenylcholine-activated protein kinase activity from heart cytosol fractionated on a DE-52 column was identified as cAMP-dependent protein kinase (PKA) based on the following: (1) protein kinase activity stimulated by cAMP and lysoplasmenylcholine co-eluted on sequential chromatographic steps; (2) lysoplasmenylcholine-activated protein kinase activity was inhibited by the PKA inhibitor, PKI; and (3) the unprimed PKA form generated from the primed form of PKA was activated by cAMP and lysoplasmenylcholine. These results demonstrate a novel mechanism for PKA activation by lysoplasmenylcholine.

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.

A role for ultraspiracle, the Drosophila RXR, in morphogenetic furrow movement and photoreceptor cluster formation

Many of the same genes needed for proper eye and limb development in vertebrates, such as hairy, hedgehog, patched and cyclic AMP-dependent protein kinase A, are responsible for patterning Drosophila imaginal discs, the tissues that will give rise to the adult cuticle structures. This is well demonstrated in the control of morphogenetic furrow movement and differentiation in the eye imaginal disc. We report that ultraspiracle, the gene encoding the Drosophila cognate of the Retinoid X Receptor, is required for normal morphogenetic furrow movement and ommatidial cluster formation. Examination of the expression of genes involved in regulating the furrow suggests that ultraspiracle defines a novel regulatory pathway in eye differentiation.

Purification of a regulatory subunit of type II cAMP-dependent protein kinase from Drosophila heads

The cytosolic extract from Drosophila heads was separated using anion-exchange column chromatography. Two types of cAMP-dependent protein kinase (PKA), type I and type II, were detected, and type II PKA was found to be a major isozyme. The regulatory subunit of type II PKA (RII) was purified, and only one isoform was observed. The purified protein had an apparent molecular mass of 51 kDa on SDS gel electrophoresis. Partial amino acid sequences of the protein were almost identical with the RII alpha subunit of human. Since PKA has been implicated to be especially important for learning and memory in Drosophila, the RII subunit may play an essential role in the regulation of neuronal activity in the brain of Drosophila, and possibly in human.

The transcriptional activity of NF-kappaB is regulated by the IkappaB-associated PKAc subunit through a cyclic AMP-independent mechanism

Stimulation of cells with inducers of NF-kappaB such as LPS and IL-1 leads to the degradation of IkappaB-alpha and IkappaB-beta proteins and translocation of NF-kappaB to the nucleus. We now demonstrate that, besides the physical partitioning of inactive NF-kappaB to the cytosol, the transcriptional activity of NF-kappaB is regulated through phosphorylation of NF-kappaB p65 by protein kinase A (PKA). The catalytic subunit of PKA (PKAc) is maintained in an inactive state through association with IkappaB-alpha or IkappaB-beta in an NF-kappaB-IkappaB-PKAc complex. Signals that cause the degradation of IkappaB result in activation of PKAc in a cAMP-independent manner and the subsequent phosphorylation of p65. Therefore, this pathway represents a novel mechanism for the cAMP-independent activation of PKA and the regulation of NF-kappaB activity.

A low molecular weight substance purified from human placenta inhibits cAMP-dependent protein kinase and activates protein kinase C

We have purified from human placenta a low molecular mass substance that inhibits cAMP-dependent protein kinase and activates protein kinase C. This protein kinase regulator was purified in three steps: (1) homogenizing placentas in chloroform/methanol and extracting the regulator into water; (2) eluting a strong anion exchange high performance liquid chromatography (HPLC) column with a quaternary gradient; and (3) eluting a reversed-phase HPLC column with a binary gradient. The regulator was found to be highly purified by HPLC, thin-layer chromatography (TLC) and laser desorption ionization mass spectrometry with a molecular mass of 703 Daltons by the latter procedure. The physical and biochemical properties of this protein kinase regulator suggest that it is a phospholipid but it did not co-elute by HPLC or by TLC with any of the known phospholipid activators of protein kinase C.

Receptor and G betagamma isoform-specific interactions with G protein-coupled receptor kinases

The G protein-coupled receptor (GPCR) kinases (GRKs) phosphorylate and desensitize agonist-occupied GPCRs. GRK2-mediated receptor phosphorylation is preceded by the agonist-dependent membrane association of this enzyme. Previous in vitro studies with purified proteins have suggested that this translocation may be mediated by the recruitment of GRK2 to the plasma membrane by its interaction with the free betagamma subunits of heterotrimeric G proteins (G betagamma). Here we demonstrate that this mechanism operates in intact cells and that specificity is imparted by the selective interaction of discrete pools of G betagamma with receptors and GRKs. Treatment of Cos-7 cells transiently overexpressing GRK2 with a beta-receptor agonist promotes a 3-fold increase in plasma membrane-associated GRK2. This translocation of GRK2 is inhibited by the carboxyl terminus of GRK2, a known G betagamma sequestrant. Furthermore, in cells overexpressing both GRK2 and G beta1 gamma2, activation of lysophosphatidic acid receptors leads to the rapid and transient formation of a GRK/G betagamma complex. That G betagamma specificity exists at the level of the GPCR and the GRK is indicated by the observation that a GRK2/G betagamma complex is formed after agonist occupancy of the lysophosphatidic acid and beta-adrenergic but not thrombin receptors. In contrast to GRK2, GRK3 forms a G betagamma complex after stimulation of all three GPCRs. This G betagamma binding specificity of the GRKs is also reflected at the level of the purified proteins. Thus the GRK2 carboxyl terminus binds G beta1 and G beta2 but not G beta3, while the GRK3 fusion protein binds all three G beta isoforms. This study provides a direct demonstration of a role for G betagamma in mediating the agonist-stimulated translocation of GRK2 and GRK3 in an intact cellular system and demonstrates isoform specificity in the interaction of these components.