Chromatographic separation of two heterogeneous forms of the catalytic subunit of cyclic AMP-dependent protein kinase holoenzyme type I and type II from striated muscle of different mammalian species

Identification of multiple isoforms of the cAMP-dependent protein kinase catalytic subunit in the bivalve mollusc Mytilus galloprovincialis

Several isoforms of the cAMP-dependent protein kinase catalytic subunit (C-subunit) were separated from the posterior adductor muscle and the mantle tissues of the sea mussel Mytilus galloprovincialis by cation exchange chromatography, and identified by: (a) protein kinase activity; (b) antibody recognition; and (c) peptide mass fingerprinting. Some of the isozymes seemed to be tissue-specific, and all them were phosphorylated at serine and threonine residues and showed slight but significant differences in their apparent molecular mass values, which ranged from 41.3 to 44.5 kDa. The results from the MS analysis suggest that at least some of the mussel C-subunit isoforms arise as a result of alternative splicing events. Furthermore, several peptide sequences from mussel C-subunits, determined by de novo sequencing, showed a high degree of homology with the mammalian Calpha-isoform, and contained some structural motifs that are essential for catalytic function. On the other hand, no significant differences were observed in the kinetic parameters of C-subunit isoforms, determined by using synthetic peptides as substrate and inhibitor. However, the C-subunit isoforms separated from the mantle tissue differed in their ability to phosphorylate in vitro some proteins present in a mantle extract.

Analysis of posttranslational modifications exemplified using protein kinase A

With the completion of the major genome projects, one focus in biomedical research has shifted from the analysis of the rather static genome to the highly dynamic proteome. The sequencing of whole genomes did not lead to much anticipated insights into disease mechanisms; however, it paved the way for proteomics by providing the databases for protein identification by peptide mass fingerprints. The relative protein distribution within a cell or tissue is subject to change upon external and internal stimuli. Signal transduction events extend beyond a simple change in protein levels; rather they are governed by posttranslational modifications (PTMs), which provide a quick and efficient way to modulate cellular signals. Because most PTMs change the mass of a protein, they are amenable to analysis by mass spectrometry. Their investigation adds a level of functionality to proteomics, which can be expected to greatly aid in the understanding of the complex cellular machinery involved in signal transduction, metabolism, differentiation or in disease. This review provides an overview on posttranslational modifications exemplified on the model system cAMP-dependent protein kinase. Strategies for detection of selected PTMs are described and discussed in the context of protein kinase function.

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.

The amino terminus of PKA catalytic subunit--a site for introduction of posttranslational heterogeneities by deamidation: D-Asp2 and D-isoAsp2 containing isozymes

Conserved deamidation of PKA catalytic subunit isozymes Calpha and Cbeta--more than 25% at Asn2 in vivo in both cases--has been shown to yield Asp2- and isoAsp2-containing isozymes (Jedrzejewski PT, Girod A, Tholey A, König N, Thullner S, Kinzel V, Bossemeyer D, 1998, Protein Sci 7:457-469). Isoaspartate formation in proteins in vivo is indicative of succinimide intermediates involved in both the initial deamidation reaction as well as the "repair" of isoAsp to Asp by the action of protein L-isoaspartyl (D-aspartyl) O-methyl transferase (PIMT). L-Succinimide is prone to racemization to D-succinimide, which may hydrolyze to D-isoAsp- and D-Asp-containing diastereomers with, respectively, no and poor substrate character for PIMT. To analyze native PKA catalytic subunit from cardiac muscle for these isomers the N-terminal tryptic peptides (T1) of the enzyme were analyzed following procedures refined specifically with a set of corresponding synthetic peptides. The methods combined high resolution high-performance liquid chromatography and a new mass spectrometric procedure for the discrimination between Asp- and isoAsp-residues in peptides (Lehmann et al., 2000). The results demonstrate the occurrence of D-isoAsp- and D-Asp-containing T1 fragments in addition to the L-isomers. The small amount of the L-isoAsp isomer, representing only part of the D-isoAsp isomer, and the relatively large amounts of the L-Asp and D-Asp isomers argues for an effective action of PIMT present in cardiac tissue.

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.

Intracellular distribution of mammalian protein kinase A catalytic subunit altered by conserved Asn2 deamidation

The catalytic (C) subunit of protein kinase A functions both in the cytoplasm and the nucleus. A major charge variant representing about one third of the enzyme in striated muscle results from deamidation in vivo of the Asn2 residue at the conserved NH(2)-terminal sequence myrGly-Asn-Ala (Jedrzejewski, P.T., A. Girod, A. Tholey, N. König, S. Thullner, V. Kinzel, and D. Bossemeyer. 1998. Protein Sci. 7:457-469). Because of the increase of electronegativity by generation of Asp2, it is reminiscent of a myristoyl-electrostatic switch. To compare the intracellular distribution of the enzymes, both forms of porcine or bovine heart enzyme were microinjected into the cytoplasm of mouse NIH 3T3 cells after conjugation with fluorescein, rhodamine, or in unlabeled form. The nuclear/cytoplasmic fluorescence ratio (N/C) was analyzed in the presence of cAMP (in the case of unlabeled enzyme by antibodies). Under all circumstances, the N/C ratio obtained with the encoded Asn2 form was significantly higher than that with the deamidated, Asp2 form; i.e., the Asn2 form reached a larger nuclear concentration than the Asp2 form. Comparable data were obtained with a human cell line. The differential intracellular distribution of both enzyme forms is also reflected by functional data. It correlates with the degree of phosphorylation of the key serine in CREB family transcription factors in the nucleus. Microinjection of myristoylated recombinant bovine Calpha and the Asn2 deletion mutant of it yielded N/C ratios in the same range as encoded native enzymes. Thus, Asn2 seems to serve as a potential site for modulating electronegativity. The data indicate that the NH(2)-terminal domain of the PKA C-subunit contributes to the intracellular distribution of free enzyme, which can be altered by site-specific in vivo deamidation. The model character for other signaling proteins starting with myrGly-Asn is discussed.

A conserved deamidation site at Asn 2 in the catalytic subunit of mammalian cAMP-dependent protein kinase detected by capillary LC-MS and tandem mass spectrometry

The N-terminal sequence myr-Gly-Asn is conserved among the myristoylated cAPK (protein kinase A) catalytic subunit isozymes Calpha, Cbeta, and Cgamma. By capillary LC-MS and tandem MS, we show that, in approximately one third of the Calpha and Cbeta enzyme populations from cattle, pig, rabbit, and rat striated muscle, Asn 2 is deamidated to Asp 2. This deamidation accounts for the major isoelectric variants of the cAPK C-subunits formerly called CA and CB. Deamidation also includes characteristic isoaspartate isomeric peptides from Calpha and Cbeta. Asn 2 deamidation does not occur during C-subunit preparation and is absent in recombinant myristoylated Calpha (rCalpha) from Escherichia coli. Deamidation appears to be the exclusive pathway for introduction of an acidic residue adjacent to the myristoylated N-terminal glycine, verified by the myristoylation negative phenotype of an rCalpha(Asn 2 Asp) mutant. This is the first report thus far of a naturally occurring myr-Gly-Asp sequence. Asp 2 seems to be required for the well-characterized (auto)phosphorylation of the native enzyme at Ser 10. Our results suggest that the myristoylated N terminus of cAPK is a conserved site for deamidation in vivo. Comparable myr-Gly-Asn sequences are found in several signaling proteins. This may be especially significant in view of the recent knowledge that negative charges close to myristic acid in some proteins contribute to regulating their cellular localization.

Promoter of the gene encoding the bovine catalytic subunit of cAMP-dependent protein kinase isoform C beta 2

Genomic sequences flanking the 5' end of the cDNA encoding isoform C beta 2 of the catalytic subunit of bovine cAMP-dependent protein kinase were cloned, sequenced and analyzed for promoter activity and transcription initiation sites. A region of 913 bp upstream the translation initiator ATG was amplified from genomic DNA by vectorette polymerase chain reaction. In primer extension reactions and RNase protection assays, residues C (at position -91), T (-71) and G (-70) were found to serve as transcription initiation sites of the gene. Amplification products and sub-fragments thereof were ligated upstream of the reporter gene chloramphenicol acetyltransferase to test for promoter activity. Constructs were transiently transfected into a Chinese hamster ovary cell line which was shown to express endogenous C beta 2 mRNA. The genomic sequence upstream the C beta 2 cDNA does have promoter activity. The region from position -51 to -292 proved sufficient to drive efficient transcription of the reporter gene. The promoter is AT rich (68%), does not contain a TATA box within 50 bp upstream of the first initiation site and possesses putative binding sites for several transcription factors such as PEA-3 and a glucocorticoid receptor.

Anti-head and anti-tail antibodies against distinct epitopes in the catalytic subunit of protein kinase A. Use in the study of the kinase splitting membranal proteinase KSMP

Protein kinases share a considerable sequence homology in their catalytic core (residues 40-300 in PKA). Each core is flanked by "head" and "tail" segments at its amino- and carboxy-termini, which are different in the various kinases. These end segments may play an important role in creating the preferential affinity of each kinase for its physiological substrates or regulatory ligands. Here we describe three anti-peptide antibodies (alpha P-1, alpha P-2, and alpha P-3) that specifically recognize the head and tail segments of the catalytic subunit (C) of PKA. (i) alpha P-1 (against 6A-K23) react with C when denatured but not when in its native structure; (ii) alpha P-2 (against 319K-I335), bind to the site in C cleaved by the kinase splitting membranal proteinase (KSMP) and inhibit this cleavage of C; (iii) alpha P-3 (against 338S-F350) react with C but not with the KSMP cleavage product C', useful for detecting a KSMP-like activity in different tissues and subcellular loci. The combined use of the antibodies described here provides a strict definition of C, and thus a high degree of fidelity in its biorecognition.

Characterization of the catalytic subunit of Trypanosoma cruzi cyclic AMP-dependent protein kinase

The catalytic subunit of cyclic AMP-dependent protein kinase from Trypanosoma cruzi epimastigote forms was purified by ionic-exchange chromatography, affinity chromatography and sucrose gradient centrifugation. Upon polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate, the purified preparations showed a main polypeptide band with a mobility of about 40 kDa. In Western blots this band immunoreacted with a polyclonal antibody specific for the catalytic subunit of bovine heart protein kinase A. Hydrodynamic and molecular parameters of this subunit are as follows: molecular weight, 40,000 +/- 3000; sedimentation constant, 2.8 +/- 0.3 S; Stokes' radius, 2.8 +/- 0.2 nm; frictional ratio, 1.28 +/- 0.05. Purified preparations of T. cruzi catalytic and regulatory subunits reconstitute a holoenzyme with a sedimentation constant. 8.6 +/- 1.17 S. This data together with those previously reported by Ulloa et al. [8] indicate that the T. cruzi cyclic AMP-dependent protein kinase holoenzyme is a tetramer with the structure R2C2 of about 200 kDa. The apparent Km of the catalytic subunit for ATP and histone IIA or kemptide as phosphate donor and acceptor, respectively, were 40 microM, 48.6 microM and 26 microM, respectively.

Cloning of the C alpha catalytic subunit of the bovine cAMP-dependent protein kinase

The bovine C alpha type catalytic subunit of the cAMP-dependent protein kinase was cloned. A partial cDNA was isolated from a bovine heart cDNA library. This clone contained 120 bp of the coding sequence and the entire 3' untranslated region of 1431 bp. The complete coding region was cloned by PCR amplification from total bovine heart and skeletal muscle RNA. The sequence of the 3' oligonucleotide was taken from the partial cDNA clone whereas the 5' oligonucleotide was chosen by comparison of sequences of published C alpha subunits from other species. In the deduced amino acid sequence there is one deviation from the published bovine C alpha protein sequence, aspartic acid 286 is exchanged by an asparagine. The C alpha mRNA was found to be expressed differentially in various bovine tissues.

Atrial natriuretic peptide is phosphorylated by intact cells through cAMP-dependent ecto-protein kinase

Recently we demonstrated the presence of cell-surface-located cAMP-dependent protein kinase (ecto-PK A) activity in a number of different cell types [Kübler, D., Pyerin, W., Bill, O., Hotz, A., Sonka, J. and Kinzel, V. (1989) J. Biol. Chem. 264, 14549-14555]. The question of the physiological role of externally directed kinase activity prompted a search for potential natural substrates present in the intercellular fluid. In the present study we have investigated the phosphorylation by ecto-PK A of the human atrial natriuretic peptide ANP99-126, a hormone released by cardiac cells. This 28-amino-acid peptide carries the phosphorylation consensus sequence Arg-Arg-Ser-Ser for the PK A. Incubation of various cell lines (including epithelial, epidermal, myoblast and lymphoma cells) or freshly isolated blood cells (macrophages, erythrocytes and platelets) with ANP in the presence of low micromolar concentrations of ATP resulted in the phosphorylation of ANP at Ser residues. The ANP phosphorylation reaction proved strictly dependent on cAMP; cAMP could not be replaced by cGMP. The phosphorylation was inhibited by the PK A-specific inhibitory peptide and increased linearily for up to 15 min and with a Km value of 3-5 microM for ANP. At higher ATP concentrations (greater than 100 microM) the incorporation rates amounted to about 0.3 mmol P (mol ANP)-1 min-1. The rise of intracellular cAMP in HEL30 (an epidermal cell line) after application of the beta-adrenergic receptor agonist isoproterenol led to an approximately three-fold stimulation of ANP phosphorylation which appears to be brought about by an efflux of intracellular cAMP. Employing cell supernatant fluids and cell sonicates, it could be shown that the phosphorylation of ANP results from the ecto-PK A. Comparison of ANP with ANP phosphorylated in vitro using purified catalytic subunit of PK A showed that phosphorylation is accompanied by certain changes in the average solution conformation of the peptide, consistent with the changes known to occur in its biological activity. Our results demonstrate cAMP-dependent phosphorylation of the peptide hormone analogue ANP99-126 by intact cells through ecto-PK A, an intriguing mechanism for post-translational processing of ANP.

Rat C alpha catalytic subunit of the cAMP-dependent protein kinase: cDNA sequence and evidence that it is the only isoform expressed in myoblasts

A full length cDNA clone encoding the C alpha type catalytic subunit of cAMP-dependent protein kinase was isolated from a cDNA library of differentiated rat myoblast L6 cell line. The 2137 bp clone codes for a protein of 351 amino acid residues having more than 90% sequence identity to C alpha subunits of other mammalians. The C alpha isoform was found to be the only isoform of catalytic subunits expressed in myoblast cells as was determined in Northern blot analysis.

Phosphorylation and dephosphorylation of the natriuretic peptide urodilatin (CDD-/ANP-95-126) and the effect on biological activity

Urodilatin (CDD-/ANP-95-126), a new peptide hormone from human urine, is comprised of the same amino acid sequence as cardiodilatin (CDD-99-126/alpha-hANP) except for N-terminal extention by four amino acid residues. The presence of the recognition sequence Arg101-Arg-Ser-Ser104 for the cyclic AMP-dependent protein kinase enables rapid phosphorylation in the Ser104-position. Phosphorylation of urodilatin is associated with decreased vasorelaxant potency, while dephosphorylation of "phospho-urodilatin" by acidic phosphatase completely restores bioactivity.

Catalytic subunit of cAMP-dependent protein kinase from bovine heart: several isoforms demonstrated by high resolution focusing in immobilized pH gradient

The catalytic subunit (C) of cAMP-dependent protein kinase holoenzyme type II from bovine cardiac muscle was separated by isoelectric focusing in Immobiline polyacrylamide gels into 9 protein forms. The major forms (i) appeared at pH 7.1, 7.4, 7.5, and 7.7, (ii) exhibited protein kinase activity and were inhibited by heat and acid stable inhibitor, (iii) represented approx. 30%, 4%, 64%, and 1% of the protein respectively, (iv) refocused in the same position from which they had been eluted from the first gel. Antibodies against C detected additional proteins at approx. pH 7.55, 7.75, and 7.8. Two more bands became detectable at approx. pH 7.3 and 7.45 by application of antibody against C beta (Uhler, M.D. & McKnight G.S. 1987, J.Biol.Chem. 262, 15202-15207). The relation of the different forms of C to the fractions CA and CB (Kinzel V. et al. 1987 Arch. Biochem. Biophys. 253, 341-349) is demonstrated.

The inhibitor protein of the cyclic AMP-dependent protein kinase-catalytic subunit interaction. Composition of multiple complexes

It has been previously demonstrated that the combination of pure preparations of the inhibitor protein of the cyclic AMP-dependent protein kinase and the catalytic subunit of this enzyme resulted in the formation of multiple complexes [Van Patten, Fletcher & Walsh (1986) J. Biol. Chem. 261, 5514-5523]. In the present study it is demonstrated that these multiple species occur because the bovine heart protein kinase preparation contains multiple forms of catalytic subunit [Kinzel, Hotz, König, Gagelmann, Pyerin, Reed, Köbler, Hofmann, Obst, Gensheimer, Goldblatt & Shaltiel (1987) Arch. Biochem. Biophys. 253, 341-349].

Relaxation of smooth muscle by cardiodilatin/atrial natriuretic peptide is inhibited by cAMP-dependent phosphorylation

Cardiodilatins/atrial natriuretic peptides (CDD/ANP) exhibit a common amino acid sequence: Arg101-Arg102-Ser103-Ser104. Cyclic AMP-dependent phosphorylation of Ser104 of atrial peptides with [gamma-32P]ATP enables rapid identification of cardiac hormones. The biological activity of in vitro phosphorylated cardiodilatin (CDD-28/alpha-hANP) is dramatically altered compared to the unphosphorylated peptide: the vaso-relaxant effect of cardiodilatin 28 is inhibited upon phosphorylation.