Activity of recombinant alpha and beta subunits of casein kinase II from Xenopus laevis

The Catalytic Subunit of Schizosaccharomyces pombe CK2 (Cka1) Negatively Regulates RNA Polymerase II Transcription through Phosphorylation of Positive Cofactor 4 (PC4)

Positive cofactor 4 (PC4) is a transcriptional coactivator that plays important roles in transcription and DNA replication. In mammals, PC4 is phosphorylated by CK2, and this event downregulates its RNA polymerase II (RNAPII) coactivator function. This work describes the effect of fission yeast PC4 phosphorylation on RNAPII transcription in a cell extract, which closely resembles the cellular context. We found that fission yeast PC4 is strongly phosphorylated by the catalytic subunit of CK2 (Cka1), while the regulatory subunit (Ckb1) downregulates the PC4 phosphorylation. The addition of Cka1 to an in vitro transcription assay can diminish the basal transcription from the Ad-MLP promoter; however, the addition of recombinant fission yeast PC4 or Ckb1 can stimulate the basal transcription in a cell extract. Fission yeast PC4 is phosphorylated in a domain which has consensus phosphorylation sites for CK2, and two serine residues were identified as critical for CK2 phosphorylation. Mutation of one of the serine residues in PC4 does not completely abolish the phosphorylation; however, when the two serine residues are mutated, CK2 is no longer able to phosphorylate PC4. The mutant which is not phosphorylated is able to stimulate transcription even though it is previously phosphorylated by Cka1, while the wild type and the point mutant are inactivated by Cka1 phosphorylation, and they cannot stimulate transcription by RNAPII in cell extracts. Those results demonstrate that CK2 can regulate the coactivator function of fission yeast PC4 and suggests that this event could be important in vivo as well.

In-vitro phosphorylation activity by recombinant alpha and beta subunits of Bombyx mori casein kinase 2

To clarify the control mechanism of the catalytic activity of casein kinase 2 (CK2) during early embryonic development in the silkworm, Bombyx mori, we attempted an in-vitro functional analysis by using the recombinant alpha and beta subunits of B. mori CK2 (rBmCK2alpha and rBmCK2beta) produced in a bacterial system. The renatured rBmCK2alpha possessed protein kinase activity. When rBmCK2alpha and rBmCK2beta were reconstituted in an approximate 1:1 molar ratio, the catalytic activity was almost the same as that of rBmCK2alpha alone. The catalytic activity of rBmCK2alpha was inhibited by polylysine, which is one of the activators of CK2 activity. However, when using the reconstituted rBmCK2alpha and rBmCK2beta (rBmCK2), activation by polylysine was observed. We examined the influence of sorbitol and 3-hydroxykynurenine (3-OHK), which are contained mainly in diapause eggs, on the phosphorylation activity of rBmCK2. Three-OHK inhibited rBmCK2 activity, but sorbitol had no effect on it. Furthermore, a functional analysis using rBmCK2alpha and beta subunits of Drosophila melanogaster CK2 revealed that a difference in the C-terminal amino acid of the CK2beta subunit influenced the phosphorylation activity of rBmCK2alpha. These results may provide new insights for clarifying the control mechanism of B. mori casein kinase 2 in eggs.

CK2alpha/CK1alpha chimeras are sensitive to regulation by the CK2beta subunit

The effect of CK2beta on the activity of CK2alpha and other protein kinases that can bind this regulatory subunit is not fully understood. In an attempt to improve our understanding of this effect, chimeras of CK2alpha and CK1alpha have been constructed. These chimeras contain different portions of the CK2alpha amino terminal region that are involved in the interaction with CK2beta to form CK2 tetramers. In the case of chimeras 1 and 2, the portions of CK2alpha replace the corresponding segments of CK1alpha. In the case of chimera 3, the fragment of CK2alpha is added to the whole CK1alpha molecule with the exception of the initial methionine. Chimera 3 has 8% of the activity of CK1alphaWT, while chimeras 1 and 2 are 3 orders of magnitude less active than CK1alphaWT. All three chimeras bind tightly to CK2beta, but only chimeras 1 and 2 are significantly stimulated in their capacity to phosphorylate casein and canonical peptide substrates by addition of the regulatory subunit. No stimulation was observed with phosvitin or non-canonical peptides derived from beta-catenin. CK2beta protects chimeras 1 and 2 from thermal inactivation. Chimera 2 can phosphorylate CK2beta and autophosphorylate; however, salt concentrations above 150 mM NaCl eliminate the phosphorylation of CK2beta but not the autophosphorylation of chimera 2. Similarly, high salt decrease the stimulatory effect of CK2beta on the phosphorylation of casein.

Protein kinase CK2 as an ectokinase: the role of the regulatory CK2beta subunit

Protein kinase CK2 (also known as casein kinase 2) is present in the cytoplasm, nuclei, and several other organelles. In addition, this enzyme has been found bound to the external side of the cell membrane where it acts as an ectokinase phosphorylating several extracellular proteins. Previous experiments with transfection of HEK-293T cells demonstrated that expression of both subunits, CK2alpha (catalytic) and CK2beta (regulatory), was necessary for the appearance of the ectopic enzyme as an ectokinase. In this work, using deletion and point mutations of CK2beta, it was possible to demonstrate that the region between amino acids 20 and 33 was necessary for the export of the enzyme as an ectokinase. Phenylalanines 21 and 22 and acidic residues in positions 26-28 are involved in the structural aspects that are required for export. However, the region encompassing amino acids 20-33 of CK2beta is not sufficient to make the carboxyl half of this subunit functional in bringing CK2 to the ectokinase locus. In cells transfected with only CK2beta, it was demonstrated that 3-4% of the subunit is exported to the cell medium, but the subunit is not bound to the external membrane.

Protein kinase casein kinase 2 holoenzyme produced ectopically in human cells can be exported to the external side of the cellular membrane

Ectokinases can phosphorylate extracellular proteins and external domains of membrane proteins influencing cell adhesion, movement, and cellular interactions. An ectokinase with the properties of casein kinase 2 (CK2) has been previously described, but little is known about the structural characteristics that allow this enzyme to be exported from the cell. Transfection of human embryonic kidney-293 cells with cDNAs coding for the catalytic (CK2alpha or CK2alpha') and regulatory (CK2beta) subunits with hemaglutinin tags allowed us to study the export of ectopically synthesized enzyme. When the catalytic (CK2alpha or CK2alpha') and the CK2beta regulatory subunits are cotransfected, the tetrameric enzyme composed of both subunits (holoenzyme) is detected outside the cell. This observation has been confirmed by assaying protein kinase activity in immunoprecipitates obtained with antihemaglutinin antibody by using a CK2-specific peptide substrate and by Western blots as well as by immunofluorescence of nonpermeabilized cells. Transfection with cDNA of catalytic or regulatory subunit alone does not result in export of these subunits. A study of the kinetics of appearance of the ectopically synthesized protein at different times after transfection indicates that a 5- to 7-h delay after the synthesis of the protein before it appears in the extracellular compartment. Using mutations of CK2alpha that eliminate phosphorylating activity [CK2alpha(Asp-156-Ala)] or that make it less sensitive to heparin inhibition [CK2alpha(Lys-75-Glu,Lys-76-Glu)] demonstrated that these mutations do not prevent the holoenzyme to be exported from the cells.

Effects of phosphorylation by protein kinase CK2 on the human basal components of the RNA polymerase II transcription machinery

We have investigated the role of phosphorylation by vertebrate protein kinase CK2 on the activity of the General Transcription Factors TFIIA, TFIIE, TFIIF, and RNAPII. The largest subunits of TFIIA, TFIIE, and TFIIF were phosphorylated by CK2 holoenzyme. Also, RNA polymerase II was phosphorylated by CK2 in the 214,000 and 20,500 daltons subunits. Our results show that phosphorylation of TFIIA, TFIIF, and RNAPII increase the formation of complexes on the TATA box of the Ad-MLP promoter. Also, phosphorylation of TFIIF increases the formation of transcripts, where as phosphorylation of RNA polymerase II dramatically inhibits transcript formation. Furthermore, we demonstrate that CK2 beta directly interacts with RNA polymerase II, TFIIA, TFIIF, and TBP. These results strongly suggest that CK2 may play a role in regulating transcription of protein coding genes.

Cell cycle regulatory protein p27KIP1 is a substrate and interacts with the protein kinase CK2

The protein kinase CK2 is constituted by two catalytic (alpha and/or alpha') and two regulatory (beta) subunits. CK2 phosphorylates more than 300 proteins with important functions in the cell cycle. This study has looked at the relation between CK2 and p27(KIP1), which is a regulator of the cell cycle and a known inhibitor of cyclin-dependent kinases (Cdk). We demonstrated that in vitro recombinant Xenopus laevis CK2 can phosphorylate recombinant human p27(KIP1), but this phosphorylation occurs only in the presence of the regulatory beta subunit. The principal site of phosphorylation is serine-83. Analysis using pull down and surface plasmon resonance (SPR) techniques showed that p27(KIP1) interacts with the beta subunit through two domains present in the amino and carboxyl ends, while CD spectra showed that p27(KIP1) phosphorylation by CK2 affects its secondary structure. Altogether, these results suggest that p27(KIP1) phosphorylation by CK2 probably involves a docking event mediated by the CK2beta subunit. The phosphorylation of p27(KIP1) by CK2 may affect its biological activity.

Role of the histidine triad-like motif in nucleotide hydrolysis by the rotavirus RNA-packaging protein NSP2

Octamers formed by the nonstructural protein NSP2 of rotavirus are proposed to function as molecular motors in the packaging of the segmented double-stranded RNA genome. The octamers have RNA binding, helix unwinding, and Mg(2+)-dependent NTPase activities and play a crucial role in assembly of viral replication factories (viroplasms). Comparison of x-ray structures has revealed significant structural homology between NSP2 and the histidine triad (HIT) family of nucleotidyl hydrolases, which in turn has suggested the location of the active site for NTP hydrolysis in NSP2. Consistent with the structural predictions, we show here using site-specific mutagenesis and ATP docking simulations that the active site for NTP hydrolysis is localized to residues within a 25-A-deep cleft between the C- and N-terminal domains of the NSP2 monomer. Although lacking the precise signature HIT motif (HØHØHØØ where Ø is a hydrophobic residue), our analyses demonstrate that histidines (His(221) and His(225)) represent critical residues of the active site. Similar to events occurring during nucleotide hydrolysis by HIT proteins, NTP hydrolysis by NSP2 was found to produce a short lived phosphorylated intermediate. Evaluation of the biological importance of the NTPase activity of NSP2 by transient expression in mammalian cells showed that such activity has no impact on the ability of NSP2 to induce the hyperphosphorylation of NSP5 or to interact with NSP5 to form viroplasm-like structures. Hence the NTPase activity of NSP2 probably has a role subsequent to the formation of viroplasms, consistent with its suspected involvement in RNA packaging and/or replication.

Pea DNA topoisomerase I is phosphorylated and stimulated by casein kinase 2 and protein kinase C

DNA topoisomerase I catalyzes the relaxation of superhelical DNA tension and is vital for DNA metabolism; therefore, it is essential for growth and development of plants. Here, we have studied the phosphorylation-dependent regulation of topoisomerase I from pea (Pisum sativum). The purified enzyme did not show autophosphorylation but was phosphorylated in an Mg(2+)-dependent manner by endogenous protein kinases present in pea nuclear extracts. This phosphorylation was abolished with calf intestinal alkaline phosphatase and lambda phosphatase. It was also phosphorylated by exogenous casein kinase 2 (CK2), protein kinase C (PKC; from animal sources), and an endogenous pea protein, which was purified using a novel phorbol myristate acetate affinity chromatography method. All of these phosphorylations were inhibited by heparin (inhibitor of CK2) and calphostin (inhibitor of PKC), suggesting that pea topoisomerase I is a bona fide substrate for these kinases. Spermine and spermidine had no effect on the CK2-mediated phosphorylation, suggesting that it is polyamine independent. Phospho-amino acid analysis showed that only serine residues were phosphorylated, which was further confirmed using antiphosphoserine antibody. The topoisomerase I activity increased after phosphorylation with exogenous CK2 and PKC. This study shows that these kinases may contribute to the physiological regulation of DNA topoisomerase I activity and overall DNA metabolism in plants.

Dual effect of lysine-rich polypeptides on the activity of protein kinase CK2

Protein kinase CK2 (casein kinase II) is normally a heterotetramer composed of catalytic (alpha, alpha') and regulatory subunits (beta). CK2 is able to phosphorylate a large number of protein substrates but the physiological mechanisms of its regulation are still unresolved. Lysine-rich peptides such as polylysine and histone H1 are known to stimulate the catalytic activity of the holoenzyme. This activation is mediated through the CK2beta regulatory subunit. In this communication, we report that the same concentrations of lysine-rich peptides or proteins that activate the holoenzyme cause strong inhibition of the phosphorylation of proteins catalyzed by the free catalytic CK2alpha subunit. The inhibitory effect of polylysine and histone H1 is observed with several protein substrates of CK2alpha (casein, adeno E1A, transcription factor II A, and CK2beta itself). With calmodulin, however, the inhibition of CK2alpha phosphorylation caused by polylysine is much lower while with a model peptide substrate of CK2 the inhibition caused by this polycation is negligible. The inhibition of CK2alpha by polylysine is observed only at limiting concentrations of the target substrate proteins. The dual effect of polylysine and of histone H1, which results in the inhibition of CK2alpha and stimulation of the CK2 alpha(2)beta(2) tetrameric holoenzyme, has the consequence that the addition of the CK2beta, in the presence of polylysine and low concentrations of substrate protein, can cause a 242-fold stimulation of the activity of CK2alpha. Other polycationic compounds such as polyarginine and spermine do not inhibit the phosphorylation of casein by CK2alpha, indicating that the effect is specific for lysine-rich peptides. Since there is evidence that there may be free CK2alpha subunits in the nuclei of cells, where there is abundant histone H1, the inhibition of CK2alpha by this lysine-rich protein may have physiological relevance.

Protein kinase CK2: structure, regulation and role in cellular decisions of life and death

Protein kinase CK2 ('casein kinase II') has traditionally been classified as a messenger-independent protein serine/threonine kinase that is typically found in tetrameric complexes consisting of two catalytic (alpha and/or alpha') subunits and two regulatory beta subunits. Accumulated biochemical and genetic evidence indicates that CK2 has a vast array of candidate physiological targets and participates in a complex series of cellular functions, including the maintenance of cell viability. This review summarizes current knowledge of the structural and enzymic features of CK2, and discusses advances that challenge traditional views of this enzyme. For example, the recent demonstrations that individual CK2 subunits exist outside tetrameric complexes and that CK2 displays dual-specificity kinase activity raises new prospects for the precise elucidation of its regulation and cellular functions. This review also discusses a number of the mechanisms that contribute to the regulation of CK2 in cells, and will highlight emerging insights into the role of CK2 in cellular decisions of life and death. In this latter respect, recent evidence suggests that CK2 can exert an anti-apoptotic role by protecting regulatory proteins from caspase-mediated degradation. The mechanistic basis of the observation that CK2 is essential for viability may reside in part in this ability to protect cellular proteins from caspase action. Furthermore, this anti-apoptotic function of CK2 may contribute to its ability to participate in transformation and tumorigenesis.

Role of the carboxyl terminus on the catalytic activity of protein kinase CK2alpha subunit

Protein kinase CK2 (also known as casein kinase 2) has catalytic (alpha, alpha') and regulatory (beta) subunits. The role of carboxyl amino acids in positions from 324 to 328 was studied for Xenopus laevis CK2alpha. Deletions and mutations of these residues were produced in recombinant CK2alpha, which was assayed for kinase activity. Activity dropped 7000-fold upon deletion of amino acids 324-328. The key residues are isoleucine 327 and phenylalanine 324. A three dimensional model of CK2alpha indicates that these hydrophobic residues of helix alphaN may interact with hydrophobic residues in helix alphaE which is linked to the catalytic center.

Human papillomavirus-16 E7 protein inhibits the DNA interaction of the TATA binding transcription factor

Previous studies have shown that the HPV-16 E7 protein interacts with TBP. This interaction was found to take place through residues in the carboxy terminal half of E7, mutation of which resulted in weaker transforming activity. In addition, binding of E7 to TBP was found to be increased following protein kinase CK2 (casein kinase II) phosphorylation of E7, and mutation of this CK2 site also reduces E7's transforming activity. To date, however, there is no information on the effects of E7 upon TBP function. In order to address this we have performed a series of assays to investigate the effects of E7 upon the ability of human and S. pombe TBP to bind DNA. We show that HPV-16 E7 is indeed a potent inhibitor of TBP DNA binding activity. Further, this activity of E7 is increased following CK2 phosphorylation of E7, consistent with it having an increased affinity for TBP. Finally, a mutant E7 protein defective in its ability to bind TBP, has no effect upon TBP binding to DNA. These results demonstrate that one consequence of the E7-TBP interaction is abolition of TBP DNA binding activity, and may provide an explanation for the transcriptional inhibitory effects of E7.

Protein p21(WAF1/CIP1) is phosphorylated by protein kinase CK2 in vitro and interacts with the amino terminal end of the CK2 beta subunit

Protein kinase CK2 is a ubiquitous protein that phosphorylates multiple substrates and is composed of catalytic (alpha, alpha') and regulatory (beta) subunits. Abundant evidence relates CK2 to the regulation of cell division. p21(WAF1/CIP1) is a potent inhibitor of cyclin-dependent kinases and of DNA replication and acts as a key inhibitor of cell cycle progression. In this work we examine the relation between these two important proteins. The interaction between the CK2 beta regulatory subunit of CK2 and p21(WAF1/CIP1) has been confirmed. Using a pull-down assay and fusion constructs of glutathione transferase with fragments of CK2 beta and other mutants, it was possible to define that the N-terminal (1-44) portion of CK2 beta contains a p21(WAF1/CIP1) binding site. CK2 reconstituted from recombinant alpha and beta subunits can phosphorylate p21(WAF1/CIP1) in vitro. This phosphorylation is greatly enhanced by histone H1. p21(WAF1/CIP1) can inhibit the phosphorylation of substrate casein by CK2. This inhibition, however, seems to be due to competition by p21(WAF1/CIP1) as an alternate substrate since in order to observe inhibition it is necessary that the concentration of p21 be of the same order of magnitude as the casein substrate concentration. This competition is not related to the binding of p21(WAF1/CIP1) to CK2 beta because it can also be observed when, in the absence of CK beta, CK alpha is used to phosphorylate casein in the presence of the p21.

Casein kinase 2 as a potentially important enzyme in the nervous system

Protein kinase CK2 is a ubiquitous and pleiotropic seryl/threonyl protein kinase which is highly conserved in evolution indicating a vital cellular role for this kinase. The holoenzyme is generally composed of two catalytic (alpha and/or alpha') and two regulatory (beta) subunits, but the free alpha/alpha' subunits are catalytically active by themselves and can be present in cells under some circumstances. Special attention has been devoted to phosphorylation status and structure of these enzymic molecules, however, their regulation and roles remain intriguing. Until recently, CK2 was believed to represent a kinase especially required for cell cycle progression in non-neural cells. At present, with respect to recent findings, four essential features suggest potentially important roles for this enzyme in specific neural functions: (1) CK2 is much more abundant in brain than in any other tissue; (2) there appear to be a myriad of substrates for CK2 in both synaptic and nuclear compartments that have clear implications in development, neuritogenesis, synaptic transmission, synaptic plasticity, information storage and survival; (3) CK2 seems to be associated with mechanisms underlying long-term potentiation in hippocampus; and (4) neurotrophins stimulate activity of CK2 in hippocampus. In addition, some data are suggestive that CK2 might play a role in processes underlying progressive disorders due to Alzheimer's disease, ischemia, chronic alcohol exposure or immunodeficiency virus HIV. The present review focuses mainly on the latest data concerning the regulatory mechanisms and the possible neurophysiological functions of this enzyme.

Involvement of asparagine 118 in the nucleotide specificity of the catalytic subunit of protein kinase CK2

Protein kinase CK2 is a heteromeric enzyme with catalytic (alpha) and regulatory (beta) subunits which form an alpha2beta2 holoenzyme and utilizes both ATP and GTP as nucleotide substrate. Site-directed mutagenesis of CK2alpha subunit was used to study this capacity to use GTP. Deletion of asparagine 118 (alpha(deltaN118)) or the mutant alphaN118E gives a 5-6-fold increase in apparent Km for GTP with little effect on the affinity for ATP. Mutants alphaN118A and alphaD120N did not alter significantly the Km for either nucleotide. CK2alphaN118 has an apparent Ki for inosine 5' triphosphate 5-fold higher than wild-type and is very heat labile. These studies complement recent crystallographic data indicating a role for CK2alpha asparagine 118 in binding the guanine base.

Phosphorylation protects sperm-specific histones H1 and H2B from proteolysis after fertilization

At intermediate stages of male pronucleus formation, sperm-derived chromatin is composed of hybrid nucleoprotein particles formed by sperm H1 (SpH1), dimers of sperm H2A-H2B (SpH2A-SpH2B), and a subset of maternal cleavage stage (CS) histone variants. At this stage in vivo, the CS histone variants are poly(ADP-ribosylated), while SpH2B and SpH1 are phosphorylated. We have postulated previously that the final steps of sperm chromatin remodeling involve a cysteine-protease (SpH-protease) that degrades sperm histones in a specific manner, leaving the maternal CS histone variants unaffected. More recently we have reported that the protection of CS histones from degradation is determined by the poly(ADP-ribose) moiety of these proteins. Because of the selectivity displayed by the SpH-protease, the coexistence of a subset of SpH together with CS histone variants at intermediate stages of male pronucleus remodeling remains intriguing. Consequently, we have investigated the phosphorylation state of SpH1 and SpH2B in relation to the possible protection of these proteins from proteolytic degradation. Histones H1 and H2B were purified from sperm, phosphorylated in vitro using the recombinant alpha-subunit of casein kinase 2, and then used as substrates in the standard assay of the SpH-protease. The phosphorylated forms of SpH1 and SpH2B were found to remain unaltered, while the nonphosphorylated forms were degraded. On the basis of this result, we postulate a novel role for the phosphorylation of SpH1 and SpH2B that occurs in vivo after fertilization, namely to protect these histones against degradation at intermediate stages of male chromatin remodeling.

Structural interpretation of site-directed mutagenesis and specificity of the catalytic subunit of protein kinase CK2 using comparative modelling

The catalytic subunit of protein kinase casein kinase 2 (CK2alpha), which has specificity for both ATP and GTP, shows significant amino acid sequence similarity to the cyclin-dependent kinase 2 (CDK2). We constructed site-directed mutants of CK2alpha and used a three-dimensional model to investigate the basis for the dual specificity. Introduction of Phe and Gly at positions 50 and 51, in order to restore the pattern of the glycine-rich motif, did not seriously affect the specificity for ATP or GTP. We show that the dual specificity probably originates from the loop situated around the position His115 to Asp120 (HVNNTD). The insertion of a residue in this loop in CK2 alpha subunits, compared with CDK2 and other kinases, might orient the backbone to interact with the base A and G; this insertion is conserved in all known CK2alpha. The mutant deltaN118, the design of which was based on the modelling, showed reduced affinity for GTP as predicted from the model. Other mutants were intended to probe the integrity of the catalytic loop, alter the polarity of a buried residue and explore the importance of the carboxy terminus. Introduction of Arg to replace Asn189, which is mapped on the activation loop, results in a mutant with decreased k(cat), possibly as a result of disruption of the interaction between this residue and basic residues in the vicinity. Truncation at position 331 eliminates the last 60 residues of the alpha subunit and this mutant has a reduced catalytic efficiency compared with the wild-type. Catalytic efficiency is restored in the truncation mutant by the replacement of a potentially buried Glu at position 252 by Lys, probably owing to a higher stability resulting from the formation of a salt bridge between Lys252 and Asp208.

Phosphorylation of yeast TBP by protein kinase CK2 reduces its specific binding to DNA

Protein kinase CK2 is a ubiquitous Ser/Thr kinase which phosphorylates a large number of proteins including several transcription factors. Recombinant Xenopus laevis CK2 phosphorylates both recombinant Saccharomyces cerevisiae and Schizosaccharomyces pombe TATA binding protein (TBP). The phosphorylation of TBP by CK2 reduces its binding activity to the TATA box. CK2 copurifies with the transcription factor IID (TFIID) complex from HeLa cell extracts and phosphorylates several of the TBP-associated factors within TFIID. Taken together these findings argue for a role of CK2 in the control of transcription by RNA polymerase II through the modulation of the binding activity of TBP to the TATA box.

An inactive mutant of the alpha subunit of protein kinase CK2 that traps the regulatory CK2beta subunit

Protein kinase CK2 (casein kinase 2) is a ubiquitous Ser/Thr protein kinase involved in cell proliferation. Mutation of the alpha subunit of the Xenopus laevis CK2 to change aspartic acid 156 to alanine (CK2alphaA156) resulted in an inactive enzyme. The CK2alphaA156 mutant, however, binds the regulatory subunit as measured by retention of beta on a nickel chelating column mediated by (His)6-tagged CK2alphaA156. Addition of CK2alphaA156 also caused beta to shift sedimentation in a sucrose gradient from a beta2 dimer (52 kDa) to an alpha2beta2 tetramer (130,000 kDa). CK2alphaA156 can trap the beta subunit in an inactive complex reducing the stimulation of casein phosphorylation caused by addition of beta to wild-type alpha. This competitive effect depends on the ratio of alpha/alphaA156 and on the amount of beta available. Since beta inhibits the phosphorylation of calmodulin by CK2alpha, the addition of CK2alphaA156, in this case, increases calmodulin phosphorylation by the alpha and beta combination. These results suggest that CK2alphaA156 may be a useful dominant-negative mutant that can serve to explore the multiple functions of CK2beta.

The recombinant alpha isoform of protein kinase CK1 from Xenopus laevis can phosphorylate tyrosine in synthetic substrates

The cDNA coding for protein kinase CK1 alpha has been cloned from a Xenopus laevis cDNA library. The derived amino acid sequence of the protein contains 337 amino acids and has a calculated molecular mass of 38874 Da. The sequence is identical to that of the human CK1 alpha and to the bovine CK1 alpha, except that it is 12 amino acids longer than the latter protein. Southern blotting with a 264-bp probe demonstrates that four or more fragments are obtained upon digestion of genomic DNA with EcoR1 and Hind3, suggesting that X. laevis possesses a family of related CK1 genes. CK1 alpha was expressed in Escherichia coli as a glutathione transferase fusion protein (GT-CK1 alpha) and certain of its characteristics were determined. The recombinant GT-CK1 alpha fusion protein was found to have apparent Km values for ATP (12 microM), casein (1.5 mg/ml) and the specific peptide substrate RRKDLHDDEEDEAMSITA (180 microM) which are similar to those of the rat liver CK1 enzyme. The recombinant CK1 alpha activity is weakly inhibited by heparin, but strongly inhibited by poly(Glu80:Tyr20). This inhibition is competitive and shows an approximate K1 of 5 microM. CK1 alpha can phosphorylate the tyrosine residues of poly(Glu80:Tyr20) and the tyrosine residue in the synthetic peptide RRREEEYEEEE. This kinase preparation also autophosphorylates in serine, threonine and weakly in tyrosine.

Cloning, expression and properties of the alpha' subunit of casein kinase 2 from zebrafish (Danio rerio)

The protein kinase casein kinase 2 (CK2) is ubiquitous in eukaryotic cells and is apparently involved in the control of cell division. The holoenzyme is a tetramer composed of two catalytic subunits (alpha and/or alpha') and regulatory subunits (beta 2). The alpha and alpha' subunits are encoded by different genes but are very similar in amino acid sequence, except that alpha' is normally considerably shorter. There have been extensive biochemical studies with recombinant alpha and beta subunits of many species, but only one previous description of the activity of an isolated recombinant alpha' subunit from human CK2 (Bodenbach, L., Fauss, J., Robitzki, A., Krehan, A., Lorenz, P., Lozeman, F. J. & Pyerin, W. (1994) Recombinant human casein kinase II. A study with the complete set of subunits (alpha, alpha', and beta), site-directed autophosphorylation mutants and a bicistronically expressed holoenzyme, Eur. J. Biochem. 220, 263-273). In the present work, the isolation and bacterial expression of a cDNA coding for the alpha' subunit of zebrafish (Danio rerio) is reported. The clone covers the complete coding region that generates a protein of 348 amino acids that is 86% identical to the alpha' subunits of human and chicken, and 82% identical to the sequenced portion of the CK2 alpha subunit of zebrafish. The recombinant alpha' subunit has apparent K(m) values for ATP (6 microM), GTP (20 microM), casein (2.0 mg/ml) and the model peptide RRRDDDSEDD (0.3 mM) which are very similar to those of the recombinant alpha subunit of Xenopus laevis. The alpha' subunit kcat was 7.2 min-1 which is again similar to that of Xenopus laevis alpha subunit (7.5 min-1). The alpha' subunit also behaved similarly to CK2 alpha with regard to optimal concentrations for Mg+2 or Mn+2 and to the inhibition by heparin and the poly(Glu80Tyr20) peptide. However alpha' kinase activity was less sensitive to poly(U) inhibition than alpha, it was more heat stable than alpha, and alpha' was slightly more sensitive to KCl inhibition than alpha. The difference in salt sensitivity, however, was enhanced by the presence of the regulatory beta subunit which shifted the optimal salt concentration of the phosphorylating activity. The alpha' 2 beta 2 holoenzyme was inhibited by KCl concentrations above 100 mM, while the alpha 2 beta 2 enzyme was stimulated by KCl concentrations up to 150 mM and required 180 mM for inhibition. Another important difference between alpha and alpha' is seen in the degree of the stimulation of casein phosphorylation activity in the presence of the regulatory beta subunit. When assayed at 100 mM KCl stoichiometric amounts of CK2 beta produced maximal stimulation of both alpha' (D. rerio) and alpha (X. laevis), however the activity levels with alpha' were stimulated 20-fold by beta while the addition of beta stimulated alpha (X. laevis) only 7-8-fold.

Expression of the subunits of protein kinase CK2 during oogenesis in Xenopus laevis

Northern-blot analysis of RNAs from different tissues demonstrated that the mRNA for the protein kinase CK2 alpha subunit is very abundant in the ovary of Xenopus laevis. The competitive reverse-PCR technique has been used to quantitate the mRNA for both CK2 alpha and CK2 beta subunits during oogenesis. The results obtained using eight different animals consistently show an increment of 2-3-fold in the mRNA for both subunits in vitellogenic oocytes (stages II-VI). Each stage-VI oocyte contains approximately 5 x 10(-7) molecules CK2 alpha mRNA and 1 x 10(-7) molecules CK2 beta mRNA. These amounts are considerably higher than many other mRNAs analyzed in these cells. Activity measurements of CK2 using casein or a specific model peptide revealed increments of about 10-12-fold during oogenesis, and also indicated that the amount of enzyme in the nucleus accounted for 15-30% of the total enzyme in the oocyte at all stages. Western-blot analysis of CK2 alpha indicated that the amount of this protein subunit also increased during oogenesis in a parallel fashion with the increment of enzymic activity.

Site-directed mutants of the beta subunit of protein kinase CK2 demonstrate the important role of Pro-58

The following amino acids of the Xenopus laevis beta subunit of protein kinase CK2 (casein kinase 2) were changed to alanine: Pro-58 (beta P-->A); Asp-59 and Glu-60 and Glu-61 (beta DEE-->AAA); His-151-153 (beta HHH-->AAA). The last 37 amino acids of the carboxyl end were deleted (beta delta 179-215). Stimulation of CK2 alpha catalytic subunit activity was measured with casein as substrate and the following relative activities were observed: beta P-->A > beta DEE-->AAA >>> beta WT > beta HHH-->AAA >>> beta delta 179-215. The beta DEE-->AAA and beta P-->A were similar to beta WT in reducing CD2 alpha binding to DNA but beta delta 179-215 was less active. The results indicate that both Pro-58 and the surrounding acidic cluster play roles in dampening the activation of CK2 alpha and that the carboxyl end of beta is involved in the interaction with CK2 alpha.

Phosphorylation and activation of protein kinase CK2 by p34cdc2 are independent events

Recombinant isolated beta-subunit of protein kinase CK2 is readily phosphorylated by p34cdc2/cyclin B kinase at Ser209 with favourable kinetic constants (Km = 1.7 microM, Vmax = 20 nmol.min-1.mg-1). Two synthetic peptides reproducing the 170-215 and the 206-215 C-terminal fragments of the beta-subunit are also phosphorylated though with tenfold higher Km values (19.5 and 28.0 microM, respectively). In contrast, both the beta-subunit associated with the alpha-subunit to give the heterotetrameric holoenzyme and the native CK2 are not appreciably phosphorylated by p34cdc2. These data suggest that the Ser209 beta-subunit phosphorylation observed in intact cells occurs prior to beta-subunit incorporation into the holoenzyme. The isolated CK2 alpha-subunit is not phosphorylated to any appreciable extent by p34cdc2 kinase. Its catalytic activity is nevertheless increased up to fivefold upon incubation with p34cdc2/cyclin B kinase complex. Such a stimulation of activity is comparable to that induced by the beta-subunit and it is paralleled by a 40% decrease of p34cdc2/cyclin B catalytic activity. Similar to beta-subunit, p34cdc2/cyclin B also protects the alpha-subunit against thermal inactivation. CK2 holoenzyme is also stimulated by p34cdc2/cyclin B, albeit less dramatically than the isolated alpha-subunit. Such an effect is also evident with CK2 holoenzyme reconstituted with a mutated beta-subunit lacking the p34cdc2 phosphorylation site and it is not accompanied by any appreciable phosphorylation of either the beta or the alpha-subunit. These data indicate that in vitro CK2 alpha-subunit interacts with and is activated by p34cdc2/cyclin B kinase by a mechanism that does not imply the phosphorylation of CK2.

Expression of recombinant human casein kinase II and recombinant heat shock protein 90 in Escherichia coli and characterization of their interactions

To assess the interaction of human casein kinase II (CKII) with the heat shock protein 90 (HSP90) class of chaperone proteins, human CKII alpha and beta subunits and beta S2A mutant were expressed and purified separately or from a tandem coexpression construct in Escherichia coli. Recombinant human HSP90 beta and recombinant yeast HSP90 as His6 constructs were also expressed in and purified from E. coli. The rhCKII S2A mutant removed the regulatory beta subunit autophosphorylation site but had no effect on catalytic efficiency with peptide or protein substrates. As a CKII substrate, recombinant hHSP90 beta displayed a Km of 9.8 microM and a kcat of 4.1 min-1 and was phosphorylated to 1.5 mol/mol, whereas ryHSP90, lacking the known serine CKII sites of hHSP90, was phosphorylated at a 19-fold lower kcat/Km ratio to levels of 0.8 mol/mol. The endoplasmic reticulum HSP90 family member Grp94 was phosphorylated to 1.4 mol/mol but, in contrast, HSC70 and FKBP25 chaperones were phosphorylated to < 0.01 mol/mol. Neither phospho nor dephospho forms of hHSP90 showed significant activation of CKII toward the peptide substrate RRREEETEEE in contrast to a previous report that activation was observed at high molar ratios of chaperone to kinase.

Recombinant human casein kinase II. A study with the complete set of subunits (alpha, alpha' and beta), site-directed autophosphorylation mutants and a bicistronically expressed holoenzyme

Human casein kinase II (CKII) is a ubiquitous and multipotential Ser/Thr kinase involved in the regulation of cell growth and differentiation. Biochemically, two characteristics are particularly notable; first, the tetrameric composition of two catalytic subunits (alpha and/or alpha') and two regulatory subunits (beta); second, the autophosphorylation of the holoenzyme at the N-terminus of CKII beta, suspected to be involved in tuning of the kinase activity. Whether CKII alpha and CKII alpha' reconstitute comparably with CKII beta to form holoenzyme is unclear. For a systematic investigation, the complete set of recombinant CKII subunits and of autophosphorylation mutants of CKII beta were expressed in Escherichia coli and comparative reconstitutions carried out. At 1:1 molar ratio, CKII beta stimulated both catalytic subunits roughly fivefold with phosvitin as a substrate. The level of activity reached with both of the reconstituted CKII isoforms was of the same order of magnitude as that of holoenzyme isolated from human placenta. It was also similar to a recombinant alpha 2 beta 2 holoenzyme whose expression had been attained in E. coli with a bicistronic construct containing the coding regions of CKII beta and CKII alpha in a tandem arrangement. Both Ser2 and Ser3 were identified as the autophosphorylation sites; replacement of one of these with Ala by oligonucleotide-mediated site-directed mutagenesis influenced only the extent of CKII beta autophosphorylation, replacement of both resulted in a loss of autophosphorylation. Despite these differences, the stimulatory effect of all the CKII beta mutants was comparable both to each other and to that of wild-type CKII beta. This was also obtained when substrates other than phosvitin were employed such as tubulin, or upstream-binding factor (UBF). However, the degree of stimulation was substrate specific and ranged from 2-5-fold with no major differences between CKII alpha and CKII alpha' stimulation. Calmodulin phosphorylation by both CKII alpha and CKII alpha' was decreased similarly by CKII beta and the CKII beta mutants. Proteins such as cAMP-responsive-element-binding protein (CREB), HPV16 E7 or Jun were not phosphorylated by either catalytic subunit but became substrates of both in the presence of CKII beta or CKII beta mutants. The data suggest that CKII alpha and CKII alpha' form similar CKII holoenzymes and that the tuning of holoenzyme activity is independent of the autophosphorylation status of CKII beta.

Activity of the E75E76 mutant of the alpha subunit of casein kinase II from Xenopus laevis

The cDNA gene coding for the alpha subunit of Xenopus laevis casein kinase II was mutated using the overlap extension PCR method. The mutation substituted glutamic acids for Lys75 and Lys76, changing the charge distribution of a very basic sequence found in the alpha subunit. Expression of the mutated cDNA in a pT7-7 vector in E. coli yielded an active mutant recombinant protein that was extensively purified. This mutant was not significantly affected in its app. Km for casein or a model peptide substrate, nor in its interaction with the activating beta subunit. Inhibition by quercetin and by 5,6-dichloro-1-beta-D-ribofuranosyl benzimidazole was also the same for mutant and wild type subunits. However, the CKII alpha E75E76 mutant was at least one order of magnitude less sensitive to inhibition by polyanionic inhibitors such as heparin, poly U, copolyglutamic acid:tyrosine (4:1) and 2,3 diphosphoglycerate.