Protein kinase CK2 in health and disease: Protein kinase CK2: from structures to insights.

Within the last decade, 40 crystal structures corresponding to protein kinase CK2 (former name 'casein kinase 2'), to its catalytic subunit CK2alpha and to its regulatory subunit CK2beta were published. Together they provide a valuable, yet by far not complete basis to rationalize the biochemical features of the enzyme, such as its constitutive activity, acidophilic substrate specificity, dual-cosubstrate specificity and its heterotetrameric quarternary structure. Comprehensive sets of structural superimpositions reveal that both CK2alpha and CK2beta are relatively rigid molecules. In CK2beta the critical region of CK2alpha recruitment is pre-formed in the unbound state. In CK2alpha the activation segment - a key element of protein kinase regulation - adapts invariably the typical conformation of the active enzymes. Recent structures of human CK2alpha revealed a surprising plasticity in the ATP-binding region, suggesting an alternative mode of activity control.

UV light blocks EGFR signalling in human cancer cell lines.

UV light excites aromatic residues, causing these to disrupt nearby disulphide bridges. The EGF receptor is rich in aromatic residues near the disulphide bridges. Herein we show that laser-pulsed UV illumination of two different skin-derived cancer cell lines i.e. Cal-39 and A431, which both overexpress the EGF receptor, leads to arrest of the EGFR signaling pathway. The phosphorylation status of the receptor and the level of phosphorylated downstream signaling molecules i.e. AKT and the mitogen activated protein kinases (MAPKs) ERK1 and 2 is detected by Western blotting using phosphospecific antibodies. There was a threshold level, below which the receptor could not be blocked. In addition, illumination caused the cells to upregulate the cyclin-dependent kinase inhibitor p21WAF1, irrespective of the p53 status. Since the EGF receptor is often overexpressed in cancers and other proliferative skin disorders, it might be possible to significantly reduce the proliferative potential of these cells making them good targets for laser-pulsed UV light treatment.

The carboxy terminus of p53 mimics the polylysine effect of protein kinase CK2-catalyzed MDM2 phosphorylation.

The oncogene product MDM2 can be phosphorylated by protein kinase CK2 in vitro 0.5-1 mol of phosphate were incorporated per mol MDM2 protein. The catalytic subunit of protein kinase CK2 (alpha-subunit) catalyzed the incorporation of twice as much phosphate into the MDM2 protein as it was obtained with the holoenzyme. Polylysine stimulated MDM2 phosphorylation by CK2 holoenzyme threefold in contrast to the alpha-subunit-catalyzed MDM2 phosphorylation which was reduced by about 66% when polylysine was added. Full length p53, but also a peptide representing a C-terminal fragment of the tumor suppressor gene product p53 (amino acids 264-393 which also harbors the CK2beta interaction site at amino acids 287-340) mimicked the polylysine effect in all respects, ie. stimulation of phosphate incorporation by CK2 holoenzyme and inhibition in the presence of the catalytic CK2 alpha-subunit. Stimulation by p53(264-393) was on the average close to twofold and inhibition in the case of the alpha-subunit-catalyzed MDM2 phosphorylation was about 40%. Phosphorylation of MDM2 by CK2 holoenzyme in the presence of the p21(WAF1/CIP1), known to be a potent inhibitor of cyclin-dependent protein kinases, also led to a significant reduction of phosphate incorporation into MDM2 indicating that p21(WAF1/CIP1) does not exclusively inhibit cell cycle kinases. Furthermore, these data add new insight into the autoregulatory loop which include p21(WAF1/CIP1), MDM2 protein, CK2 and p53.

p21WAF1/CIP1 interacts with protein kinase CK2.

p21WAF1/CIP1 which belongs to a class of regulatory proteins that interact with cyclin dependent kinases is a potent inhibitor of these kinases. The inhibition of the cyclin dependent kinases induces an arrest of cells in the G phase of the cell cycle. In addition p21WAF1/CIP1 associates with PCNA and inhibits DNA replication. Here, we show that p21WAF1/CIP1 binds to the regulatory beta-subunit of protein kinase CK2 but not to the catalytic alpha-subunit. Binding of p21WAF1/CIP1 down regulates the kinase activity of CK2 with respect to the phosphorylation of the beta-subunit of CK2, casein and the C-terminus of p53. This study demonstrates a new binding partner for the regulatory beta-subunit of protein kinase CK2 which regulates the activity of the holoenzyme.

Mapping of the interaction sites of the growth suppressor protein p53 with the regulatory beta-subunit of protein kinase CK2.

p53 plays an essential role in cellular growth control. Some of its distinct biological functions are regulated by interaction with cellular proteins. We have previously (Wagner et al., 1994) shown that p53 binds to the regulatory subunit of protein kinase CK2. Using C-terminal protein fragments of p53 we now demonstrate that the region between amino acids 287 and 340 on the polypeptide chain of p53 is critical for binding of p53 to the beta-subunit of CK2. Neither phosphorylation at the p34cdc2 site (aa315) nor at the CK2 site (aa392) is necessary for binding of p53 to the beta-subunit of CK2. Using deletion mutants of the beta-subunit of CK2 we also show that an internal region between amino acids 72 and 149 of the beta-subunit of CK2 is necessary for binding to p53. Thus, this study defines new functional regions on the polypeptide chains of p53 and of protein kinase CK2.

Phosphorylation of acidic ribosomal proteins from rabbit reticulocytes by a ribosome-associated casein kinase.

Two acidic proteins from 80-S ribosomes were isolated and purified to homogeneity. The purified acidic proteins could be phosphorylated by casein kinase using [gamma-32P]ATP and [gamma-32P]GTP as a phosphoryl donor. The proteins became phosphorylated in situ, too. Sodium dodecyl sulfate polyacrylamide gel analysis of the purified acidic proteins and 80-S particles showed identical phosphoproteins in the 16 000 dalton region.

Characterization of native 40 S particles from Krebs II mouse ascites tumor cells: resolution, nomenclature and molecular weights of the nonribosomal proteins.

Native 40 S particles from Krebs II mouse ascites tumor cells were isolated on a large scale. A nonribosomal protein moiety of about 30 proteins could be removed from the ribosomal particles by treatment with 250 mM KCl. These proteins were analysed by two-dimensional polyacrylamide gel electrophoresis and turned out to be mostly acidic in nature. The molecular weights of about 17 proteins were determined by three-dimensional gel electrophoresis. Radioactively labelled nonribosomal protein spots were excised from two-dimensional gel electrophoresis. Radioactively labelled nonribosomal protein spots were excised from two-dimensional gels and transferred directly or after electrodialysis onto the third dimension gel. The proteins fell into a molecular weight range from about 20,000 to 300,000.

Growth-dependent modulation of casein kinase II and its substrate nucleolin in primary human cell cultures and HeLa cells.

We have previously provided evidence that casein kinase II (CKII) and its substrate nucleolin increase concomitantly during certain development stages during embryogenesis (Schneider et al., Eur. J. Biochem. 161, 733-738). We now show that during normal growth of primary cell cultures and HeLa cells CKII activity is increased concomitant with cellular growth and that the activity declines when confluency is reached. Parallel to the CKII activity increase, nucleolin, which has been shown to be a potential substrate of CKII changes its phosphorylation status, reaching a maximum at the time when CKII activity is highest and decreasing again when CKII activity declines.

Synthetic peptides and ribosomal proteins as substrate for 60S ribosomal protein kinase from yeast cells.

Kinetic studies on the 60S protein kinase were conducted with synthetic peptides and ribosomal proteins as substrate. Peptide RRREEESDDD proved to be the best synthetic substrate for this enzyme. The peptide has a sequence of amino acids which most closely resembles the structure of potential phosphorylation sites in natural substrates, i.e., acidic ribosomal proteins. The superiority of certain kinetic parameters for 60S kinase obtained with the native whole 80S ribosomes over those of the isolated fraction of acidic ribosomal proteins indicates that the affinity of 60S kinase to the specific protein substrate not only depends on the structure of the polypeptide chain around the target amino acid but also on its native structure within the 80S ribosome.

Cloning and sequencing of the casein kinase 2 alpha subunit from Zea mays.

The nucleotide sequence of the cDNA coding for the alpha subunit of casein kinase 2 of Zea mays has been determined. The cDNA clone contains an open reading frame of 996 nucleotides encoding a polypeptide comprising 332 amino acids. The primary amino acid sequence exhibits 75% identity to the alpha subunit and 71% identity to the alpha' subunit of human casein kinase 2.

Ribosomal protein S6 phosphorylation and morphological changes in response to the tumour promoter 12-O-tetradecanoylphorbol 13-acetate in primary human tumour cells, established and transformed cell lines.

The phosphorylation of ribosomal protein S6 in fibroblasts, primary human tumour cells, established and SV40-transformed human cell lines was compared after the addition of 12-O-tetradecanoylphorbol 13-acetate (TPA). In fibroblasts and primary tumour cell cultures, stimulation of S6 phosphorylation was about 4-6-fold. Established and transformed cell lines showed enhanced S6 phosphorylation which was not further stimulated by the addition of TPA. These findings indicated that the influence of TPA on the metabolic pathway, that finally leads to the phosphorylation of protein S6 in cells with a limited lifespan (fibroblasts, primary human tumour cells) can be mimicked by unknown steps also associated with immortalization (establishment function) and the transformed state of the tumour cells. Another interesting observation were morphological changes of the established and SV40-transformed cells which were visible as early as 20 min after the addition of TPA. In fibroblasts and primary tumour cells no changes in morphology were observed, even after 8h incubation.

Expression and characterization of a recombinant maize CK-2 alpha subunit.

CKIIB, one of the CK-2 like enzymes which have been isolated from maize, has been shown to be a monomeric enzyme that cross-reacts with anti CK-2 alpha specific antibodies suggesting a possible relationship between the two proteins (Dobrowolska et al. (1992) Eur. J. Biochem. 204, 299-303). In order to support the immunological data also by biochemical and biophysical experiments the availability of a recombinant CK-2 alpha from maize was a prerequisite. A maize cDNA clone of maize CK-2 alpha was expressed in the bacterial strain BL21 (DE3). The recombinant protein was purified to homogeneity; its molecular mass on one-dimensional SDS PAGE was estimated to be 36.5 kDa. The calculated molecular mass according to the amino acid composition is 39,228 Da (332 amino acids). The recombinant maize CK-2 alpha (rmCK-2 alpha) exhibited mostly the same properties as the recombinant human CK-2 alpha (rhCK-2 alpha). In several respects it behaved differently from CKIIB, thus supporting the notion that either CKIIB is encoded by another gene or it undergoes extensive posttranscriptional and/or posttranslational alterations. Three observations in particular disprove any close relatedness between CKIIB and rmCK-2 alpha, namely: (a) the phosphorylation of calmodulin by CKIIB is dramatically stimulated by polylysine, whereas polylysine inhibits rather than stimulating the phosphorylation of calmodulin by rmCK-2 alpha (and by rhCK-2 alpha). (b) Addition of rhCK-2 beta has no significant influence on the stimulation of the calmodulin phosphorylation by CKIIB whereas in the case of rmCK-2 alpha and rhCK-2 alpha addition of rhCK-2 beta is required for optimal stimulation by polylysine. (c) CKIIB does not self-assemble with rhCK-2 beta to form a high molecular mass complex as it is demonstrated for rmCK-2 alpha.

Characterization of the cDNA and three processed pseudogenes from the murine protein kinase CK2 alpha subunit.

Seven protein kinase CK2 alpha clones were isolated from a murine genomic DNA library. They were assigned to four different genomic loci (A,B,C,D). Locus D was previously identified as a processed pseudogene (Boldyreff et al. (1992) Biochem. Biophys. Res. Commun. 186, 723-730). Here we present sequences of genomic loci B and C and the murine CK2 alpha cDNA. Loci B and C are like locus D processed pseudogenes, however, with considerable differences among each other and to the cDNA, especially with respect to the lengths of the putative gene products. Genomic locus D would code for a protein of 82 amino acids, locus B for a protein of 132 amino acids and locus C product for the full length product of 391 amino acids as the murine cDNA.

The beta subunit of casein kinase II: cloning of cDNAs from murine and porcine origin and expression of the porcine sequence as a fusion protein.

cDNAs encoding the beta subunit of pig and mouse CKII were isolated. The porcine cDNA was expressed as a fusion protein in Escherichia coli and used for the production of anti-CKII-beta subunit specific antibodies.

The autophosphorylation and p34cdc2 phosphorylation sites of casein kinase-2 beta-subunit are not essential for reconstituting the fully-active heterotetrameric holoenzyme.

Two mutants of human casein kinase-2 beta-subunit with short deletions at either their amino (delta 1-4) or carboxy (delta 209-215) terminal side have been created that have lost the capability to undergo autophosphorylation and p34cdc2 mediated phosphorylation, respectively. Both mutants give rise to reconstituted CK2 holoenzymes displaying basal catalytic activity, thermostability and responsiveness to polylysine, identical to those of wild-type holoenzyme, whose reconstitution, moreover, is not affected by previous phosphorylation of the beta-subunit at either its N-terminal or C-terminal sites. Unlike the wild-type beta and beta(delta 209-215), however, beta(delta 1-4) fails to confer to the reconstituted holoenzyme the typical responsiveness to NaCl stimulation. These results suggest that while neither the autophosphorylation nor the p34cdc2 phosphorylation sites are required for conferring a stable structure and full catalytic activity to CK2, the autophosphorylation site is implicated in the NaCl-dependent fine tuning of CK2 activity.

Casein kinases: pleiotropic mediators of cellular regulation.

The present review on casein kinases focuses mainly on the possible metabolic role of CK-2, with special emphasis on its behavior in pathological tissues. From these data at least three ways to regulate CK-2 activity emerge: (i) CK-2 activity changes during embryogenesis, being high at certain stages of development and showing basal activity values at others; (ii) CK-2 activity can be enhanced in vitro by treatment of tissue culture cells with various growth factors and serum and (iii) CK-2 activity is constitutively enhanced in rapidly proliferating cells. The regulated CK-2 activity changes during embryogenesis cannot be explained as yet. In the case of the constitutive high expression of CK-2 in tumors, genetic changes may be responsible, e.g. through alterations of the regulatory genetic elements and/or regulation by specific transcription factors. In the case of serum induction, no genetic changes are necessarily involved; the observed changes may be entirely due to a signal transduction pathway where CK-2 could be phosphorylated by another kinase(s). CK-2 cDNAs from various organisms have been isolated and characterized. From the deduced amino acid sequence it turns out that CK-2 subunits are highly conserved during evolution. The relationship between CK-2 alpha from humans and plants is still 73%. Similar relationships are reported for the beta-subunit. Chromosomal assignment of CK-2 alpha shows two gene loci, one of which is a pseudogene. They are located on different chromosomes. Expression of the CK-2 subunits in Escherichia coli and the Baculo expression system is shown. The recombinant subunits can self-assemble to a functional holoenzyme in vitro. Biochemical and biophysical analysis of the recombinant beta-subunit suggests it to be trifunctional in association with the alpha-subunit affecting: (i) stability, (ii) enzyme specificity and (iii) enzyme activity. The question where CK-2 and its subunits are located throughout the cell cycle has also been addressed, mainly because of the large discrepancies that still exist between results obtained by different investigators. Tissue-specific expression of CK-2 at the mRNA and at the protein level has also been given attention. The fact that the enzyme activity is surprisingly high in brain and low in heart and lung may be indicative of involvement of CK-2 in processes other than proliferation.(ABSTRACT TRUNCATED AT 400 WORDS)

CK2: a protein kinase in need of control.

Protein kinase CK2 is a heterotetrameric alpha2beta2 Ser/Thr protein kinase with some features unusual among the eukaryotic protein kinases: (1) CK2 recognizes phosphoacceptor sites specified by several acidic determinants; (2) CK2 can use both ATP and GTP as phosphoryl donors; and (3) the regulatory properties of CK2 are poorly understood; it is insensitive to any known second messenger and displays high basal activity. To gain insight into CK2 regulation and to understand its unusual properties, site-directed mutagenesis experiments on both subunits and X-ray crystallographic studies of the catalytic alpha-subunit were performed. The noncatalytic beta-subunit has at least three functions: (1) it protects the alpha-subunit against denaturing agents or conditions; (2) it alters the substrate specificity of the alpha-subunit; and (3) it modulates the activity of the enzyme, i.e., depending on the substrate, it increases or decreases the activity of the alpha-subunit. Mutagenesis experiments revealed that an acidic stretch between amino acids 55 and 64 has a down-regulatory and autoinhibitory function. Mutational analysis of the alpha-subunit has revealed a network of unique basic residues that are responsible for the recognition of phosphoacceptor substrates and for down-regulation by the beta-subunit and by polyanionic inhibitors. The resolution of the crystal structure of Zea mays CK2 alpha-subunit has disclosed the structural features that are responsible for high basal activity and for unusual response to nucleotide analogs. The increasing knowledge of CK2 structure-function relationships will allow the design of highly selective inhibitors of this pleiotropic kinase with oncogenic potential.

Determination of mRNA, and protein levels of p53, MDM2 and protein kinase CK2 subunits in F9 cells after treatment with the apoptosis-inducing drugs cisplatin and carboplatin.

Protein kinase CK2 is a pleiotropic serine/threonine kinase which has been shown to phosphorylate numerous substrates. Evidence is accumulating that CK2 may exist complexed to a variety of cellular proteins, e.g. p53, MDM2, and A-Raf. Here, we explored the effects of the chemotherapeutic drugs cisplatin and carboplatin on the mRNA and protein levels of p53, MDM2 and CK2 in a murine teratocarcinoma cell line F9. Northern and Western blot analyses were performed and the CK2 activity was determined. The degree of apoptosis after drug treatment was assessed using the TUNEL test. Six hours after cisplatin and carboplatin treatment, the RNA level of p53 dropped by 59% +/- 9% and 86% +/- 8% respectively, whereas the observed level of p53 protein rose to 7 and 10 times over the untreated control, respectively. Treatment with 33 microM cisplatin prompted apoptosis as early as 4 h after drug treatment. More than 50% apoptotic cells were seen after 6 h. We conclude that cisplatin and its second generation drug carboplatin act similarly i.e. both drugs cause a concomitant decrease in p53 mRNA and an increase in p53 protein level. After 4 h treatment with either of the two drugs, p53 levels reach a threshold which leads to the initiation of apoptosis.

A-Raf kinase is a new interacting partner of protein kinase CK2 beta subunit.

In a search for protein kinase CK2 beta subunit binding proteins using the two-hybrid system, more than 1000 positive clones were isolated. Beside clones for the alpha' and beta subunit of CK2, there were clones coding for a so far unknown protein, whose partial cDNA sequence was already deposited in the EMBL database under the accession numbers R08806 and Z17360, for the ribosomal protein L5 and for A-Raf kinase. All isolated clones except the one for CK2 beta showed no interaction with the catalytic alpha subunit of CK2. A-Raf kinase is a new interesting partner of CK2 beta. The isolated A-Raf clone represented amino acids 268-606, but also a full length A-Raf clone interacted with CK2 beta. At the site of CK2 beta, residue 175 and amino acids between residues 194 and 200 are likely to be involved in direct interaction.

p53 and the ribosomal protein L5 participate in high molecular mass complex formation with protein kinase CK2 in murine teratocarcinoma cell line F9 after serum stimulation and cisplatin treatment.

Using the murine teratocarcinoma cell line F9 we investigated the influence of serum stimulation and cisplatin treatment on the p53, CK2, MDM2 levels. Both treatments led to an increase of p53, though with different kinetics; the other proteins investigated were not affected. We present direct evidence by immunoprecipitation for an association of protein kinase CK2 holoenzyme (alpha2beta2), p53, and the ribosomal protein L5. The results suggest complexes between the CK2 holoenzyme and p53 but also p53/CKbeta complexes. Furthermore we provide evidence for the existence of high molecular mass complexes of CK2 in vivo. This is the first evidence that, under physiological conditions, protein kinase CK2 does not exist solely as a heterotetramer, but predominantly in association with other proteins.