A majority of casein kinase II alpha subunit is tightly bound to intranuclear components but not to the beta subunit

Design of CK2β-Mimicking Peptides as Tools To Study the CK2α/CK2β Interaction in Cancer Cells

The ubiquitously expressed Ser/Thr kinase CK2 is a key regulator in a variety of key processes in normal and malignant cells. Due to its distinctive anti-apoptotic and tumor-driving properties, elevated levels of CK2 have frequently been found in tumors of different origin. In recent years, development of CK2 inhibitors has largely been focused on ATP-competitive compounds; however, targeting the CK2α/CK2β interface has emerged as a further concept that might avoid selectivity issues. To address the CK2 subunit interaction site, we have synthesized halogenated CK2β-mimicking cyclic peptides modified with the cell-penetrating peptide sC18 to mediate cellular uptake. We investigated the binding of the resulting chimeric peptides to recombinant human CK2α using a recently developed fluorescence anisotropy assay. The iodinated peptide sC18-I-Pc was identified as a potent CK2α ligand (K_i =0.622 μm). It was internalized in cells to a high extent and exhibited significant cytotoxicity toward cancerous HeLa cells (IC₅₀ =37 μm) in contrast to non-cancerous HEK-293 cells. The attractive features and functionalities of sC18-I-Pc offer the opportunity for further improvement.

Protein kinase CK2 in breast cancer: the CK2β regulatory subunit takes center stage in epithelial plasticity

Structurally, protein kinase CK2 consists of two catalytic subunits (α and α') and two regulatory subunits (β), which play a critical role in targeting specific CK2 substrates. Compelling evidence shows the complexity of the CK2 cellular signaling network and supports the view that this enzyme is a key component of regulatory protein kinase networks that are involved in several aspects of cancer. CK2 both activates and suppresses the expression of a number of essential oncogenes and tumor suppressors, and its expression and activity are upregulated in blood tumors and virtually all solid tumors. The prognostic significance of CK2α expression in association with various clinicopathological parameters highlighted this kinase as an adverse prognostic marker in breast cancer. In addition, several recent studies reported its implication in the regulation of the epithelial-to-mesenchymal transition (EMT), an early step in cancer invasion and metastasis. In this review, we briefly overview the contribution of CK2 to several aspects of cancer and discuss how in mammary epithelial cells, the expression of its CK2β regulatory subunit plays a critical role in maintaining an epithelial phenotype through CK2-mediated control of key EMT-related transcription factors. Importantly, decreased CK2β expression in breast tumors is correlated with inefficient phosphorylation and nuclear translocation of Snail1 and Foxc2, ultimately leading to EMT induction. This review highlights the pivotal role played by CK2β in the mammary epithelial phenotype and discusses how a modest alteration in its expression may be sufficient to induce dramatic effects facilitating the early steps in tumor cell dissemination through the coordinated regulation of two key transcription factors.

The CK2 kinase stabilizes CLOCK and represses its activity in the Drosophila circadian oscillator

Phosphorylation is a pivotal regulatory mechanism for protein stability and activity in circadian clocks regardless of their evolutionary origin. It determines the speed and strength of molecular oscillations by acting on transcriptional activators and their repressors, which form negative feedback loops. In Drosophila, the CK2 kinase phosphorylates and destabilizes the PERIOD (PER) and TIMELESS (TIM) proteins, which inhibit CLOCK (CLK) transcriptional activity. Here we show that CK2 also targets the CLK activator directly. Downregulating the activity of the catalytic α subunit of CK2 induces CLK degradation, even in the absence of PER and TIM. Unexpectedly, the regulatory β subunit of the CK2 holoenzyme is not required for the regulation of CLK stability. In addition, downregulation of CK2α activity decreases CLK phosphorylation and increases per and tim transcription. These results indicate that CK2 inhibits CLK degradation while reducing its activity. Since the CK1 kinase promotes CLK degradation, we suggest that CLK stability and transcriptional activity result from counteracting effects of CK1 and CK2.

BID, an interaction partner of protein kinase CK2alpha

Recombinant murine BID protein was used as an in vitro substrate for the CK2 holoenzyme and the catalytic CK2alpha subunit. The results obtained show that BID can only serve as a substrate for the catalytic CK2alpha subunit. Phosphorylation of BID using the CK2 holoenzyme was only possible in the presence of polylysine, supporting the notion that BID behaves similarly to calmodulin. Co-immunoprecipitation of BID and CK2 subunits revealed that BID is preferentially associated with the CK2alpha subunit. Enzyme kinetic analyses yielded a Km value for BID that is a level of magnitude lower than that measured for casein and the synthetic peptide, suggesting more specific and tight binding of BID to CK2alpha. In contrast are the Vmax values observed, with a significantly higher phosphorylation rate measured for casein and the synthetic peptide than for BID. When BID was phosphorylated by polylysine-stimulated CK2 holoenzyme prior to caspase-8 cleavage, the formation of tC-BID was reduced in comparison to treatment with caspase-8 in the absence of protein kinase. Mass spectrometric analysis of BID phosphorylated by CK2alpha before and after cleavage with caspase-8 showed phosphorylation of residues Thr58 and Ser76.

Discrimination between the activity of protein kinase CK2 holoenzyme and its catalytic subunits

The acronym CK2 denotes a highly pleiotropic Ser/Thr protein kinase whose over-expression correlates with neoplastic growth. A vexed question about the enigmatic regulation of CK2 concerns the actual existence in living cells of the catalytic (alpha and/or alpha') and regulatory beta-subunits of CK2 not assembled into the regular heterotetrameric holoenzyme. Here we take advantage of novel reagents, namely a peptide substrate and an inhibitor which discriminate between the holoenzyme and the catalytic subunits, to show that CK2 activity in CHO cells is entirely accounted for by the holoenzyme. Transfection with individual subunits moreover does not give rise to holoenzyme formation unless the catalytic and regulatory subunits are co-transfected together, arguing against the existence of free subunits in CHO cells.

p53-dependent inhibition of mammalian cell survival by a genetically selected peptide aptamer that targets the regulatory subunit of protein kinase CK2

Based on the perturbation of its expression in human cancers and on its involvement in transformation and tumorigenesis, protein kinase CK2 has recently attracted attention as a potential therapeutic target. To assess the value of CK2 as a target for antiproliferative strategies, we have initiated a program aiming to develop inhibitors targeting specifically the regulatory CK2beta subunit. Here, we use a two-hybrid approach to isolate from combinatorial libraries, peptide aptamers that specifically interact with CK2beta. One of these (P1), which has significant sequence homology to the cytomegalovirus IE2 protein, binds with high affinity to the N-terminal domain of CK2beta without disrupting the formation of the CK2 holoenzyme. Expression of green fluorescent protein (GFP)-P1 in different mammalian cell lines activates p53 phosphorylation on serine 15, induces an upregulation of p21 and the release of the Cyt-C and apoptosis-inducing factor proapoptotic proteins triggering caspase-dependent and caspase-independent apoptosis. GFP-P1-induced apoptosis is associated with a p53-dependent pathway as cell death was abrogated in p53 knocked out cells. In summary, our data show that genetically selected peptide aptamers that specifically target CK2beta can induce apoptosis in mammalian cells through the recruitment of a p53-dependent apoptosis pathway. They also emphasize the critical role of CK2beta for cell survival and might allow the design of novel proapoptotic agents targeting this protein.

The 'regulatory' beta-subunit of protein kinase CK2 negatively influences p53-mediated allosteric effects on Chk2 activation

The 'regulatory' beta-subunit of protein kinase CK2 has previously been shown to interact with protein kinases such as A-Raf, c-Mos, Lyn and Chk1 in addition to the catalytic subunit of CK2. Sequence alignments suggest that these interactions have a structural basis, and hence other protein kinases harboring corresponding sequences may be potential interaction partners for CK2beta. We show here that Chk2 specifically interacts with CK2beta in vitro and in cultured cells, and that activation of Chk2 leads to a reduction of this interaction. Additionally, we show that the presence of the CK2beta-subunit significantly reduces the Chk2-catalysed phosphorylation of p53 in vitro. These findings support the notion that CK2beta can act as a general modulator of remote docking sites in protein kinase--substrate interactions.

Autophosphorylation at the regulatory β subunit reflects the supramolecular organization of protein kinase CK2

Among the features of protein kinase CK2, autophosphorylation at its beta-subunit(s) upon incubation with ATP/Mg++ was early detected as a rapid and stoichiometric event occurring through an intramolecular mechanism as judged from kinetic analyses. The autophosphorylation site was mapped to Ser2 and, to a lesser extent, Ser3 both fulfilling the CK2 consensus sequence (MSSSEEV). The crystal structure of the heterotetrameric holoenzyme, however, is not compatible with an intramolecular autophosphorylation of the N-terminal stretch of either of the two beta subunits. Here we show that efficient "intramolecular" autophosphorylation of the beta subunit is crucially dependent on the formation of oligomers composed by several holoenzyme heterotetrameric protomers. Increasing ionic strength of the incubation medium promoting dissociation of the supramolecular oligomers abrogates beta subunit autophosphorylation, although CK2 catalytic activity, as judged from the phosphorylation of exogenous substrates, is still quite evident. These findings, in conjunction with graphic modelization, support the view that CK2 autophosphorylation at its beta subunits takes place through an "intraoligomeric" mechanism where the beta subunits of a protomer are phosphorylated by the catalytic subunits of another adjacent protomer. It appears therefore that in vivo beta autophosphorylation is symptomatic of supramolecular CK2 oligomers.

The multiple personalities of the regulatory subunit of protein kinase CK2: CK2 dependent and CK2 independent roles reveal a secret identity for CK2beta

Protein kinase CK2 (formerly casein kinase II), an enzyme that participates in a wide variety of cellular processes, has traditionally been classified as a stable tetrameric complex consisting of two catalytic CK2α or CK2α' subunits and two regulatory CK2β subunits. While consideration of CK2 as a tetrameric complex remains relevant, significant evidence has emerged to challenge the view that its individual subunits exist exclusively within these complexes. This review will summarize biochemical and genetic evidence indicating that the regulatory CK2β subunit exists and performs functions independently of CK2 tetramers. For example, unbalanced expression of catalytic and regulatory CK2 subunits has been observed in a variety of tissues and tumors. Furthermore, localization studies including live cell imaging have demonstrated that while the catalytic and regulatory subunits of CK2 exhibit extensive co-localization, independent mobility of the individual CK2 subunits can also be observed within cells. Identification of proteins that interact with CK2β in the absence of catalytic CK2 subunits reinforces the notion that CK2β has functions distinct from CK2 and begins to offer insights into these CK2-independent functions. In this respect, the discovery that CK2β can interact with and modulate the activity of a number of other serine/threonine protein kinases including A-Raf, c-Mos and Chk1 is particularly striking. This review will discuss the interactions between CK2β and these protein kinases with special emphasis on the properties of CK2β that mediate these interactions and on the implications of these interactions in yielding new prospects for elucidation of the cellular functions of CK2β.

Protein kinase CK2: a new view of an old molecular complex

Protein kinase CK2 (formerly known as casein kinase II) has been viewed traditionally as a stable heterotetrameric complex, but new analytical techniques are bringing a different picture into focus. The transient nature of this complex has been highlighted by the elucidation of its structure. Furthermore, analysis of the spatiotemporal organization of individual CK2 subunits in living cells has shown that they are dynamic and that they integrate into different multimolecular assemblies. These new studies give an additional dimension to the challenge of determining the cellular regulation of this protein kinase.

Casein kinase II-mediated phosphorylation of NF-kappaB p65 subunit enhances inducible nitric-oxide synthase gene transcription in vivo

Nitric oxide (NO) produced by inducible nitric-oxide synthase (NOSII) is mainly regulated at the transcriptional level by the nuclear factor-kappaB (NF-kappaB). In the present study, we further analyzed the role of NF-kappaB in the in vivo transcriptional regulation of NOSII gene by comparing two clones isolated from the EMT-6 mouse mammary cancer cell line. In response to interleukin (IL)-1beta or lipopolysaccharide (LPS), EMT-6 clone J (EMT-6J) cells produce 3-fold more NO than EMT-6 clone H (EMT-6H) cells, an effect correlated with enhanced activation of NF-kappaB in EMT-6J cells. In response to IL-1beta, the kinetics of degradation of NF-kappaB inhibitors IkappaB-alpha and IkappaB-beta, the nucleo-cytoplasmic shuttling of the transcription factor and its binding to a specific DNA sequence were similar in both clones. In contrast, an IL-1beta-induced phosphorylation of serine residues in NF-kappaB p65 subunit was observed in EMT-6J, but not in EMT-6H, cells. This IL-1beta-induced phosphorylation of p65 was specifically prevented by pretreatment of EMT-6J cells with the casein kinase II inhibitor DRB. Small interfering RNA-mediated depletion of casein kinase II-alpha subunit also decreased NF-kappaB transcriptional activity and NOSII gene transcription in IL-1beta and LPS-stimulated EMT-6J cells to the levels observed in EMT-6H cells treated in the same conditions. Altogether, these data indicate that casein kinase II-mediated phosphorylation of p65 subunit can enhance the transcriptional activity of NF-kappaB in vivo. This post-translational modification of the transcription factor can be responsible for increased NOSII gene transcription and NO production in tumor cells exposed to either IL-1beta or LPS.

Eukaryotic translation-initiation factor eIF2beta binds to protein kinase CK2: effects on CK2alpha activity

eIF2 (eukaryotic translation-initiation factor 2) is a substrate and an interacting partner for CK2 (protein kinase CK2). Co-immuno-precipitation of CK2 with eIF2beta has now been observed in HeLa cells, overexpressing haemagglutinin-tagged human recombinant eIF2beta. A direct association between His6-tagged human recombinant forms of eIF2beta subunit and both the catalytic (CK2alpha) and the regulatory (CK2beta) subunits of CK2 has also been shown by using different techniques. Surface plasmon resonance analysis indicated a high affinity in the interaction between eIF2beta and CK2alpha, whereas the affinity for the association with CK2beta is much lower. Free CK2alpha is unable to phosphorylate eIF2beta, whereas up to 1.2 mol of phosphate/mol of eIF2beta was incorporated by the reconstituted CK2 holoenzyme. The N-terminal third part of eIF2beta is dispensable for binding to either CK2alpha or CK2beta, although it contains the phosphorylation sites for CK2. The remaining central/C-terminal part of eIF2beta is not phosphorylated by CK2, but is sufficient for binding to both CK2 subunits. The presence of eIF2beta inhibited CK2alpha activity on calmodulin and beta-casein, but it had a minor effect on that of the reconstituted CK2 holoenzyme. The truncated forms corresponding to the N-terminal or central/C-terminal regions of eIF2beta were much less inhibitory than the intact subunit. The results demonstrate that the ability to associate with CK2 subunits and to serve as a CK2 substrate are confined to different regions in eIF2beta and that it may act as an inhibitor on CK2alpha.

Spermatogenesis of mice lacking CK2alpha': failure of germ cell survival and characteristic modifications of the spermatid nucleus

Csnk2a2 encodes the CK2alpha'catalytic subunit of CK2 that is predominantly expressed in testis. Male mice in which Csnk2a2 has been disrupted were infertile and displayed oligozoospermia with an abnormal shape of the spermatid nucleus. In this study, Csnk2a2 null testes revealed extensive germ cell degenerative processes at all stages of spermatogenesis, including the first spermatogenesis wave. Nuclear envelope (NE) protrusions with loss of nuclear pores, swelling of the outer membrane, and disruption of the inner membrane were observed in cells ranging from spermatogonia to early spermatids. Most early round spermatids were depleted, and DNA-specific fluorescent dyes showed a large chromatin-free nuclear domain near the chromocenter. Spermatids that were not eliminated retained NE defects that could explain the acrosomal and nuclear abnormalities of Csnk2a2 null spermatozoa. Data suggest that CK2alpha' deficiency could impair the phosphorylation of nuclear proteins of male germ cells leading to a particular cell-death pathway characterized by NE protrusions and an unusual pattern of chromatin modifications in spermatids.

The protein of a new gene, Tctex4, interacts with protein kinase CK2beta subunit and is highly expressed in mouse testis

Casein kinase 2 (CK2) is a ubiquitous, multifunctional eukaryotic serine/threonine kinase that phosphorylates an array of proteins. CK2 is a heterotetramer composed of two catalytic (alpha,alpha(')) and two regulatory (beta) subunits. CK2 plays an essential role in regulatory pathways in cell transformation and proliferation. But the role and function of the individual subunits of CK2, which are not in the holoenzyme, are not yet clear. Northern blot analysis reveals the highest CK2beta activity in mouse testicles and brain. By employing a yeast two-hybrid screen to identify the proteins that interact with CK2beta, we have isolated a cDNA clone encoding a 14-kDa protein with homology to dynein light chains and have designated it as Tctex4. CK2beta interacts specifically with Tctex4 both in a yeast two-hybrid system and in an in vitro interaction assay. Northern blot and in situ hybridization showed that Tctex4 is a novel gene that is expressed in mouse testis.

Tyrosine phosphorylation of protein kinase CK2 by Src-related tyrosine kinases correlates with increased catalytic activity

Casein kinase-2 (CK2) is a pleiotropic and constitutively active serine/threonine protein kinase composed of two catalytic (alpha and/or alpha') and two regulatory beta-subunits, whose regulation is still not well understood. In the present study, we show that the catalytic subunits of human CK2, but not the regulatory beta-subunits, are readily phosphorylated by the Src family protein tyrosine kinases Lyn and c-Fgr to a stoichiometry approaching 2 mol phosphotyrosine/mol CK2alpha with a concomitant 3-fold increase in catalytic activity. We also show that endogenous CK2alpha becomes tyrosine-phosphorylated in pervanadate-treated Jurkat cells. Both tyrosine phosphorylation and stimulation of activity are suppressed by the specific Src inhibitor 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4- d ]pyrimidine. By comparison, mutations giving rise to inactive forms of CK2alpha do not abrogate and, in some cases, stimulate Lyn and c-Fgr-dependent tyrosine phosphorylation of CK2. Several radiolabelled phosphopeptides could be resolved by HPLC, following tryptic digestion of CK2alpha that had been phosphoradiolabelled by incubation with [(32)P]ATP and c-Fgr. The most prominent phosphopeptide co-migrates with a synthetic peptide encompassing the 248-268 sequence, phosphorylated previously by c-Fgr at Tyr(255) in vitro. The identification of Tyr(255) as a phosphorylated residue was also supported by MS sequencing of both the phosphorylated and non-phosphorylated 248-268 tryptic fragments from CK2alpha and by on-target phosphatase treatment. A CK2alpha mutant in which Tyr(255) was replaced by phenylalanine proved less susceptible to phosphorylation and refractory to stimulation by c-Fgr.

Live-cell fluorescence imaging reveals the dynamics of protein kinase CK2 individual subunits

Protein kinase CK2 is a multifunctional enzyme which has long been described as a stable heterotetrameric complex resulting from the association of two catalytic (alpha or alpha') and two regulatory (beta) subunits. To track the spatiotemporal dynamics of CK2 in living cells, we fused its catalytic alpha and regulatory beta subunits with green fluorescent protein (GFP). Both CK2 subunits contain nuclear localization domains that target them independently to the nucleus. Imaging of stable cell lines expressing low levels of GFP-CK2alpha or GFP-CK2beta revealed the existence of CK2 subunit subpopulations exhibiting differential dynamics. Once in the nucleus, they diffuse randomly at different rates. Unlike CK2beta, CK2alpha can shuttle, showing the dynamic nature of the nucleocytoplasmic trafficking of the kinase. When microinjected in the cytoplasm, the isolated CK2 subunits are rapidly translocated into the nucleus, whereas the holoenzyme complex remains in this cell compartment, suggesting an intramolecular masking of the nuclear localization sequences that suppresses nuclear accumulation. However, binding of FGF-2 to the holoenzyme triggers its nuclear translocation. Since the substrate specificity of CK2alpha is dramatically changed by its association with CK2beta, the control of the nucleocytoplasmic distribution of each subunit may represent a unique potential regulatory mechanism for CK2 activity.

Autocatalytic tyrosine-phosphorylation of protein kinase CK2 alpha and alpha' subunits: implication of Tyr182

CK2 is a pleiotropic and constitutively active serine/threonine protein kinase composed of two catalytic (alpha and/or alpha') and two regulatory beta-subunits, whose mechanism of modulation is still obscure. Here we show that CK2 alpha/alpha' subunits undergo intermolecular (trans) tyrosine-autophosphorylation, which is dependent on intrinsic catalytic activity and is suppressed by the individual mutation of Tyr182, a crucial residue of the activation loop, to phenylalanine. At variance with serine-autophosphorylation, tyrosine-autophosphorylation of CK2alpha is reversed by ADP and GDP and is counteracted by the beta-subunit and by a peptide reproducing the activation loop of CK2alpha/alpha' (amino acids 175-201). These results disclose new perspectives about the mode of regulation of CK2 catalytic subunits.

Identification of a novel stretch-responsive skeletal muscle gene (Smpx)

Skeletal muscle is able to respond to a range of stimuli, including stretch and increased load, by increasing in diameter and length in the absence of myofiber division. This type of cellular growth (hypertrophy) is a highly complex process involving division of muscle precursor cells (myoblasts) and their fusion to existing muscle fibers as well as increased protein synthesis and decreased protein degradation. Underlying the alterations in protein levels are increases in a range of specific mRNAs including those coding for structural proteins and proteins that regulate the hypertrophic process. Seven days of passive stretch in vivo of tibialis anterior (TA) muscle has been shown to elicit muscle hypertrophy. We have identified a cDNA corresponding to an mRNA that exhibits increased expression in response to 7 days of passive stretch imposed on TA muscles in vivo. This 944-bp novel murine transcript is expressed primarily in cardiac and skeletal muscle and to a lesser extent in brain. Translation of the transcript revealed an open reading frame of 85 amino acids encoding a nuclear localization signal and two overlapping casein kinase II phosphorylation sites. This gene has been called "small muscle protein (X chromosome)" (Smpx; HGMW-approved human gene symbol SMPX) and we hypothesize that it plays a role in skeletal muscle hypertrophy.

A potential role of nuclear matrix-associated protein kinase CK2 in protection against drug-induced apoptosis in cancer cells

Protein kinase CK2 (CK2) has long been implicated in the regulation of cell growth and proliferation. Its activity is generally elevated in rapidly proliferating tissues, and nuclear matrix (NM) is an important subnuclear locale of its functional signaling. In the prostate, nuclear CK2 is rapidly lost commensurate with induction of receptor-mediated apoptosis after growth stimulus withdrawal. By contrast, chemical-induced apoptosis in prostate cancer and other cells (by etoposide and diethylstilbestrol) evokes an enhancement in CK2 associated with the NM that appears to be because of translocation of CK2 from the cytoplasmic to the nuclear compartment. This shuttling of CK2 to the NM may reflect a protective response to chemical-mediated apoptosis. Supporting evidence for this was obtained by employing cells that were transiently transfected with various expression plasmids of CK2 (thereby expressing additional CK2) prior to treatment with etoposide or diethylstilbestrol. Cells transfected with the CK2alpha or CK2alphabeta showed significant resistance to chemical-mediated apoptosis commensurate with the corresponding elevation in CK2 in the NM. Transfection with CK2beta did not demonstrate this effect. These results suggest, for the first time, that besides the commonly appreciated function of CK2 in cell growth, it may also have a role in protecting cells against apoptosis.

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.

pCMB treatment reveals the essential role of cysteinyl residues in conferring functional competence to the regulatory subunit of protein kinase CK2

To assess the functional role of the four conserved cysteinyl residues in the regulatory beta-subunit of protein kinase CK2, the effect of pCMB and other reagents of sulfhydryl groups has been investigated. The pCMB-treated beta-subunit has lost its ability to form either homodimers or regular alpha(2)beta(2) heterotetramers with the catalytic subunit. It also fails to increase catalytic activity toward peptide substrates and to mediate the stimulatory effect of polylysine. The pCMB-treated beta-subunit, however, is still able to prevent calmodulin phosphorylation and to physically interact with the alpha-subunit to form inactive complexes whose sedimentation coefficient is lower than that of CK2 holoenzyme. These inactive complexes upon treatment with reducing agents like DTT are converted into a fully active heterotetrameric holoenzyme.

Protein kinase CK2: evidence for a protein kinase CK2beta subunit fraction, devoid of the catalytic CK2alpha subunit, in mouse brain and testicles

The highest CK2 activity was found in mouse testicles and brain, followed by spleen, liver, lung, kidney and heart. The activity values were directly correlated with the protein expression level of the CK2 subunits alpha (catalytic) and beta (regulatory). The alpha' subunit was only detected in brain and testicles. By contrast, Northern blot analyses of the CK2alpha mRNA revealed a somewhat different picture. Here, the strongest signals were obtained for brain, liver, heart and lung. In kidney, spleen and testicles mRNAs were only weakly detectable. For CK2alpha' mRNA distribution strong signals were observed for lung, liver and testicles. In the case of CK2beta mRNA the highest signals were found for testicles, kidney, brain and liver. The amount of CK2beta mRNA in testicles was estimated to be about 6-fold higher than in brain. The strongest CK2beta signals in the Western blot were found for testicles and brain. The amount of CK2beta protein in brain in comparison to the other organs (except testicles) was estimated to be ca. 2-3-fold higher whereas the ratio of CK2beta between testicles and brain was estimated to be 3-4-fold. Results from the immunoprecipitation experiments support the notion for the existence of free CK2beta population and/or CK2beta in complex with other protein(s) present in brain and testicles. In all other mouse organs investigated, i.e. heart, lung, liver, kidney and spleen, no comparable amount of free CK2beta was observed. This is the first physiological evidence for the existence of a 'free CK2beta' (or in complex with proteins other than CK2a) in normal animal tissue apart from the hitherto dogmatic association with CK2alpha in a tetrameric holoenzyme complex.

Isotype-specific functions of Raf kinases

The family of Raf-protein kinases consisting of A-Raf, B-Raf, and c-Raf-1 is involved in cellular processes which regulate proliferation, differentiation, and apoptosis. Cell-culture experiments and the knockout of individual Raf genes suggested that the three Raf isoforms have overlapping and unique regulatory functions. However, it is not known how these isotype-specific functions of Raf kinases occur in the cell. Published data suggest that Raf proteins might differ in the regulation of their activation as well as in their ability to connect to downstream signaling pathways. Since Raf is part of a multiprotein complex and protein-protein interactions are important for Raf signaling, we propose that isotype-specific functions can be achieved by isotype-restricted protein binding. Recently we were able to identify candidates for such Raf-isoform-specific interaction partners.

Heterogeneous nuclear ribonucleoprotein A2 interacts with protein kinase CK2

The catalytic subunit of protein kinase CK2 (CK2alpha) was found associated with heterogeneous nuclear ribonucleoprotein particles (hnRNPs) that contain the core proteins A2 and C1-C2. High levels of CK2 activity were also detected in these complexes. Phosphopeptide patterns of hnRNP A2 phosphorylated in vivo and in vitro by protein kinase CK2 were similar, suggesting that this kinase can phosphorylate hnRNPA2 in vivo. Binding experiments using human recombinant hnRNP A2, free human recombinant CK2alpha or CK2beta subunits, reconstituted CK2 holoenzyme and purified native rat liver CK2 indicated that hnRNP A2 associated with both catalytic and regulatory CK2 subunits, and that the interaction was independent of the presence of RNA. However, the capability of hnRNP A2 to bind to CK2 holoenzyme was lower than its binding to the isolated subunits. These data indicate that the association of CK2alpha with CK2beta interferes with the subsequent binding of hnRNP A2. HnRNP A2 inhibited the autophosphorylation of CK2beta. This effect was stronger with reconstituted human recombinant CK2 than with purified native rat liver CK2.

Serum-stimulated cell cycle entry of fibroblasts requires undisturbed phosphorylation and non-phosphorylation interactions of the catalytic subunits of protein kinase CK2

Protein kinase CK2 is a pleiotropic Ser/Thr kinase occurring as alpha2beta2, alpha'2beta2, or alphaalpha'beta2 tetramers. A requirement in serum-stimulated cell cycle entry in both the cytoplasm and the nucleus of human fibroblasts for phosphorylation(s) by CK2 has been concluded from stimulation inhibition by microinjected antibodies against the regulatory subunit (beta). We have now examined this idea more directly by microinjection-mediated perturbation of phosphorylation and non-phosphorylation interactions of the catalytic subunits (alpha and alpha'), and by verifying the supposed matching of the cellular partition of CK2 subunits in the fibroblasts employed. While immunostaining and cell fractionation indicate that the partitions of subunits indeed match each other (with their predominant location in the nucleus in both quiescent and serum-stimulated cells), microinjection of substrate or pseudosubstrate peptides competing for the CK2-mediated phosphorylation in vitro resulted in significant inhibition of serum stimulation when placed into the nucleus but not when placed into the cytoplasm. Also inhibitory were nuclear but not cytoplasmic injections of antibodies against alpha and alpha' that affect neither their kinase activity in vitro nor their complexing to beta. The data indicate that the role played by CK2 in serum-stimulated cell cycle entry is predominantly nuclear and more complex than previously assumed, involving not only phosphorylation but also experimentally separable non-phosphorylation interactions by the catalytic subunits.

Protein kinase CK2 and its role in cellular proliferation, development and pathology

Protein kinase CK2 is a pleiotropic, ubiquitous and constitutively active protein kinase that can use both ATP and GTP as phosphoryl donors with specificity for serine/threonine residues in the vicinity of acidic amino acids. Recent results show that the enzyme is involved in transcription, signaling, proliferation and in various steps of development. The tetrameric holoenzyme (alpha2beta2) consists of two catalytic alpha-subunits and two regulatory beta-subunits. The structure of the catalytic subunit with the fixed positioning of the activation segment in the active conformation through its own aminoterminal region suggests a regulation at the transcriptional level making a regulation by second messengers unlikely. The high conservation of the catalytic subunit from yeast to man and its role in the tetrameric complex supports this notion. The regulatory beta-subunit has been far less conserved throughout evolution. Furthermore the existence of different CK2beta-related proteins together with the observation of deregulated CK2beta levels in tumor cells and the reported association of CK2beta protein with key proteins in signal transduction, e.g. A-Raf, Mos, pg90rsk etc. are suggestive for an additional physiological role of CK2beta protein beside being the regulatory compound in the tetrameric holoenzyme.

Protein kinase CK2 ("casein kinase-2") and its implication in cell division and proliferation

Protein kinase CK2 (also termed casein kinase-2 or -II) is a ubiquitous Ser/Thr-specific protein kinase required for viability and for cell cycle progression. CK2 is especially elevated in proliferating tissues, either normal or transformed, and the expression of its catalytic subunit in transgenic mice is causative of lymphomas. CK2 is highly pleiotropic: more than 160 proteins phosphorylated by it at sites specified by multiple acidic residues are known. Despite its heterotetrameric structure generally composed by two catalytic (alpha and/or alpha') and two non catalytic beta-subunits, the regulation of CK2 is still enigmatic. A number of functional features of the beta-subunit which could cooperate to the modulation of CK2 targeting/activity will be discussed.

Regulation of protein phosphatase 2A by direct interaction with casein kinase 2alpha

Timely deactivation of kinase cascades is crucial to the normal control of cell signaling and is partly accomplished by protein phosphatase 2A (PP2A). The catalytic (alpha) subunit of the serine-threonine kinase casein kinase 2 (CK2) bound to PP2A in vitro and in mitogen-starved cells; binding required the integrity of a sequence motif common to CK2alpha and SV40 small t antigen. Overexpression of CK2alpha resulted in deactivation of mitogen-activated protein kinase kinase (MEK) and suppression of cell growth. Moreover, CK2alpha inhibited the transforming activity of oncogenic Ras, but not that of constitutively activated MEK. Thus, CK2alpha may regulate the deactivation of the mitogen-activated protein kinase pathway.

The casein kinase II beta subunit binds to Mos and inhibits Mos activity

Mos is a germ cell-specific serine/threonine kinase and is required for Xenopus oocyte maturation. Active Mos stimulates a mitogen-activated protein kinase (MAPK) by directly phosphorylating and activating MAPK kinase (MKK). We report here that the Xenopus homolog of the beta subunit of casein kinase II (CKII beta) binds to and regulates Mos. The Mos-interacting region of CKII beta was mapped to the C terminus. Mos bound to CKII beta in somatic cells ectopically expressing Mos and CKII beta as well as in unfertilized Xenopus eggs. CKII beta inhibited Mos-mediated MAPK activation in rabbit reticulocyte lysates and repressed MKK activation by v-Mos in a coupled kinase assay. In addition, microinjection of CKII beta mRNA into Xenopus oocytes inhibited progesterone-induced meiotic maturation and MAPK activation, presumably by binding of CKII beta to Mos and thereby inhibiting MAPK activation. Moreover, this inhibitory phenotype could be rescued by another protein that binds to CKII beta, CKII alpha. The ability of ectopic CKII beta to inhibit meiotic maturation and the detection of a complex between endogenous Mos and CKII beta suggest that CKII beta may act as an inhibitor of Mos during oocyte maturation, perhaps setting a threshold beyond which Mos protein must accumulate before it can activate the MAPK pathway.

Casein kinase II associates with Egr-1 and acts as a negative modulator of its DNA binding and transcription activities in NIH 3T3 cells

Although the activation domains within early growth response gene protein 1 (Egr-1) have been mapped, little is known of the kinases which phosphorylate Egr-1 and how phosphorylation correlates with the transcriptional activity of Egr-1. In this study we report that casein kinase II (CKII) co-immunoprecipitates with Egr-1 from NIH 3T3 cell lysates. The association of Egr-1 and CKII requires the C terminus of Egr-1 and CKII phosphorylates Egr-1 in vitro. The in vitro phosphorylation of Egr-1 by CKII and that induced by serum in vivo was compared by examining the CNBr-digested fragments of the phosphorylated Egr-1. CKII strongly phosphorylates fragments 7 and 10 which cover part of the activation/nuclear localization and DNA binding domains of Egr-1. CKII also phosphorylates, albeit weakly, fragments 5 and 8 which cover part of activation domain and the entire repression domain of Egr-1, respectively. Strong phosphorylation on fragment 10 as well as fragment 5 was also observed in Egr-1 immunoprecipitated from serum-induced, 32P-labeled cells. CKII phosphorylation of Egr-1 resulted in a decrease of its DNA binding as well as its transcriptional activities.

Casein kinase II increases the transcriptional activities of MRF4 and MyoD independently of their direct phosphorylation

The myogenic regulatory factors (MRFs) are a subclass of a much larger group of basic helix-loop-helix transcription factors which includes members of the E protein such as E47, E2-2, and HEB. Although the MRFs are unique in their ability to confer a myogenic phenotype on nonmuscle cells, they require E protein partners to form a MRF-E protein heterodimer, which represents the functional myogenesis-inducing complex. The mechanisms controlling homodimer and heterodimer formation in vivo remain largely unknown, although it is likely that posttranslational modification of one or both basic helix-loop-helix partners is critical to this regulatory event. In this respect, MyoD and MRF4, both members of the MRF family, exist in vivo as phosphoproteins and contains multiple consensus phosphorylation sites, including sites for casein kinase II (CKII) phosphorylation. In this study, we demonstrate that overexpression of CKII increases the transcriptional activities of MRF4 and MyoD in vivo. Interestingly, mutation of the individual CKII sites within MRF4 and MyoF does not alter the ability of CKII to enhance MRF transcriptional activity, suggesting that the effect of CKII expression on the MRFs is indirect. Given that the MRFs require dimerization with E protein partners to activate muscle-specific transcription, the effects of CKII expression on E protein function also were examined. Our studies show that E47 serves as an in vitro substrate for CKII and that CKII-phosphorylated E-47 proteins no longer bind to DNA. These observations were confirmed by in vivo experiments showing that overexpressing of CKII produces a dramatic reduction in E47 homodimer-directed transcription. We conclude from these studies that CKII may act as a positive regulator of myogenesis by preventing E protein homodimers from binding to muscle gene regulatory elements.

Isolation and characterization of a monoclonal anti-protein kinase CK2 beta-subunit antibody of the IgG class for the direct detection of CK2 beta-subunit in tissue cultures of various mammalian species and human tumors

A murine monoclonal anti-protein kinase CK2 beta antibody was isolated and characterized. The antibody detects 1 pmol of purified recombinant CK2 beta-subunit after analysis on SDS-PAGE. Alternatively undenatured CK2 beta-subunit was detected by an ELISA assay either as recombinant CK2 beta-subunit or in the CK2 holoenzyme (alpha 2 beta 2). Here, concentrations of the first antibody of 1 ng/ml still allowed the detection of the subunit. Immunoblotting of crude cellular extracts from various tissue cultures (man, mouse, and hamster), from human tumors, and the nonneoplastic tissue allowed the detection of the CK2 beta-subunit. The detected epitope of this antibody was, as determined by the epitope analysis technique, 123GLSDI127.

Two distinct signaling pathways activate the latent DNA binding function of p53 in a casein kinase II-independent manner

Post-translational modification of a carboxyl-terminal negative regulatory domain in vitro by either casein kinase II or protein kinase C allosterically activates the latent sequence-specific DNA binding function of p53. Reported here is a biochemical approach to determine the types of signaling pathways and enzymes that are involved in p53 activation in cells. Using a novel chromatographic method, we have been able to separate three distinct biochemical forms of p53 that have been synthesized in vivo; two are in an activated state, and one is in a latent state for sequence-specific DNA binding. The two activated forms of p53 appear to be controlled individually by either a constitutive or a UV-inducible signaling pathway. p53 lacking the COOH-terminal casein kinase II site (p53 delta 4) was characterized biochemically and used to determine the affects of deletion of the casein kinase II motif on the production of the two activated forms of p53 in vivo. As observed with full-length p53, the production of two distinct chromatographic forms of activated p53 delta 4 occurs in vivo, indicating that p53 activation can occur through a casein kinase II-independent pathway and suggesting that two other factors are involved in activation of p53 in vivo.

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.

Developmental changes in the activity and cellular localization of hepatic casein kinase II in the rat

The activity and cellular localization of hepatic casein kinase II (CKII) was examined during late fetal development in the rat. Cultured fetal hepatocytes displayed constitutive CKII activity which was not further activated by growth factor exposure. Similarly, fetal liver CKII showed approximately fivefold greater activity than adult liver. The fetal hepatic activity was, to a large degree, localized to a nuclear fraction. Postnuclear cytosol preparations from fetal and adult liver showed similar CKII activity. In all cases, FPLC ion exchange chromatography followed by Western immunoblotting showed that immunoreactive CKII coincided with kinase activity. However, parallel determinations of CKII activity and immunoreactive CKII levels showed a higher (five- to sixfold) CKII specific activity in nuclear extracts compared to cytosol. In summary, fetal hepatic CKII demonstrates coincident nuclear localization and activation. We hypothesize that the regulation of hepatic CKII is relevant to the mitogen-independent proliferation displayed by fetal rat hepatocytes.

Phosphorylation of rat liver heterogeneous nuclear ribonucleoproteins A2 and C can be modulated by calmodulin

It was previously reported that the phosphorylation of three proteins of 36, 40 to 42, and 50 kDa by casein kinase 2 is inhibited by calmodulin in nuclear extracts from rat liver cells (R. Bosser, R. Aligué, D. Guerini, N. Agell, E. Carafoli, and O. Bachs, J. Biol. Chem. 268:15477-15483, 1993). By immunoblotting, peptide mapping, and endogenous phosphorylation experiments, the 36- and 40- to 42-kDa proteins have been identified as the A2 and C proteins, respectively, of the heterogeneous nuclear ribonucleoprotein particles. To better understand the mechanism by which calmodulin inhibits the phosphorylation of these proteins, they were purified by using single-stranded DNA chromatography, and the effect of calmodulin on their phosphorylation by casein kinase 2 was analyzed. Results revealed that whereas calmodulin inhibited the phosphorylation of purified A2 and C proteins in a Ca(2+)-dependent manner, it did not affect the casein kinase 2 phosphorylation of a different protein substrate, i.e., beta-casein. These results indicate that the effect of calmodulin was not on casein kinase 2 activity but on specific protein substrates. The finding that the A2 and C proteins can bind to a calmodulin-Sepharose column in a Ca(2+)-dependent manner suggests that this association could prevent the phosphorylation of the proteins by casein kinase 2. Immunoelectron microscopy studies have revealed that such interactions could also occur in vivo, since calmodulin and A2 and C proteins colocalize on the ribonucleoprotein particles in rat liver cell nuclei.

Proteins of the Xenopus laevis zinc finger multigene family as targets for CK II phosphorylation

Zn finger proteins (ZFPs) of the C2/H2 type in Xenopus laevis are encoded by a multigene family comprising several hundred members. Based upon conserved sequence features outside the Zn finger region, ZFPs can be subdivided into distinct subfamilies. Two of such subfamilies are characterized by conserved, N-terminal amino acid sequences termed the FAX and the FAR Domain. Here we present data suggesting that the zinc finger proteins of the FAR-ZFP subfamily are targets for CK II mediated phosphorylation. Expression of these proteins during oogenesis coincides with CK II activity in unfertilized eggs. Additionally, we have found that XlcOF 7.1, a member of the FAX-ZFP subfamily, is also phosphorylated by CK II. The target sites for in vitro phosphorylation are localized within the conserved N-terminal domains but not within the Zn finger regions. However, amino acid sequence comparison revealed that individual phosphoacceptor sites are not generally conserved among all members of the respective ZFP subfamilies. The relevance of a potential CK II phosphorylation for the regulation of ZFP activity in vivo is discussed.

The salivary gland 42-kDa phosphoprotein is a single-stranded DNA-binding protein with characteristics of the epithelial casein kinase N42 in Chironomus tentans

The DNA-binding and phosphorylation properties of a rapidly phosphorylated nuclear 42-kDa phosphoprotein and of its two structurally related proteins, pp43 and pp44 in Chironomus tentans salivary glands were investigated. pp42, pp43 and pp44 bind promoter probes of the ecdysterone controlled I-18C gene and of the joint histone H2A/H2B genes in a sequence-selective and single-stranded DNA (ssDNA) specific manner. Rapid phosphorylation appears to give pp42 and pp43 uniquely hydrophilic characters making them soluble in the aqueous phase during phenol treatment. Dephosphorylation of the nuclear proteins markedly stimulates the ssDNA-binding activity of pp42 but not of pp43 and pp44. All three phosphoproteins are sensitive to heparin and the transcription inhibitor 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB) in vitro, but their sensitivity to heparin is more than one order of magnitude lower than that of casein kinase II. The heparin sensitivity of pp42 and pp43 is, however, similar to that described for a previously identified nuclear 42-kDa phosphoprotein in a Chironomus tentans epithelial cell line, casein kinase N42 (CKN42). pp42 and pp43 bind with high affinity to a Phosvitin-Sepharose matrix, like casein kinase I, II and N42, and can be eluted with high salt buffers from the affinity column. In intact salivary gland cells, microinjected (gamma-32P)GTP labels pp42 in a heparin sensitive manner, and this GTP-phosphorylation of pp42 could be competed out by a large excess of phosvitin. (gamma-32P)ATP-based phosphorylation of pp42 was uninfluenced by phosvitin in intact cells. The experimental data suggest that the salivary gland 42-kDa phosphoprotein, pp42, is a ssDNA-binding protein with characteristics of the epithelial CKN42.