Casein kinase II and the tumor suppressor protein P53 associate in a molecular complex that is negatively regulated upon P53 phosphorylation

Design of 2-amino-6-methyl-pyrimidine benzoic acids as ATP competitive casein kinase-2 (CK2) inhibitors using structure- and fragment-based design, docking and molecular dynamic simulation studies

Overexpression of casein kinase-2 (CK2) has been implicated in several carcinomas, mainly lung, prostate and acute myeloid leukaemia. The smaller nucleotide pocket compared to related kinases provides a great opportunity to discover newer ATP-competitive CK2 inhibitors. In this study, we have employed an integrated structure- and fragment-based design strategy to design 2-amino-6-methyl-pyrimidine benzoic acids as ATP-competitive CK2 inhibitors. A statistically significant four features-based E-pharmacophore (ARRR) model was used to screen 780,092 molecules. Further, the retrieved hits were considered for molecular docking study to identify essential binding interactions. At the same time, fragment-based virtual screening was performed using a dataset of 1,542,397 fragments. The identified hits and fragments were used as structure templates to rationalize the design of 2-amino-6-methyl-pyrimidine benzoic acids as newer CK2 inhibitors. Finally, the binding interactions of the designed hits were identified using an induced fit docking (IFD) study, and their stability was estimated by a molecular dynamics (MD) simulation study of 100 ns.

Comparing Two Neurodevelopmental Disorders Linked to CK2: Okur-Chung Neurodevelopmental Syndrome and Poirier-Bienvenu Neurodevelopmental Syndrome—Two Sides of the Same Coin?

In recent years, variants in the catalytic and regulatory subunits of the kinase CK2 have been found to underlie two different, yet symptomatically overlapping neurodevelopmental disorders, termed Okur-Chung neurodevelopmental syndrome (OCNDS) and Poirier-Bienvenu neurodevelopmental syndrome (POBINDS). Both conditions are predominantly caused by de novo missense or nonsense mono-allelic variants. They are characterized by a generalized developmental delay, intellectual disability, behavioral problems (hyperactivity, repetitive movements and social interaction deficits), hypotonia, motricity and verbalization deficits. One of the main features of POBINDS is epilepsies, which are present with much lower prevalence in patients with OCNDS. While a role for CK2 in brain functioning and development is well acknowledged, these findings for the first time clearly link CK2 to defined brain disorders. Our review will bring together patient data for both syndromes, aiming to link symptoms with genotypes, and to rationalize the symptoms through known cellular functions of CK2 that have been identified in preclinical and biochemical contexts. We will also compare the symptomatology and elaborate the specificities that distinguish the two syndromes.

Spatial proteomics reveals subcellular reorganization in human keratinocytes exposed to UVA light

The effects of UV light on the skin have been extensively investigated. However, systematic information about how the exposure to ultraviolet-A (UVA) light, the least energetic but the most abundant UV radiation reaching the Earth, shapes the subcellular organization of proteins is lacking. Using subcellular fractionation, mass-spectrometry-based proteomics, machine learning algorithms, immunofluorescence, and functional assays, we mapped the subcellular reorganization of the proteome of human keratinocytes in response to UVA light. Our workflow quantified and assigned subcellular localization for over 1,600 proteins, of which about 200 were found to redistribute upon UVA exposure. Reorganization of the proteome affected modulators of signaling pathways, cellular metabolism, and DNA damage response. Strikingly, mitochondria were identified as one of the main targets of UVA-induced stress. Further investigation demonstrated that UVA induces mitochondrial fragmentation, up-regulates redox-responsive proteins, and attenuates respiratory rates. These observations emphasize the role of this radiation as a potent metabolic stressor in the skin.

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.

Regulation of Cop9 signalosome activity by the EF-hand Ca2+-binding protein tescalcin

The Ca(2+)-binding protein tescalcin is known to be involved in hematopoietic cell differentiation; however, this mechanism is poorly understood. Here, we identify CSN4 (subunit 4 of the COP9 signalosome) as a novel binding partner of tescalcin. The COP9 signalosome (CSN) is a multiprotein complex that is essential for development in all eukaryotes. This interaction is selective, Ca(2+)-dependent and involves the PCI domain of CSN4 subunit. We then investigated tescalcin and CSN activity in human erythroleukemia HEL and promyelocytic leukemia K562 cells and find that phorbol 12-myristate 13-acetate (PMA)-induced differentiation, resulting in the upregulation of tescalcin, coincides with reduced deneddylation of cullin-1 (Cul1) and stabilization of p27(Kip1) - molecular events that are associated with CSN activity. The knockdown of tescalcin led to an increase in Cul1 deneddylation, expression of F-box protein Skp2 and the transcription factor c-Jun, whereas the levels of cell cycle regulators p27(Kip1) and p53 decreased. These effects are consistent with the hypothesis that tescalcin might play a role as a negative regulator of CSN activity towards Cul1 in the process of induced cell differentiation.

Protein kinase CK2α subunit over-expression correlates with metastatic risk in breast carcinomas: quantitative immunohistochemistry in tissue microarrays

BACKGROUND

CK2α is a signalling molecule that participates in major events in solid tumour progression. The aim of this study was to evaluate the prognostic significance of the immunohistochemical expression of CK2α in breast carcinomas.

METHODS

Quantitative measurements of immunohistochemical expression of 33 biomarkers using high-throughput densitometry, assessed on digitised microscopic tissue micro-array images were correlated with clinical outcome in 1000 breast carcinomas using univariate and multivariate analyses.

RESULTS

In univariate analysis, CK2α was a significant prognostic indicator (p<0.001). Moreover, a multivariable model allowed the selection of the best combination of the 33 biomarkers to predict patients' outcome through logistic regression. A nine-marker signature highly predictive of metastatic risk, associating SHARP-2, STAT1, eIF4E, pmapKAPk-2, pAKT, caveolin, VEGF, FGF-1 and CK2α permitted to classify well 82.32% of patients (specificity 81.59%, sensitivity 92.55%, area under ROC curve 0.939). Importantly, in a node negative subset of patients an even more (86%) clinically relevant association of eleven markers was found predictive of poor outcome.

CONCLUSION

A strong quantitative CK2α immunohistochemical expression in breast carcinomas is individually a significant indicator of poor prognosis. Moreover, an immunohistochemical signature of 11 markers including CK2α accurately (86%) well classifies node negative patients in good and poor outcome subsets. Our results suggest that CK2α evaluation together with key downstream CK2 targets might be a useful tool to identify patients at high risk of distant metastases and that CK2 can be considered as a relevant target for potential specific therapy.

Cellular regulators of protein kinase CK2

Protein phosphorylation is a key regulatory post-translational modification and is involved in the control of many cellular processes. Protein kinase CK2, formerly known as casein kinase II, which is a ubiquitous and highly conserved protein serine/threonine kinase, plays a central role in the control of a variety of pathways in cell proliferation, transformation, apoptosis and senescence. An understanding of the regulation of such a central protein kinase would greatly help our comprehension of the regulation of many pathways in cellular regulation. A number of reviews have addressed the detection, the development, and the characterization of inhibitors of CK2. The present review focuses on possible natural regulators of CK2, i.e. proteins and other cellular factors that bind to CK2 and thereby regulate its activity.

How druggable is protein kinase CK2?

CK2 is a pleiotropic, ubiquitous, and constitutively active protein kinase (PK), with both cytosolic and nuclear localization in most mammalian cells. 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. CK2 catalyzes the phosphorylation of more than 300 substrates characterized by multiple acidic residues surrounding the phosphor-acceptor amino acid, and, consequently, it plays a key role in several physiological and pathological processes. But how can one kinase orchestrate all these tasks faithfully? How is it possible that one kinase can, despite all pleiotropic characteristics of PKs in general, be involved in so many different biochemical events? Is CK2 a druggable target? Several questions are still to be clearly answered, and this review is an occasion for a fruitful discussion.

A Novel Protein Kinase CK2 Substrate Indicates CK2 Is Not Directly Stimulated by Polyamines in Vivo

The activity of the protein kinase (CK2) is enhanced in vitro by the binding of polyamines to the CK2beta regulatory subunit. The overexpression of ornithine decarboxylase (ODC), the rate-limiting enzyme in polyamine biosynthesis, also elevates CK2 kinase activity in primary keratinocytes and tissues of K6/ODC transgenic mice. In an effort to better characterize the mechanisms by which polyamines may affect CK2 in vivo, we constructed a transfectable CK2 substrate cDNA consisting of the enhanced green fluorescence protein appended with a canonical CK2 phosphorylation sequence (EGFP-S). In contrast to unmodified EGFP, the EGFP-S protein was extensively phosphorylated by CK2, and this phosphorylation was stimulated by the polyamine spermine in a dose-dependent manner. The in vivo phosphorylation of EGFP-S was examined in cell lines which inducibly express either wild-type CK2 holoenzyme or a CK2 holoenzyme which contains activating mutations in the polyamine-binding region of its CK2beta regulatory subunit. Neither the overexpression of ODC in either cell line nor the mutation of the CK2beta subunit conferred an increase in CK2 kinase activity as measured by the in vivo phosphorylation of EGFP-S. Rather, our data indicate that polyamines increase total CK2 kinase activity through increases in steady-state levels of both CK2alpha and CK2beta subunits. The overexpression of ODC resulted in a 3-fold increase in steady-state levels of both exogenous and endogenous CK2 transcripts but did not increase the half-life of wild-type or mutated CK2 protein. These data suggest that the regulation of intracellular CK2 by the polyamines may occur through mechanisms distinct from the direct stimulation of CK2 by polyamines in vitro as previously described.

Cdk9 phosphorylates p53 on serine 392 independently of CKII

The tumor suppressor p53 is an important cellular protein, which controls cell cycle progression. Phosphorylation is one of the mechanisms by which p53 is regulated. Here we report the interaction of p53 with another key regulator, cdk9, which together with cyclin T1 forms the positive transcription elongation complex, p-TEFb. This complex cooperates with the HIV-1 Tat protein to cause the phosphorylation of the carboxyl terminal domain (CTD) of RNA polymerase II and this facilitates the elongation of HIV-1 transcription. We demonstrate that cdk9 phosphorylates p53 on serine 392 through their direct physical interaction. Results from protein-protein interaction assays revealed that cdk9 interacts with the C-terminal domain (aa 361-393) of p53, while p53 interacts with the N-terminal domain of cdk9. Transfection and protein binding assays (EMSA and ChIP) demonstrated the ability of p53 to bind and activate the cdk9 promoter. Interestingly, cdk9 phosphorylates serine 392 of p53, which could be also phosphorylated by casein kinase II. Kinase assays demonstrated that cdk9 phosphorylates p53 independently of CKII. These studies demonstrate the existence of a feedback-loop between p53 and cdk9, pinpointing a novel mechanism by which p53 regulates the basal transcriptional machinery.

Global Phosphoproteome of HT-29 Human Colon Adenocarcinoma Cells

Phosphorylation events in cellular signaling cascades triggered by a variety of cellular stimuli modulate protein function, leading to diverse cellular outcomes including cell division, growth, death, and differentiation. Abnormal regulation of protein phosphorylation due to mutation or overexpression of signaling proteins often results in various disease states. We provide here a list of protein phosphorylation sites identified from HT-29 human colon adenocarcinoma cell line by immobilized metal affinity chromatography (IMAC) combined with liquid chromatography (LC)-tandem mass spectrometry (MS/MS) analysis. In this study, proteins extracted from HT-29 whole cell lysates were digested with trypsin and carboxylate groups on the resulting peptides were converted to methyl esters. Derivatized phosphorylated peptides were enriched using Fe(3+)-chelated metal affinity resin. Phosphopeptides retained by IMAC were separated by high performance liquid chromatography (HPLC) and analyzed by electrospray ionization-quadrupole-time-of-flight (ESI-Q-TOF) mass spectrometry. We identified 238 phosphorylation sites, 213 of which could be conclusively localized to a single residue, from 116 proteins by searching MS/MS spectra against the human protein database using MASCOT. Peptide identification and phosphorylation site assignment were confirmed by manual inspection of the MS/MS spectra. Many of the phosphorylation sites identified in our results have not been described previously in the scientific literature. We attempted to ascribe functionality to the sites identified in this work by searching for potential kinase motifs with Scansite (http://scansite.mit.edu) and obtaining information on kinase substrate selectivity from Pattern Explorer (http://scansite.mit.edu/pe). The list of protein phosphorylation sites identified in the present experiment provides broad information on phosphorylated proteins under normal (asynchronous) cell culture conditions. Sites identified in this study may be utilized as surrogate bio-markers to assess the activity of selected kinases and signaling pathways from different cell states and exogenous stimuli.

Binding of RNA to p53 regulates its oligomerization and DNA-binding activity

The C-terminus of p53 is responsible for maintaining the latent, non-DNA-binding form of p53. However, the mechanism by which the C-terminus regulates DNA binding is not yet fully understood. We show here that p53 interacts with RNA via its C-terminal domain and that disruption of this interaction, by RNase A treatment, truncation or phosphorylation of the C-terminus, restores DNA-binding activity. Furthermore, the oligomerization of p53 is significantly enhanced by disrupting the interaction between p53 and RNA. These findings suggest that binding of RNA to p53 is involved in the mechanism of p53 latency.

Modulation of p53 transcription regulatory activity and post-translational modification by hepatitis C virus core protein

Oncogenic virus proteins often target to tumor suppressor p53 during virus life cycle. In the case of hepatitis C virus (HCV) core protein, it has been shown to affect p53-dependent transcription. Here, we further characterized the in vitro and in vivo interactions between HCV core protein and p53 and showed that these two proteins colocalized in subnuclear granular structures and the perinuclear area. By use of a reporter assay, we observed that while low level of HCV core protein enhanced the transactivational activity of p53, high level of HCV core protein inhibited this activity. In both cases, however, HCV core protein increased the p53 DNA-binding affinity in gel retardation analyses, likely due to the hyperacetylation of p53 Lys(373) and Lys(382) residues. Additionally, HCV core protein, depending on its expression level, had differential effects on the Ser(15) phosphorylation of p53. Moreover, HCV core protein could rescue p53-mediated suppressive effects on both RNA polymerase I and III transcriptions. Collectively, our results indicate that HCV core protein targets to p53 pathway via at least three means: physical interaction, modulation of p53 gene regulatory activity and post-translational modification. This feature of HCV core protein, may potentially contribute to the HCV-associated pathogenesis.

[Protein kinase CK2: an enzyme that likes to be different]

Protein kinase CK2 (formerly known as casein kinase 2) was among the first protein kinases to be identified and characterized. Surprisingly, in spite of intense efforts, the regulation and cellular functions of CK2 remain obscure. However, recent data on its molecular structure, its signal-mediated intracellular dynamic localization and its unexpected function in cell survival have raised new interest in this enzyme. These studies reveal unique features of CK2 and highlight its importance in the transduction of survival signals.

Protein kinase CK2 regulates CDC25B phosphatase activity

Human dual-specificity phosphatases CDC25 (A, B and C) play an important role in the control of cell cycle progression by activating the cyclin-dependent kinases (CDKs). Regulation of these phosphatases during the cell cycle involves post-translational modifications such as phosphorylation and protein-protein interactions. Given the suspected involvement of the protein kinase CK2 at the G2/M transition, we have investigated its effects on the CDC25B phosphatase. We show that in vitro CK2 phosphorylates CDC25B, but not CDC25C. Mass spectrometry analysis demonstrates that at least two serine residues, Ser-186 and Ser-187, are phosphorylated in vivo. We also report that CDC25B interacts with CK2, and this interaction, mediated by the CK2beta regulatory subunit, involves domains that are located within the first 55 amino acids of CK2beta and between amino acids 122 and 200 on CDC25B. This association was confirmed in vivo, in Sf9 insect cells and in U(2)OS human cells expressing an HA epitope-tagged CDC25B. Finally, we demonstrate that phosphorylation of CDC25B by protein kinase CK2 increases the catalytic activity of the phosphatase in vitro as well as in vivo. We discuss the possibility that CDC25B phosphorylation by CK2 could play a role in the regulation of the activity of CDC25B as a starter of mitosis.

Molecular cloning and characterization of spurt, a human novel gene that is retinoic acid-inducible and encodes a secretory protein specific in upper respiratory tracts

Retinoids, such as all-trans-retinoic acid, play an essential role in the regulation of airway epithelial cell growth, differentiation, and gene expression. Using cDNA microarray, we identified a clone, DD4, that contains the cDNA of a novel gene, spurt (secretory protein in upper respiratory tracts) that was significantly induced by all-trans-retinoic acid in primary cultured human tracheobroncheal epithelia. Two alternatively spliced spurt transcripts of 1090 and 1035 base pairs exist that contain the same open reading frame expressing a 256-amino acid peptide. The full-length spurt cDNA sequence spans a genomic DNA fragment of 7,313 bp, and the gene is located on chromosome 20q11.21. spurt mRNA is notably expressed at high levels in human nasal, tracheal, and lung tissues. In situ hybridization demonstrated that spurt message is often present in secretory cell types. The human spurt gene product is a secretory protein that contains a distinct signal peptide sequence in its first 19 amino acids. Mono-specific antibodies were generated to characterize spurt expression. Our data demonstrate that spurt is secreted onto the apical side of primary human airway epithelial cultures and is present in clinical sputum samples. spurt gene expression is higher in sputum and tissue samples obtained from patients with chronic obstructive lung disease. Our results provide the cloning and characterization of this tissue-specific novel gene and its possible relationship with airway diseases.

Solution NMR structure of S100B bound to the high-affinity target peptide TRTK-12

The solution NMR structure is reported for Ca(2+)-loaded S100B bound to a 12-residue peptide, TRTK-12, from the actin capping protein CapZ (alpha1 or alpha2 subunit, residues 265-276: TRTKIDWNKILS). This peptide was discovered by Dimlich and co-workers by screening a bacteriophage random peptide display library, and it matches exactly the consensus S100B binding sequence ((K/R)(L/I)XWXXIL). As with other S100B target proteins, a calcium-dependent conformational change in S100B is required for TRTK-12 binding. The TRTK-12 peptide is an amphipathic helix (residues W7 to S12) in the S100B-TRTK complex, and helix 4 of S100B is extended by three or four residues upon peptide binding. However, helical TRTK-12 in the S100B-peptide complex is uniquely oriented when compared to the three-dimensional structures of other S100-peptide complexes. The three-dimensional structure of the S100B-TRTK peptide complex illustrates that residues in the S100B binding consensus sequence (K4, I5, W7, I10, L11) are all involved in the S100B-peptide interface, which can explain its orientation in the S100B binding pocket and its relatively high binding affinity. A comparison of the S100B-TRTK peptide structure to the structures of apo- and Ca(2+)-bound S100B illustrates that the binding site of TRTK-12 is buried in apo-S100B, but is exposed in Ca(2+)-bound S100B as necessary to bind the TRTK-12 peptide.

Cyclin H is a new binding partner for protein kinase CK2

The protein kinase CK2 holoenzyme is composed of two regulatory beta- and two catalytic alpha- or alpha(')-subunits. There is ample evidence for the binding of individual subunits of CK2 to various cellular proteins and, moreover, for functions of the individual subunits, which are different from their roles in the holoenzyme. Here, we report that the regulatory cyclin H subunit of the cyclin H/cdk7/Mat1 complex was associated with a protein kinase activity, which shows some similarity with protein kinase CK2. Coimmunoprecipitation experiments supported the existence of complexes of cyclin H and CK2 in mammalian cells. Far Western blot experiments revealed that cyclin H bound to the alpha-subunit but not the alpha(')- and beta-subunits of CK2. Immunofluorescence analysis showed that cyclin H and CK2alpha were colocated in the nucleus. Although cyclin H functions as the regulatory subunit for the cyclin H/cdk7/Mat1 complex, it could not substitute the regulatory beta-subunit of CK2 in its regulatory function of the CK2 activity.

Engrailed homeoprotein secretion is a regulated process

Chicken Engrailed 2 homeoprotein is transported between cells in culture. This intercellular transfer is based on unconventional secretion and internalisation mechanisms: Engrailed 2 has access to vesicles but lacks a signal sequence for secretion and is internalised by a non-endocytic process. We show that phosphorylation of a serine-rich domain within Engrailed 2 by the protein kinase CK2 specifically inhibits Engrailed 2 secretion. The availability of the serine-rich domain to CK2 is highly increased when it is displaced from its normal position to the C terminus of Engrailed 2, leading to a constitutive blockage of Engrailed 2 intercellular transfer. This indicates that intercellular transfer of Engrailed 2 is a highly regulated process.

Phosphorylation regulates the stability of the regulatory CK2beta subunit

Protein kinase CK2 is a protein serine/threonine kinase that exhibits elevated expression in a number of cancers and displays oncogenic activity in mice. The regulatory CK2beta subunit has a central role in assembly of functional tetrameric CK2 complexes where it participates in modulation of catalytic activity and substrate specificity. Since overexpression of CK2beta results in elevated levels of CK2 activity, we investigated the molecular mechanisms that control its degradation since perturbations in these pathways could contribute to elevated CK2 in cancer. In this study, we demonstrate that CK2beta is degraded by a proteasome-dependent pathway and that it is ubiquitinated. We have also investigated the role of phosphorylation and a putative destruction box in regulating its stability in cells. Importantly, replacement of three serine residues within the autophosphorylation site of CK2beta with glutamic acid residues resulted in a significant decrease in its degradation indicating that autophosphorylation is involved in regulating its stability. Notably, although the autophosphorylation site of CK2beta is remarkably conserved between species, this is the first functional role ascribed to this site. Furthermore, based on these results, we speculate that alterations in the phosphorylation or dephosphorylation of the regulatory CK2beta subunit could underlie the elevated expression of CK2 that is observed in cancer cells.

Human p53 phosphorylation mimic, S392E, increases nonspecific DNA affinity and thermal stability

DNA binding is crucial to the protective role of the tumor suppressor protein p53, a nuclear phosphoprotein and transcription factor. The mutant human p53 protein S392E is a phosphorylation mimic that has been previously demonstrated to represent an "activated" form of p53 in both in vivo and in vitro assays [Hupp and Lane (1995) J. Biol. Chem. 270, 18165; Hao et al. (1996) J. Biol. Chem. 271, 29380]. Herein, we describe an analysis of structural and functional differences between this mutant and the wild-type protein. Structurally, the S392E protein exhibits increased thermal stability compared to wild-type p53, as monitored by circular dichroism and conformational antibody Ab1620 reactivity. These structural effects include alterations to the core DNA binding domain, remote in sequence space from the site of mutation. Functionally, the S392E mutation does not increase p53 binding to its 20 bp consensus DNA sequence in the absence of nonspecific DNA additives. In contrast, affinity of S392E for a 20 bp nonspecific DNA sequence is enhanced. Embedding 20 bp consensus DNA in the context of longer DNA sequences does not substantially alter S392E affinity, whereas wild-type affinity for these DNAs decreases with increased proportion of nonspecific DNA. These differences may account for the S392E "activated" phenotype and illuminate the role of this modified p53 in vivo.

Identification and characterization of CKIP-1, a novel pleckstrin homology domain-containing protein that interacts with protein kinase CK2

The catalytic subunits of protein kinase CK2, CK2alpha and CK2alpha', are closely related to each other but exhibit functional specialization. To test the hypothesis that specific functions of CK2alpha and CK2alpha' are mediated by specific interaction partners, we used the yeast two-hybrid system to identify CK2alpha- or CK2alpha'-binding proteins. We report the identification and characterization of a novel CK2-interacting protein, designated CKIP-1, that interacts with CK2alpha, but not CK2alpha', in the yeast two-hybrid system. CKIP-1 also interacts with CK2alpha in vitro and is co-immunoprecipitated from cell extracts with epitope-tagged CK2alpha and an enhanced green fluorescent protein fusion protein encoding CKIP-1 (i.e. EGFP-CKIP-1) when they are co-expressed. CK2 activity is detected in anti-CKIP-1 immunoprecipitates performed with extracts from non-transfected cells indicating that CKIP-1 and CK2 interact under physiological conditions. The CKIP-1 cDNA is broadly expressed and encodes a protein with a predicted molecular weight of 46,000. EGFP-CKIP-1 is localized within the nucleus and at the plasma membrane. The plasma membrane localization is dependent on the presence of an amino-terminal pleckstrin homology domain. We postulate that CKIP-1 is a non-enzymatic regulator of one isoform of CK2 (i.e. CK2alpha) with a potential role in targeting CK2alpha to a particular cellular location.

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.

Expression of the casein kinase 2 subunits in Chinese hamster ovary and 3T3 L1 cells provides information on the role of the enzyme in cell proliferation and the cell cycle

In order to investigate the in vivo functions of protein kinase CK2 (CK2), the expression of Myc-tagged versions of the subunits, Myc-CK2alpha and Myc-CK2beta, was carried out in Chinese hamster ovary cells (CHO cells) and in 3T3 L1 fibroblasts. Cell proliferation in these cells was examined. CHO cells that transiently overexpressed the Myc-CK2beta subunit exhibited a severe growth defect, as shown by a much lower value of [(3)H]thymidine incorporation than the vector controls, and a rounded shrunken morphology. In contrast, cells overexpressing Myc-tagged CK2alpha showed a slightly but consistently higher value of [(3)H]thymidine incorporation than the controls. The defect in cell growth and changes in morphology caused by Myc-CK2beta overexpression were partially rescued by coexpression of Myc-tagged CK2alpha. In parallel to the studies in CHO cells, the stable transfection of Myc-CK2alpha and Myc-CK2beta subunits was achieved in 3T3 L1 fibroblast cells. Similarly, the ectopic expression of Myc-CK2beta, but not Myc-CK2alpha, caused a growth defect. By measuring [(3)H]thymidine incorporation, it was found that expression of Myc-CK2beta prolonged the G(1) phase and inhibited up-regulation of cyclin D1 expression during G(1). In addition, a lower mitotic index and lower mitotic cyclin-dependent kinase activities were detected in Myc-CK2beta-expressing cells. Detailed analysis of stable cells that were synchronously released into the cell cycle revealed that the expression of Myc-CK2beta inhibited cells entering into mitosis and prevented the activation of mitotic cyclin-dependent kinases. Taken together, results from both transient and stable expression of CK2 subunits strongly suggest that CK2 may be involved in the control of cell growth and progression of the cell cycle.

Casein kinase 2 binds and phosphorylates the nucleosome assembly protein-1 (NAP1) in Drosophila melanogaster

The nucleosome assembly protein-1 (NAP1) was originally identified in HeLa cells as a factor facilitating the in vitro assembly of nucleosomes. However, in yeast cells NAP1 is required in the control of mitotic events induced by the Clb2/p34(CDC28). Here, we show that Drosophila NAP1 is a phosphoprotein that is associated with a kinase able to phosphorylate NAP1. By using an in-gel kinase assay we found that this kinase displays a molecular mass of 38 kDa. Following purification and peptide microsequencing, we identified the kinase phosphorylating NAP1 as the alpha subunit of casein kinase 2 (CK2). With the help of a series of NAP1 segments and synthetic peptides, we assigned the CK2 phosphorylation sites to residues Ser118, Thr120, and Ser284. Interestingly, Ser118 and Thr120 are located within a PEST domain, while Ser284 is adjacent to the nuclear localization signal. Substitution of the identified phosphoresidues by alanine was found to reduce considerably the ability of CK2 to phosphorylate NAP1. The enhanced ability of CK2 to phosphorylate phosphatase-treated NAP1 extracted from Drosophila embryos and the similar tryptic phospho-peptide pattern of in vivo labelled NAP1 and in vitro labelled NAP1 with CK2 indicate that NAP1 is a natural substrate of CK2. Further analysis revealed that both CK2alpha and beta subunits are associated with NAP1 but we found that only the catalytic alpha subunit establishes direct contact with NAP1 on two distinct domains of this protein. The location of CK2 phosphorylation sites in NAP1 suggests that their phosphorylation can contribute to a PEST-mediated protein degradation of NAP1 and the translocation of NAP1 between cytoplasm and nucleus.

Protein kinase CKII interacts with and phosphorylates the SAG protein containing ring-H2 finger motif

To investigate the biological function of CKII, we have identified proteins that interact with the subunits of CKII using the yeast two-hybrid system. Here we report that SAG, an antioxidant protein containing Ring-H2 finger motif, is a cellular partner associating with the beta subunit of CKII. SAG does not interact with the alpha subunit of CKII. Analysis of SAG deletion mutants indicates that the Ring-H2 motif of SAG is necessary and sufficient for its binding to the beta subunit of CKII. Recombinant SAG can be phosphorylated by CKII in vitro, providing evidence that the beta subunit mediates the interaction of CKII enzyme with substrate proteins. Overlay experiment shows that SAG and the beta subunit of CKII associate directly in vitro and that CKII-mediated phosphorylation of SAG does not affect the interaction between SAG and the beta subunit of CKII. Northern blot analysis indicates that both SAG and the beta subunit of CKII were relatively rich in human heart, liver, skeletal muscle, and pancreas, but were detected in only trace amounts in brain, placenta, and lung. Our present results suggest that CKII may play a role in the regulation of SAG function.

Tumour suppressor protein p53 released by nuclease digestion increases at the onset of rat liver regeneration

BACKGROUND/AIMS

Protein kinase CK2 (CK2) increases when cells are committed to proliferate, as in liver regeneration. This enzyme phosphorylates the tumour suppressor protein p53, whose expression controls the levels of many other cell cycle proteins. The aim of this study was to determine if CK2 was affected by p53.

METHODS

Male Sprague-Dawley rats (200-250 g) were subjected to either partial hepatectomy or laparotomy and the levels and subcellular distribution of p53 were studied, following the approach used earlier for CK2. The levels of both proteins were also studied in the human cell lines HL-60 (devoid of p53) and HepG2 (with normal p53 levels) and in fibroblasts from transgenic p53-deficient mice (p53-/-) or homozygous for wild-type p53 (p53+/+). Computer-assisted search was used to detect p53 consensus sequences in genes for CK2 subunits Binding of p53 protein to some of these sequences was assayed by electrophoretic mobility shift assay.

RESULTS

Rat liver p53 protein was present mainly in the fraction extracted from intact nuclei by nucleases (S1) and showed a transient increase at 6 h post partial hepatectomy, as observed previously with nuclear CK2. The human CK2a gene presents the consensus sequence for trans-activation by p53 and specific binding of p53 protein to some of these sequences was detected in vitro. Total CK2a was higher in HepG2 than in HL-60 cells but total CK2 and its cytosolic/ nuclear distribution was similar in mice (p53+/+) fibroblasts and (p53-/-) fibroblasts.

CONCLUSIONS

p53 is present in the nuclease-extracted S1 fraction from liver cells, as described for CK2, and undergoes similar changes at the beginning of rat liver regeneration. However, the data on cultured cells suggest that the expression of CK2 and its subcellular localization are p53-independent events.

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.

Cutting Edge: Differential CD5-Dependent Regulation of CD5-Associated CK2 Activity in Mature and Immature T Cells: Implication on TCR/CD3-Mediated Activation

CD5 attenuates TCR-induced signals in immature thymocytes but functions as a costimulatory molecule potentiating TCR/CD3-mediated activation in mature, peripheral T cells. We have recently shown that the serine/threonine kinase, casein kinase 2 (CK2), a major regulator of cell growth and signaling, associates with and is activated by CD5. Therefore, we tested the possibility that differential regulation of CK2 activity by CD5 may be associated with these differences in CD5 modulation of TCR signaling. Consistent with our hypothesis, CD5-specific cross-linking activated associated CK2 in thymocytes but not active in mature splenocytes. Differential regulation of CD5-associated CK2 provides, for the first time, a potential mechanism for the differential effects of CD5 signaling in immature and mature cells. We propose that CD5 modulates Ag receptor activity through developmentally regulated activation of CD5-associated CK2.

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.

The tight association of protein kinase CK2 with plasma membranes is mediated by a specific domain of its regulatory beta-subunit

Previous immunocytochemical studies have shown that protein kinase CK2 is mostly detected both in the cytoplasm and the nucleus of most cells. In the present study, CK2 was detected in highly purified plasma membrane preparations from rat liver. The protein kinase could be released from the membranes by high salt extraction (>1 M NaCl). Plasma membranes prepared from SF9 insect cells expressing the alpha- and beta-subunits of CK2 also contained a significant amount of oligomeric CK2. Furthermore, it was demonstrated in this cell system as well as in rat liver plasma membranes, that the beta-subunit of the kinase is the targeting subunit which mediates the tight association of the enzyme to plasma membrane components. Binding studies using membranes and recombinant proteins corresponding to different regions of the beta-subunit suggest that a functional domain previously shown to be involved in the binding of polyamines may also participate to the binding of CK2 to membranes. Modification of membranes by trypsin and phospholipases indicated that the binding process may require both membrane protein(s) and phospholipids. Interestingly, it was observed that the amount of membrane-bound CK2 in liver of embryos and new born rats increases dramatically after birth and persists during the postnatal stages of development.

Characterization of pICln phosphorylation state and a pICln-associated protein kinase

pICln is a ubiquitous cellular protein that has been proposed to be a volume-sensitive Cl- channel or a channel regulator. Detailed biochemical, cellular and molecular characterization of pICln is required to understand its function. Our goal in the present investigation was to define further the biochemical properties of pICln and the proteins that associate with it. Immunoprecipitation of pICln from 32P-orthophosphoric acid-labeled C6 glioma cells revealed that the protein is phosphorylated constitutively, primarily on serine residues. Protein kinase activity was detected in pICln immunoprecipitates, revealing that a constitutively active protein kinase co-precipitates with pICln. A specific association between pICln and a protein kinase was also observed in affinity assays using a recombinant GST-pICln fusion protein. The pICln-associated kinase displayed broad substrate specificity and was inhibited in a concentration-dependent manner by heparin, zinc and 5,6-dichloro-1-beta-D-ribofuranosylbenose (DRB). These characteristics resembled those of casein kinase I and II. The pICln-associated kinase was not recognized, however, by antibodies against these two enzymes. Association of the kinase with pICln was disrupted by increasing concentrations of NaCl in the washing buffer, suggesting that electrostatic interactions are involved in kinase binding. Mutagenesis experiments corroborated this observation. Truncation of pICln demonstrated that two highly charged clusters of acidic amino acid residues are both necessary and sufficient for kinase binding. Phosphopeptide mapping demonstrated that pICln contains at least two phosphorylated serine residues that are located on trypsin cleavage fragments rich in acidic amino acid residues. We propose that the kinase or a kinase binding protein binds to acidic amino acids located between D101 and Y156 and phosphorylates nearby serine residues.

Dynamics of protein kinase CK2 association with nucleosomes in relation to transcriptional activity

Protein kinase CK2 has been implicated in control of cell growth and proliferation. Since growth stimuli evoke its preferential association with chromatin and nuclear matrix, we examined the dynamics of CK2 in nucleosomes fractionated on the basis of their transcriptional activity in the rat prostate. In this model, androgens induce expression of androgen-dependent genes but inhibit the androgen-repressed genes, whereas absence of androgens has the reverse effect. The level of CK2 was higher in the active than in inactive nucleosomes from normal prostate. Differential alterations in the levels of CK2 activity in the transcriptionally active versus inactive nucleosomes were evoked by androgen deprivation or administration. Comparison of the distribution of CK2 in active and inactive nucleosomes under varying androgenic conditions showed that the relative CK2 activity intrinsic to the transcriptionally active nucleosomes remained fairly stable, concordant with gene activity specific to the androgenic status. However, CK2 associated with inactive nucleosomes declined to a minimal level on androgen deprivation but increased rapidly on androgen administration (reflecting expression of multiple androgen-dependent genes). We suggest a role for CK2 in promoting the conformational transition of inactive nucleosomes to the active form and in the function of transcriptionally active nucleosomes.

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.

Multisite phosphorylation and the integration of stress signals at p53

The p53 tumour suppressor protein is a potent transcription factor that plays a major role in the defence against tumour development. p53 exists in a latent form that can be activated by a range of stresses including DNA damage, hypoxia, cytokines, metabolic changes, viral infection, and activated oncogenes. Activation of p53 can lead to cellular growth arrest prior to entry into either S phase or mitosis or can trigger cell death through apoptosis. The modification of p53 by multisite phosphorylation provides a potential link between stress signalling and the regulation of p53 activity, and there is now striking evidence that agents that activate p53 can lead to selective changes in its phosphorylation status. Topologically, the phosphorylation sites in p53 fall into two discrete functional domains. Four phosphorylation events take place within the N-terminal 83 amino acids containing the transactivation domain and a region involved in transcription-independent growth suppression. At least three of these modifications occur in response to agents that cause cellular stress such as DNA damage. At the C-terminus, there are three phosphorylation events, each of which can independently regulate the specific DNA-binding function of p53, suggesting convergent control by different signalling pathways. The multiplicity of these covalent modifications and their responsiveness to a wide range of signals suggest that p53 activity is tightly and coordinately controlled in response to stresses and changes in the cellular environment.

Murine protein kinase CK2 alpha': cDNA and genomic cloning and chromosomal mapping

Protein kinase CK2 (casein kinase II) is a heterotetrameric enzyme implicated in many essential regulatory pathways in cells. We have determined the sequence of the murine CK2 alpha' cDNA that encodes a 350-amino-acid protein that would have 99 and 98% homology with the human and chicken proteins, respectively, and is also highly homologous to murine CK2 alpha. To clarify the sequence of the 5' end of the cDNA and to elucidate the structure and regulation of the gene, we obtained a bacterial artificial chromosome clone that contains the 35-kb CK2 alpha' gene. The gene consists of 12 small exons; the 5' end, including the first exon and intron, is extremely GC rich and contains a CpG island. The putative promoter contains potential binding sites for a variety of transcriptional factors but appears to lack CCAAT- or TATA-like elements. A polymorphic dinucleotide repeat in the fifth intron allowed us to map the CK2 alpha' gene to murine Chromosome 8.

Cysteine oxidation in the mitogenic S100B protein leads to changes in phosphorylation by catalytic CKII-alpha subunit

The glial-derived calcium-binding protein S100B can be secreted to act as a neurotrophic factor or a mitogen, stimulating proliferation of glial cells. The extracellular S100B activities rely on the oxidation of the protein cysteine residues (Kligman, D., and Marshak, D. R. (1985) Proc. Natl. Acad. Sci. U. S. A. 82, 7136-7139; Winningham-Major, F., Staecker, J. L., Barger, S. W., Coats, S., and Van Eldik, L. J. (1989) J. Cell Biol. 109, 3063-3071). Here we show that oxidation of the S100B cysteine residues, Cys-68 and Cys-84, induces a conformational change in the protein structure, unmasking a canonical CKII phosphorylation site located within the typical EF-hand calcium-binding site IIbeta. Intrasubunit disulfide-bridged S100B monomer and disulfide-bonded S100B dimer are phosphorylated by the catalytic CKII-alpha subunit on Ser-62 with a Km of 0.5 microM and a Vmax of 10 pmol/min/100 pmol of S100B. Oxidized S100B is the best in vitro CKII-alpha substrate identified so far. Next we show that intrasubunit disulfide-bridged S100B monomer is the most potent S100B species to stimulate [3H]thymidine uptake by C6 glial cells in culture. In addition, the phosphorylated intrasubunit disulfide-bridged S100B monomer retains apparent mitogenic activity toward C6 glial cells, and hence, 32P-labeled S100B should be a useful probe for characterizing the mechanisms by which extracellular oxidized S100B functions. Finally, we show that formation of intrasubunit disulfide-bridged S100B monomer is stimulated by peroxynitrite anion, suggesting that production of mitogenic S100B species could be enhanced in neuropathology associated with peroxynitrite anion production.

Casein kinase II stabilizes the activity of human topoisomerase IIalpha in a phosphorylation-independent manner

Previous reports have indicated that topoisomerase II (topo II) co-purifies with and is a substrate for casein kinase II. We have carried out a detailed study of the effect that purified casein kinase II has on the activity of purified recombinant human topo IIalpha. Co-incubation of topo IIalpha and casein kinase II led to an apparent activation of the topo IIalpha; however, in experiments in which topo IIalpha was preincubated at 37 degrees C with or without native casein kinase II prior to assaying for decatenation activity, it emerged that the kinase was exerting its "activating" function via a decrease in the rate of topo IIalpha enzyme inactivation during the incubation period. This stabilization of topo IIalpha by casein kinase II was ATP-independent and was observed in both mutated and truncated derivatives of topo IIalpha lacking the major casein kinase II phospho-acceptor sites, indicating the lack of a requirement for phosphorylation. Consistent with a nonenzymatic role for casein kinase II, stoichiometric quantities of kinase were required for topo IIalpha stabilization. These data indicate that casein kinase II plays a significant role in regulating human topo IIalpha protein action via stabilization against thermal inactivation.

Hepatitis C virus NS5A protein is phosphorylated in vitro by a stably bound protein kinase from HeLa cells and by cAMP-dependent protein kinase A-alpha catalytic subunit

Hepatitis C virus (HCV) has a positive-strand RNA genome that codes for a polyprotein precursor, which is processed co- and post-translationally by cellular and viral proteinases into three structural and at least six non-structural (NS) proteins. The NS5A protein, expressed in mammalian cells, exists in two phosphorylated forms of 56-kDa and 58-kDa. In this study, we provide evidence for a stable association between NS5A and a protein kinase from HeLa cells and hepatocellular carcinoma (HepG2) cells by co-immunoprecipitation and by affinity to immobilized glutathione-S-transferase (GST)-NS5A fusion protein produced in E. coli. This protein kinase could phosphorylate in vitro the native NS5A on serine residues, (GST)-NS5A, histone H1, and casein as substrates. In addition, the GST-NS5A was also phosphorylated in vitro by the cAMP-dependent protein kinase A-alpha catalytic subunit.

Substrates for protein kinase CK2 in insulin receptor preparations from rat liver membranes: identification of a 210-kDa protein substrate as the dimeric form of endoplasmin

Chromatography of extracts from rat liver membranes on wheat-germ lectin-Sepharose resulted in a partial resolution of the insulin receptor from other phosphorylatable proteins. Among the latter, a protein (p210, with an apparent M(r) of 210 kDa on SDS/PAGE under nonreducing conditions) was found to be phosphorylated by protein kinase CK2 on Thr and Ser residues. Under reducing conditions p210 was resolved into two phosphopolypeptides with apparent M(r) of 95 and 105 kDa. Neither the 95-kDa nor the 105-kDa polypeptides were recognized by antibodies against the beta-subunit of the insulin receptor. Both polypeptides gave identical phosphopeptide maps after protease V8 digestion and contained the same N-terminal amino acid sequence. This sequence coincided with that of endoplasmin, and both polypeptides as well as p210 were recognized by antibodies against this protein. This shows that p210 corresponds to the dimeric form of rat liver endoplasmin. DEAE-Sepharose chromatography of p210 preparations removed most other contaminating proteins and revealed the presence of a protein kinase activity that coeluted with p210. This protein kinase possessed the properties (substrate specificity and inhibition by heparin) that are characteristic of the protein kinase CK2 enzymes. Furthermore, phosphoamino acid analysis and phosphopeptide maps of the 95/105-kDa polypeptides phosphorylated either by the endogenous protein kinase or by exogenous protein kinase CK2 gave similar results. The phosphorylation of p210/endoplasmin by protein kinase CK2 and its coelution gives support to the involvement of this protein kinase in membrane-associated processes.

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.

Casein kinase II binds to and phosphorylates cytoplasmic dynein

We have isolated a 27-kDa protein that binds to cytoplasmic dynein. Microsequencing of a 17-amino acid peptide of this polypeptide yielded a sequence which completely matched the predicted sequence of the beta subunit of casein kinase II, a highly conserved serine/threonine kinase. Affinity chromatography using a dynein column indicates that both the alpha and beta subunits of casein kinase II are retained by the column from rat brain cytosol. Although dynactin is also bound to the column, casein kinase II is not a dynactin subunit. Casein kinase II does not co-immunoprecipitate with dynactin, and it binds to a dynein intermediate chain column which has been preblocked with excess p150(Glued), a treatment that inhibits the binding of dynactin from cytosol. Bacterially expressed and purified rat dynein intermediate chain can be phosphorylated by casein kinase II in vitro. Further, native cytoplasmic dynein purified from rat brain can also be phosphorylated by casein kinase II in vitro. We propose that CKII may be involved in the regulation of dynein function possibly by altering its cargo specificity or its ability to interact with dynactin.

Specific interaction between casein kinase 2 and the nucleolar protein Nopp140

Casein kinase 2 (CK2) is a multifunctional second messenger-independent protein serine/threonine kinase that phosphorylates many different proteins. To understand the function and regulation of this enzyme, biochemical methods were used to search for CK2-interacting proteins. Using immobilized glutathione S-transferase fusion proteins of CK2, the nucleolar protein Nopp140 was identified as a CK2-associated protein. It was found that Nopp140 binds primarily to the CK2 regulatory subunit, beta. The possible in vivo association of Nopp140 with CK2 was also suggested from a coimmunoprecipitation experiment in which Nopp140 was detected in immunoprecipitates of CK2 prepared from cell extracts. Further studies using an overlay technique with radiolabeled CK2 as a probe revealed a direct CK2-Nopp140 interaction. Using deletion mutants of CK2beta subunits, the binding region of the CK2beta subunit to Nopp140 has been mapped. It was found that the NH2-terminal 20 amino acids of CK2beta are involved. Since Nopp140 has been identified as a nuclear localization sequence-binding protein and has been shown to shuttle between the cytoplasm and the nucleus, the finding of a CK2-Nopp140 interaction could shed light on our understanding of the function and regulation of CK2 and Nopp140.

p53 Phosphorylation: biochemical and functional consequences

The p53 protein plays a vital role in suppressing the development of cancer. Posttranslational modification through phosphorylation has been postulated to be an important regulatory mechanism of p53 function. Data describing the role of phosphorylation in terms of its effects on several biochemical properties and cellular functions of p53 are examined in the context of how p53 might be "phospho-regulated."

Fibroblast growth factor-2 binds to the regulatory beta subunit of CK2 and directly stimulates CK2 activity toward nucleolin

The presence of fibroblast growth factor-2 (FGF-2) in the nucleus has now been reported both in vitro and in vivo, but its nuclear functions are unknown. Here, we show that FGF-2 added to nuclear extract binds to protein kinase CK2 and nucleolin, a CK2 natural substrate. Added to baculovirus-infected cell extracts overexpressing CK2 or its isolated subunits, FGF-2 binds to the enzyme through its regulatory beta subunit. Using purified proteins, FGF-2 is shown to directly interact with CK2 and to stimulate CK2 activity toward nucleolin. Furthermore, a mitogenic-deficient FGF-2 mutant protein has an impaired ability to interact with CK2 and to stimulate CK2 activity using nucleolin as substrate. We propose that in growing cells, one function of nuclear FGF-2 is to modulate CK2 activity through binding to its regulatory beta subunit.