Selectivity of 4,5,6,7-tetrabromobenzotriazole, an ATP site-directed inhibitor of protein kinase CK2 ('casein kinase-2')

CDK phosphorylation inhibits the DNA-binding and ATP-hydrolysis activities of the Drosophila origin recognition complex

Faithful propagation of eukaryotic chromosomes usually requires that no DNA segment be replicated more than once during one cell cycle. Cyclin-dependent kinases (Cdks) are critical for the re-replication controls that inhibit the activities of components of the pre-replication complexes (pre-RCs) following origin activation. The origin recognition complex (ORC) initiates the assembly of pre-RCs at origins of replication and Cdk phosphorylation of ORC is important for the prevention of re-initiation. Here we show that Drosophila melanogaster ORC (DmORC) is phosphorylated in vivo and is a substrate for Cdks in vitro. Cdk phosphorylation of DmORC subunits DmOrc1p and DmOrc2p inhibits the intrinsic ATPase activity of DmORC without affecting ATP binding to DmOrc1p. Moreover, Cdk phosphorylation inhibits the ATP-dependent DNA-binding activity of DmORC in vitro, thus identifying a novel determinant for DmORC-DNA interaction. DmORC is a substrate for both Cdk2 x cyclin E and Cdk1 x cyclin B in vitro. Such phosphorylation of DmORC by Cdk2 x cyclin E, but not by Cdk1 x cyclin B, requires an "RXL" motif in DmOrc1p. We also identify casein kinase 2 (CK2) as a kinase activity in embryonic extracts targeting DmORC for modification. CK2 phosphorylation does not affect ATP hydrolysis by DmORC but modulates the ATP-dependent DNA-binding activity of DmORC. These results suggest molecular mechanisms by which Cdks may inhibit ORC function as part of re-replication control and show that DmORC activity may be modulated in response to phosphorylation by multiple kinases.

Mammalian Groucho homologs: redundancy or specificity?

The proteins termed TLE in humans, Grg in mice and Groucho in Drosophila constitute a family of transcriptional corepressors. In mammalians there are five different genes encoding an even larger number of proteins. Interactions between these TLE/Grg proteins and an array of transcription factors has been described. But is there any specificity? This review tries to make a case for a non-redundant function of individual TLE/Grg proteins. The specificity may be brought about by a tightly controlled temporo-spatial expression pattern, post-translational modifications, and subtle structural differences leading to distinct preferences for interacting transcription factors. A confirmation of this concept will ultimately need to come from genetic experiments.

Tetrabromobenzotriazole (TBBt) and tetrabromobenzimidazole (TBBz) as selective inhibitors of protein kinase CK2: evaluation of their effects on cells and different molecular forms of human CK2

The development of selective cell-permeable inhibitors of protein kinase CK2 has represented an important advance in the field. However, it is important to not overlook the existence of discrete molecular forms of CK2 that arise from the presence of distinct isozymic forms, and the existence of the catalytic CK2 subunits as free subunits and in complexes with the regulatory CK2beta subunits and, possibly, other proteins. This review examines two recently developed, and presently widely applied, CK2 inhibitors, 4,5,6,7-tetrabromobenzotriazole (TBBt) and the related 4,5,6,7-tetrabromobenzimidazole (TBBz), the latter of which was previously shown to discriminate between different molecular forms of CK2 in yeast. We have shown, by spectrophotometric titration, that TBBt, with a pK(a) approximately 5, exists in solution at physiological pH almost exclusively (>99%) as the monoanion; whereas TBBz, with a pKa approximately 9, is predominantly (>95%) in the neutral form, both of obvious relevance to their modes of binding. In vitro, TBBt inhibits different forms of CK2 with Ki values ranging from 80 to 210 nM. TBBz better discriminates between CK2 forms, with Ki values ranging from 70 to 510 nM. Despite their general similar in vitro activities, TBBz is more effective than TBBt in inducing apoptosis and, to a lesser degree, necrosis, in transformed human cell lines. Finally, development of shRNA strategies for the selective knockdown of the CK2alpha and CK2alpha' isoforms reinforces the foregoing results, indicating that inhibition of CK2 leads to attenuation of proliferation.

Features and potentials of ATP-site directed CK2 inhibitors

A panel of quite specific, fairly potent and cell-permeable inhibitors of protein kinase CK2 belonging to the classes of condensed polyphenolic compounds, tetrabromobenzimidazole/triazole derivatives and indoloquinazolines have been developed, with K(i) values in the submicromolar range. Nine structures have been solved to date of complexes between the catalytic alpha subunit of CK2 and a number of these compounds, many of which display a remarkable specificity toward CK2 as compared to a panel of >30 kinases tested. The structural basis for such selectivity appears to reside in the shape and size of a hydrophobic pocket adjacent to the ATP binding site where these ATP competitive ligands are entrapped mainly by van der Waals interactions and by an energy contribution derived from the hydrophobic effect. In CK2, this cavity is smaller than in the majority of other protein kinases due to a number of unique bulky apolar residues. Consequently, the replacement of two of these residues (V66 and I174) in human CK2 alpha with alanines gives rise to mutants, which are markedly less susceptible than wild type to these classes of inhibitors. Cell-permeable CK2 inhibitors have been successfully employed, either alone or in combination with CK2 mutants refractory to inhibition, to dissect signalling pathways affected by CK2 and/or to validate the identification of in vivo targets of this pleiotropic kinase. Moreover, the remarkable pro-apoptotic efficacy of these compounds toward cell lines derived from a wide spectrum of tumors, disclose the possibility that in perspective CK2 inhibitors might become leads for the development of anti-cancer drugs.

The shape of things to come: an emerging role for protein kinase CK2 in the regulation of cell morphology and the cytoskeleton

Protein kinase CK2 is a highly conserved, pleiotropic, protein serine/threonine kinase that is essential for life in eukaryotes. CK2 has been implicated in diverse cellular processes such as cell cycle regulation, circadian rhythms, apoptosis, transformation and tumorigenesis. In addition, there is increasing evidence that CK2 is involved in the maintenance of cell morphology and cell polarity, and in the regulation of the actin and tubulin cytoskeletons. Accordingly, this review will highlight published evidence in experimental models ranging from yeast to mammals documenting the emerging roles of protein kinase CK2 in the regulation of cell polarity, cell morphology and the cytoskeleton.

A Role for Proapoptotic BID in the DNA-Damage Response

The BCL-2 family of apoptotic proteins encompasses key regulators proximal to irreversible cell damage. The BH3-only members of this family act as sentinels, interconnecting specific death signals to the core apoptotic pathway. Our previous data demonstrated a role for BH3-only BID in maintaining myeloid homeostasis and suppressing leukemogenesis. In the absence of Bid, mice accumulate chromosomal aberrations and develop a fatal myeloproliferative disorder resembling chronic myelomonocytic leukemia. Here, we describe a role for BID in preserving genomic integrity that places BID at an early point in the path to determine the fate of a cell. We show that BID plays an unexpected role in the intra-S phase checkpoint downstream of DNA damage distinct from its proapoptotic function. We further demonstrate that this role is mediated through BID phosphorylation by the DNA-damage kinase ATM. These results establish a link between proapoptotic Bid and the DNA-damage response.

C-terminal splicing of NTPDase2 provides distinctive catalytic properties, cellular distribution and enzyme regulation

The present study provides functional characterization of alternative splicing of the NTPDase2 (ecto-nucleoside triphosphate diphosphohydrolase-2) involved in the regulation of extracellular nucleotide concentrations in a range of organ systems. A novel NTPDase2beta isoform produced by alternative splicing of the rat NTPDase2 gene provides an extended intracellular C-terminus and distinguishes itself from NTPDase2alpha isoform in gaining several intracellular protein kinase CK2 (casein kinase 2) phosphorylation sites and losing the intracellular protein kinase C motif. The plasmids containing NTPDase2alpha or NTPDase2beta cDNA were used to stably transfect Chinese-hamster ovary-S cells. Imaging studies showed that NTPDase2alpha was predominantly membrane-bound, whereas NTPDase2beta had combined cell surface and intracellular localization. alpha and beta isoforms showed variations in divalent cation dependence and substrate specificity for nucleoside-5'-triphosphates and nucleoside-5'-diphosphates. NTPDase2beta exhibited reduced ATPase activity and no apparent ADPase activity. NTPDase2 isoforms demonstrated similar sensitivity to inhibitors such as suramin and pyridoxal phosphate-6-azophenyl-2',4'-disulphonic acid, and differential regulation by protein kinases. NTPDase2beta was up-regulated by intracellular protein kinase CK2 phosphorylation, whereas NTPDase2alpha activity was down-regulated by protein kinase C phosphorylation. The results demonstrate that alternative coding of the intracellular C-terminal domain contributes distinctive phenotypic variation with respect to extracellular nucleotide specificity, hydrolysis kinetics, protein kinase-dependent intracellular regulation and protein trafficking. These findings advance the molecular physiology of this enzyme system by characterizing the contribution of the C-terminal domain to many of the enzyme's signature properties.

Extracellular phosphorylation of C9 by protein kinase CK2 regulates complement-mediated lysis

Ecto-protein kinases (ecto-PK) are expressed on many cell types, both normal and malignant, yet their functions are largely unknown. An ecto-PK capable of phosphorylating the C9 component of the complement system is described. This C9 ecto-PK could be inhibited by TBB, Emodin and DRB, selective inhibitors of protein kinase CK2. Treatment of Raji human B lymphoma cells with these CK2 inhibitors augmented cell killing by Rituximab (anti-CD20 antibodies) and human complement. Analysis of C5b-7-bearing Raji cells showed that extracellular inhibition of the ecto-CK2 enhanced cell lysis by C8 and C9. Blocking of the membrane complement regulator CD59 with monoclonal antibodies further enhanced the effect of the CK2 inhibitors on Raji cell death by complement. C9 ecto-CK2 activity was increased on cancer cells relative to normal fibroblasts and blood cells. Therefore, ecto-CK2 appears to be an additional factor protecting cells from complement-mediated lysis, probably by phosphorylation/inhibition of complement C9.

Rapid computational identification of the targets of protein kinase inhibitors

We describe a method for rapidly computing the relative affinities of an inhibitor for all individual members of a family of homologous receptors. The approach, implemented in a new program, SCR, models inhibitor-receptor interactions in full atomic detail with an empirical energy function and includes an explicit account of flexibility in homology-modeled receptors through sampling of libraries of side chain rotamers. SCR's general utility was demonstrated by application to seven different protein kinase inhibitors: for each inhibitor, relative binding affinities with panels of approximately 20 protein kinases were computed and compared with experimental data. For five of the inhibitors (SB203580, purvalanol B, imatinib, H89, and hymenialdisine), SCR provided excellent reproduction of the experimental trends and, importantly, was capable of identifying the targets of inhibitors even when they belonged to different kinase families. The method's performance in a predictive setting was demonstrated by performing separate training and testing applications, and its key assumptions were tested by comparison with a number of alternative approaches employing the ligand-docking program AutoDock (Morris et al. J. Comput. Chem. 1998, 19, 1639-1662). These comparison tests included using AutoDock in nondocking and docking modes and performing energy minimizations of inhibitor-kinase complexes with the molecular mechanics code GROMACS (Berendsen et al. Comput. Phys. Commun. 1995, 91, 43-56). It was found that a surprisingly important aspect of SCR's approach is its assumption that the inhibitor be modeled in the same orientation for each kinase: although this assumption is in some respects unrealistic, calculations that used apparently more realistic approaches produced clearly inferior results. Finally, as a large-scale application of the method, SB203580, purvalanol B, and imatinib were screened against an almost full complement of 493 human protein kinases using SCR in order to identify potential new targets; the predicted targets of SB203580 were compared with those identified in recent proteomics-based experiments. These kinome-wide screens, performed within a day on a small cluster of PCs, indicate that explicit computation of inhibitor-receptor binding affinities has the potential to promote rapid discovery of new therapeutic targets for existing inhibitors.

Spinal CK2 regulates nociceptive signaling in models of inflammatory pain

Casein kinase 2 (CK2) is a widely expressed protein kinase. Over the last several years a long list of protein substrates has evolved, many of which have proven or hypothesized roles in nociceptive signal transmission. However, CK2 has not itself been demonstrated to participate in nociception prior to this time. We set out to test the hypothesis that spinal CK2 regulates nociception using several pain models. Our first studies focused on the ability of the selective CK2 inhibitors 4,5,6,7-tetrabromobenzotriazole (TBBT) and 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB) to reduce formalin-stimulated pain behaviors in mice. Both phases of the response to subcutaneous formalin were strongly inhibited by intrathecal administration of TBBT or DRB in dose-dependent fashion. Likewise, using the complete Freund's adjuvant (CFA) model of chronic inflammatory pain, TBBT was observed to strongly reduce mechanical allodynia. The inhibition of spinal CK2 with either inhibitor did not, however, alter withdrawal latencies in the hotplate thermal pain model while intrathecal morphine was very effective. Immunohistochemical studies demonstrated all three known CK2 subunits, alpha, alpha' and beta to be expressed in spinal cord tissue as did real-time PCR experiments. While mRNA levels for each of the subunits was transiently enhanced after formalin or CFA hindpaw injection, overall spinal cord protein levels were not elevated in a sustained fashion. Our results indicate that CK2 participates in inflammatory nociception both in the acute and chronic phases. Simple changes in the abundance of spinal CK2 subunits do not likely underlie these phenomena, however.

The pleckstrin homology domain-containing protein CKIP-1 is involved in regulation of cell morphology and the actin cytoskeleton and interaction with actin capping protein

CKIP-1 is a pleckstrin homology domain-containing protein that interacts with protein kinase CK2. To elucidate the functions of CKIP-1, we generated human osteosarcoma cell lines with tetracycline-regulated expression of Flag-CKIP-1. Flag-CKIP-1 expression resulted in distinct changes in cellular morphology. Therefore, we examined the actin profile by immunofluorescence, quantitative measurement of phalloidin binding, and immunoblot analysis. These studies demonstrate that Flag-CKIP-1 expression resulted in increases in F-actin staining and protein levels of beta-actin. To elucidate the mechanisms behind the observed phenotype, we utilized tandem affinity purification to isolate CKIP-1 interacting proteins. Mass spectrometry analysis led to the identification of the actin capping protein subunits, CPalpha and CPbeta, as novel CKIP-1 interaction partners. Interactions were confirmed by coimmunoprecipitation and by colocalization. Furthermore, we demonstrate that Ser9 of CPalpha is phosphorylated by protein kinase CK2 in vitro, that CPalpha is phosphorylated in vivo, and that treatment with a CK2-specific inhibitor results in a decrease in CPalpha phosphorylation. Finally, we demonstrate that CKIP-1 and CK2 inhibit the activity of actin capping protein at the barbed ends of actin filaments. Overall, our results are consistent with CKIP-1 playing a role in the regulation of the actin cytoskeleton through its interactions with actin capping protein.

Regulation of the NMDA receptor complex and trafficking by activity-dependent phosphorylation of the NR2B subunit PDZ ligand

Interactions between NMDA receptors (NMDARs) and the PDZ [postsynaptic density-95 (PSD-95)/Discs large/zona occludens-1] domains of PSD-95/SAP90 (synapse-associated protein with a molecular weight of 90 kDa) family proteins play important roles in the synaptic targeting and signaling of NMDARs. However, little is known about the mechanisms that regulate these PDZ domain-mediated interactions. Here we show that casein kinase II (CK2) phosphorylates the serine residue (Ser1480) within the C-terminal PDZ ligand (IESDV) of the NR2B subunit of NMDAR in vitro and in vivo. Phosphorylation of Ser1480 disrupts the interaction of NR2B with the PDZ domains of PSD-95 and SAP102 and decreases surface NR2B expression in neurons. Interestingly, activity of the NMDAR and Ca2+/calmodulin-dependent protein kinase II regulates CK2 phosphorylation of Ser1480. Furthermore, CK2 colocalizes with NR1 and PSD-95 at synaptic sites. These results indicate that activity-dependent CK2 phosphorylation of the NR2B PDZ ligand regulates the interaction of NMDAR with PSD-95/SAP90 family proteins as well as surface NMDAR expression and may be a critical mechanism for modulating excitatory synaptic function and plasticity.

Phosphorylation of the acidic domain of Mdm2 by protein kinase CK2

The Murine double-minute clone 2 (Mdm2) onco-protein is the principal regulator of the tumour suppressor, p53. Mdm2 acts as an E3-type ubiquitin ligase that mediates the ubiquitylation and turnover of p53 under normal, unstressed circumstances. In response to cellular stress, such as DNA damage, the Mdm2-p53 interaction is disrupted. Part of the mechanism of uncoupling p53 from Mdm2-mediated degradation involves hypo-phosphorylation of a cluster of phosphorylated serine residues in the central acidic domain of Mdm2. Here, we show that two of the residues within this domain that are phosphorylated in vivo, Ser-260 and Ser-269, are phosphorylated by CK2 in vitro. Treatment of cells with the CK2 inhibitor, 4,5,6,7-tetrabromo-2-azabenzimidazole (TBB), leads to the induction of p53 and downstream targets of p53 including Mdm2 itself and p21. These data are consistent with the idea that CK2-mediated phosphorylation of Mdm2 may regulate Mdm2-mediated p53 turnover.

Optimization of protein kinase CK2 inhibitors derived from 4,5,6,7-tetrabromobenzimidazole

Casein kinase 2 (CK2) is a ubiquitous, essential, and highly pleiotropic protein kinase whose abnormally high constitutive activity is suspected to underlie its pathogenic potential in neoplasia and infective diseases. Thus, CK2 inhibitors designed to dissect the signaling pathways affected by this kinase, in perspective, may give rise to pharmacological tools. One of the most successful CK2 inhibitors is TBB (4,5,6,7-tetrabromobenzotriazole). Here we show that its inhibitory properties can be markedly improved by generating adducts in which N(2) is replaced by a carbon atom bound to a variety of polar functions. The most efficient inhibitor is 4,5,6,7-tetrabromo-2-(dimethylamino)benzimidazole (2c) followed by the methylsulfanyl (8), isopropylamino (2e), and amino (2a) congeners. All these compounds display K(i) values <100 nM (40 nM in the case of 2c). 2c induces apoptosis of Jurkat cells more readily than TBB (DC(50) value 2.7 vs 17 microM) and, unlike TBB, it does not display any side effect on mitochondria polarization up to 10 microM concentration. Molecular modeling of the CK2-2c complex, based on the crystal structure of the CK2-TBB complex suggests that a number of additional apolar contacts between its two methyl groups and hydrophobic residues nearby could account for its superior inhibitory properties. Consequently, 2c is even more susceptible than TBB to mutations of the unique hydrophobic residues V66 and/or I174 to alanine. We propose to adopt 2c as first choice CK2 inhibitor instead of TBB, especially for in cell studies.

Involvement of protein kinase CK2 in angiogenesis and retinal neovascularization

PURPOSE

The purpose of the study was to characterize signaling intermediates involved in angiogenic responses of retinal endothelial cells (RECs) to the extracellular matrix and growth factors, by using specific inhibitors.

METHODS

Tubelike structure formation and the development of secondary sprouts on a basement membrane (BM) matrix, cell proliferation, and cell migration were studied in cultures of bovine and human RECs. Specific inhibitors were tested for inhibition of retinal neovascularization in a mouse model of oxygen-induced retinopathy (OIR).

RESULTS

In initial experiments, the broad-spectrum protein kinase inhibitors, H7 and H89, stabilized REC tubes on BM matrix and inhibited secondary sprouting, cell migration, and cell proliferation. Among more specific kinase inhibitors tested, only inhibitors of protein kinase CK2 (formerly, casein kinase II), such as emodin and DRB, were able to duplicate the effects of H7 and H89. Actinomycin D caused only minor changes in angiogenic assays, suggesting that CK2's effects on REC did not involve its known impact on transcription. The extent of retinal neovascularization in a mouse OIR model was reduced >70% (versus untreated or vehicle-treated groups) after treatment with emodin (6 days at 60 mg/kg per day) and by approximately 60% after treatment at the same dose with TBB, the most specific CK2 inhibitor known. In the treated retinas, the main vascular tree had minimal changes, but the neovascular tufts were greatly reduced in number or absent.

CONCLUSIONS

This is the first demonstration of the involvement of ubiquitous protein kinase CK2 in angiogenesis. Naturally derived CK2 inhibitors may be useful for treatment of proliferative retinopathies.

Synthesis and activity of 1H-benzimidazole and 1H-benzotriazole derivatives as inhibitors of Acanthamoeba castellanii

Chloro-, bromo- and methyl- analogues of 1H-benzimidazole and 1H-benzotriazole and their N-alkyl derivatives have been synthesized and tested in vitro against the protozoa Acanthamoeba castellanii. The results indicate that 5,6-dimethyl-1H-benzotriazole (11) and 5,6-dibromo-1H-benzotriazole (14) have higher efficacy than the antiprotozoal agent chlorohexidine.

The DEK protein--an abundant and ubiquitous constituent of mammalian chromatin

The protein DEK is an abundant and ubiquitous chromatin protein in multicellular organisms (not in yeast). It is expressed in more than a million copies/nucleus of rapidly proliferating mammalian cells. DEK has two DNA binding modules of which one includes a SAP box, a sequence motif that DEK shares with a number of other chromatin proteins. DEK has no apparent affinity to specific DNA sequences, but preferentially binds to superhelical and cruciform DNA, and induces positive supercoils into closed circular DNA. The available evidence strongly suggests that DEK could function as an architectural protein in chromatin comparable to the better known classic architectural chromatin proteins, the high-mobility group or HMG proteins.

CK2 phosphorylation of Bdp1 executes cell cycle-specific RNA polymerase III transcription repression

RNA polymerase III (pol III) transcription from the human U6 snRNA promoter can be reconstituted with the recombinant factors SNAPc and Brf2-TFIIIB combined with purified pol III. In this system, CK2 treatment of the pol III complex is required for transcription, whereas treatment of Brf2-TFIIIB is inhibitory. Here we show that CK2 inhibits Brf2-TFIIIB by specifically phosphorylating its Bdp1 component. Bdp1 is phosphorylated by CK2 during mitosis, and this is accompanied by Bdp1 dissociation from the U6 promoter and from chromatin in general and by transcription repression. Remarkably, whereas inhibition of CK2 in mitotic extracts restores pol III transcription, inhibition of CK2 in active S phase extracts debilitates transcription. Thus, CK2 is directed to phosphorylate different targets within the basal pol III transcription machinery at different times during the cell cycle, with opposite transcriptional effects.

CK2 is responsible for phosphorylation of human La protein serine-366 and can modulate rpL37 5'-terminal oligopyrimidine mRNA metabolism

La protein binds precursors to 5S rRNA, tRNAs, and other transcripts that contain 3' UUU-OH and also promotes their maturation in the nucleus. Separate from this function, human La has been shown to positively modulate the translation of mRNAs that contain complex 5' regulatory motifs that direct internal initiation of translation. Nonphosphorylated La (npLa) inhibits pre-tRNA processing, while phosphorylation of human La serine-366 (S(366)) promotes pre-tRNA processing. npLa was found specifically associated with a class of mRNAs that have unusually short 5' untranslated regions comprised of terminal oligopyrimidine (5'TOP) tracts and that encode ribosomal proteins and translation elongation factors. Although La S(366) represents a CK2 phosphorylation site, there was no evidence that CK2 phosphorylates it in vivo. We used the CK2-specific inhibitor, 4,5,6,7-tetrabromo-2-azabenzimidazole (TBB), and antisense-mediated knockdown to demonstrate that CK2 is responsible for La S(366) phosphorylation in vivo. Hypophosphorylation was not associated with significant change in total La levels or proteolytic cleavage. Quantitative reverse transcription-PCR revealed increased association of the 5'TOP-mRNA encoding ribosomal protein L37 (rpL37) with La after TBB treatment. Transfection revealed more rpL37 mRNA associated with nonphosphorylatable La A(366) than with La S(366), concomitant with La A(366)-specific shift of a fraction of L37 mRNA off polysomes. The data indicate that CK2 phosphorylates La S(366) in vivo, that this limits 5'TOP mRNA binding, and that increasing npLa leads to greater association with potentially negative effects on TOP mRNA translation. Consistent with data that indicate that phosphorylation reverses negative effects of npLa on tRNA production, the present data suggest that CK2 phosphorylation of La can affect production of the translational machinery.

Phosphorylation of serine 239 of Groucho/TLE1 by protein kinase CK2 is important for inhibition of neuronal differentiation

Transcriptional corepressors of the Groucho (Gro)/TLE family play important roles during a variety of developmental pathways, including neuronal differentiation. In particular, they act as negative regulators of neurogenesis, together with Hairy/Enhancer of split (Hes) DNA-binding proteins. The interaction with Hes1 leads to Gro/TLE hyperphosphorylation and increased transcription repression activity in mammalian cells, but the underlying molecular mechanisms are poorly characterized. We now show that Gro/TLE1 is phosphorylated in vivo by protein kinase CK2. This phosphorylation occurs at serine 239 within the conserved CcN domain present in all Gro/TLE family members. Mutation of serine 239 into alanine decreases Hes1-induced hyperphosphorylation of Gro/TLE1 and also reduces its nuclear association and transcription repression activity. We demonstrate further that Gro/TLE1 inhibits the transition of cortical neural progenitors into neurons and that its antineurogenic activity is inhibited by a serine-239-alanine mutation but not by a serine-239-glutamate mutation. These results suggest that CK2 phosphorylation of serine 239 of Gro/TLE1 is important for its function during neuronal differentiation.

A new XRCC1-containing complex and its role in cellular survival of methyl methanesulfonate treatment

DNA single-strand break repair (SSBR) is important for maintaining genome stability and homeostasis. The current SSBR model derived from an in vitro-reconstituted reaction suggests that the SSBR complex mediated by X-ray repair cross-complementing protein 1 (XRCC1) is assembled sequentially at the site of damage. In this study, we provide biochemical data to demonstrate that two preformed XRCC1 protein complexes exist in cycling HeLa cells. One complex contains known enzymes that are important for SSBR, including DNA ligase 3 (DNL3), polynucleotide kinase 3'-phosphatase, and polymerase beta; the other is a new complex that contains DNL3 and the ataxia with oculomotor apraxia type 1 (AOA) gene product aprataxin. We report the characterization of the new XRCC1 complex. XRCC1 is phosphorylated in vivo and in vitro by CK2, and CK2 phosphorylation of XRCC1 on S518, T519, and T523 largely determines aprataxin binding to XRCC1 though its FHA domain. An acute loss of aprataxin by small interfering RNA renders HeLa cells sensitive to methyl methanesulfonate treatment by a mechanism of shortened half-life of XRCC1. Thus, aprataxin plays a role to maintain the steady-state protein level of XRCC1. Collectively, these data provide insights into the SSBR molecular machinery in the cell and point to the involvement of aprataxin in SSBR, thus linking SSBR to the neurological disease AOA.

2-Dimethylamino-4,5,6,7-tetrabromo-1H-benzimidazole: a novel powerful and selective inhibitor of protein kinase CK2

Protein kinase CK2 is a highly pleiotropic enzyme whose high constitutive activity is suspected to be instrumental to the enhancement of the tumour phenotype and to the propagation of infectious diseases. Here we describe a novel compound, 2-dimethylamino-4,5,6,7-tetrabromo-1H-benzimidazole (DMAT), which is superior to the commonly used specific CK2 inhibitor 4,5,6,7-tetrabromobenzotriazole (TBB) in several respects. DMAT displays the lowest K(i) value ever reported for a CK2 inhibitor (40 nM); it is cell permeable and its efficacy on cultured cells, both in terms of endogenous CK2 inhibition and induction of apoptosis, is several fold higher than that of TBB. The selectivity of DMAT assayed on a panel of >30 protein kinases is comparable to that of TBB, with the additional advantage of being ineffective on protein kinase CK1 up to 200 microM. These properties make DMAT the first choice CK2 inhibitor for in vivo studies available to date.

Flow-Dependent Regulation of Endothelial Toll-Like Receptor 2 Expression Through Inhibition of SP1 Activity

Innate immune system activation is associated with atherosclerotic lesion development. The specific sites of lesion development are believed to be defined by the shear stress of blood flow. Consequently, we investigated the responsiveness of human coronary artery endothelial cells (HCAECs) to Toll-like receptor (TLR) 2 and 4 agonists in an in vitro model of chronic laminar flow. HCAECs under chronic laminar flow were found to be normally responsive to lipopolysaccharide (and tumor necrosis factor) in terms of E-selectin expression but were found to be hyporesponsive to stimulation with the specific TLR2 ligands macrophage activating lipopeptide-2, PAM 2 -Cys, and Lip19; this was observed to be attributable to downregulation of TLR2 transcription and protein expression. We found that laminar flow induced SP1 serine phosphorylation by protein kinase CK2 and thereby blocked SP1 binding to the TLR2 promoter, which is required for TLR2 expression. This regulatory mechanism also blocked lipopolysaccharide- and tumor necrosis factor–induced TLR2 upregulation in HCAECs and could be important for suppression of other flow-sensitive endothelial proteins. These results extend the role of flow in controlling endothelial responsiveness. Given the current evidence that TLRs are proatherogenic, flow suppression of TLR2 expression may be atheroprotective.

The kinase Grk2 regulates Nedd4/Nedd4-2-dependent control of epithelial Na+ channels

Epithelial Na(+) channels mediate the transport of Na across epithelia in the kidney, gut, and lungs and are required for blood pressure regulation. They are inhibited by ubiquitin protein ligases, such as Nedd4 and Nedd4-2, with loss of this inhibition leading to hypertension. Here, we report that these channels are maintained in the active state by the G protein-coupled receptor kinase, Grk2, which has been previously implicated in the development of essential hypertension. We also show that Grk2 phosphorylates the C terminus of the channel beta subunit and renders the channels insensitive to inhibition by Nedd4-2. This mechanism has not been previously reported to regulate epithelial Na(+) channels and provides a potential explanation for the observed association of Grk2 overactivity with hypertension. Here, we report a G protein-coupled receptor kinase regulating a membrane protein other than a receptor and provide a paradigm for understanding how the interaction between membrane proteins and ubiquitin protein ligases is controlled.

Phosphorylation by protein kinase CK2 changes the DNA binding properties of the human chromatin protein DEK

We have examined the posttranslational modification of the human chromatin protein DEK and found that DEK is phosphorylated by the protein kinase CK2 in vitro and in vivo. Phosphorylation sites were mapped by quadrupole ion trap mass spectrometry and found to be clustered in the C-terminal region of the DEK protein. Phosphorylation fluctuates during the cell cycle with a moderate peak during G(1) phase. Filter binding assays, as well as Southwestern analysis, demonstrate that phosphorylation weakens the binding of DEK to DNA. In vivo, however, phosphorylated DEK remains on chromatin. We present evidence that phosphorylated DEK is tethered to chromatin throughout the cell cycle by the un- or underphosphorylated form of DEK.

CK2 controls multiple protein kinases by phosphorylating a kinase-targeting molecular chaperone, Cdc37

Cdc37 is a kinase-associated molecular chaperone whose function in concert with Hsp90 is essential for many signaling protein kinases. Here, we report that mammalian Cdc37 is a pivotal substrate of CK2 (casein kinase II). Purified Cdc37 was phosphorylated in vitro on a conserved serine residue, Ser13, by CK2. Moreover, Ser13 was the unique phosphorylation site of Cdc37 in vivo. Crucially, the CK2 phosphorylation of Cdc37 on Ser13 was essential for the optimal binding activity of Cdc37 toward various kinases examined, including Raf1, Akt, Aurora-B, Cdk4, Src, MOK, MAK, and MRK. In addition, nonphosphorylatable mutants of Cdc37 significantly suppressed the association of Hsp90 with protein kinases, while the Hsp90-binding activity of the mutants was unchanged. The treatment of cells with a specific CK2 inhibitor suppressed the phosphorylation of Cdc37 in vivo and reduced the levels of Cdc37 target kinases. These results unveil a regulatory mechanism of Cdc37, identify a novel molecular link between CK2 and many crucial protein kinases via Cdc37, and reveal the molecular basis for the ability of CK2 to regulate pleiotropic cellular functions.

Biochemical and functional characterization of protein kinase CK2 in ascidian Ciona intestinalis oocytes at fertilization. Cloning and sequence analysis of cDNA for alpha and beta subunits

The ubiquitous and pleiotropic dual specificity protein kinase CK2 has been studied and characterized in many organisms, from yeast to mammals. Generally, the enzyme is composed of two catalytic (alpha and/or alpha') and two regulatory (beta) subunits, forming a differently assembled tetramer. Although prone to controversial interpretation, the function of CK2 has been associated with fundamental biological processes such as signal transduction, cell cycle progression, cell growth, apoptosis, and transcription. Less known is the role of CK2 during meiosis and the early phase of embryogenesis. In this work, we studied CK2 activity during oocyte activation, a process occurring at the end of oocyte maturation and triggered by fertilization. In ascidian Ciona intestinalis, an organism whose complete genome has been published recently, CK2 was constitutively active in unfertilized and fertilized oocytes. The enzymatic activity oscillated through meiosis showing three major peaks: soon after fertilization (metaphase I exit), before metaphase II, and at the exit from metaphase II. Biochemical analysis of CK2 subunit composition in activated oocytes indicated that CK2-alpha was catalytically active as a monomer, independently from its regulatory subunit beta; however, CK2-beta was only detectable in unfertilized oocytes where it was associated with a bona fide identified ascidian mitogen-activated protein kinase. After fertilization, CK2-beta was undetectable, suggesting its rapid degradation. Protein sequence analysis of CK2-alpha and -beta cDNA indicated a high identity compared with vertebrate homologs. In addition, the absence of putative phosphorylation sites for Cdc2 kinase on both alpha and beta subunits suggested an important role for CK2 in regulating meiotic cell cycle in C. intestinalis oocytes.

Protein kinase CK2 phosphorylates the cell cycle regulatory protein Geminin

Geminin contributes to cell cycle regulation by a timely inhibition of Cdt1p, the loading factor required for the assembly of pre-replication complexes. Geminin is expressed during S and G2 phase of the HeLa cell cycle and phosphorylated soon after its synthesis. We show here that Geminin is an excellent substrate for protein kinase CK2 in vitro; and that the highly specific CK2 inhibitor tetrabromobenzotriazole (TBB) blocks the phosphorylation of Geminin in HeLa protein extracts and HeLa cells in vivo. The sites of CK2 phosphorylation are located in the carboxyterminal region of Geminin, which carries several consensus sequence motifs for CK2. We also show that a minor phosphorylating activity in protein extracts can be attributed to glycogen synthase kinase 3 (GSK3), which most likely targets a central peptide in Geminin. Treatment of HeLa cells with TBB does not interfere with the ability of Geminin to interact with the loading factor Cdt1.

The Protein Kinase CK2 Facilitates Repair of Chromosomal DNA Single-Strand Breaks

CK2 was the first protein kinase identified and is required for the proliferation and survival of mammalian cells. Here, we have identified an unanticipated role for CK2. We show that this essential protein kinase phosphorylates the scaffold protein XRCC1 and thereby enables the assembly and activity of DNA single-strand break repair protein complexes in vitro and at sites of chromosomal breakage. Moreover, we show that inhibiting XRCC1 phosphorylation by mutation of the CK2 phosphorylation sites or preventing CK2 activity using a highly specific inhibitor ablates the rapid repair of cellular DNA single-strand breaks by XRCC1. These data identify a direct role for CK2 in the repair of chromosomal DNA strand breaks and in maintaining genetic integrity.

Protein kinase CKIIalpha interacts with the Bcr moiety of Bcr/Abl and mediates proliferation of Bcr/Abl-expressing cells

The Bcr protein was originally identified because of its fusion to Abl as a consequence of the Philadelphia chromosome translocation found in chronic myelogenous and acute lymphoblastic leukemias. The Bcr moiety is essential for the transforming activity of the Bcr/Abl oncogene. In search of physiologically relevant Bcr and Bcr/Abl-interacting proteins, we performed an interaction screen in yeast using the entire Bcr protein as bait. We here report that the alpha catalytic subunit of protein kinase CKII strongly and specifically forms a complex with Bcr in yeast in mouse lysates. The region in Bcr responsible for CKIIalpha binding was localized to residues 242-413. CKIIalpha was previously shown to be involved in leukemogenesis and tumorigenesis using different experimental approaches including mouse models. Inhibition of Bcr/Abl P190 in lymphoma cells from Bcr/Abl transgenic mice using imatinib reduced CKIIalpha activity. A highly selective inhibitor of CKIIalpha, 4,5,6,7-tetrabromo-2-benzotriazole, inhibited the growth of murine lymphoid cells with induced P210 Bcr/Abl expression and of P190 lymphoma cells. Our results demonstrate that CKIIalpha plays an important role in the proliferation of Bcr/Abl expressing cells, and suggests that inhibitors of CKIIalpha may have therapeutic potential in the treatment of Bcr/Abl-positive leukemia patients.

Carbon monoxide neurotransmission activated by CK2 phosphorylation of heme oxygenase-2

Carbon monoxide (CO) is a putative gaseous neurotransmitter that lacks vesicular storage and must be synthesized rapidly following neuronal depolarization. We show that the biosynthetic enzyme for CO, heme oxygenase-2 (HO2), is activated during neuronal stimulation by phosphorylation by CK2 (formerly casein kinase 2). Phorbol ester treatment of hippocampal cultures results in the phosphorylation and activation of HO2 by CK2, implicating protein kinase C (PKC) in CK2 stimulation. Odorant treatment of olfactory receptor neurons augments HO2 phosphorylation and activity as well as cyclic guanosine monophosphate (cGMP) levels, with all of these effects selectively blocked by CK2 inhibitors. Likewise, CO-mediated nonadrenergic, noncholinergic (NANC) relaxation of the internal anal sphincter requires CK2 activity. Our findings provide a molecular mechanism for the rapid neuronal activation of CO biosynthesis, as required for a gaseous neurotransmitter.

Biochemical and three-dimensional-structural study of the specific inhibition of protein kinase CK2 by [5-oxo-5,6-dihydroindolo-(1,2-a)quinazolin-7-yl]acetic acid (IQA)

IQA [[5-oxo-5,6-dihydro-indolo(1,2-a)quinazolin-7-yl]acetic acid] is a novel ATP/GTP site-directed inhibitor of CK2 ('casein kinase 2'), a pleiotropic and constitutively active protein kinase whose activity is abnormally high in transformed cells. The K (i) value of IQA (0.17 microM) is lower than those of other CK2 inhibitors reported so far. Tested at 10 microM concentration in the presence of 100 microM ATP, IQA almost suppresses CK2 activity in vitro, whereas it is ineffective or weakly effective on a panel of 44 protein kinases and on phosphoinositide 3-kinase. In comparison, other CK2 inhibitors, notably apigenin and quercetin, are more promiscuous. The in vivo efficacy of IQA has been assessed by using the fact that treatment of Jurkat cells with IQA inhibits endogenous CK2 in a dose-dependent manner. IQA has been co-crystallized with maize CK2alpha, which is >70% identical with its human homologue, and the structure of the complex has been determined at 1.68 A (1 A=0.1 nm) resolution. The inhibitor lies in the same plane occupied by the purine moiety of ATP with its more hydrophobic side facing the hinge region. Major contributions to the interaction are provided by hydrophobic forces and non-polar interactions involving the aromatic portion of the inhibitor and the hydrophobic residues surrounding the ATP-binding pocket, with special reference to the side chains of V53 (Val53), I66, M163 and I174. Consequently, mutants of human CK2alpha in which either V66 (the homologue of maize CK2alpha I66) or I174 is replaced by alanine are considerably less sensitive to IQA inhibition when compared with wild-type. These results provide new tools for deciphering the enigmatic role of CK2 in living cells and may pave the way for the development of drugs depending on CK2 activity.

Polyhalogenobenzimidazoles: synthesis and their inhibitory activity against casein kinases

A series of novel polyhalogenated benzimidazoles have been prepared by exhaustive bromination of a variety of 2-substituted benzimidazoles. The efficacy of both new compounds and a number of their previously described cognates as inhibitors of casein kinases CK1, CK2 and G-CK was investigated. The type of N-1 alkyl substituent as well as introduction of a polyfluoroalkyl moiety at position 2 did not markedly influence the inhibitory efficacy toward CK2 of the respective 4,5,6,7-tetrabromobenzimidazole derivatives which conversely were almost ineffective toward CK1 and G-CK. However, 4,5,6,7-tetrabromobenzimidazoles substituted at position 2 with either chlorine, bromine or sulfur atom, while manifesting a still considerable inhibitory activity against CK2 (IC(50) in the 0.49-0.93 microM range) proved to be potentially powerful inhibitors also against CK1 (IC(50) in the 18.4-2.2 microM range).

Alternative binding modes of an inhibitor to two different kinases

Protein kinases are targets for therapeutic agents designed to intervene in signaling processes in the diseased state. Most kinase inhibitors are directed towards the conserved ATP binding site. Because the essential features of this site are conserved in all eukaryotic protein kinases, it is generally assumed that the same compound will bind in a similar manner to different protein kinases. The inhibitor 4,5,6,7-tetrabromobenzotriazole (TBB) is a selective inhibitor for the protein kinase CK2 (IC50 1.6 micro m) (Sarno et al. (2001) FEBS Letts.496, 44-48). Three other kinases [cyclin-dependent protein kinase 2 (CDK2), phosphorylase kinase and glycogen synthase kinase 3beta] exhibit approximately 10-fold weaker affinity for TBB than CK2. We report the crystal structure of TBB in complex with phospho-CDK2-cyclin A at 2.2 A resolution and compare the interactions with those observed for TBB bound to CK2. TBB binds at the ATP binding site of both kinases. In CDK2, each of the four bromine atoms makes polar contacts either to main chain oxygens in the hinge region of the kinase or to water molecules, in addition to several van der Waals contacts. The mode of binding of TBB to CDK2 is different from that to CK2. TBB in CDK2 is displaced more towards the hinge region between the N- and C-terminal lobes and rotated relative to TBB in CK2. The ATP binding pocket is wider in CDK2 than in CK2 resulting in fewer van der Waals contacts but TBB in CK2 does not contact the hinge. The structures show that, despite the conservation of the ATP binding pocket, the inhibitor is able to exploit different recognition features so that the same compound can bind in different ways to the two different kinases.

Determination of a large number of kinase activities using peptide substrates, P81 phosphocellulose paper arrays and phosphor imaging

To perform phosphoproteomics and signal transduction studies, a number of protein kinase activities and levels must be simultaneously analyzed in different cell samples and correlated with phosphoprotein patterns to obtain conclusions with regard to the regulation of kinase networks. We describe here a miniaturized format of the classical phosphocellulose (P81) paper binding assay with which up to 594 kinase reactions can be simultaneously analyzed. Kinase peptide substrates possessing a minimum of three consecutive basic residues were subjected to phosphorylation in 96-well plates and aliquots of the phosphorylation reactions were spotted on arrays printed on P81 papers. Phosphorylation levels were quantified using a storage phosphor system imager. The versatility of the procedure was validated by analyzing casein kinase 2, protein kinase C, and p34cdc2/cyclin B in cell extracts and testing the effect of known inhibitors and activators on kinase activities. This improved, miniaturized version of the classical P81 paper method combines simplicity, high sensitivity, high reproducibility, high reliability, and optimal Z factors and takes into account possible sources of background signals. We discuss the possibility of automation and the advantages over other methods.

Protein kinase CK2: a challenge to canons

CK2 is an extremely conserved pleiotropic protein kinase with a growing list of more than 300 substrates, the majority of which are proteins implicated in signal transduction, gene expression and other nuclear functions. The CK2 phosphoacceptor sites are specified by multiple acidic residues, with the one at position +3 relative to the target residue being of crucial relevance. The CK2 holoenzyme is composed of two catalytic subunits (alphaalpha, alpha'alpha' or alphaalpha'), which are essential for cell viability, and a dimer of two non-catalytic beta subunits, whose precise function is still poorly understood. Although the beta subunits deeply affect many properties of CK2, both the isolated catalytic subunits and the holoenzyme are constitutively active, which is probably responsible for the oncogenic potential of CK2. Given the structure of the holoenzyme, the beta subunits could undergo reversible dissociation under physiological conditions and play a role as anchoring elements and/or as a docking platform for protein substrates and effectors. These unusual features are likely to be instrumental in the involvement of CK2 in a number of key biological functions, notably RNA synthesis, Wnt signaling, ubiquitination and cell survival.

Selectivity of 4,5,6,7-tetrabromobenzimidazole as an ATP-competitive potent inhibitor of protein kinase CK2 from various sources

Like the previously reported 4,5,6,7-tetrabromobenzotriazole (TBBt), the structurally related 4,5,6,7-tetrabromobenzimidazole (TBBz) is a selective ATP-competitive inhibitor of protein kinase CK2 from such divergent sources as yeast, rat liver, Neurospora crassa and Candida tropicalis, with K(i) values in the range 0.5-1 microM. It is virtually inactive vs. PKA, PKC, and a very weak inhibitor of protein kinase CK1. The corresponding tetrachlorobenzimidazole (TCBz) is a much weaker inhibitor of CK2, like tetrachlorobenzotriazole (TCBt) relative to TBBt. Bearing in mind the similarity of the van der Waals radii of Br (1.95 A) and CH(3) (2.0 A), the corresponding much less hydrophobic 4,5,6,7-tetramethylbenzotriazole (TMeBt) was prepared and found to be a very weak inhibitor of CK2, as well as of CK1. An unexpected, and significant, difference between TBBt and TBBz are their inhibitory activities vs. the yeast protein kinase PK60S, which phosphorylates, both in vitro and in intact yeast cells, three of the five pp13 kDa ribosomal surface acidic proteins in yeast cells. TBBt was previously noted to be a more effective inhibitor of PK60S than of yeast CK2; by contrast, TBBz is a relatively feeble inhibitor of PK60S, hence more selective than TBBt vs. CK2 in yeast cells. TMeBt was virtually inactive vs PK60S. Like TBBt, TBBz is an additional lead compound for development of more potent inhibitors of CK2.

Discovery of a potent and selective protein kinase CK2 inhibitor by high-throughput docking

To assess the potential of protein kinase CK2 as a target for developing new antitumor agents, we have undertaken a search for inhibitors of this enzyme. As part of this effort, we report here the discovery of the potent and selective CK2 inhibitor (5-oxo-5,6-dihydroindolo[1,2-a]quinazolin-7-yl)acetic acid. We identified this inhibitor of a novel structural type by high-throughput docking of our corporate compound collection in the ATP binding site of a homology model of human CK2, using an appropriate protocol. The synthesis of the inhibitor as well as that of related analogues whose CK2 inhibitory activities give support to the binding mode proposed by the docking program is described. The results obtained suggest that virtual screening of a 3D database by molecular docking is a useful approach for lead finding provided that adapted postdocking filtering and reranking procedures are applied to the primary hit list.

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.

The raison d'être of constitutively active protein kinases: the lesson of CK2

Protein kinases generally are tightly controlled signaling molecules that are switched on only in response to specific stimuli. Exceptionally few protein kinases are constitutively active, the most striking example being provided by CK2 (formerly "casein kinase 2"). Owing to unique structural features, the catalytic activity of CK2 is constantly on, although its targeting can be deeply influenced by the association of its two catalytic subunits (alpha and/or alpha') with a dimeric non catalytic beta subunit. Constitutive activity of CK2 reflects its extraordinary pleiotropy documented by its growing list of >300 protein substrates and is consistent with emerging evidence that CK2 plays an essential role in the cell by counteracting premature and/or unscheduled apoptosis, thus ensuring cell survival under stress conditions.

Halogenated benzimidazoles and benzotriazoles as inhibitors of the NTPase/helicase activities of hepatitis C and related viruses

A search has been initiated for lead inhibitors of the nonstructural protein 3 (NS3)-associated NTPase/helicase activities of hepatitis C virus, the related West Nile virus, Japanese encephalitis virus and the human mitochondrial Suv3 enzyme. Random screening of a broad range of unrelated low-molecular mass compounds, employing both RNA and DNA substrates, revealed that 4,5,6,7-tetrabromobenzotriazole (TBBT) hitherto known as a potent highly selective inhibitor of protein kinase 2, is a good inhibitor of the helicase, but not NTPase, activity of hepatitis C virus NTPase/helicase. The IC50 is approximately 20 micro m with a DNA substrate, but only 60 micro m with an RNA substrate. Several related analogues of TBBT were enzyme- and/or substrate-specific inhibitors. For example, 5,6-dichloro-1-(beta-d-ribofuranosyl)benzotriazole (DRBT) was a good, and selective, inhibitor of the West Nile virus enzyme with an RNA substrate (IC50 approximately 0.3 micro m), but much weaker with a DNA substrate (IC50 approximately 3 micro m). Preincubation of the enzymes, but not substrates, with DRBT enhanced inhibitory potency, e.g. the IC50 vs the hepatitis C virus helicase activity was reduced from 1.5 to 0.1 micro m. No effect of preincubation was noted with TBBT, suggesting a different mode of interaction with the enzyme. The tetrachloro congener of TBBT, 4,5,6,7,-tetrachlorobenzotriazole (TCBT; a much weaker inhibitor of casein kinase 2) is also a much weaker inhibitor than TBBT of all four helicases. Kinetic studies, supplemented by comparison of ATP-binding sites, indicated that, unlike the case with casein kinase 2, the mode of action of the inhibitors vs the helicases is not by interaction with the catalytic ATP-binding site, but rather by occupation of an allosteric nucleoside/nucleotide binding site. The halogeno benzimidazoles and benzotriazoles included in this study are excellent lead compounds for the development of more potent inhibitors of hepatitis C virus and other viral NTPase/helicases.

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

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

CK2 constitutively associates with and phosphorylates chicken erythroid ankyrin and regulates its ability to bind to spectrin

Previous analyses have shown that the phosphorylation state of chicken erythroid ankyrin regulates its association with the spectrin cytoskeleton in vivo. Treatment of erythroid cells with serine and threonine phosphatase inhibitors stimulates the hyperphosphorylation of ankyrin and its dissociation from spectrin. In this study, we demonstrate that a kinase that directs the phosphorylation of ankyrin in vivo coprecipitates with ankyrin-containing complexes and has properties identical to CK2. Studies using CK2-specific inhibitors have indicated that all of the phosphorylation events associated with erythroid ankyrin in vivo are CK2 dependent. Furthermore, inhibitor studies combined with in vitro binding analyses have indicated that the phosphorylation of erythroid ankyrin by CK2 regulates its ability to associate with spectrin. Additional analyses revealed that CK2 coprecipitates with ankyrin-3-containing complexes isolated from Madin Darby canine kidney epithelial cells and phosphorylates this epithelial ankyrin isoform in vivo. These results are the first demonstration of a kinase constitutively associating with the ankyrin-spectrin cytoskeleton in erythroid and kidney epithelial cells. This association provides a mechanism for rapidly reorganizing the membrane cytoskeleton in these cell types through the phosphorylation of ankyrin.

Identification of protein kinase CK2 as a potent kinase of Epstein-Barr virus latent membrane protein 1

The C-terminus of latent membrane protein 1 (LMP1) can be phosphorylated in vivo. However, the protein kinase responsible for LMP1 phosphorylation has not yet been identified. In this study, GST fusion proteins containing the C-terminus of LMP1 were generated and used as substrates to survey the kinases that phosphorylate LMP1. Among several purified protein kinases tested, only protein kinase CK2 (CK2) could specifically phosphorylate LMP1. Using the in-gel kinase assay in the absence and presence of a selective CK2 inhibitor, 4,5,6,7-tetrabromobenzotriazole, CK2 was determined to be the major kinase to phosphorylate LMP1 in lymphoma and epithelial cell lines. This is the first study to show that CK2 is a potent kinase to phosphorylate LMP1 in vitro.

CK2-dependent phosphorylation of the E2 ubiquitin conjugating enzyme UBC3B induces its interaction with beta-TrCP and enhances beta-catenin degradation

Protein kinase CK2 is a ubiquitous and pleiotropic Ser/Thr protein kinase involved in cell growth and transformation. Here we report the identification by yeast interaction trap of a CK2 interacting protein, UBC3B, which is highly homologous to the E2 ubiquitin conjugating enzyme UBC3/CDC34. UBC3B complements the yeast cdc34-2 cell cycle arrest mutant in S. cerevisiae and transfers ubiquitin to a target substrate in vitro. UBC3B is specifically phosphorylated by CK2 in vitro and in vivo. We mapped by deletions and site directed mutagenesis the phosphorylation site to a serine residue within the C-terminal domain in position 233 of UBC3B and in the corresponding serine residue of UBC3. Following CK2-dependent phosphorylation both UBC3B and UBC3 bind to the F-box protein beta-TrCP, the substrate recognition subunit of an SCF (Skp1, Cul1, F-box) ubiquitin ligase. Furthermore, we observed that co-transfection of CK2alpha' together with UBC3B, but not with UBC3DeltaC, enhances the degradation of beta-catenin. Taken together these data suggest that CK2-dependent phosphorylation of UBC3 and UBC3B functions by regulating beta-TrCP substrate recognition.

Protein kinase CK2 inhibitor 4,5,6,7-tetrabromobenzotriazole (TBB) induces apoptosis and caspase-dependent degradation of haematopoietic lineage cell-specific protein 1 (HS1) in Jurkat cells

Incubation of Jurkat cells with 4,5,6,7-tetrabromobenzotriazole (TBB), a specific inhibitor of protein kinase CK2, induces dose-and time-dependent apoptosis as judged by several criteria. TBB-promoted apoptosis is preceded by inhibition of Ser/Thr phosphorylation of haematopoietic lineage cell-specific protein 1 (HS1) and is accompanied by caspase-dependent fragmentation of the same protein. Both effects are also observable if apoptosis is promoted by anti-Fas antibodies and by etoposide. Moreover, in vitro experiments show that HS1, once phosphorylated by CK2, becomes refractory to cleavage by caspase-3. These findings, in conjunction with similar data in the literature concerning two other CK2 protein substrates, Bid and Max, suggest that CK2 may play a general anti-apoptotic role through the generation of phosphorylated sites conferring resistance to caspase cleavage.

Toward the rational design of protein kinase casein kinase-2 inhibitors

Casein kinase-2 (CK2) probably is the most pleiotropic member of the protein kinase family, with more than 200 substrates known to date. Unlike the great majority of protein kinases, which are tightly regulated enzymes, CK2 is endowed with high constitutive activity, a feature that is suspected to underlie its oncogenic potential and possible implication in viral infections. This makes CK2 an attractive target for anti-neoplastic and antiviral drugs. Here, we present an overview of our present knowledge about CK2 inhibitors, with special reference to the information drawn from two recently solved crystal structures of CK2alpha in complex with emodin and with 4,5,6,7-tetrabromo-2-azabenzimidazole (TBB), this latter being the most specific CK2 inhibitor known to date. A comparison with a series of anthraquinone and xanthenone derivatives highlights the crucial relevance of the hydroxyl group at position 3 for inhibition by emodin, and discloses the possibility of increasing the inhibitory potency by placing an electron withdrawing group at position 5. We also present mutational data corroborating the relevance of two hydrophobic residues unique to CK2, Val66 and Ile174, for the interactions with emodin and TBB, but not with the flavonoid inhibitors quercetin and fisetin. In particular, the CK2alpha mutant V66A displays 27- and 11-fold higher IC(50) values with emodin and TBB, respectively, as compared with the wild-type, while the IC(50) value with quercetin is unchanged. The data presented pave the road toward the rational design of more potent and selective inhibitors of CK2 and the generation of CK2 mutants refractory to inhibition, useful to probe the implication of CK2 in specific cellular functions.

Ca(2+)-dependent interaction of triadin with histidine-rich Ca(2+)-binding protein carboxyl-terminal region

A direct binding of HRC (histidine-rich Ca(2+)-binding protein) to triadin, the main transmembrane protein of the junctional sarcoplasmic reticulum (SR) of skeletal muscle, seems well supported. Opinions are still divided, however, concerning the triadin domain involved, either the cytoplasmic or the lumenal domain, and the exact role played by Ca(2+), in the protein-to-protein interaction. Further support for colocalization of HRC with triadin cytoplasmic domain is provided here by experiments of mild tryptic digestion of tightly sealed TC vesicles. Accordingly, we show that HRC is preferentially phosphorylated by endogenous CaM K II, anchored to SR membrane on the cytoplasmic side, and not by lumenally located casein kinase 2. We demonstrate that HRC can be isolated as a complex with triadin, following equilibrium sucrose-density centrifugation in the presence of mM Ca(2+). Here, we characterized the COOH-terminal portion of rabbit HRC, expressed and purified as a fusion protein (HRC(569-852)), with respect to Ca(2+)-binding properties, and to the interaction with triadin on blots, as a function of the concentration of Ca(2+). Our results identify the polyglutamic stretch near the COOH terminus, as the Ca(2+)-binding site responsible, both for the acceleration in mobility of HRC on SDS-PAGE in the presence of millimolar concentrations of Ca(2+), and for the enhancement by high Ca(2+) of the interaction between HRC and triadin cytoplasmic segment. (c)2001 Elsevier Science.

Structural features underlying selective inhibition of protein kinase CK2 by ATP site-directed tetrabromo-2-benzotriazole

Two novel crystal structures of Zea mays protein kinase CK2alpha catalytic subunit, one in complex with the specific inhibitor 4,5,6,7-tetrabromobenzotriazole (TBB) and another in the apo-form, were solved at 2.2 A resolution. These structures were compared with those of the enzyme in presence of ATP and GTP (the natural cosubstrates) and the inhibitor emodin. Interaction of TBB with the active site of CK2alpha is mainly due to van der Waals contacts, with the ligand fitting almost perfectly the cavity. One nitrogen of the five-membered ring interacts with two charged residues, Glu 81 and Lys 68, in the depth of the cavity, through two water molecules. These are buried in the active site and are also generally found in the structures of CK2alpha enzyme analyzed so far, with the exception of the complex with emodin. In the N-terminal lobe, the position of helix alphaC is particularly well preserved in all the structures examined; the Gly-rich loop is displaced from the intermediate position it has in the apo-form and in the presence of the natural cosubstrates (ATP/GTP) to either an upper (with TBB) or a lower position (with emodin). The selectivity of TBB for CK2 appears to be mainly dictated by the reduced size of the active site which in most other protein kinases is too large for making stable interactions with this inhibitor.