Quaternary structure of casein kinase 2. Characterization of multiple oligomeric states and relation with its catalytic activity

Analyzing the interactome of human CK2β in prostate carcinoma cells reveals HSP70-1 and Rho guanin nucleotide exchange factor 12 as novel interaction partners

CK2β is the non-catalytic modulating part of the S/T-protein kinase CK2. However, the overall function of CK2β is poorly understood. Here, we report on the identification of 38 new interaction partners of the human CK2β from lysates of DU145 prostate cancer cells using photo-crosslinking and mass spectrometry, whereby HSP70-1 was identified with high abundance. The K_D value of its interaction with CK2β was determined as 0.57 μM by microscale thermophoresis, this being the first time, to our knowledge, that a K_D value of CK2β with another protein than CK2α or CK2α' was quantified. Phosphorylation studies excluded HSP70-1 as a substrate or activity modulator of CK2, suggesting a CK2 activity independent interaction of HSP70-1 with CK2β. Co-immunoprecipitation experiments in three different cancer cell lines confirmed the interaction of HSP70-1 with CK2β in vivo. A second identified CK2β interaction partner was Rho guanin nucleotide exchange factor 12, indicating an involvement of CK2β in the Rho-GTPase signal pathway, described here for the first time to our knowledge. This points to a role of CK2β in the interaction network affecting the organization of the cytoskeleton.

Predictive functional, statistical and structural analysis of CSNK2A1 and CSNK2B variants linked to neurodevelopmental diseases

Okur-Chung Neurodevelopmental Syndrome (OCNDS) and Poirier-Bienvenu Neurodevelopmental Syndrome (POBINDS) were recently identified as rare neurodevelopmental disorders. OCNDS and POBINDS are associated with heterozygous mutations in the CSNK2A1 and CSNK2B genes which encode CK2α, a serine/threonine protein kinase, and CK2β, a regulatory protein, respectively, which together can form a tetrameric enzyme called protein kinase CK2. A challenge in OCNDS and POBINDS is to understand the genetic basis of these diseases and the effect of the various CK2⍺ and CK2β mutations. In this study we have collected all variants available to date in CSNK2A1 and CSNK2B, and identified hotspots. We have investigated CK2⍺ and CK2β missense mutations through prediction programs which consider the evolutionary conservation, functionality and structure or these two proteins, compared these results with published experimental data on CK2α and CK2β mutants, and suggested prediction programs that could help predict changes in functionality of CK2α mutants. We also investigated the potential effect of CK2α and CK2β mutations on the 3D structure of the proteins and in their binding to each other. These results indicate that there are functional and structural consequences of mutation of CK2α and CK2β, and provide a rationale for further study of OCNDS and POBINDS-associated mutations. These data contribute to understanding the genetic and functional basis of these diseases, which is needed to identify their underlying mechanisms.

How can a traffic light properly work if it is always green? The paradox of CK2 signaling

CK2 is a constitutively active protein kinase that assuring a constant level of phosphorylation to its numerous substrates supports many of the most important biological functions. Nevertheless, its activity has to be controlled and adjusted in order to cope with the varying needs of a cell, and several examples of a fine-tune regulation of its activity have been described. More importantly, aberrant regulation of this enzyme may have pathological consequences, e.g. in cancer, chronic inflammation, neurodegeneration, and viral infection. Our review aims at summarizing our current knowledge about CK2 regulation. In the first part, we have considered the most important stimuli shown to affect protein kinase CK2 activity/expression. In the second part, we focus on the molecular mechanisms by which CK2 can be regulated, discussing controversial aspects and future perspectives.

Structures, functions, and mechanisms of filament forming enzymes: a renaissance of enzyme filamentation

Filament formation by non-cytoskeletal enzymes has been known for decades, yet only relatively recently has its wide-spread role in enzyme regulation and biology come to be appreciated. This comprehensive review summarizes what is known for each enzyme confirmed to form filamentous structures in vitro, and for the many that are known only to form large self-assemblies within cells. For some enzymes, studies describing both the in vitro filamentous structures and cellular self-assembly formation are also known and described. Special attention is paid to the detailed structures of each type of enzyme filament, as well as the roles the structures play in enzyme regulation and in biology. Where it is known or hypothesized, the advantages conferred by enzyme filamentation are reviewed. Finally, the similarities, differences, and comparison to the SgrAI endonuclease system are also highlighted.

Magnaporthe oryzae CK2 Accumulates in Nuclei, Nucleoli, at Septal Pores and Forms a Large Ring Structure in Appressoria, and Is Involved in Rice Blast Pathogenesis

Magnaporthe oryzae (Mo) is a model pathogen causing rice blast resulting in yield and economic losses world-wide. CK2 is a constitutively active, serine/threonine kinase in eukaryotes, having a wide array of known substrates, and involved in many cellular processes. We investigated the localization and role of MoCK2 during growth and infection. BLAST search for MoCK2 components and targeted deletion of subunits was combined with protein-GFP fusions to investigate localization. We found one CKa and two CKb subunits of the CK2 holoenzyme. Deletion of the catalytic subunit CKa was not possible and might indicate that such deletions are lethal. The CKb subunits could be deleted but they were both necessary for normal growth and pathogenicity. Localization studies showed that the CK2 holoenzyme needed to be intact for normal localization at septal pores and at appressorium penetration pores. Nuclear localization of CKa was however not dependent on the intact CK2 holoenzyme. In appressoria, CK2 formed a large ring perpendicular to the penetration pore and the ring formation was dependent on the presence of all CK2 subunits. The effects on growth and pathogenicity of deletion of the b subunits combined with the localization indicate that CK2 can have important regulatory functions not only in the nucleus/nucleolus but also at fungal specific structures such as septa and appressorial pores.

ATM directs DNA damage responses and proteostasis via genetically separable pathways

The protein kinase ATM is a master regulator of the DNA damage response but also responds directly to oxidative stress. Loss of ATM causes ataxia telangiectasia, a neurodegenerative disorder with pleiotropic symptoms that include cerebellar dysfunction, cancer, diabetes, and premature aging. We genetically separated the activation of ATM by DNA damage from that by oxidative stress using separation-of-function mutations. We found that deficient activation of ATM by the Mre11-Rad50-Nbs1 complex and DNA double-strand breaks resulted in loss of cell viability, checkpoint activation, and DNA end resection in response to DNA damage. In contrast, loss of oxidative activation of ATM had minimal effects on DNA damage-related outcomes but blocked ATM-mediated initiation of checkpoint responses after oxidative stress and resulted in deficiencies in mitochondrial function and autophagy. In addition, expression of a variant ATM incapable of activation by oxidative stress resulted in widespread protein aggregation. These results indicate a direct relationship between the mechanism of ATM activation and its effects on cellular metabolism and DNA damage responses in human cells and implicate ATM in the control of protein homeostasis.

Unexpected Binding Mode of a Potent Indeno[1,2-b]indole-Type Inhibitor of Protein Kinase CK2 Revealed by Complex Structures with the Catalytic Subunit CK2α and Its Paralog CK2α'

Protein kinase CK2, a member of the eukaryotic protein kinase superfamily, is associated with cancer and other human pathologies and thus an attractive drug target. The indeno[1,2-b]indole scaffold is a novel lead structure to develop ATP-competitive CK2 inhibitors. Some indeno[1,2-b]indole-based CK2 inhibitors additionally obstruct ABCG2, an ABC half transporter overexpressed in breast cancer and co-responsible for drug efflux and resistance. Comprehensive derivatization studies revealed substitutions of the indeno[1,2-b]indole framework that boost either the CK2 or the ABCG2 selectivity or even support the dual inhibition potential. The best indeno[1,2-b]indole-based CK2 inhibitor described yet (IC₅₀ = 25 nM) is 5-isopropyl-4-(3-methylbut-2-enyl-oxy)-5,6,7,8-tetrahydroindeno[1,2-b]indole-9,10-dione (4p). Herein, we demonstrate the membrane permeability of 4p and describe co-crystal structures of 4p with CK2α and CK2α′, the paralogs of human CK2 catalytic subunit. As expected, 4p occupies the narrow, hydrophobic ATP site of CK2α/CK2α′, but surprisingly with a unique orientation: its hydrophobic substituents point towards the solvent while its two oxo groups are hydrogen-bonded to a hidden water molecule. An equivalent water molecule was found in many CK2α structures, but never as a critical mediator of ligand binding. This unexpected binding mode is independent of the interdomain hinge/helix αD region conformation and of the salt content in the crystallization medium.

Characterization of the oligomeric states of the CK2 α2β2 holoenzyme in solution

The regulatory mechanism of protein kinase CK2 has still to be fully clarified. The prevailing hypothesis is that CK2 is controlled by a self-polymerisation mechanism leading to inactive supramolecular assemblies that, when needed, can be disassembled into the α2β2 monomer, the active form of the holoenzyme. In vitro, monomeric α2β2 seems present only at high ionic strengths, typically 0.35–0.50 M NaCl, while at lower salt concentrations oligomers are formed. In the present study, size-exclusion chromatography (SEC), dynamic light scattering (DLS), small-angle X-ray scattering (SAXS) and mutagenesis have been employed for the characterization of the oligomeric states of CK2 in solution. SAXS measurements at 0.35 M NaCl show for the first time the shape of the α2β2 active monomer in solution. At 0.25 M salt, despite single average properties indicating an aggregated holoenzyme, deconvolution analysis of SAXS data reveals an equilibrium involving not only circular trimeric and linear oligomeric (3–4 units) forms of α2β2, but also considerable amounts of the monomer. Together SAXS and mutagenesis confirm the presence in solution of the oligomers deduced by crystal structures. The lack of intermediate species such as αβ2, α or β2 indicates that the holoenzyme is a strong complex that does not spontaneously dissociate, challenging what was recently proposed on the basis of mass spectrometry data. A significant novel finding is that a considerable amount of monomer, the active form of CK2, is present also at low salt. The solution properties of CK2 shown in the present study complement the model of regulation by polymerization.

Protein kinase CK2 in development and differentiation

Among the human kinomes, protein kinase CK2 (formerly termed casein kinase II) is considered to be essential, as it is implicated in the regulation of various cellular processes. Experiments with pharmacological inhibitors of the kinase activity of CK2 provide evidence that CK2 is essential for development and differentiation. Therefore, the present review addresses the role of CK2 during embryogenesis, neuronal, adipogenic, osteogenic and myogenic differentiation in established model cell lines, and in embryonic, neural and mesenchymal stem cells. CK2 kinase activity appears to be essential in the early stages of differentiation, as CK2 inhibition at early time points generally prevents differentiation. In addition, the present review reports on target proteins of CK2 in embryogenesis and differentiation.

Mass Spectrometry Reveals Protein Kinase CK2 High-Order Oligomerization via the Circular and Linear Assembly

CK2 is an intrinsically active protein kinase that is crucial for cellular viability. However, conventional kinase regulatory mechanisms do not apply to CK2, and its mode of regulation remains elusive. Interestingly, CK2 is known to undergo reversible ionic-strength-dependent oligomerization. Furthermore, a regulatory mechanism based on autoinhibitory oligomerization has been postulated on the basis of the observation of circular trimeric oligomers and linear CK2 assemblies in various crystal structures. Here, we employ native mass spectrometry to monitor the assembly of oligomeric CK2 species in an ionic-strength-dependent manner. A subsequent combination of ion mobility spectrometry-mass spectrometry and hydrogen-deuterium exchange mass spectrometry techniques was used to analyze the conformation of CK2 oligomers. Our findings support ionic-strength-dependent CK2 oligomerization, demonstrate the transient nature of the α/β interaction, and show that CK2 oligomerization proceeds via both the circular and linear assembly.

Benzotriazole: An overview on its versatile biological behavior

Discovered in late 1960, azoles are heterocyclic compounds class which constitute the largest group of available antifungal drugs. Particularly, the imidazole ring is the chemical component that confers activity to azoles. Triazoles are obtained by a slight modification of this ring and similar or improved activities as well as less adverse effects are reported for triazole derivatives. Consequently, it is not surprising that benzimidazole/benzotriazole derivatives have been found to be biologically active. Since benzimidazole has been widely investigated, this review is focused on defining the place of benzotriazole derivatives in biomedical research, highlighting their versatile biological properties, the mode of action and Structure Activity Relationship (SAR) studies for a variety of antimicrobial, antiparasitic, and even antitumor, choleretic, cholesterol-lowering agents.

Functional display of heterotetrameric human protein kinase CK2 on Escherichia coli: a novel tool for drug discovery

Background

Human protein kinase CK2 represents a novel therapeutic target for neoplastic diseases. Inhibitors are in need to explore the druggability and the therapeutic options of this enzyme. A bottleneck in the search for new inhibitors is the availability of the target for testing. Therefore an assay was developed to provide easy access to CK2 for discovery of novel inhibitors.

Results

Autodisplay was used to present human CK2 on the surface of Escherichia coli. Heterotetrameric CK2 consists of two subunits, α and β, which were displayed individually on the surface. Co-display of CK2α and CK2β on the cell surface led to the formation of functional holoenzyme, as demonstrated by NaCl dependency of enzymatic activity, which differs from that of the catalytic subunit CK2α without β. In addition interaction of CK2α and CK2β at the cell surface was confirmed by co-immunoprecipitation assays. Surface displayed CK2 holoenzyme enabled an easy IC₅₀ value determination. The IC₅₀ values for the known CK2 inhibitors TBB and Silmitasertib were determined to be 50 and 3.3 nM, respectively.

Conclusion

Surface-displayed CK2α and CK2β assembled on the cell surface of E. coli to an active tetrameric holoenzyme. The whole-cell CK2 autodisplay assay as developed is suitable for inhibition studies. Furthermore, it can be used to determine quantitative CK2 inhibition data such as IC₅₀ values. In summary, this is the first report on the functional surface display of a heterotetrameric enzyme on E. coli.

Identification of a novel potent, selective and cell permeable inhibitor of protein kinase CK2 from the NIH/NCI Diversity Set Library

The anti-apoptotic protein kinase CK2 increasingly becomes an attractive target in cancer research with great therapeutic potential. Here, we have performed an in vitro screening of the Diversity Set III of the DTP program from the NCI/NIH, comprising 1600 compounds. We have identified 1,3-Dichloro-6-[(E)-((4-methoxyphenyl)imino)methyl] dibenzo(b,d) furan-2,7-diol (referred to as D11) to be a potent and selective inhibitor of protein kinase CK2. The D11 compound was tested against 354 eukaryotic protein kinases. By setting the threshold for inhibition to <2% remaining kinase activity, only DYRK1B, IRAK1 and PIM3 were inhibited to an extent as the tetrameric CK2 holoenzyme and its catalytic subunits α and α'. The IC50 values for the CK2α and CK2α' were on average 1-2 nM in comparison to the DYRK1B, IRAK1 and PIM3 kinases, which ranged from 18 to 49 nM. Cell permeability and efficacy of D11 were tested with cells in culture. In MIA PaCa-2 cells (human pancreatic carcinoma cell line), the phosphorylation of the CK2 biomarker CDC37 at S13 was almost completely inhibited in the presence of D11. This was observed both under normoxia and hypoxia. In the case of the human non-small cell lung carcinoma cell line, H1299, increasing amounts of D11 led to an inhibition of S380/T382/383 phosphorylation in PTEN, another biomarker for CK2 activity.

Evidence for aggregation of protein kinase CK2 in the cell: a novel strategy for studying CK2 holoenzyme interaction by BRET(2)

Protein kinase CK2 is a ubiquitous pro-survival kinase whose substrate targets are involved in various cellular processes. Crystal structure analysis confirmed constitutive activity of the kinase, yet CK2 activity regulation in the cell is still obscure. In-vitro studies suggest autoinhibitory aggregation of the hetero-tetrameric CK2 holoenzyme as a basis for CK2 regulation. In this study, we applied bioluminescent resonance energy transfer (BRET) technology to investigate CK2 holoenzyme aggregation in living cells. We designed a BRET(2) pair consisting of the fusion proteins CK2α-Rluc8 and CK2α-GFP(2). This BRET(2) sensor reported specific interaction of CK2 holoenzyme complexes. Furthermore, the BRET(2) sensor was applied to study modulators of CK2 aggregation. We found that CK2 aggregation is not static and can be influenced by the CK2-binding protein alpha subunit of the heterotrimeric G-protein that stimulates adenylyl cyclase (Gαs) and the polycationic compound polylysine. Gαs, but not the CK2 substrate β-arrestin2, decreased the BRET(2) signal by up to 50%. Likewise polylysine, but not the CK2 inhibitor DRB, decreased the signal in a dose-dependent manner up to 50%. For the first time, we present direct experimental evidence for CK2 holoenzyme aggregates in the cell. Our data suggest that CK2 activity may be controlled by holoenzyme aggregation, to our knowledge a novel mechanism for protein kinase regulation. Moreover, the BRET(2) sensor used in our study is a novel tool for studying CK2 regulation by aggregation and pharmacological screening for novel allosteric CK2 effectors.

Active form of the protein kinase CK2 α2β2 holoenzyme is a strong complex with symmetric architecture

CK2 is a protein kinase essential for cell viability whose activity is altered in several cancers. Its mechanisms of regulation differ from those common to other eukaryotic protein kinases and are not entirely established yet. Here we present crystal structures of the monomeric form of the α2β2 holoenzyme that allow refining a formerly proposed structural model for activity regulation by oligomerization. Previous crystal structures of the CK2 holoenzyme show an asymmetric arrangement of the two α catalytic subunits around the obligate β2 regulatory subunits. Asymmetric α2β2 tetramers are organized in trimeric rings that correspond to inactive forms of the enzyme. The new crystal structures presented here reveal the symmetric architecture of the isolated active tetramers. The dimension and the nature of the α/β interfaces configure the holoenzyme as a strong complex that does not spontaneously dissociate in solution, in accordance with the low dissociation constant (∼4 nM).

Inhibition of protein kinase CK2 by flavonoids and tyrphostins. A structural insight

Sixteen flavonoids and related compounds have been tested for their ability to inhibit three acidophilic Ser/Thr protein kinases: the Golgi apparatus casein kinase (G-CK) recently identified with protein FAM20C, protein kinase CK1, and protein kinase CK2. While G-CK is entirely insensitive to all compounds up to 40 μM concentration, consistent with the view that it is not a member of the kinome, and CK1 is variably inhibited in an isoform-dependent manner by fisetin and luteolin, and to a lesser extent by myricetin and quercetin, CK2 is susceptible to drastic inhibition by many flavonoids, displaying with six of them IC(50) values < 1 μM. A common denominator of these compounds (myricetin, quercetin, fisetin, kaempferol, luteolin, and apigenin) is a flavone scaffold with at least two hydroxyl groups at positions 7 and 4'. Inhibition is competitive with respect to the phospho-donor substrate ATP. The crystal structure of apigenin and luteolin in complex with the catalytic subunit of Zea mays CK2 has been solved, revealing their ability to interact with both the hinge region (Val116) and the positive area near Lys68 and the conserved water W1, the two main polar ligand anchoring points in the CK2 active site. Modeling experiments account for the observation that luteolin but not apigenin inhibits also CK1. The observation that luteolin shares its pyrocatechol moiety with tyrphostin AG99 prompted us to solve also the structure of this compound in complex with CK2. AG99 was found inside the ATP pocket, consistent with its mode of inhibition competitive with respect to ATP. As in the case of luteolin, the pyrocatechol group of AG99 is critical for binding, interacting with the positive area in the deepest part of the CK2 active site.

Structural determinants of protein kinase CK2 regulation by autoinhibitory polymerization

CK2 is a Ser/Thr protein kinase essential for cell viability whose activity is anomalously high in several cancers. CK2 is a validated target for cancer therapy with one small molecule inhibitor in phase I clinical trials. This enzyme is not regulated by mechanisms common to other protein kinases, and how its activity is controlled is still unclear. We present a new crystal structure of the CK2 holoenzyme that supports an autoinhibitory mechanism of regulation whereby the β-subunit plays an essential role in the formation of inactive polymeric assemblies. The derived structural model of (down)regulation by aggregation contributes to the interpretation of biochemical and functional data and paves the way for new strategies in the modulation of CK2 activity and for the design of non-ATP-competitive inhibitors targeting the interaction between the α catalytic and the β regulatory subunits.

Phosphorylation of CRN2 by CK2 regulates F-actin and Arp2/3 interaction and inhibits cell migration

CRN2 (synonyms: coronin 1C, coronin 3) functions in the re-organization of the actin network and is implicated in cellular processes like protrusion formation, secretion, migration and invasion. We demonstrate that CRN2 is a binding partner and substrate of protein kinase CK2, which phosphorylates CRN2 at S463 in its C-terminal coiled coil domain. Phosphomimetic S463D CRN2 loses the wild-type CRN2 ability to inhibit actin polymerization, to bundle F-actin, and to bind to the Arp2/3 complex. As a consequence, S463D mutant CRN2 changes the morphology of the F-actin network in the front of lamellipodia. Our data imply that CK2-dependent phosphorylation of CRN2 is involved in the modulation of the local morphology of complex actin structures and thereby inhibits cell migration.

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.

Effects of buffer loading for electrospray ionization mass spectrometry of a noncovalent protein complex that requires high concentrations of essential salts

Electrospray ionization (ESI) mass spectrometry (MS) is a powerful method for analyzing the active forms of macromolecular complexes of biomolecules. However, these solutions often contain high concentrations of salts and/or detergents that adversely effect ESI performance by making ion formation less reproducible, causing severe adduction or ion suppression. Many methods for separating complexes from nonvolatile additives are routinely used with ESI-MS, but these methods may not be appropriate for complexes that require such stabilizers for activity. Here, the effects of buffer loading using concentrations of ammonium acetate ranging from 0.22 to 1.41 M on the ESI mass spectra of a solution containing a domain truncation mutant of a sigma(54) activator from Aquifex aeolicus were studied. This 44.9 kDa protein requires the presence of millimolar concentrations of Mg(2+), BeF(3)(-), and ADP, (at approximately 60 degrees C) to assemble into an active homo-hexamer. Addition of ammonium acetate can improve signal stability and reproducibility, and can significantly lower adduction and background signals. However, at higher concentrations, the relative ion abundance of the hexamer is diminished, while that of the constituent monomer is enhanced. These results are consistent with loss of enzymatic activity as measured by ATP hydrolysis and indicate that the high concentration of ammonium acetate interferes with assembly of the hexamer. This shows that buffer loading with ammonium acetate is effective for obtaining ESI signal for complexes that require high concentrations of essential salts, but can interfere with formation of, and/or destabilize complexes by disrupting crucial electrostatic interactions at high concentration.

Structural basis of the constitutive activity of protein kinase CK2

Protein kinase CK2 (formerly referred to as casein kinase II) is an evolutionary conserved, ubiquitous protein kinase. In mammals, there are two paralog catalytic subunits, that is, CK2α (A1) and CK2α' (A2), and one CK2β dimer, which together form the heterotetrameric holoenzyme. The presence of full functioning CK2α and CK2β subunits are absolutely mandatory for embryonic development. Total knockouts are lethal. The CK2α' paralog seems to be an exception inasmuch as a total knockout only leads to sterility in male mice. The catalytic subunits are distantly related to the CMGC subfamily of protein kinases, such as the cyclin-dependent kinases (CDKs). There are some peculiarities associated with protein kinase CK2, which are not found with most of the other protein kinases: the enzyme is constitutively active, it can use ATP and GTP as phosphoryl donors, and it is found elevated in most tumors investigated and rapidly proliferating tissues. In this review, we explain (i) its constitutive activity at the intramolecular level, and (ii) come forward with a model how this protein kinase could be regulated in cells by a mechanism involving intermolecular interactions.

Identification of polyoxometalates as nanomolar noncompetitive inhibitors of protein kinase CK2

Protein kinase CK2 is a multifunctional kinase of medical importance that is dysregulated in many cancers. In this study, polyoxometalates were identified as original CK2 inhibitors. [P2Mo18O62](6-) has the most potent activity. It inhibits the kinase in the nanomolar range by targeting key structural elements located outside the ATP- and peptide substrate-binding sites. Several polyoxometalate derivatives exhibit strong inhibitory efficiency, with IC50 values < or = 10 nM. Furthermore, these inorganic compounds show a striking specificity for CK2 when tested in a panel of 29 kinases. Therefore, polyoxometalates are effective CK2 inhibitors in terms of both efficiency and selectivity and represent nonclassical kinase inhibitors that interact with CK2 in a unique way. This binding mode may provide an exploitable mechanism for developing potent drugs with desirable properties, such as enhanced selectivity relative to ATP-mimetic inhibitors.

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

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

Validation of protein kinase CK2 as oncological target

Protein kinase CK2 is a highly conserved enzyme composed of two catalytic subunits alpha and/or alpha' and two regulatory subunits beta whose activity is elevated in diverse tumour types as well as in highly proliferating tissues. Several results suggest that the overexpression of either CK2 catalytic subunits or the CK2 holoenzyme contributes to cellular transformation. In a similar vein, experiments performed compromising the intracellular expression of CK2 has led to somehow contradictory results with respect to the ability of this enzyme to control survival and apoptosis. To better elucidate the role of CK2 in programmed cell death, we have depleted cells of CK2 catalytic subunits by the application of antisense oligodeoxynucleotides and siRNAs techniques, respectively. Our results indicate that protein kinase CK2 is characterized by an extremely high stability that might be due to its association with other intracellular proteins, enhanced half-life or lower vulnerability towards proteolytic degradation. In addition, we show that despite the effectiveness of the methods applied in lowering CK2 kinase activity in all cells investigated, CK2 might not by itself be sufficient to trigger enhanced drug-induced apoptosis in cells.

Characterization of protein kinase CK2 from Trypanosoma brucei

CK2 is a ubiquitous but enigmatic kinase. The difficulty in assigning a role to CK2 centers on the fact that, to date, no biologically relevant modulator of its function has been identified. One common theme revolves around a constellation of known substrates involved in growth control, compatible with its concentration in the nucleus and nucleolus. We had previously described the identification of two catalytic subunits of CK2 in Trypanosoma brucei and characterized one of them. Here we report the characterization of the second catalytic subunit, CK2alpha', and the identification and characterization of the regulatory subunit CK2beta. All three subunits are primarily localized to the nucleolus in T. brucei. We also show that CK2beta interacts with the nucleolar protein NOG1, adding to the interaction map which previously linked CK2alpha to the nucleolar protein NOPP44/46, which in turn associates with the rRNA binding protein p37. CK2 activity has four distinctive features: near equal affinity for GTP and ATP, heparin sensitivity, and stimulation by polyamines and polybasic peptides. Sequence comparison shows that the parasite orthologues have mutations in residues previously mapped as important in specifying affinity for GTP and stimulation by both polyamines and polybasic peptides. Studies of the enzymatic activity of the T. brucei CK2s show that both the affinity for GTP and stimulation by polyamines have been lost and only the features of heparin inhibition and stimulation by polybasic peptides are conserved.

Highlighting protein kinase CK2 movement in living cells

Protein kinase CK2 has traditionally been described as a stable heterotetrameric complex (alpha2beta2) but new approaches that effectively capture the dynamic behavior of proteins, are bringing a new picture of this complex into focus. To track the spatio-temporal dynamics of CK2 in living cells, we fused its catalytic alpha and regulatory beta subunits with GFP and analog proteins. Beside the mostly nuclear localization of both subunits, and the identification of specific domains on each subunit that triggers their localization, the most significant finding was that the association of both CK2 subunits in a stable tetrameric holoenzyme eliminates their nuclear import (Mol Cell Biol 23: 975-987, 2003). Molecular movements of both subunits in the cytoplasm and in the nucleus were analyzed using different new and updated fluorescence imaging methods such as: fluorescence recovery after photo bleaching (FRAP), fluorescence loss in photo bleaching (FLIP), fluorescence correlation spectroscopy (FCS), and photoactivation using a biphoton microscope. These fluorescence-imaging techniques provide unprecedented ways to visualize and quantify the mobility of each individual CK2 subunit with high spatial and temporal resolution. Visualization of CK2 heterotetrameric complex formation could also be recorded using the fluorescence resonance energy transfer (FRET) technique. FRET imaging revealed that the assembling of this molecular complex can take place both in the cytoplasmic and nuclear compartments. The spatio-temporal organization of individual CK2 subunits and their dynamic behavior remain now to be correlated with the functioning of this kinase in the complex environment of the cell.

Casein kinase 2, circadian clocks, and the flight from mutagenic light

Circadian clocks play a fundamental role in biology and disease. Much has been learned about the molecular underpinnings of these biological clocks from genetic studies in model organisms, such as the fruit fly, Drosophila melanogaster. Here we review the literature from our lab and others that establish a role for the protein kinase CK2 in Drosophila clock timing. Among the clock genes described thus far, CK2 is unique in its involvement in plant, fungal, as well as animal circadian clocks. We propose that this reflects an ancient, conserved function for CK2 in circadian clocks. CK2 and other clock genes have been implicated in cellular responses to DNA damage, particularly those induced by ultraviolet (UV) light. The finding of a dual function of CK2 in clocks and in UV responses supports the notion that clocks evolved to assist organisms in avoiding the mutagenic effects of daily sunlight.

A global view of CK2 function and regulation

The wealth of biochemical, molecular, genetic, genomic, and bioinformatic resources available in S. cerevisiae make it an excellent system to explore the global role of CK2 in a model organism. Traditional biochemical and genetic studies have revealed that CK2 is required for cell viability, cell cycle progression, cell polarity, ion homeostasis, and other functions, and have identified a number of potential physiological substrates of the enzyme. Data mining of available bioinformatic resources indicates that (1) there are likely to be hundreds of CK2 targets in this organism, (2) the majority of predicted CK2 substrates are involved in various aspects of global gene expression, (3) CK2 is present in several nuclear protein complexes predicted to have a role in chromatin structure and remodeling, transcription, or RNA metabolism, and (4) CK2 is localized predominantly in the nucleus. These bioinformatic results suggest that the observed phenotypic consequences of CK2 depletion may lie downstream of primary defects in chromatin organization and/or global gene expression. Further progress in defining the physiological role of CK2 will almost certainly require a better understanding of the mechanism of regulation of the enzyme. Beginning with the crystal structure of the human CK2 holoenzyme, we present a molecular model of filamentous CK2 that is consistent with earlier proposals that filamentous CK2 represents an inactive form of the enzyme. The potential role of filamentous CK2 in regulation in vivo is discussed.

Primary and secondary interactions between CK2alpha and CK2beta lead to ring-like structures in the crystals of the CK2 holoenzyme

Protein kinase CK2 predominantly exists as a heterotetrameric holoenyzme consisting of two catalytic subunits (CK2alpha) and two non-catalytic subunits (CK2beta). Early investigations which we review here had revealed the presence of two types of contacts between CK2alpha and CK2beta: a primary interaction responsible for the stability of the CK2 holoenzyme and stimulatory for the catalytic activity, and a secondary interaction which is inhibitory and in which the acidic loop of CK2beta associates with the basic stretch and the (p+1)-loop of CK2alpha. At the end of the last decade both types of interactions were assumed to occur within the same tetrameric complex. The CK2 holoenyzme structure, however, suggested that the secondary interactions must happen between different CK2 tetramers. Such a behaviour should lead to higher-ordered aggregates consistent with several previous reports about a distinct aggregation propensity of CK2. We demonstrate here that in the CK2 holoenzyme crystals contacts between different CK2 tetramers exists which provide structural details of the secondary CK2alpha/CK2beta interactions. These mainly ionic interactions lead to trimeric rings of CK2 holoenzymes in the crystal. In these rings each CK2 tetramer possesses one CK2alpha subunit open for substrate binding and another one whose active site is blocked by a secondary contact with CK2beta from a neighbouring tetramer. This observation fits to previous findings that salt-sensitive ring-like aggregates of CK2 holoenzymes can exist which possess significant catalytic activity. Furthermore it suggests that earlier ideas about a regulatory role of the enzyme's aggregation propensity may be worth to be revitalised.

A comprehensive view of polyamine and histamine metabolism to the light of new technologies

Polyamines and histamine are biogenic amines with multiple biological roles. In spite of the evidence for the involvement of both polyamines and histamine metabolism impairment in several highly prevalent pathological conditions, multiple questions concerning the molecular processes behind these effects remain to be elucidated. More comprehensive and systemic studies integrating molecular biology, biophysical and bioinformatics tools could contribute to accelerate the advances in this research area. This review is designed to underscore the main questions to be answered in polyamine and histamine research and how these new systemic approaches could help to find these answers.

Purification and characterization of the CK2alpha'-based holoenzyme, an isozyme of CK2alpha: a comparative analysis

Protein kinase CK2 (former name: "casein kinase 2") is a pivotal and ubiquitously expressed member of the eukaryotic protein kinase superfamily. It predominantly exists as a heterotetrameric holoenzyme composed of two catalytic subunits (CK2alpha) and two regulatory subunits (CK2beta). In higher animals two paralog catalytic chains-abbreviated CK2alpha and CK2alpha'--exist which can combine with CK2beta to three isoforms of the holoenzyme: CK2alpha(2)beta(2), CK2alpha(2)(')beta(2), and CK2alphaalpha(')beta(2). While CK2alpha and the "normal" holoenzyme CK2alpha(2)beta(2) have been extensively characterized in vitro and in vivo, little is known about the enzymological properties of CK2alpha' and the "alternative" holoenzyme CK2alpha(2)(')beta(2) and about their specific physiological roles. A major reason for this lack of knowledge is the fact that so far CK2alpha' rather than CK2alpha has caused serious stability and solubility problems during standard heterologous expression procedures. To overcome them, we developed a preparation scheme for CK2alpha(2)(')beta(2) from Homo sapiens in catalytically active form based on two critical steps: first expression of human CK2alpha' as a well soluble fusion protein with the maltose binding protein (MBP) and second proteolytic cleavage of CK2alpha'-MBP in the presence of human CK2beta so that CK2alpha' subunits are incorporated into holoenzyme complexes directly after their release from MBP. This successful strategy which may be adopted in comparably difficult cases of protein/protein complex preparation is presented here together with evidence that the CK2alpha'-based and the CK2alpha-based holoenzymes are similar concerning their catalytic activities but are significantly different with respect to some well-known CK2 properties like autophosphorylation and supra-molecular aggregation.

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.

Order or chaos? An evaluation of the regulation of protein kinase CK2

CK2 is a highly conserved, ubiquitously expressed protein serine/threonine kinase present in all eukaryotes. Circumscribed as having a vast array of substrates located in a number of cellular compartments, CK2 has been implicated in critical cellular processes such as proliferation, apoptosis, differentiation, and transformation. Despite advances in elucidating its substrates and involvement in cellular regulation, its precise mode of regulation remains poorly defined. In this respect, there are currently conflicting views as to whether CK2 is constitutively active or modulated in response to specific stimuli. Perhaps an important consideration in resolving these apparent discrepancies is recognition of the existence of many discrete CK2 subpopulations that are distinguished from one another by localization or association with distinct cellular components. The existence of these subpopulations brings to light the possibility of each population being regulated independently rather than the entire cellular CK2 content being regulated globally. Logically, each local population may then be regulated in a distinct manner to carry out its precise function(s). This review will examine those mechanisms including regulated expression and assembly of CK2 subunits, phosphorylation of CK2, and interactions with small molecules or cellular proteins that could contribute to the local regulation of distinct CK2 populations.

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

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

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

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

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

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

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

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