Protein kinase CK2 catalyzes tyrosine phosphorylation in mammalian cells

The effect of phosphorylation efficiency on the oncogenic properties of the protein E7 from high-risk HPV

The Human papillomavirus (HPV) causes tumors in part by hijacking the host cell cycle and forcing uncontrolled cellular division. While there are >200 genotypes of HPV, 15 are classified as high-risk and have been shown to transform infected cells and contribute to tumor formation. The remaining low-risk genotypes are not considered oncogenic and result in benign skin lesions. In high-risk HPV, the oncoprotein E7 contributes to the dysregulation of cell cycle regulatory mechanisms. High-risk E7 is phosphorylated in cells at two conserved serine residues by Casein Kinase 2 (CK2) and this phosphorylation event increases binding affinity for cellular proteins such as the tumor suppressor retinoblastoma (pRb). While low-risk E7 possesses similar serine residues, it is phosphorylated to a lesser degree in cells and has decreased binding capabilities. When E7 binding affinity is decreased, it is less able to facilitate complex interactions between proteins and therefore has less capability to dysregulate the cell cycle. By comparing E7 protein sequences from both low- and high-risk HPV variants and using site-directed mutagenesis combined with NMR spectroscopy and cell-based assays, we demonstrate that the presence of two key nonpolar valine residues within the CK2 recognition sequence, present in low-risk E7, reduces serine phosphorylation efficiency relative to high-risk E7. This results in significant loss of the ability of E7 to degrade the retinoblastoma tumor suppressor protein, thus also reducing the ability of E7 to increase cellular proliferation and reduce senescence. This provides additional insight into the differential E7-mediated outcomes when cells are infected with high-risk verses low-risk HPV. Understanding these oncogenic differences may be important to developing targeted treatment options for HPV-induced cancers.

Casein kinase 2 complex: a central regulator of multiple pathobiological signaling pathways in Cryptococcus neoformans

The casein kinase 2 (CK2) complex has garnered extensive attention over the past decades as a potential therapeutic target for diverse human diseases, including cancer, diabetes, and obesity, due to its pivotal roles in eukaryotic growth, differentiation, and metabolic homeostasis. While CK2 is also considered a promising antifungal target, its role in fungal pathogens remains unexplored. In this study, we investigated the functions and regulatory mechanisms of the CK2 complex in Cryptococcus neoformans, a major cause of fungal meningitis. The cryptococcal CK2 complex consists of a single catalytic subunit, Cka1, and two regulatory subunits, Ckb1 and Ckb2. Our findings show that Cka1 plays a primary role as a protein kinase, while Ckb1 and Ckb2 have major and minor regulatory functions, respectively, in growth, cell cycle control, morphogenesis, stress response, antifungal drug resistance, and virulence factor production. Interestingly, triple mutants lacking all three subunits (cka1Δ ckb1Δ ckb2Δ) exhibited more severe phenotypic defects than the cka1Δ mutant alone, suggesting that Ckb1/2 may have Cka1-independent functions. In a murine model of systemic cryptococcosis, cka1Δ and cka1Δ ckb1Δ ckb2Δ mutants showed severely reduced virulence. Transcriptomic, proteomic, and phosphoproteomic analyses further revealed that the CK2 complex controls a wide array of effector proteins involved in transcriptional regulation, cell cycle control, nutrient metabolisms, and stress responses. Most notably, CK2 disruption led to dysregulation of key signaling cascades central to C. neoformans pathogenicity, including the Hog1, Mpk1 MAPKs, cAMP/PKA, and calcium/calcineurin signaling pathways. In summary, our study provides novel insights into the multifaceted roles of the fungal CK2 complex and presents a compelling case for targeting it in the development of new antifungal drugs.IMPORTANCEThe casein kinase 2 (CK2) complex, crucial for eukaryotic growth, differentiation, and metabolic regulation, presents a promising therapeutic target for various human diseases, including cancer, diabetes, and obesity. Its potential as an antifungal target is further highlighted in this study, which explores CK2's functions in C. neoformans, a key fungal meningitis pathogen. The CK2 complex in C. neoformans, comprising the Cka1 catalytic subunit and Ckb1/2 regulatory subunits, is integral to processes like growth, cell cycle, morphogenesis, stress response, drug resistance, and virulence. Our findings of CK2's role in regulating critical signaling pathways, including Hog1, Mpk1 MAPKs, cAMP/PKA, and calcium/calcineurin, underscore its importance in C. neoformans pathogenicity. This study provides valuable insights into the fungal CK2 complex, reinforcing its potential as a target for novel antifungal drug development and pointing out a promising direction for creating new antifungal agents.

Yeast Protein Asf1 Possesses Modulating Activity towards Protein Kinase CK2

Protein kinase CK2 plays an important role in cell survival and protects regulatory proteins from caspase-mediated degradation during apoptosis. The consensus sequence of proteins phosphorylated by CK2 contains a cluster of acidic amino acids around the phosphorylation site. The poly-acidic sequence in yeast protein Asf1 is similar to the acidic loop in CK2β, which possesses a regulatory function. We observed that the overexpression of Asf1 in yeast cells influences cell growth. Experiments performed in vitro and in vivo indicate that yeast protein Asf1 inhibits protein kinase CK2. Our data suggest that each CK2 isoform might be regulated in a different way. Deletion of the amino or carboxyl end of Asf1 reveals that the acidic cluster close to the C-terminus is responsible for the activation or inhibition of CK2 activity.

Protein kinase CK2 – diverse roles in cancer cell biology and therapeutic promise

The association of protein kinase CK2 (formerly casein kinase II or 2) with cell growth and proliferation in cells was apparent at early stages of its investigation. A cancer-specific role for CK2 remained unclear until it was determined that CK2 was also a potent suppressor of cell death (apoptosis); the latter characteristic differentiated its function in normal versus malignant cells because dysregulation of both cell growth and cell death is a universal feature of cancer cells. Over time, it became evident that CK2 exerts its influence on a diverse range of cell functions in normal as well as in transformed cells. As such, CK2 and its substrates are localized in various compartments of the cell. The dysregulation of CK2 is documented in a wide range of malignancies; notably, by increased CK2 protein and activity levels with relatively moderate change in its RNA abundance. High levels of CK2 are associated with poor prognosis in multiple cancer types, and CK2 is a target for active research and testing for cancer therapy. Aspects of CK2 cellular roles and targeting in cancer are discussed in the present review, with focus on nuclear and mitochondrial functions and prostate, breast and head and neck malignancies.

The Role of Protein Kinase CK2 in Development and Disease Progression: A Critical Review

Protein kinase CK2 (CK2) is a ubiquitous holoenzyme involved in a wide array of developmental processes. The involvement of CK2 in events such as neurogenesis, cardiogenesis, skeletogenesis, and spermatogenesis is essential for the viability of almost all organisms, and its role has been conserved throughout evolution. Further into adulthood, CK2 continues to function as a key regulator of pathways affecting crucial processes such as osteogenesis, adipogenesis, chondrogenesis, neuron differentiation, and the immune response. Due to its vast role in a multitude of pathways, aberrant functioning of this kinase leads to embryonic lethality and numerous diseases and disorders, including cancer and neurological disorders. As a result, CK2 is a popular target for interventions aiming to treat the aforementioned diseases. Specifically, two CK2 inhibitors, namely CX-4945 and CIBG-300, are in the early stages of clinical testing and exhibit promise for treating cancer and other disorders. Further, other researchers around the world are focusing on CK2 to treat bone disorders. This review summarizes the current understanding of CK2 in development, the structure of CK2, the targets and signaling pathways of CK2, the implication of CK2 in disease progression, and the recent therapeutics developed to inhibit the dysregulation of CK2 function in various diseases.

Casein Kinase 2 Signaling in White Matter Stroke

The growth of the aging population, together with improved stroke care, has resulted in an increase in stroke survivors and a rise in recurrent events. Axonal injury and white matter (WM) dysfunction are responsible for much of the disability observed after stroke. The mechanisms of WM injury are distinct compared to gray matter and change with age. Therefore, an ideal stroke therapeutic must restore neuronal and axonal function when applied before or after a stroke, and it must also protect across age groups. Casein kinase 2 (CK2), is expressed in the brain, including WM, and is regulated during the development and numerous disease conditions such as cancer and ischemia. CK2 activation in WM mediates ischemic injury by activating the Cdk5 and AKT/GSK3β signaling pathways. Consequently, CK2 inhibition using the small molecule inhibitor CX-4945 (Silmitasertib) correlates with preservation of oligodendrocytes, conservation of axon structure, and axonal mitochondria, leading to improved functional recovery. Remarkably, CK2 inhibition promotes WM function when applied after ischemic injury by specifically regulating the AKT/GSK3β pathways. The blockade of the active conformation of AKT confers post-ischemic protection to young and old WM by preserving mitochondria, implying AKT as a common therapeutic target across age groups. Using a NanoString nCounter miRNA expression profiling, comparative analyses of ischemic WM with or without CX-4945 treatment reveal that miRNAs are expressed at high levels in WM after ischemia, and CX-4945 differentially regulates some of these miRNAs. Therefore, we propose that miRNA regulation may be one of the protective actions of CX-4945 against WM ischemic injury. Silmitasertib is FDA approved and currently in use for cancer and Covid patients; therefore, it is plausible to repurpose CK2 inhibitors for stroke patients.

CK2β Is a Gatekeeper of Focal Adhesions Regulating Cell Spreading

CK2 is a hetero-tetrameric serine/threonine protein kinase made up of two CK2α/αʹ catalytic subunits and two CK2β regulatory subunits. The free CK2α subunit and the tetrameric holoenzyme have distinct substrate specificity profiles, suggesting that the spatiotemporal organization of the individual CK2 subunits observed in living cells is crucial in the control of the many cellular processes that are governed by this pleiotropic kinase. Indeed, previous studies reported that the unbalanced expression of CK2 subunits is sufficient to drive epithelial to mesenchymal transition (EMT), a process involved in cancer invasion and metastasis. Moreover, sub-stoichiometric expression of CK2β compared to CK2α in a subset of breast cancer tumors was correlated with the induction of EMT markers and increased epithelial cell plasticity in breast carcinoma progression. Phenotypic changes of epithelial cells are often associated with the activation of phosphotyrosine signaling. Herein, using phosphotyrosine enrichment coupled with affinity capture and proteomic analysis, we show that decreased expression of CK2β in MCF10A mammary epithelial cells triggers the phosphorylation of a number of proteins on tyrosine residues and promotes the striking activation of the FAK1-Src-PAX1 signaling pathway. Moreover, morphometric analyses also reveal that CK2β loss increases the number and the spatial distribution of focal adhesion signaling complexes that coordinate the adhesive and migratory processes. Together, our findings allow positioning CK2β as a gatekeeper for cell spreading by restraining focal adhesion formation and invasion of mammary epithelial cells.

Chemical Genetic Validation of CSNK2 Substrates Using an Inhibitor-Resistant Mutant in Combination with Triple SILAC Quantitative Phosphoproteomics

Casein Kinase 2 (CSNK2) is an extremely pleiotropic, ubiquitously expressed protein kinase involved in the regulation of numerous key biological processes. Mapping the CSNK2-dependent phosphoproteome is necessary for better characterization of its fundamental role in cellular signalling. While ATP-competitive inhibitors have enabled the identification of many putative kinase substrates, compounds targeting the highly conserved ATP-binding pocket often exhibit off-target effects limiting their utility for definitive kinase-substrate assignment. To overcome this limitation, we devised a strategy combining chemical genetics and quantitative phosphoproteomics to identify and validate CSNK2 substrates. We engineered U2OS cells expressing exogenous wild type CSNK2A1 (WT) or a triple mutant (TM, V66A/H160D/I174A) with substitutions at residues important for inhibitor binding. These cells were treated with CX-4945, a clinical-stage inhibitor of CSNK2, and analyzed using large-scale triple SILAC (Stable Isotope Labelling of Amino Acids in Cell Culture) quantitative phosphoproteomics. In contrast to wild-type CSNK2A1, CSNK2A1-TM retained activity in the presence of CX-4945 enabling identification and validation of several CSNK2 substrates on the basis of their increased phosphorylation in cells expressing CSNK2A1-TM. Based on high conservation within the kinase family, we expect that this strategy can be broadly adapted for identification of other kinase-substrate relationships.

Writing and Erasing O-GlcNAc on Casein Kinase 2 Alpha Alters the Phosphoproteome

O-GlcNAc is an essential carbohydrate modification that intersects with phosphorylation signaling pathways via crosstalk on protein substrates or by direct modification of the kinases that write the phosphate modification. Casein kinase 2 alpha (CK2α), the catalytic subunit of the ubiquitously expressed and constitutively active kinase CK2, is modified by O-GlcNAc, but the effect of this modification on the phosphoproteome in cells is unknown. Here, we apply complementary targeted O-GlcNAc editors, nanobody-OGT and -splitOGA, to selectively write and erase O-GlcNAc from a tagged CK2α to measure the effects on the phosphoproteome in cells. These tools effectively and selectively edit the Ser347 glycosite on CK2α. Using quantitative phosphoproteomics, we report 51 phosphoproteins whose enrichment changes as a function of editing O-GlcNAc on CK2α, including HDAC1, HDAC2, ENSA, SMARCAD1, and PABPN1. Specific phosphosites on HDAC1 Ser393 and HDAC2 Ser394, both reported CK2 substrates, are significantly enhanced by O-GlcNAcylation of CK2α. These data will propel future studies on the crosstalk between O-GlcNAc and phosphorylation.

The Okur-Chung Neurodevelopmental Syndrome Mutation CK2K198R Leads to a Rewiring of Kinase Specificity

Okur-Chung Neurodevelopmental Syndrome (OCNDS) is caused by heterozygous mutations to the CSNK2A1 gene, which encodes the alpha subunit of protein kinase CK2. The most frequently occurring mutation is lysine 198 to arginine (K198R). To investigate the impact of this mutation, we first generated a high-resolution phosphorylation motif of CK2^WT, including the first characterization of specificity for tyrosine phosphorylation activity. A second high resolution motif representing CK2^K198R substrate specificity was also generated. Here we report the impact of the OCNDS associated CK2^K198R mutation. Contrary to prior speculation, the mutation does not result in a complete loss of function, but rather shifts the substrate specificity of the kinase. Broadly speaking the mutation leads to 1) a decreased preference for acidic residues in the +1 position, 2) a decreased preference for threonine phosphorylation, 3) an increased preference for tyrosine phosphorylation, and 4) an alteration of the tyrosine phosphorylation specificity motif. To further investigate the result of this mutation we have developed a probability-based scoring method, allowing us to predict shifts in phosphorylation in the K198R mutant relative to the wild type kinase. As an initial step we have applied the methodology to the set of axonally localized ion channels in an effort to uncover potential alterations of the phosphoproteome associated with the OCNDS disease condition.

ZWC complex-mediated SPT5 phosphorylation suppresses divergent antisense RNA transcription at active gene promoters

The human genome encodes large numbers of non-coding RNAs, including divergent antisense transcripts at transcription start sites (TSSs). However, molecular mechanisms by which divergent antisense transcription is regulated have not been detailed. Here, we report a novel ZWC complex composed of ZC3H4, WDR82 and CK2 that suppresses divergent antisense transcription. The ZWC complex preferentially localizes at TSSs of active genes through direct interactions of ZC3H4 and WDR82 subunits with the S5p RNAPII C-terminal domain. ZC3H4 depletion leads to increased divergent antisense transcription, especially at genes that naturally produce divergent antisense transcripts. We further demonstrate that the ZWC complex phosphorylates the previously uncharacterized N-terminal acidic domain of SPT5, a subunit of the transcription-elongation factor DSIF, and that this phosphorylation is responsible for suppressing divergent antisense transcription. Our study provides evidence that the newly identified ZWC-DSIF axis regulates the direction of transcription during the transition from early to productive elongation.

Protein Kinase CK2 Acts as a Molecular Brake to Control NADPH Oxidase 1 Activation and Colon Inflammation

BACKGROUND & AIMS

NADPH oxidase 1 (NOX1) has emerged as a prime regulator of intestinal mucosa immunity and homeostasis. Dysregulation of NOX1 may cause inflammatory bowel disease (IBD). It is not clear how NOX1 is regulated in vivo under inflammatory conditions. We studied the role of CK2 in this process.

METHODS

The NOX1 organizer subunit, NADPH oxidase organizer 1 (NOXO1), was immunoprecipitated from cytokine-treated colon epithelial cells, and bound proteins were identified by mass spectrometry analysis. Sites on NOXO1 phosphorylated by CK2 were identified by nanoscale liquid chromatography coupled to tandem mass spectrometry. NOX1 activity was determined in colon epithelial cells and colonoids in the presence or absence of CX-4945, a CK2 specific inhibitor. Acute colitis was induced by administration of trinitrobenzenesulfonic acid in mice treated or not with CX-4945. Colon tissues were analyzed by histologic examination, quantitative polymerase chain reaction, and Western blots. CK2 activity, markers of inflammation, and oxidative stress were assessed.

RESULTS

We identified CK2 as a major partner of NOXO1 in colon epithelial cells under inflammatory conditions. CK2 directly binds NOXO1 at the C-terminus containing the Phox homology domain and phosphorylates NOXO1 on several sites. CX-4945 increased ROS generation by NOX1 in human colon epithelial cells and organoids. Strikingly, CK2 activity was reduced in trinitrobenzenesulfonic acid-induced acute colitis, and CX-4945 exacerbated colitis inflammation as shown by increased levels of CXCL1, ROS generation, lipid peroxidation, and colon damage.

CONCLUSIONS

The ubiquitous protein kinase CK2 limits NOX1 activity via NOXO1 binding and phosphorylation in colonic epithelial cells and lessens experimental colitis. Loss of CK2 activity during acute colitis results in excessive ROS production, contributing to the pathogenesis. Strategies to activate CK2 could be an effective novel therapeutic approach in IBD.

Tyrosine 136 phosphorylation of α-synuclein aggregates in the Lewy body dementia brain: involvement of serine 129 phosphorylation by casein kinase 2

Serine 129 (S129) phosphorylation of α-synuclein (αSyn) is a central feature of Lewy body (LB) disease pathology. Although the neighboring tyrosine residues Y125, Y133, and Y136 are also phosphorylation sites, little is known regarding potential roles of phosphorylation cross-talk between these sites and its involvement in the pathogenesis of LB disease. Here, we found that αSyn aggregates are predominantly phosphorylated at Y136 in the Lewy body dementia brain, which is mediated by unexpected kinase activity of Casein kinase 2 (CK2). Aggregate formation with S129 and Y136 phosphorylation of recombinant αSyn (r-αSyn) were induced by CK2 but abolished by replacement of S129 with alanine (S129A) in vitro. Mutation of Y136 to alanine (Y136A) promoted aggregate formation and S129 phosphorylation of r-αSyn by CK2 in vitro. Introduction of Y136A r-αSyn oligomers into cultured cells exhibited increased levels of aggregates with S129 phosphorylation compared to wild-type r-αSyn oligomers. In addition, aggregate formation with S129 phosphorylation induced by introduction of wild-type r-αSyn oligomers was significantly attenuated by CK2 inhibition, which resulted in an unexpected increase in Y136 phosphorylation in cultured cells. Our findings suggest the involvement of CK2-related αSyn Y136 phosphorylation in the pathogenesis of LB disease and its potential as a therapeutic target.

Natural Compounds Isolated from Stachybotrys chartarum Are Potent Inhibitors of Human Protein Kinase CK2

A large number of secondary metabolites have been isolated from the filamentous fungus Stachybotrys chartarum and have been described before. Fourteen of these natural compounds were evaluated in vitro in the present study for their inhibitory activity towards the cancer target CK2. Among these compounds, stachybotrychromene C, stachybotrydial acetate and acetoxystachybotrydial acetate turned out to be potent inhibitors with IC₅₀ values of 0.32 µM, 0.69 µM and 1.86 µM, respectively. The effects of these three compounds on cell proliferation, growth and viability of MCF7 cells, representing human breast adenocarcinoma as well as A427 (human lung carcinoma) and A431 (human epidermoid carcinoma) cells, were tested using EdU assay, IncuCyte^® live-cell imaging and MTT assay. The most active compound in inhibiting MCF7 cell proliferation was acetoxystachybotrydial acetate with an EC₅₀ value of 0.39 µM. In addition, acetoxystachybotrydial acetate turned out to inhibit the growth of all three cell lines completely at a concentration of 1 µM. In contrast, cell viability was impaired only moderately, to 37%, 14% and 23% in MCF7, A427 and A431 cells, respectively.

Working day and night: plastid casein kinase 2 catalyses phosphorylation of proteins with diverse functions in light- and dark-adapted plastids

Casein kinase 2 is a ubiquitous protein kinase that has puzzled researchers for several decades because of its pleiotropic activity. Here, we set out to identify the in vivo targets of plastid casein kinase 2 (pCK2) in Arabidopsis thaliana. Survey phosphoproteome analyses were combined with targeted analyses with wild-type and pck2 knockdown mutants to identify potential pCK2 targets by their decreased phosphorylation state in the mutant. To validate potential substrates, we complemented the pck2 knockdown line with tandem affinity tag (TAP)-tagged pCK2 and found it to restore growth parameters, as well as many, but not all, putative pCK2-dependent phosphorylation events. We further performed a targeted analysis at the end-of-night to increase the specificity of target protein identification. This analysis confirmed light-independent phosphorylation of several pCK2 target proteins. Based on the aforementioned data, we define a set of in vivo pCK2-targets that span different chloroplast functions, such as metabolism, transcription, translation and photosynthesis. The pleiotropy of pCK2 functions is also manifested by altered state transition kinetics during short-term acclimation and significant alterations in the mutant metabolism, supporting its function in photosynthetic regulation. Thus, our data expand our understanding on chloroplast phosphorylation networks and provide insights into kinase networks in the regulation of chloroplast functions.

CK2 inhibition protects white matter from ischemic injury

Strokes occur predominantly in the elderly and white matter (WM) is injured in most strokes, contributing to the disability associated with clinical deficits. Casein kinase 2 (CK2) is expressed in neuronal cells and was reported to be neuroprotective during cerebral ischemia. Recently, we reported that CK2 is abundantly expressed by glial cells and myelin. However, in contrast to its role in cerebral (gray matter) ischemia, CK2 activation during ischemia mediated WM injury via the CDK5 and AKT/GSK3β signaling pathways (Bastian et al., 2018). Subsequently, CK2 inhibition using the small molecule inhibitor CX-4945 correlated with preservation of oligodendrocytes as well as conservation of axon structure and axonal mitochondria, leading to improved functional recovery. Notably, CK2 inhibition promoted WM function when applied before or after ischemic injury by differentially regulating the CDK5 and AKT/GSK3β pathways. Specifically, blockade of the active conformation of AKT conferred post-ischemic protection to young, aging, and old WM, suggesting a common therapeutic target across age groups. CK2 inhibitors are currently being used in clinical trials for cancer patients; therefore, it is important to consider the potential benefits of CK2 inhibitors during an ischemic attack.

Protein kinase D displays intrinsic Tyr autophosphorylation activity: insights into mechanism and regulation

The protein kinase D (PKD) family is regulated through multi-site phosphorylation, including autophosphorylation. For example, PKD displays in vivo autophosphorylation on Ser-742 (and Ser-738 in vitro) in the activation loop and Ser-910 in the C-tail (hPKD1 numbering). In this paper, we describe the surprising observation that PKD also displays in vitro autocatalytic activity towards a Tyr residue in the P + 1 loop of the activation segment. We define the molecular determinants for this unusual activity and identify a Cys residue (C705 in PKD1) in the catalytic loop as of utmost importance. In cells, PKD Tyr autophosphorylation is suppressed through the association of an inhibitory factor. Our findings provide important novel insights into PKD (auto)regulation.

A Selective Biligand Inhibitor of CK2 Increases Caspase-3 Activity in Cancer Cells and Inhibits Platelet Aggregation

Cancer cells express high levels of CK2, and its inhibition leads to apoptosis. CK2 has therefore emerged as a new drug target for cancer therapy. A biligand inhibitor ARC-772 was constructed by conjugating 4-(2-amino-1,3-thiazol-5-yl)benzoic acid and a carboxylate-rich peptoid. ARC-772 was found to bind CK2 with a K_d value of 0.3 nm and showed remarkable CK2 inhibitory selectivity in a panel of 140 protein kinases (Gini coefficient: 0.75 at c=100 nm). ARC-775, the acetoxymethyl ester prodrug of ARC-772, was efficiently taken up by cells. Once internalized, the inhibitor is activated by cellular esterase activity. In HeLa cancer cells ARC-775 was found to activate caspase-3 (an apoptosis marker) at sub-micromolar concentrations (EC₅₀ =0.3 μm), a 20-fold lower extracellular concentration than CX-4945, the only CK2 inhibitor under clinical trials. At micromolar concentrations, ARC-775 was also found to inhibit ADP-induced aggregation of human platelets. The overall results of this study demonstrate that oligo-anionic biligand inhibitors have good potential for drug development.

Selected flavonoid compounds as promising inhibitors of protein kinase CK2α and CK2α', the catalytic subunits of CK2

CK2 is a ubiquitous protein kinase involved in many cell functions. During the last years it became an interesting target in cancer research. A series of flavonoid compounds was tested as inhibitors of protein kinase CK2. Several substances were found to be highly active against both catalytic subunits with IC₅₀ values below 1 μM in case of CK2α'. The most promising inhibitor we identified is chrysoeriol with IC₅₀ values of 250 and 34 nM for CK2α and CK2α', respectively.

Protein Kinase CK2: Intricate Relationships within Regulatory Cellular Networks

Protein kinase CK2 is a small family of protein kinases that has been implicated in an expanding array of biological processes. While it is widely accepted that CK2 is a regulatory participant in a multitude of fundamental cellular processes, CK2 is often considered to be a constitutively active enzyme which raises questions about how it can be a regulatory participant in intricately controlled cellular processes. To resolve this apparent paradox, we have performed a systematic analysis of the published literature using text mining as well as mining of proteomic databases together with computational assembly of networks that involve CK2. These analyses reinforce the notion that CK2 is involved in a broad variety of biological processes and also reveal an extensive interplay between CK2 phosphorylation and other post-translational modifications. The interplay between CK2 and other post-translational modifications suggests that CK2 does have intricate roles in orchestrating cellular events. In this respect, phosphorylation of specific substrates by CK2 could be regulated by other post-translational modifications and CK2 could also have roles in modulating other post-translational modifications. Collectively, these observations suggest that the actions of CK2 are precisely coordinated with other constituents of regulatory cellular networks.

Plasmodium falciparum Adhesins Play an Essential Role in Signalling and Activation of Invasion into Human Erythrocytes

The most severe form of malaria in humans is caused by the protozoan parasite Plasmodium falciparum. The invasive form of malaria parasites is termed a merozoite and it employs an array of parasite proteins that bind to the host cell to mediate invasion. In Plasmodium falciparum, the erythrocyte binding-like (EBL) and reticulocyte binding-like (Rh) protein families are responsible for binding to specific erythrocyte receptors for invasion and mediating signalling events that initiate active entry of the malaria parasite. Here we have addressed the role of the cytoplasmic tails of these proteins in activating merozoite invasion after receptor engagement. We show that the cytoplasmic domains of these type 1 membrane proteins are phosphorylated in vitro. Depletion of PfCK2, a kinase implicated to phosphorylate these cytoplasmic tails, blocks P. falciparum invasion of red blood cells. We identify the crucial residues within the PfRh4 cytoplasmic domain that are required for successful parasite invasion. Live cell imaging of merozoites from these transgenic mutants show they attach but do not penetrate erythrocytes implying the PfRh4 cytoplasmic tail conveys signals important for the successful completion of the invasion process.

Arabidopsis casein kinase 2 α4 subunit regulates various developmental pathways in a functionally overlapping manner

Casein kinase 2 (CK2) is an essential and well-conserved Ser/Thr kinase that regulates proteins in a posttranslational manner. CK2 has been shown to affect a large number of developmental processes across eukaryotes. It is a tetrameric protein composed of a dimer of alpha (catalytic) and beta (regulatory) subunit each. In our previous study we showed that three of the four CK2 α subunits in Arabidopsis act in a functionally redundant manner to regulate various developmental pathways. In this study we constructed two independent CK2 α4 RNAi lines in the CK2 alpha triple mutant background. Through functional characterization of these RNAi lines we show that the fourth α subunit in Arabidopsis also functions redundantly in regulating ABA response, lateral root formation and flowering time. CK2 α4-GFP localizes to the chloroplast in transgenic Arabidopsis seedlings, consistent with the presence of a chloroplast localization signal at the amino-terminus of CK2 α4 subunit. Taken together, our results suggest a functionally overlapping role for the CK2 α4 subunit in regulating various developmental processes in plants.

Systematic investigation of hierarchical phosphorylation by protein kinase CK2

Although multiple phosphorylation sites are often clustered in substrates, the mechanism of phosphorylation within clusters has not been systematically investigated. Intriguingly, in addition to acidic residues, protein kinase CK2 can use phosphoserine residues as consensus determinants suggesting that CK2 may act in concert with other kinases. We used a peptide array approach to outline optimal consensus sequences for hierarchical phosphorylation by CK2, both in the context of processive, multisite phosphorylation, and in concert with a priming proline-directed kinase. Results suggest that hierarchical phosphorylation involving CK2 requires precise positioning of either multiple phosphodeterminant residues or specific combinations of canonical determinants and phosphodeterminants, and can be as enzymatically favorable as canonical CK2 phosphorylation. Over 1600 human proteins contain at least one CK2 hierarchical consensus motif, and ~20% of these motifs contain at least one reported in vivo phosphorylation site. These motifs occur non-randomly in the human proteome, with significant enrichment in proteins controlling specific cellular processes. Taken together, our results provide strong in vitro evidence that hierarchical phosphorylation may contribute to the regulation of crucial biological processes. In addition, the results suggest a mechanism by which CK2, a constitutively active kinase, can be a regulatory participant in cellular processes.

BIOLOGICAL SIGNIFICANCE

Phosphorylation is a crucial regulatory mechanism governing cellular signal transduction pathways, and despite the large number of identified sites to date, most mechanistic studies remain focused on individual phosphorylation sites. This study is the first to systematically determine specific consensus sequences for hierarchical phosphorylation events. The results indicate that individual phosphorylation sites should not be studied in isolation, and that larger, multisite phosphorylation motifs may have profound impact on cellular signaling. This article is part of a Special Issue entitled: Protein dynamics in health and disease. Guest Editors: Pierre Thibault and Anne-Claude Gingras.

Tyrosine phosphorylation of histone H2A by CK2 regulates transcriptional elongation

Post-translational histone modifications play critical roles in regulating transcription, the cell cycle, DNA replication and DNA damage repair¹. The identification of new histone modifications critical for transcriptional regulation at initiation, elongation, or termination is of particular interest. Here, we report a new layer of regulation in transcriptional elongation that is conserved from yeast to mammals, based on a phosphorylation of a highly-conserved tyrosine residue, Y57, in histone H2A that is mediated by an unsuspected tyrosine kinase activity of casein kinase 2 (CK2). Mutation of H2A-Y57 in yeast or inhibition of CK2 activity impairs transcriptional elongation in yeast as well as in mammalian cells. Genome-wide binding analysis reveals that CK2α, the catalytic subunit of CK2, binds across RNA polymerase II-transcribed coding genes and active enhancers. Mutation of Y57 causes a loss of H2B mono-ubiquitylation as well as H3K4me3 and H3K79me3, histone marks associated with active transcription. Mechanistically, both CK2 inhibition and H2A-Y57F mutation enhance the H2B deubiquitylation activity of the SAGA complex, suggesting a critical role of this phosphorylation in coordinating the activity of the SAGA during transcription. Together, these results identify a new component of regulation in transcriptional elongation based on CK2-dependent tyrosine phosphorylation of the globular domain of H2A.

CK2 phosphorylation of human centrins 1 and 2 regulates their binding to the DNA repair protein XPC, the centrosomal protein Sfi1 and the phototransduction protein transducin β

Centrins are calcium-binding proteins that can interact with several cellular targets (Sfi1, XPC, Sac3 and transducin β) through the same hydrophobic triad. However, two different orientations of the centrin-binding motif have been observed: W(1)xxL(4)xxxL(8) for XPC (xeroderma pigmentosum group C protein) and the opposite orientation L(8)xxxL(4)xxW(1) for Sfi1 (suppressor of fermentation-induced loss of stress resistance protein 1), Sac3 and transducin β. Centrins are also phosphorylated by several protein kinases, among which is CK2. The purpose of this study was to determine the binding mechanism of human centrins to three targets (transducin β, Sfi1 and XPC), and the effects of in vitro phosphorylation by CK2 of centrins 1 and 2 with regard to this binding mechanism. We identified the centrin-binding motif at the COOH extremity of transducin β. Human centrin 1 binds to transducin β only in the presence of calcium with a binding constant lower than the binding constant observed for Sfi1 and for XPC. The affinity constants of centrin 1 were 0.10 10(6) M(-1), 249 10(6) M(-1) and 52.5 10(6) M(-1) for Trd, R17-Sfi1 and P17-XPC respectively. CK2 phosphorylates human centrin 1 at residue T138 and human centrin 2 at residues T138 and S158. Consequently CK2 phosphorylation abolished the binding of centrin 1 to transducin β and reduced the binding to Sfi1 and XPC. CK2 phosphorylation of centrin 2 at T138 and S158 abolished the binding to Sfi1 as assessed using a C-HsCen2 T138D-S158D phosphomimetic form of centrin 2.

Identification of a major determinant for serine-threonine kinase phosphoacceptor specificity

Eukaryotic protein kinases are generally classified as being either tyrosine or serine-threonine specific. Though not evident from inspection of their primary sequences, many serine-threonine kinases display a significant preference for serine or threonine as the phosphoacceptor residue. Here we show that a residue located in the kinase activation segment, which we term the “DFG+1” residue, acts as a major determinant for serine-threonine phosphorylation site specificity. Mutation of this residue was sufficient to switch the phosphorylation site preference for multiple kinases, including the serine-specific kinase PAK4 and the threonine-specific kinase MST4. Kinetic analysis of peptide substrate phosphorylation and crystal structures of PAK4-peptide complexes suggested that phosphoacceptor residue preference is not mediated by stronger binding of the favored substrate. Rather, favored kinase-phosphoacceptor combinations likely promote a conformation optimal for catalysis. Understanding the rules governing kinase phosphoacceptor preference allows kinases to be classified as serine or threonine specific based on their sequence.

•

A single active site residue can determine kinase phosphoacceptor specificity

•

Favored and disfavored substrates promote distinct kinase-bound conformations

•

A simple rule predicts kinase phosphoacceptor preference from its DFG+1 residue

Casein kinase 2 associates with the yeast chromatin reassembly factor Spt2/Sin1 to regulate its function in the repression of spurious transcription

Spt2/Sin1 is a DNA binding protein with HMG-like domains. It plays a role in chromatin modulations associated with transcription elongation in Saccharomyces cerevisiae. Spt2 maintains the nucleosome level in coding regions and is important for the inhibition of spurious transcription in yeast. In this work, we undertook a biochemical approach to identify Spt2-interacting partners. Interestingly, casein kinase 2 (CK2) interacts with Spt2 and phosphorylates it in vitro as well as in vivo on two small regions, region I (RI) (amino acids 226 to 230) and RII (amino acids 277 to 281), located in its essential C-terminal domain. Mutation of the phosphorylation sites in RI and RII to acidic residues, thereby mimicking CK2 phosphorylation, leads to the inhibition of Spt2 function in the repression of spurious transcription and to a loss of its recruitment to coding regions. Inversely, depleting cells of CK2 activity leads to an increased Spt2 association with genes. We further show that Spt2 physically interacts with the essential histone chaperone Spt6 and that this association is inhibited in vitro and in vivo by CK2-dependent phosphorylation. Taken together, our data suggest that CK2 regulates the function of Spt2 by modulating its interaction with chromatin and the histone chaperone Spt6.

Aberrant signalling by protein kinase CK2 in imatinib-resistant chronic myeloid leukaemia cells: biochemical evidence and therapeutic perspectives

Chronic myeloid leukaemia (CML) is driven by the fusion protein Bcr-Abl, a constitutively active tyrosine kinase playing a crucial role in initiation and maintenance of CML phenotype. Despite the great efficacy of the Bcr-Abl-specific inhibitor imatinib, resistance to this drug is recognized as a major problem in CML treatment. We found that in LAMA84 cells, characterized by imatinib-resistance caused by BCR-ABL1 gene amplification, the pro-survival protein kinase CK2 is up-regulated as compared to the sensitive cells. CK2 exhibits a higher protein-level and a parallel enhancement of catalytic activity. Consistently, CK2-catalysed phosphorylation of Akt-Ser129 is increased. CK2 co-localizes with Bcr-Abl in the cytoplasmic fraction as judged by subcellular fractionation and fluorescence immunolocalization. CK2 and Bcr-Abl are members of the same multi-protein complex(es) in imatinib-resistant cells as demonstrated by co-immunoprecipitation and co-sedimentation in glycerol gradients. Cell treatment with CX-4945, a CK2 inhibitor currently in clinical trials, counteracts CK2/Bcr-Abl interaction and causes cell death by apoptosis. Interestingly, combination of CX-4945 with imatinib displays a synergistic effect in reducing cell viability. Consistently, knockdown of CK2α expression by siRNA restores the sensitivity of resistant LAMA84 cells to low imatinib concentrations. Remarkably, the CK2/Bcr-Abl interaction and the sensitization towards imatinib obtained by CK2-inhibition in LAMA84 is observable also in other imatinib-resistant CML cell lines. These results demonstrate that CK2 contributes to strengthen the imatinib-resistance phenotype of CML cells conferring survival advantage against imatinib. We suggest that CK2 inhibition might be a promising tool for combined strategies in CML therapy.

Identification of a novel function of the clathrin-coated structure at the plasma membrane in facilitating GM-CSF receptor-mediated activation of JAK2

It is well known that ligand binding to the high-affinity GM-CSF receptor (GMR) activates JAK2. However, how and where this event occurs in a cellular environment remains unclear. Here, we demonstrate that clathrin- but not lipid raft-mediated endocytosis is crucial for GMR signaling. Knockdown expression of clathrin heavy chain or intersectin 2 (ITSN2) attenuated GMR-mediated activation of JAK2, whereas inhibiting clathrin-coated pits or plagues to bud off the membrane by the dominant-negative mutant of dynamin enhanced such event. Moreover, unlike the wild-type receptor, an ITSN2-non-binding mutant of GMR defective in targeting to clathrin-coated pits or plagues [collectively referred to as clathrin-coated structures (CCSs) here] failed to activate JAK2 at such locations. Additional experiments demonstrate that ligand treatment not only enhanced JAK2/GMR association at CCSs, but also induced a conformational change of JAK2 which is required for JAK2 to be activated by CCS-localized CK2. Interestingly, ligand-independent activation of the oncogenic mutant of JAK2 (JAK2V617F) also requires the targeting of this mutant to CCSs. But JAK2V617F seems to be constitutively in an open conformation for CK2 activation. Together, this study reveals a novel functional role of CCSs in GMR signaling and the oncogenesis of JAK2V617F.

Global kinomic and phospho-proteomic analyses of the human malaria parasite Plasmodium falciparum

The role of protein phosphorylation in the life cycle of malaria parasites is slowly emerging. Here we combine global phospho-proteomic analysis with kinome-wide reverse genetics to assess the importance of protein phosphorylation in Plasmodium falciparum asexual proliferation. We identify 1177 phosphorylation sites on 650 parasite proteins that are involved in a wide range of general cellular activities such as DNA synthesis, transcription and metabolism as well as key parasite processes such as invasion and cyto-adherence. Several parasite protein kinases are themselves phosphorylated on putative regulatory residues, including tyrosines in the activation loop of PfGSK3 and PfCLK3; we show that phosphorylation of PfCLK3 Y526 is essential for full kinase activity. A kinome-wide reverse genetics strategy identified 36 parasite kinases as likely essential for erythrocytic schizogony. These studies not only reveal processes that are regulated by protein phosphorylation, but also define potential anti-malarial drug targets within the parasite kinome.

Casein kinase 2 regulates in vivo bone formation through its interaction with bone morphogenetic protein receptor type Ia

Approximately 7.9 million fractures occur annually in the United States with 5-10% of these resulting in delayed or impaired healing. Nearly half of the trauma cost of $56 billion per year is used for the treatment of fractures. More importantly, fracture results in a substantial reduction in the quality of life. New approaches and therapies are needed to enhance fracture healing. Only a limited number of treatments are available including bone grafting, allogeneic and autologous bone marrow transplantation, and bone morphogenetic protein (BMP). We previously identified Protein Kinase CK2 to interact with BMP receptor type Ia (BMPRIa) and as a key protein for signal activation. Peptides approximately 30 AA were developed that mimicked BMP2 action in vitro by blocking this interaction. In this paper we extended our studies to investigate if the most promising peptide could induce in vivo bone formation in mice and to elucidate this mechanism of action. The CK2 blocking peptide activated the Wnt pathway. To identify the optimal peptide concentration and peptide concentration curves for mineralization studies were performed. We designed BMPRIa mutants with a point mutation in the CK2 phosphorylation site to establish a specific effect. Mineralization was initiated with the overexpression of the BMPRIa mutants indicating CK2 is a negative regulatory protein for osteoblast differentiation. Osteoclast differentiation and activity was decreased with the CK2 blocking peptide. Further, subcutaneous calvarial bone injections of a CK2 blocking peptide increased bone area, areal bone mineral density, and bone growth. These results indicate CK2 is crucial for osteoblast differentiation and could be a target for future therapeutics of fracture healing.

Unbiased functional proteomics strategy for protein kinase inhibitor validation and identification of bona fide protein kinase substrates: application to identification of EEF1D as a substrate for CK2

Protein kinases have emerged as attractive targets for treatment of several diseases prompting large-scale phosphoproteomics studies to elucidate their cellular actions and the design of novel inhibitory compounds. Current limitations include extensive reliance on consensus predictions to derive kinase-substrate relationships from phosphoproteomics data and incomplete experimental validation of inhibitors. To overcome these limitations in the case of protein kinase CK2, we employed functional proteomics and chemical genetics to enable identification of physiological CK2 substrates and validation of CK2 inhibitors including TBB and derivatives. By 2D electrophoresis and mass spectrometry, we identified the translational elongation factor EEF1D as a protein exhibiting CK2 inhibitor-dependent decreases in phosphorylation in (32)P-labeled HeLa cells. Direct phosphorylation of EEF1D by CK2 was shown by performing CK2 assays with EEF1D -FLAG from HeLa cells. Dramatic increases in EEF1D phosphorylation following λ-phosphatase treatment and phospho- EEF1D antibody recognizing EEF1D pS162 indicated phosphorylation at the CK2 site in cells. Furthermore, phosphorylation of EEF1D in the presence of TBB or TBBz is restored using CK2 inhibitor-resistant mutants. Collectively, our results demonstrate that EEF1D is a bona fide physiological CK2 substrate for CK2 phosphorylation. Furthermore, this validation strategy could be adaptable to other protein kinases and readily combined with other phosphoproteomic methods.

A CK2-dependent mechanism for activation of the JAK-STAT signaling pathway

JAK-STAT signaling is involved in the regulation of cell survival, proliferation, and differentiation. JAK tyrosine kinases can be transiently activated by cytokines or growth factors in normal cells, whereas they become constitutively activated as a result of mutations that affect their function in tumors. Specifically, the JAK2V617F mutation is present in the majority of patients with myeloproliferative disorders (MPDs) and is implicated in the pathogenesis of these diseases. In the present study, we report that the kinase CK2 is a novel interaction partner of JAKs and is essential for JAK-STAT activation. We demonstrate that cytokine-induced activation of JAKs and STATs and the expression of suppressor of cytokine signaling 3 (SOCS-3), a downstream target, are inhibited by CK2 small interfering RNAs or pharmacologic inhibitors. Endogenous CK2 is associated with JAK2 and JAK1 and phosphorylates JAK2 in vitro. To extend these findings, we demonstrate that CK2 interacts with JAK2V617F and that CK2 inhibitors suppress JAK2V617F autophosphorylation and downstream signaling in HEL92.1.7 cells (HEL) and primary cells from polycythemia vera (PV) patients. Furthermore, CK2 inhibitors potently induce apoptosis of HEL cells and PV cells. Our data provide evidence for novel cross-talk between CK2 and JAK-STAT signaling, with implications for therapeutic intervention in JAK2V617F-positive MPDs.

Casein kinase 2 beta-subunit is a regulator of bone morphogenetic protein 2 signaling

Bone morphogenetic proteins (BMPs) play a crucial role during embryonic development and regulate processes as diverse as neurogenesis, skeletal formation, and hematopoesis. They signal through a hetero-oligomer complex of BMP receptors. Binding of the ligand to the receptors activates several pathways, including Smad and p38. BMP signaling is controlled in the extracellular space, the plasma membrane, and the intracellular space; however, the mechanism of receptor signaling at the plasma membrane and proteins that regulate this process still need to be identified. The experiments presented here identify the protein kinase casein kinase II (CK2) as a BMP receptor type Ia (BRIa) interacting protein. Fluorescence resonance energy transfer revealed that this interaction occurs at the plasma membrane. BMP2 stimulation of C2C12 cells leads to the release of CK2 from BRIa. Blocking this interaction with specific peptides that inhibit the binding sites for CK2 on BRIa demonstrated a redistribution of BRIa on the plasma membrane. Signaling was initiated once CK2 was released from BRIa, leading to the mineralization of C2C12 cells. These data suggest that CK2 is a negative regulator of BMP signaling and osteoblast differentiation.

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.

Protein kinase CK2-mediated phosphorylation of HDAC2 regulates co-repressor formation, deacetylase activity and acetylation of HDAC2 by cigarette smoke and aldehydes

Histone deacetylase 2 (HDAC2) mediates the repression of pro-inflammatory genes by deacetylating core histones, RelA/p65 and the glucocorticoid receptor. Reduced level of HDAC2 is associated with steroid resistant inflammation caused by cigarette smoke (CS)-derived oxidants and aldehydes. However, the molecular mechanisms regulating HDAC2 in response to CS and aldehydes is not known. Here, we report that CS extract, and aldehyde acrolein induced phosphorylation of HDAC2 which was abolished by mutations at serine sites S(394), S(411), S(422) and S(424). HDAC2 phosphorylation required direct interaction with serine-phosphorylated protein kinase CK2alpha and involved reduced HDAC2 deacetylase activity. Furthermore, HDAC2 phosphorylation was required for HDAC2 interaction with transcription factors, co-repressor complex formation, CBP recruitment, acetylation on lysine residues and modulates transrepression activity. Thus, phospho-acetylation of HDAC2 negatively regulates its deacetylase activity which has implications in steroid resistance in chronic inflammatory conditions.

Phosphorylation of Ser136 is critical for potent bone sialoprotein-mediated nucleation of hydroxyapatite crystals

Acidic phosphoproteins of mineralized tissues such as bone and dentin are believed to play important roles in HA (hydroxyapatite) nucleation and growth. BSP (bone sialoprotein) is the most potent known nucleator of HA, an activity that is thought to be dependent on phosphorylation of the protein. The present study identifies the role phosphate groups play in mineral formation. Recombinant BSP and peptides corresponding to residues 1-100 and 133-205 of the rat sequence were phosphorylated with CK2 (protein kinase CK2). Phosphorylation increased the nucleating activity of BSP and BSP-(133-205), but not BSP-(1-100). MS analysis revealed that the major site phosphorylated within BSP-(133-205) was Ser136, a site adjacent to the series of contiguous glutamate residues previously implicated in HA nucleation. The critical role of phosphorylated Ser136 in HA nucleation was confirmed by site-directed mutagenesis and functional analyses. Furthermore, peptides corresponding to the 133-148 sequence of rat BSP were synthesized with or without a phosphate group on Ser136. As expected, the phosphopeptide was a more potent nucleator. The mechanism of nucleation was investigated using molecular-dynamics simulations analysing BSP-(133-148) interacting with the {100} crystal face of HA. Both phosphorylated and non-phosphorylated sequences adsorbed to HA in extended conformations with alternating residues in contact with and facing away from the crystal face. However, this alternating-residue pattern was more pronounced when Ser136 was phosphorylated. These studies demonstrate a critical role for Ser136 phosphorylation in BSP-mediated HA nucleation and identify a unique mode of interaction between the nucleating site of the protein and the {100} face of HA.

Phosphorylation of thymidylate synthase from various sources by human protein kinase CK2 and its catalytic subunits

Thymidylate synthase (TS) was found to be a substrate for both catalytic subunits of human CK2, with phosphorylation by CK2alpha and CK2alpha' characterized by similar K(m) values, 4.6microM and 4.2microM, respectively, but different efficiencies, the apparent turnover number with CK2alpha being 10-fold higher. With both catalytic subunits, phosphorylation of human TS, like calmodulin and BID, was strongly inhibited in the presence of the regulatory subunit CK2beta, the holoenzyme being activated by polylysine. Phosphorylation of recombinant human, rat, mouse and Trichinella spiralis TSs proteins was compared, with the human enzyme being apparently a much better substrate than the others. Following hydrolysis and TLC, phosphoserine was detected in human and rat, and phosphotyrosine in T. spiralis, TS, used as substrates for CK2alpha. MALDI-TOF MS analysis led to identification of phosphorylated Ser(124) in human TS, within a sequence LGFS(124)TREEGD, atypical for a CK2 substrate recognition site. The phosphorylation site is located in a region considered important for the catalytic mechanism or regulation of human TS, corresponding to the loop 107-128. Following phosphorylation by CK2alpha, resulting in incorporation of 0.4mol of phosphate per mol of dimeric TS, human TS exhibits unaltered K(m) values for dUMP and N(5,10)-methylenetetrahydrofolate, but a 50% lower turnover number, pointing to a strong influence of Ser(124) phosphorylation on its catalytic efficiency.

Protein kinase CK2 is a constitutively active enzyme that promotes cell survival: strategies to identify CK2 substrates and manipulate its activity in mammalian cells

Protein kinase CK2 is a constitutively active protein serine/threonine kinase that is ubiquitously expressed and essential for the survival of eukaryotic cells. On the basis of its elevated expression in a number of human cancers and its ability to promote tumorigenesis in transgenic mice, CK2 has emerged as a promising candidate for molecular-targeted therapy. Accordingly, there has been considerable interest in identifying the cellular events that are regulated by CK2 and the cellular substrates of CK2 that are responsible for mediating its actions in cells. Large-scale phosphoproteomics studies are revealing extensive lists of candidate CK2 substrates on the basis that these proteins are phosphorylated at sites conforming to the consensus for phosphorylation by CK2. However, efforts to validate the vast majority of these candidates as bona fide physiological CK2 substrates have been hindered by the lack of systematic strategies to identify its direct substrates and manipulate its activity in intact cells. To overcome these limitations, we describe experimental procedures for isolating CK2 from bacteria and from mammalian cells to enable in vitro phosphorylation of candidate substrates. We also outline strategies for manipulating the levels and activity of CK2 in intact cells. Collectively, the methods that are presented in this chapter should enable the identification and characterization of CK2 substrates and CK2-regulated processes both in vitro and in living cells.