On the physiological role of casein kinase II in Saccharomyces cerevisiae

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.

Minor Kinases with Major Roles in Cytokinesis Regulation

Cytokinesis, the conclusive act of cell division, allows cytoplasmic organelles and chromosomes to be faithfully partitioned between two daughter cells. In animal organisms, its accurate regulation is a fundamental task for normal development and for preventing aneuploidy. Cytokinesis failures produce genetically unstable tetraploid cells and ultimately result in chromosome instability, a hallmark of cancer cells. In animal cells, the assembly and constriction of an actomyosin ring drive cleavage furrow ingression, resulting in the formation of a cytoplasmic intercellular bridge, which is severed during abscission, the final event of cytokinesis. Kinase-mediated phosphorylation is a crucial process to orchestrate the spatio-temporal regulation of the different stages of cytokinesis. Several kinases have been described in the literature, such as cyclin-dependent kinase, polo-like kinase 1, and Aurora B, regulating both furrow ingression and/or abscission. However, others exist, with well-established roles in cell-cycle progression but whose specific role in cytokinesis has been poorly investigated, leading to considering these kinases as "minor" actors in this process. Yet, they deserve additional attention, as they might disclose unexpected routes of cell division regulation. Here, we summarize the role of multifunctional kinases in cytokinesis with a special focus on those with a still scarcely defined function during cell cleavage. Moreover, we discuss their implication in cancer.

Phosphorylation-mediated regulation of the Nem1-Spo7/Pah1 phosphatase cascade in yeast lipid synthesis

The PAH1-encoded phosphatidate phosphatase, which catalyzes the dephosphorylation of phosphatidate to produce diacylglycerol, controls the divergence of phosphatidate into triacylglycerol synthesis and phospholipid synthesis. Pah1 is inactive in the cytosol as a phosphorylated form and becomes active on the nuclear/endoplasmic reticulum membrane as a dephosphorylated form by the Nem1-Spo7 protein phosphatase complex. The phosphorylation of Pah1 by protein kinases, which include casein kinases I and II, Pho85-Pho80, Cdc28-cyclin B, and protein kinases A and C, controls its cellular location, catalytic activity, and susceptibility to proteasomal degradation. Nem1 (catalytic subunit) and Spo7 (regulatory subunit), which form a protein phosphatase complex catalyzing the dephosphorylation of Pah1 for its activation, are phosphorylated by protein kinases A and C. In this review, we discuss the functions and interrelationships of the protein kinases in the control of the Nem1-Spo7/Pah1 phosphatase cascade and lipid synthesis.

Casein kinase II activates Bik to induce death of hyperplastic mucous cells in a cell cycle-dependent manner

Extensive inflammation causes epithelial cell hyperplasia in the airways and Bcl-2-interacting killer (Bik) reduces epithelial cell and mucous cell hyperplasia without affecting resting cells to restore homeostasis. These observations suggest that Bik induces apoptosis in a cell cycle-specific manner, but the mechanisms are not understood. Mice were exposed to an allergen for 3, 14, or 30 days and Bik expression was induced in airway epithelia of transgenic mice. Bik reduced epithelial and mucous cell hyperplasia when mice were exposed to an allergen for 3 or 14 days, but not when exposure lasted for 30 days, and Ki67-positivity was reduced. In culture, Bik expression killed proliferating cells but not quiescent cells. To capture the stage of the cell cycle when Bik induces cell death, airway cells that express fluorescent ubiquitin cell cycle indicators were generated that fluoresce red or green during the G0/G1 and S/G2/M phases of the cells cycle, respectively. Regardless of the cell cycle stage, Bik expression eliminated green-fluorescent cells. Also, Bik, when tagged with a blue-fluorescent protein, was only detected in green cells. Bik phosphorylation mutants at threonine 33 or serine 35 demonstrated that phosphorylation activated Bik to induce death even in quiescent cells. Immunoprecipitation and proteomic approaches identified casein kinase IIα to be responsible for phosphorylating and activating Bik to kill cells in S/G2/M. As casein kinase 2 alpha (CKIIα) is expressed only during the G2/M phase, we conclude that Bik activation in airway epithelial cells selectively targets hyperplastic epithelial cells, while leaving resting airway cells unaffected.

Casein kinase II-mediated phosphorylation of lipin 1β phosphatidate phosphatase at Ser-285 and Ser-287 regulates its interaction with 14-3-3β protein

The mammalian lipin 1 phosphatidate phosphatase is a key regulatory enzyme in lipid metabolism. By catalyzing phosphatidate dephosphorylation, which produces diacylglycerol, the enzyme plays a major role in the synthesis of triacylglycerol and membrane phospholipids. The importance of lipin 1 to lipid metabolism is exemplified by cellular defects and lipid-based diseases associated with its loss or overexpression. Phosphorylation of lipin 1 governs whether it is associated with the cytoplasm apart from its substrate or with the endoplasmic reticulum membrane where its enzyme reaction occurs. Lipin 1β is phosphorylated on multiple sites, but less than 10% of them are ascribed to a specific protein kinase. Here, we demonstrate that lipin 1β is a bona fide substrate for casein kinase II (CKII), a protein kinase that is essential to viability and cell cycle progression. Phosphoamino acid analysis and phosphopeptide mapping revealed that lipin 1β is phosphorylated by CKII on multiple serine and threonine residues, with the former being major sites. Mutational analysis of lipin 1β and its peptides indicated that Ser-285 and Ser-287 are both phosphorylated by CKII. Substitutions of Ser-285 and Ser-287 with nonphosphorylatable alanine attenuated the interaction of lipin 1β with 14-3-3β protein, a regulatory hub that facilitates the cytoplasmic localization of phosphorylated lipin 1. These findings advance our understanding of how phosphorylation of lipin 1β phosphatidate phosphatase regulates its interaction with 14-3-3β protein and intracellular localization and uncover a mechanism by which CKII regulates cellular physiology.

The role of CKIP-1 in osteoporosis development and treatment

Osteoporosis is a systemic skeletal disorder characterized by reduced bone mass and deterioration of bone microarchitecture, which results in increased bone fragility and fracture risk. Casein kinase 2-interacting protein-1 (CKIP-1) is a protein that plays an important role in regulation of bone formation. The effect of CKIP-1 on bone formation is mainly mediated through negative regulation of the bone morphogenetic protein pathway. In addition, CKIP-1 has an important role in the progression of osteoporosis. This review provides a summary of the recent studies on the role of CKIP-1 in osteoporosis development and treatment.

Cite this article: X. Peng, X. Wu, J. Zhang, G. Zhang, G. Li, X. Pan. The role of CKIP-1 in osteoporosis development and treatment. Bone Joint Res 2018;7:173–178. DOI: 10.1302/2046-3758.72.BJR-2017-0172.R1.

Identification of Candidate Casein Kinase 2 Substrates in Mitosis by Quantitative Phosphoproteomics

Protein phosphorylation is a crucial regulatory mechanism that controls many aspects of cellular signaling. Casein kinase 2 (CK2), a constitutively expressed and active kinase, plays key roles in an array of cellular events including transcription and translation, ribosome biogenesis, cell cycle progression, and apoptosis. CK2 is implicated in cancerous transformation and is a therapeutic target in anti-cancer therapy. The specific and selective CK2 ATP competitive inhibitor, CX-4945 (silmitaseratib), is currently in phase 2 clinical trials. While many substrates and interactors of CK2 have been identified, less is known about CK2 substrates in mitosis. In the present work, we utilize CX-4945 and quantitative phosphoproteomics to inhibit CK2 activity in mitotically arrested HeLa cells and determine candidate CK2 substrates. We identify 330 phosphorylation sites on 202 proteins as significantly decreased in abundance upon inhibition of CK2 activity. Motif analysis of decreased sites reveals a linear kinase motif with aspartic and glutamic amino acids downstream of the phosphorylated residues, which is consistent with known substrate preferences for CK2. To validate specific candidate CK2 substrates, we perform in vitro kinase assays using purified components. Furthermore, we identified CK2 interacting proteins by affinity purification-mass spectrometry (AP-MS). To investigate the biological processes regulated by CK2 in mitosis, we perform network analysis and identify an enrichment of proteins involved in chromosome condensation, chromatin organization, and RNA processing. We demonstrate that overexpression of CK2 in HeLa cells affects proper chromosome condensation. Previously, we found that phosphoprotein phosphatase 6 (PP6), but not phosphoprotein phosphatase 2A (PP2A), opposes CK2 phosphorylation of the condensin I complex, which is essential for chromosome condensation. Here, we extend this observation and demonstrate that PP6 opposition of CK2 is a more general cellular regulatory mechanism.

CX-4945: the protein kinase CK2 inhibitor and anti-cancer drug shows anti-fungal activity

CX-4945 is a selective inhibitor of protein kinase CK2 exhibiting clinical significance. Its antitumor properties arise from the abrogation of CK2-mediated pro-survival cellular pathways. The presented data reveal the influence of CX-4945 on the growth of yeast cells showing variable potency against Saccharomyces cerevisiae deletion strains with different contents of CK2 subunits. The catalytic subunit CK2α appears to sensitize yeast to the CX-4945 action. Moreover, the compound suppresses hyphal growth and cell adhesion of Candida albicans, thereby abolishing some hallmarks of invasiveness of the pathogen. It is known that cancer patients are more prone to fungal infections. Our data unveil the dual-activity of CX-4945; when used in anti-cancer therapy, it may simultaneously prevent cancer-associated candidiasis.

Phosphorylation of Dgk1 Diacylglycerol Kinase by Casein Kinase II Regulates Phosphatidic Acid Production in Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, Dgk1 diacylglycerol (DAG) kinase catalyzes the CTP-dependent phosphorylation of DAG to form phosphatidic acid (PA). The enzyme in conjunction with Pah1 PA phosphatase controls the levels of PA and DAG for the synthesis of triacylglycerol and membrane phospholipids, the growth of the nuclear/endoplasmic reticulum membrane, and the formation of lipid droplets. Little is known about how DAG kinase activity is regulated by posttranslational modification. In this work, we examined the phosphorylation of Dgk1 DAG kinase by casein kinase II (CKII). When phosphate groups were globally reduced using nonspecific alkaline phosphatase, Triton X-100-solubilized membranes from DGK1-overexpressing cells showed a 7.7-fold reduction in DAG kinase activity; the reduced enzyme activity could be increased 5.5-fold by treatment with CKII. Dgk1(1-77) expressed heterologously in Escherichia coli was phosphorylated by CKII on a serine residue, and its phosphorylation was dependent on time as well as on the concentrations of CKII, ATP, and Dgk1(1-77). We used site-specific mutagenesis, coupled with phosphorylation analysis and phosphopeptide mapping, to identify Ser-45 and Ser-46 of Dgk1 as the CKII target sites, with Ser-46 being the major phosphorylation site. In vivo, the S46A and S45A/S46A mutations of Dgk1 abolished the stationary phase-dependent stimulation of DAG kinase activity. In addition, the phosphorylation-deficient mutations decreased Dgk1 function in PA production and in eliciting pah1Δ phenotypes, such as the expansion of the nuclear/endoplasmic reticulum membrane, reduced lipid droplet formation, and temperature sensitivity. This work demonstrates that the CKII-mediated phosphorylation of Dgk1 regulates its function in the production of PA.

Scan-o-matic: High-Resolution Microbial Phenomics at a Massive Scale

The capacity to map traits over large cohorts of individuals—phenomics—lags far behind the explosive development in genomics. For microbes, the estimation of growth is the key phenotype because of its link to fitness. We introduce an automated microbial phenomics framework that delivers accurate, precise, and highly resolved growth phenotypes at an unprecedented scale. Advancements were achieved through the introduction of transmissive scanning hardware and software technology, frequent acquisition of exact colony population size measurements, extraction of population growth rates from growth curves, and removal of spatial bias by reference-surface normalization. Our prototype arrangement automatically records and analyzes close to 100,000 growth curves in parallel. We demonstrate the power of the approach by extending and nuancing the known salt-defense biology in baker’s yeast. The introduced framework represents a major advance in microbial phenomics by providing high-quality data for extensive cohorts of individuals and generating well-populated and standardized phenomics databases

Phosphorylation of Yeast Pah1 Phosphatidate Phosphatase by Casein Kinase II Regulates Its Function in Lipid Metabolism

Pah1 phosphatidate phosphatase in Saccharomyces cerevisiae catalyzes the penultimate step in the synthesis of triacylglycerol (i.e. the production of diacylglycerol by dephosphorylation of phosphatidate). The enzyme playing a major role in lipid metabolism is subject to phosphorylation (e.g. by Pho85-Pho80, Cdc28-cyclin B, and protein kinases A and C) and dephosphorylation (e.g. by Nem1-Spo7) that regulate its cellular location, catalytic activity, and stability/degradation. In this work, we show that Pah1 is a substrate for casein kinase II (CKII); its phosphorylation was time- and dose-dependent and was dependent on the concentrations of Pah1 (Km = 0.23 μm) and ATP (Km = 5.5 μm). By mass spectrometry, truncation analysis, site-directed mutagenesis, phosphopeptide mapping, and phosphoamino acid analysis, we identified that >90% of its phosphorylation occurs on Thr-170, Ser-250, Ser-313, Ser-705, Ser-814, and Ser-818. The CKII-phosphorylated Pah1 was a substrate for the Nem1-Spo7 protein phosphatase and was degraded by the 20S proteasome. The prephosphorylation of Pah1 by protein kinase A or protein kinase C reduced its subsequent phosphorylation by CKII. The prephosphorylation of Pah1 by CKII reduced its subsequent phosphorylation by protein kinase A but not by protein kinase C. The expression of Pah1 with combined mutations of S705D and 7A, which mimic its phosphorylation by CKII and lack of phosphorylation by Pho85-Pho80, caused an increase in triacylglycerol content and lipid droplet number in cells expressing the Nem1-Spo7 phosphatase complex.

DEK over-expression promotes mitotic defects and micronucleus formation

The DEK gene encodes a nuclear protein that binds chromatin and is involved in various fundamental nuclear processes including transcription, RNA splicing, DNA replication and DNA repair. Several cancer types characteristically over-express DEK at the earliest stages of transformation. In order to explore relevant mechanisms whereby DEK supports oncogenicity, we utilized cancer databases to identify gene transcripts whose expression patterns are tightly correlated with that of DEK. We identified an enrichment of genes involved in mitosis and thus investigated the regulation and possible function of DEK in cell division. Immunofluorescence analyses revealed that DEK dissociates from DNA in early prophase and re-associates with DNA during telophase in human keratinocytes. Mitotic cell populations displayed a sharp reduction in DEK protein levels compared to the corresponding interphase population, suggesting DEK may be degraded or otherwise removed from the cell prior to mitosis. Interestingly, DEK overexpression stimulated its own aberrant association with chromatin throughout mitosis. Furthermore, DEK co-localized with anaphase bridges, chromosome fragments, and micronuclei, suggesting a specific association with mitotically defective chromosomes. We found that DEK over-expression in both non-transformed and transformed cells is sufficient to stimulate micronucleus formation. These data support a model wherein normal chromosomal clearance of DEK is required for maintenance of high fidelity cell division and chromosomal integrity. Therefore, the overexpression of DEK and its incomplete removal from mitotic chromosomes promotes genomic instability through the generation of genetically abnormal daughter cells. Consequently, DEK over-expression may be involved in the initial steps of developing oncogenic mutations in cells leading to cancer initiation.

Differential phosphorylation of a regulatory subunit of protein kinase CK2 by target of rapamycin complex 1 signaling and the Cdc-like kinase Kns1

Transcriptional regulation of ribosome and tRNA synthesis plays a central role in determining protein synthetic capacity and is tightly controlled in response to nutrient availability and cellular stress. In Saccharomyces cerevisiae, the regulation of ribosome and tRNA synthesis was recently shown to involve the Cdc-like kinase Kns1 and the GSK-3 kinase Mck1. In this study, we explored additional roles for these conserved kinases in processes connected to the target of rapamycin complex 1 (TORC1). We conducted a synthetic chemical-genetic screen in a kns1Δ mck1Δ strain and identified many novel rapamycin-hypersensitive genes. Gene ontology analysis showed enrichment for TORC1-regulated processes (vesicle-mediated transport, autophagy, and regulation of cell size) and identified new connections to protein complexes including the protein kinase CK2. CK2 is considered to be a constitutively active kinase and in budding yeast, the holoenzyme comprises two regulatory subunits, Ckb1 and Ckb2, and two catalytic subunits, Cka1 and Cka2. We show that Ckb1 is differentially phosphorylated in vivo and that Kns1 mediates this phosphorylation when nutrients are limiting and under all tested stress conditions. We determined that the phosphorylation of Ckb1 does not detectably affect the stability of the CK2 holoenzyme but correlates with the reduced occupancy of Ckb1 on tRNA genes after rapamycin treatment. Thus, the differential occupancy of tRNA genes by CK2 is likely to modulate its activation of RNA polymerase III transcription. Our data suggest that TORC1, via its effector kinase Kns1, may regulate the association of CK2 with some of its substrates by phosphorylating Ckb1.

Synthesis, biological activity and structural study of new benzotriazole-based protein kinase CK2 inhibitors

A new series of TBB-derivatives was synthesized and characterized as CK2 inhibitors. Crystallographic analysis and docking studies were used to understand the mode of binding.

Evolutionary analysis of multidrug resistance genes in fungi - impact of gene duplication and family conservation

Although the emergence of bacterial drug resistance is of great concern to the scientific community, few studies have evaluated this phenomenon systematically in fungi by using genome-wide datasets. In the present study, we assembled a large compendium of Saccharomyces cerevisiae chemical genetic data to study the evolution of multidrug resistance genes (MDRs) in the fungal lineage. We found that MDRs typically emerge in widely conserved families, most of which containing homologs from pathogenic fungi, such as Candida albicans and Coccidioides immitis, which could favor the evolution of drug resistance in those species. By integrating data from chemical genetics with protein family conservation, genetic and protein interactions, we found that gene families rarely have more than one MDR, indicating that paralogs evolve asymmetrically with regard to multidrug resistance roles. Furthermore, MDRs have more genetic and protein interaction partners than non-MDRs, supporting their participation in complex biochemical systems underlying the tolerance to multiple bioactive molecules. MDRs share more chemical genetic interactions with other MDRs than with non-MDRs, regardless of their evolutionary affinity. These results suggest the existence of an intricate system involved in the global drug tolerance phenotypes. Finally, MDRs are more likely to be hit repeatedly by mutations in laboratory evolution experiments, indicating that they have great adaptive potential. The results presented here not only reveal the main genomic features underlying the evolution of MDRs, but also shed light on the gene families from which drug resistance is more likely to emerge in fungi.

Phosphorylation by casein kinase 2 facilitates Psh1 protein-assisted degradation of Cse4 protein

Cse4 is the centromeric histone H3 variant in budding yeast. Psh1 is an E3 ubiquitin ligase that controls Cse4 levels through proteolysis. Here we report that Psh1 is phosphorylated by the Cka2 subunit of casein kinase 2 (CK2) to promote its E3 activity for Cse4. Deletion of CKA2 significantly stabilized Cse4. Consistent with phosphorylation promoting the activity of Psh1, Cse4 was stabilized in a Psh1 phosphodepleted mutant strain in which the major phosphorylation sites were changed to alanines. Phosphorylation of Psh1 did not control Psh1-Cse4 or Psh1-Ubc3(E2) interactions. Although Cse4 was highly stabilized in a cka2Δ strain, mislocalization of Cse4 was mild, suggesting that Cse4 misincorporation was prevented by the intact Psh1-Cse4 association. Supporting this idea, Psh1 was also stabilized in a cka2Δ strain. Collectively our data suggest that phosphorylation is crucial in Psh1-assisted control of Cse4 levels and that the Psh1-Cse4 association itself functions to prevent Cse4 misincorporation.

Casein kinase: the triple meaning of a misnomer

The term 'casein kinase' has been widely used for decades to denote protein kinases sharing the ability to readily phosphorylate casein in vitro. These fall into three main classes: two of them, later renamed as protein kinases CK1 (casein kinase 1, also known as CKI) and CK2 (also known as CKII), are pleiotropic members of the kinome functionally unrelated to casein, whereas G-CK, or genuine casein kinase, responsible for the phosphorylation of casein in the Golgi apparatus of the lactating mammary gland, has only been identified recently with Fam20C [family with sequence similarity 20C; also known as DMP-4 (dentin matrix protein-4)], a member of the four-jointed family of atypical protein kinases, being responsible for the phosphorylation of many secreted proteins. In hindsight, therefore, the term 'casein kinase' is misleading in every instance; in the case of CK1 and CK2, it is because casein is not a physiological substrate, and in the case of G-CK/Fam20C/DMP-4, it is because casein is just one out of a plethora of its targets, and a rather marginal one at that. Strikingly, casein kinases altogether, albeit representing a minimal proportion of the whole kinome, appear to be responsible for the generation of up to 40-50% of non-redundant phosphosites currently retrieved in human phosphopeptides database. In the present review, a short historical explanation will be provided accounting for the usage of the same misnomer to denote three unrelated classes of protein kinases, together with an update of our current knowledge of these pleiotropic enzymes, sharing the same misnomer while playing very distinct biological roles.

The multiple nucleotide-divalent cation binding modes of Saccharomyces cerevisiae CK2α indicate a possible co-substrate hydrolysis product (ADP/GDP) release pathway

CK2 is a ubiquitous and conserved protein kinase in eukaryotic organisms and is important in many biological processes. It is unique in maintaining constitutive activity and in using both ATP and GTP as phosphor donors. In this study, crystal structures of recombinant Saccharomyces cerevisiae CK2α (scCK2α) complexed with GMPPNP, ATP and AMPPN with either Mg2+ or Mn2+ as the coordinated divalent cation are presented. The overall structure of scCK2α shows high similarity to its homologous proteins by consisting of two domains with the co-substrate lying in the cleft between them. However, three characteristic features distinguish scCK2α from its homologues. Firstly, the Lys45-Glu53 and Arg48-Glu53 interactions in scCK2α lead Lys50 to adopt a unique conformation that is able to stabilize the γ-phosphate of the co-substrate, which makes the existence of the `essential divalent cation' not so essential. The multiple nucleotide-divalent cation binding modes of the active site of scCK2α are apparently different from the two-divalent-cation-occupied active site of Zea mays CK2α and human CK2α. Secondly, conformational change of Glu53 in scCK2α-AMPPN breaks its interaction with Lys45 and Arg48; as a result, the co-substrate binding pocket becomes more open. This may suggest a clue to a possible ADP/GDP-release pathway, because the NE1 atom of the Trp in the `DWG motif' of CK2α forms a hydrogen bond to the O atom of Leu212, which seems to make ADP release by means of the `DFG-in flip to DFG-out' model found in most eukaryotic protein kinases impossible. Coincidentally, two sulfate ions which may mimic two phosphate groups were captured by Arg161 and Lys197 around the pocket. Mutagenesis and biochemical experiments on R161A and K197A mutants support the above proposal. Finally, scCK2α is unique in containing an insertion region whose function had not been identified in previous research. It is found that the insertion region contributes to maintaining the constitutively active conformation of the scCK2α catalytic site, but does not participate in interaction with the regulatory subunits.

CKIP-1: a scaffold protein and potential therapeutic target integrating multiple signaling pathways and physiological functions

The PH domain-containing casein kinase 2 interacting protein-1 (CKIP-1, also known as PLEKHO1) acts as a scaffold protein mediating interactions with multiple proteins, including CK2α, CPα, AP-1/c-Jun, Akt, ATM, IFP35/Nmi and Smurf1. CKIP-1 functions through different ways, such as plasma membrane recruitment, transcriptional activity modulation and posttranscriptional modification regulation. Moreover, the subcellular localization of CKIP-1 is determined by several key amino acids in a cell type dependent style, and the nucleus/plasma membrane shuttle of CKIP-1 is regulated by different cell stresses. As an adaptor protein, CKIP-1 is involved in various important signaling pathways, controlling cell growth, apoptosis, differentiation, cytoskeleton and bone formation. Strikingly, CKIP-1 has been recently demonstrated to be a promising target for treatment of osteoporosis in rat models. In addition, more evidences suggest that CKIP-1 might also function as a potential tumor suppressor.

Structural disorder and local order of hNopp140

Human nucleolar phosphoprotein p140 (hNopp 140) is a highly phosphorylated protein inhibitor of casein kinase 2 (CK2). As in the case of many kinase-inhibitor systems, the inhibitor has been described to belong to the family of intrinsically disordered proteins (IDPs), which often utilize transient structural elements to bind their cognate enzyme. Here we investigated the structural status of this protein both to provide distinct lines of evidence for its disorder and to point out its transient structure potentially involved in interactions and also its tendency to aggregate. Structural disorder of hNopp140 is apparent by its anomalous electrophoretic mobility, protease sensitivity, heat stability, hydrodynamic behavior on size-exclusion chromatography, (1)H NMR spectrum and differential scanning calorimetry scan. hNopp140 has a significant tendency to aggregate and the change of its circular dichroism spectrum in the presence of 0-80% TFE suggests a tendency to form local helical structures. Wide-line NMR measurements suggest the overall disordered character of the protein. In all, our data suggest that this protein falls into the pre-molten globule state of IDPs, with a significant tendency to become ordered in the presence of its partner as demonstrated in the presence of transcription factor IIB (TFIIB).

Structure of yeast kinetochore Ndc10 DNA-binding domain reveals unexpected evolutionary relationship to tyrosine recombinases

We have solved the x-ray structure of the N-terminal half of the yeast kinetochore protein Ndc10 at 1.9 Å resolution. This essential protein is a key constituent of the budding yeast centromere and is essential for the recruitment of the centromeric nucleosome and establishment of the kinetochore. The fold of the protein shows unexpected similarities to the tyrosine recombinase/λ-integrase family of proteins, most notably Cre, with some variation in the relative position of the subdomains. This finding offers new insights into kinetochore evolution and the adaptation of a well studied protein fold to a novel role. By comparison with tyrosine recombinases and mutagenesis studies, we have been able to define some of the key DNA-binding motifs.

Regulation of phospholipid synthesis in the yeast Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae, with its full complement of organelles, synthesizes membrane phospholipids by pathways that are generally common to those found in higher eukaryotes. Phospholipid synthesis in yeast is regulated in response to a variety of growth conditions (e.g., inositol supplementation, zinc depletion, and growth stage) by a coordination of genetic (e.g., transcriptional activation and repression) and biochemical (e.g., activity modulation and localization) mechanisms. Phosphatidate (PA), whose cellular levels are controlled by the activities of key phospholipid synthesis enzymes, plays a central role in the transcriptional regulation of phospholipid synthesis genes. In addition to the regulation of gene expression, phosphorylation of key phospholipid synthesis catalytic and regulatory proteins controls the metabolism of phospholipid precursors and products.

Localization of phosphorylated CK2α to the mitotic spindle requires the peptidyl-prolyl isomerase Pin1

CK2 is a serine/threonine kinase with many substrates, largely unknown modes of regulation and essential roles in mitotic progression. CK2α, a catalytic subunit of CK2, is phosphorylated in mitosis, and here we examine the effect of phosphorylation on CK2α localization. Using phosphospecific antibodies, we show that CK2α localizes to the mitotic spindle in a phosphorylation-dependent manner. Mitotic spindle localization requires the unique C-terminus of CK2α, and involves a novel regulatory mechanism in which phosphorylation of CK2α facilitates binding to the peptidyl-prolyl isomerase Pin1, which is required for CK2α mitotic spindle localization. This could explain how the constitutive activity of CK2α might be targeted towards mitotic substrates. Furthermore, because Pin1 has many important spindle substrates, this might represent a general mechanism for localization of mitotic signalling proteins.

Overlapping kinetochore targets of CK2 and Aurora B kinases in mitotic regulation

Protein kinase CK2 is one of the most conserved kinases in eukaryotic cells and plays essential roles in diverse processes. While we know that CK2 plays a role(s) in cell division, our understanding of how CK2 regulates cell cycle progression is limited. In this study, we revealed a regulatory role for CK2 in kinetochore function. The kinetochore is a multi-protein complex that assembles on the centromere of a chromosome and functions to attach chromosomes to spindle microtubules. To faithfully segregate chromosomes and maintain genomic integrity, the kinetochore is tightly regulated by multiple mechanisms, including phosphorylation by Aurora B kinase. We found that a loss of CK2 kinase activity inhibits anaphase spindle elongation and results in chromosome missegregation. Moreover, a lack of CK2 activates the spindle assembly checkpoint. We demonstrate that CK2 associates with Mif2, the Saccharomyces cerevisiae homologue of human CENP-C, which serves as an important link between the inner and outer kinetochore. Furthermore, we show Mif2 and the inner kinetochore protein Ndc10 are phosphorylated by CK2, and this phosphorylation plays antagonistic and synergistic roles with Aurora B phosphorylation of these targets, respectively.

Isolation of a CK2α subunit and the holoenzyme from the mussel Mytilus galloprovincialis and construction of the CK2α and CK2β cDNAs

Protein kinase CK2 is a ubiquitous, highly pleiotropic, and constitutively active phosphotransferase that phosphorylates mainly serine and threonine residues. CK2 has been studied and characterized in many organisms, from yeast to mammals. The holoenzyme is generally composed of two catalytic (α and/or α') and two regulatory (β) subunits, forming a differently assembled tetramer. The free and catalytically active α/α' subunits can be present in cells under some circumstances. We present here the isolation of a putative catalytic CK2α subunit and holoenzyme from gills of the mussel Mytilus galloprovincialis capable of phosphorylating the purified recombinant ribosomal protein rMgP1. For further analysis of M. galloprovincialis protein kinase CK2, the cDNA molecules of CK2α and CK2β subunits were constructed and cloned into expression vectors, and the recombinant proteins were purified after expression in Escherichia coli. The recombinant MgCK2β subunit and MgP1 were phosphorylated by the purified recombinant MgCK2α subunit. The mussel enzyme presented features typical for CK2: affinity for GTP, inhibition by both heparin and ATP competitive inhibitors (TBBt, TBBz), and sensitivity towards NaCl. Predicted amino acid sequence comparison showed that the M. galloprovincialis MgCK2α and MgCK2β subunits have similar features to their mammalian orthologs.

Assessing the components of the eIF3 complex and their phosphorylation status

The eukaryotic initiation factor 3 (eIF3) is an essential, highly conserved multiprotein complex that is a key component in the recruitment and assembly of the translation initiation machinery. To better understand the molecular function of eIF3, we examined its composition and phosphorylation status in Saccharomyces cerevisiae. The yeast eIF3 complex contains five core components: Rpg1, Nip1, Prt1, Tif34, and Tif35. 2-D LC-MS/MS analysis of affinity purified eIF3 complexes showed that several other initiation factors (Fun12, Tif5, Sui3, Pab1, Hcr1, and Sui1) and the casein kinase 2 complex (CK2) copurify. In Vivo metabolic labeling of proteins with (32)P revealed that Nip1 is phosphorylated. Using 2-D LC-MS/MS analysis of eIF3 complexes, we identified Prt1 phosphopeptides indicating phosphorylation at S22 and T707 and a Tif5 phosphopeptide with phosphorylation at T191. Additionally, we used immobilized metal affinity chromatography (IMAC) to enrich for eIF3 phosphopeptides and tandem mass spectrometry to identify phosphorylated residues. We found that three CK2 consensus sequences in Nip1 are phosphorylated: S98, S99, and S103. Using in vitro kinase assays, we showed that CK2 phophorylates Nip1 and that a synthetic Nip1 peptide containing S98, S99, and S103 competitively inhibits the reaction. Replacement of these three Nip1 serines with alanines causes a slow growth phenotype.

Coordinate regulation of histone mRNA metabolism and DNA replication: cyclin A/cdk1 is involved in inactivation of histone mRNA metabolism and DNA replication at the end of S phase

S phase is characterized by the replication of DNA and assembly of chromatin. This requires the synthesis of large amounts of histone proteins to package the newly replicated DNA. Histone mRNAs are the only mRNAs that do not have polyA tails, ending instead in a conserved stemloop sequence. The stemloop binding protein (SLBP) that binds the 3' end of histone mRNA is cell cycle regulated and SLBP is required in all steps of histone mRNA metabolism. Activation of cyclin E/cdk2 prior to entry into S-phase is critical for initiation of DNA replication and histone mRNA accumulation. At the end of S phase SLBP is rapidly degraded as a result of phosphorylation of SLBP by cyclin A/cdk1 and CK2 effectively shutting off histone mRNA biosynthesis. E2F1, which is required for expression of many S-phase genes, is regulated in parallel with SLBP and its degradation also requires a cyclin binding site, suggesting that it may also be regulated by the same pathway. It is likely that activation of cyclin A/cdk1 helps inhibit both DNA replication and histone mRNA accumulation, marking the end of S phase and entry into G(2)-phase.

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.

Halogenated imidazole derivatives block RNA polymerase II elongation along mitogen inducible genes

Background

Aberrant activation of protein kinases is one of the essential oncogenic driving forces inherent to the process of tumorigenesis. The protein kinase CK2 plays an important role in diverse biological processes, including cell growth and proliferation as well as in the governing and transduction of prosurvival signals. Increased expression of CK2 is a hallmark of some cancers, hence its antiapoptotic properties may be relevant to cancer onset. Thus, the designing and synthesis of the CK2 inhibitors has become an important pursuit in the search for cancer therapies.

Results

Using a high-throughput microarray approach, we demonstrate that two potent inhibitors of CK2, 4,5,6,7-tetrabromo-benzimidazole (TBBz) and 2-Dimethyloamino-4,5,6,7-tetrabromo-1H-benzimidazole (DMAT), blocked mitogen induced mRNA expression of immediate early genes. Given the impact of these inhibitors on the process of transcription, we investigated their effects on RNA Polymerase II (RNAPII) elongation along the mitogen inducible gene, EGR1 (early growth response 1), using chromatin immunoprecipitation (ChIP) assay. ChIP analysis demonstrated that both drugs arrest RNAPII elongation. Finally, we show that CDK9 kinase activity, essential for the triggering of RNAPII elongation, was blocked by TBBz and to lesser degree by DMAT.

Conclusions

Our approach revealed that small molecules derived from halogenated imidazole compounds may decrease cell proliferation, in part, by inhibiting pathways that regulate transcription elongation.

CK2 is the regulator of SIRT1 substrate-binding affinity, deacetylase activity and cellular response to DNA-damage

SIRT1, an NAD(+) (nicotinamide adenine dinucleotide)-dependent deacetylase, protects cells from stress-induced apoptosis, and its orthologues delay aging in lower eukaryotes. SIRT1 increases survival in response to stress such as DNA damage by deacetylating a number of substrates including pro-apoptotic protein p53. The molecular mechanism by which DNA-damage activates SIRT1 is not known. By screening a kinase inhibitor library, we identified CK2 as a SIRT1 kinase. CK2 is a pleiotropic kinase with more than 300 substrates and well-known anti-apoptotic and pro-growth activities. We find that CK2 is recruited to SIRT1 after ionizing radiation (IR) and phosphorylates conserved residues Ser 154, 649, 651 and 683 in the N- and C-terminal domains of mouse SIRT1. Phosphorylation of SIRT1 increases its deacetylation rate but not if the four Ser residues are mutated. In addition, phosphorylation of SIRT1 increases its substrate-binding affinity. CK2-mediated phosphorylation increases the ability of SIRT1 to deacetylate p53 and protect cells from apoptosis after DNA damage. Based on these findings, we propose that CK2 protects against IR-induced apoptosis partly by phosphorylating and activating SIRT1. Thus, this work suggests that SIRT1 is a component of the expansive anti-apoptotic network controlled by CK2. Since expression of both CK2 and SIRT1 is upregulated with tumorigenesis and downregulated with senescence, the CK2-SIRT1 link sheds new light on how CK2 may regulate cancer development and aging.

On the intracellular trafficking of mouse S5 ribosomal protein from cytoplasm to nucleoli

The non-ribosomal functions of mammalian ribosomal proteins have recently attracted worldwide attention. The mouse ribosomal protein S5 (rpS5) derived from ribosomal material is an assembled non-phosphorylated protein. The free form of rpS5 protein, however, undergoes phosphorylation. In this study, we have (a) investigated the potential role of phosphorylation in rpS5 protein transport into the nucleus and then into nucleoli and (b) determined which of the domains of rpS5 are involved in this intracellular trafficking. In vitro PCR mutagenesis of mouse rpS5 cDNA, complemented by subsequent cloning and expression of rpS5 truncated recombinant forms, produced in fusion with green fluorescent protein, permitted the investigation of rpS5 intracellular trafficking in HeLa cells using confocal microscopy complemented by Western blot analysis. Our results indicate the following: (a) rpS5 protein enters the nucleus via the region 38-50 aa that forms a random coil as revealed by molecular dynamic simulation. (b) Immunoprecipitation of rpS5 with casein kinase II and immobilized metal affinity chromatography analysis complemented by in vitro kinase assay revealed that phosphorylation of rpS5 seems to be indispensable for its transport from nucleus to nucleoli; upon entering the nucleus, Thr-133 phosphorylation triggers Ser-24 phosphorylation by casein kinase II, thus promoting entrance of rpS5 into the nucleoli. Another important role of rpS5 N-terminal region is proposed to be the regulation of protein's cellular level. The repetitively co-appearance of a satellite C-terminal band below the entire rpS5 at the late stationary phase, and not at the early logarithmic phase, of cell growth suggests a specific degradation balancing probably the unassembled ribosomal protein molecules with those that are efficiently assembled to ribosomal subunits. Overall, these data provide new insights on the structural and functional domains within the rpS5 molecule that contribute to its cellular functions.

Sgt1 dimerization is negatively regulated by protein kinase CK2-mediated phosphorylation at Ser361

The kinetochore, which consists of centromere DNA and structural proteins, is essential for proper chromosome segregation in eukaryotes. In budding yeast, Sgt1 and Hsp90 are required for the binding of Skp1 to Ctf13 (a component of the core kinetochore complex CBF3) and therefore for the assembly of CBF3. We have previously shown that Sgt1 dimerization is important for this kinetochore assembly mechanism. In this study, we report that protein kinase CK2 phosphorylates Ser(361) on Sgt1, and this phosphorylation inhibits Sgt1 dimerization.

Gly-46 and His-50 of yeast maltose transporter Mal21p are essential for its resistance against glucose-induced degradation

The maltose transporter gene is situated at the MAL locus, which consists of genes for a transporter, maltase, and transcriptional activator. Five unlinked MAL loci (MAL1, MAL2, MAL3, MAL4, and MAL6) constitute a gene family in Saccharomyces cerevisiae. The expression of the maltose transporter is induced by maltose and repressed by glucose. The activity of the maltose transporter is also regulated post-translationally; Mal61p is rapidly internalized from the plasma membrane and degraded by ubiquitin-mediated proteolysis in the presence of glucose. We found that S. cerevisiae strain ATCC20598 harboring MAL21 could grow in maltose supplemented with a non- assimilable glucose analogue, 2-deoxyglucose, whereas strain ATCC96955 harboring MAL61 and strain CB11 with MAL31 and AGT1 could not. These observations implied a Mal21p-specific resistance against glucose-induced degradation. Mal21p found in ATCC20598 has 10 amino acids, including Gly-46 and His-50, that are inconsistent with the corresponding residues in Mal61p. The half-life of Mal21p for glucose-induced degradation was 118 min when expressed using the constitutive TPI1 promoter, which was significantly longer than that of Mal61p (25 min). Studies with mutant cells that are defective in endocytosis or the ubiquitination process indicated that Mal21p was less ubiquitinated than Mal61p, suggesting that Mal21p remains on the plasma membrane because of poor susceptibility to ubiquitination. Mutational studies revealed that both residues Gly-46 and His-50 in Mal21p are essential for the full resistance of maltose transporters against glucose-induced degradation.

Whi5 regulation by site specific CDK-phosphorylation in Saccharomyces cerevisiae

The Whi5 transcriptional repressor is a negative regulator of G1 cell cycle progression in Saccharomyces cerevisiae and is functionally equivalent to the Retinoblastoma (Rb) tumor suppressor protein in mammals. In early G1, Whi5 binds to and inhibits SBF (Swi4/Swi6) transcriptional complexes. At Start, Cln:Cdc28 kinases phosphorylate and inactivate Whi5, causing its dissociation from SBF promoters and nuclear export, allowing activation of SBF transcription and entry into late G1. In an analysis of Whi5 phosphorylation, we found that 10 of the 12 putative CDK phosphorylation sites on Whi5 were occupied in vivo in asynchronously growing cells. In addition, we identified 6 non-CDK Whi5 phosphorylation sites. Whi5 CDK and non-CDK phosphorylation mutants were functional and able to rescue the small cell size of whi5Δ cells. However, the Whi5 CDK mutant with all 12 putative CDK sites changed to alanine causes a dramatic cell cycle phenotype when expressed with a Swi6 CDK phosphorylation mutant. Mutational analysis of Whi5 determined that only four C-terminal CDK sites were necessary and sufficient for Whi5 inactivation when Swi6 CDK sites were also mutated. Although these four Whi5 CDK sites do not wholly determine Whi5 nuclear export, they do impact regulation of cell size. Taken together, these observations begin to dissect the regulatory role of specific phosphorylation sites on Whi5.

Synthesis of new analogs of benzotriazole, benzimidazole and phthalimide--potential inhibitors of human protein kinase CK2

New derivatives of 4,5,6,7-tetrabromo-1H-1,2,3-benzotriazole (TBBt), 4,5,6,7-tetrabromo-1H-benzimidazole (TBBi), and N-substituted tetrabromophthalimides were synthesized and their effect on the activity of human protein kinase CK2 was examined. The most active were derivatives with N-hydroxypropyl substituents (IC(50) in 0.32-0.54 microM range) whereas derivatives of phthalimide were almost ineffective.

The repertoire and dynamics of evolutionary adaptations to controlled nutrient-limited environments in yeast

The experimental evolution of laboratory populations of microbes provides an opportunity to observe the evolutionary dynamics of adaptation in real time. Until very recently, however, such studies have been limited by our inability to systematically find mutations in evolved organisms. We overcome this limitation by using a variety of DNA microarray-based techniques to characterize genetic changes—including point mutations, structural changes, and insertion variation—that resulted from the experimental adaptation of 24 haploid and diploid cultures of Saccharomyces cerevisiae to growth in either glucose, sulfate, or phosphate-limited chemostats for ∼200 generations. We identified frequent genomic amplifications and rearrangements as well as novel retrotransposition events associated with adaptation. Global nucleotide variation detection in ten clonal isolates identified 32 point mutations. On the basis of mutation frequencies, we infer that these mutations and the subsequent dynamics of adaptation are determined by the batch phase of growth prior to initiation of the continuous phase in the chemostat. We relate these genotypic changes to phenotypic outcomes, namely global patterns of gene expression, and to increases in fitness by 5–50%. We found that the spectrum of available mutations in glucose- or phosphate-limited environments combined with the batch phase population dynamics early in our experiments allowed several distinct genotypic and phenotypic evolutionary pathways in response to these nutrient limitations. By contrast, sulfate-limited populations were much more constrained in both genotypic and phenotypic outcomes. Thus, the reproducibility of evolution varies with specific selective pressures, reflecting the constraints inherent in the system-level organization of metabolic processes in the cell. We were able to relate some of the observed adaptive mutations (e.g., transporter gene amplifications) to known features of the relevant metabolic pathways, but many of the mutations pointed to genes not previously associated with the relevant physiology. Thus, in addition to answering basic mechanistic questions about evolutionary mechanisms, our work suggests that experimental evolution can also shed light on the function and regulation of individual metabolic pathways.

Adaptive evolution is a central biological process that underlies diverse phenomena from the acquisition of antibiotic resistance by microbes to the evolution of niche specialization. Two unresolved questions regarding adaptive evolution are what types of genomic variation are associated with adaptation and how repeatable is the process. We evolved yeast populations for more than 200 generations in nutrient-limited chemostats. We find that the phenotype of adapted individuals, as measured using global gene expression, is much less variable in clones adapted to sulfate limitation than either glucose or phosphate limitation. We comprehensively analyzed the genomes of adapted clones and found that those adapted to sulfate limitation almost invariably carry amplifications of the gene encoding a sulfur transporter, but the mutations in individuals adapted to glucose and phosphate limitation are much more diverse. This parallelism holds true at the level of single-nucleotide mutations. Although there may be other paths to adapt to sulfate limitation, one path confers a much greater advantage than all others so it dominates. By contrast, there are a number of ways to adapt to glucose and phosphate limitation that confer similar advantages. We conclude that the reproducibility of evolution depends on the specific selective pressure experienced by the organism.

Drosophila protein kinase CK2 is rendered temperature-sensitive by mutations of highly conserved residues flanking the activation segment

CK2 is a Ser/Thr protein kinase essential for animal development. Although null alleles for CK2 are available in the mouse and Drosophila models, they are lethal when homozygous, thus necessitating conditional alleles for analysis of its developmental roles. We describe the isolation of temperature-sensitive (ts) alleles of Drosophila CK2alpha (dCK2alpha). These alleles efficiently rescue lethality of yeast lacking endogenous CK2 at 29 degrees C, but this ability is lost at higher temperatures in an allele-specific manner. These ts-variants exhibit properties akin to the wild type protein, and interact robustly with dCK2beta. Modeling of these ts-variants using the crystal structure of human CK2alpha indicates that the affected residues are in close proximity to the active site. We find that substitution of Asp(212) elicits potent ts-behavior, an important finding because this residue contributes to stability of the activation segment and is invariant in other Ser/Thr protein kinases.

The regulatory beta-subunit of protein kinase CK2 regulates cell-cycle progression at the onset of mitosis

Cell-cycle transition from the G(2) phase into mitosis is regulated by the cyclin-dependent protein kinase 1 (CDK1) in complex with cyclin B. CDK1 activity is controlled by both inhibitory phosphorylation, catalysed by the Myt1 and Wee1 kinases, and activating dephosphorylation, mediated by the CDC25 dual-specificity phosphatase family members. In somatic cells, Wee1 is downregulated by phosphorylation and ubiquitin-mediated degradation to ensure rapid activation of CDK1 at the beginning of M phase. Here, we show that downregulation of the regulatory beta-subunit of protein kinase CK2 by RNA interference results in delayed cell-cycle progression at the onset of mitosis. Knockdown of CK2beta causes stabilization of Wee1 and increased phosphorylation of CDK1 at the inhibitory Tyr15. PLK1-Wee1 association is an essential event in the degradation of Wee1 in unperturbed cell cycle. We have found that CK2beta participates in PLK1-Wee1 complex formation whereas its cellular depletion leads to disruption of PLK1-Wee1 interaction and reduced Wee1 phosphorylation at Ser53 and 121. The data reported here reinforce the notion that CK2beta has functions that are independent of its role as the CK2 regulatory subunit, identifying it as a new component of signaling pathways that regulate cell-cycle progression at the entry of mitosis.

Negative regulation of the yeast ABC transporter Ycf1p by phosphorylation within its N-terminal extension

The yeast vacuolar membrane protein Ycf1p and its mammalian counterpart, MRP1, belong to the ABCC subfamily of ATP-binding cassette (ABC) transporters that rid cells of toxic endogenous and xenobiotic compounds. Like most members of the ABCC subfamily, Ycf1p contains an N-terminal extension in addition to its ABC "core" domain and transports substrates in the form of glutathione conjugates. Ycf1p is subject to complex regulation to ensure its optimal function. Previous studies showed that Ycf1p activity is stimulated by a guanine nucleotide exchange factor, Tus1p, and is positively regulated by phosphorylation in its ABC core domain at residues Ser-908 and Thr-911. Here we provide evidence that phosphorylation of Ser-251 in the Ycf1p N-terminal extension negatively regulates activity. Mutant Ycf1p-S251A exhibits increased resistance to cadmium in vivo and increased Ycf1p-dependent transport of [(3)H]estradiol-beta-17-glucuronide in vitro as compared with wild-type Ycf1p. Activity is restored to the wild-type level for Ycf1-S251E. To identify kinase(s) that negatively regulate Ycf1p function, we conducted an integrated membrane yeast two-hybrid (iMYTH) screen and identified two kinase genes, CKA1 and HAL5, deletion of which increases Ycf1p function. Genetic evidence suggests that Cka1p may regulate Ycf1p function through phosphorylation of Ser-251 either directly or indirectly. Overall, this study provides compelling evidence that negative, as well as positive, regulation of Ycf1p is mediated by phosphorylation.

Protein kinase CK2 catalyzes tyrosine phosphorylation in mammalian cells

Protein kinase CK2 exhibits oncogenic activity in mice and is over-expressed in a number of tumors or leukemic cells. On the basis of its amino acid sequence and a wealth of experimental information, CK2 has traditionally been classified as a protein serine/threonine kinase. In contrast to this traditional view of CK2, recent evidence has shown that CK2 can also phosphorylate tyrosine residues under some circumstances in vitro and in yeast. In this study, we provide definitive evidence demonstrating that CK2 also exhibits tyrosine kinase activity in mammalian cells. Tyrosine phosphorylation of CK2 in cells and in CK2 immunoprecipitates is dependent on CK2 activity and is inhibited by the CK2 selective inhibitor 4,5,6,7-tetrabromobenzotriazole. Examination of phosphotyrosine profiles in cells reveals a number of proteins, including CK2 itself, which exhibit increased tyrosine phosphorylation when CK2 levels are increased. Peptide arrays to evaluate the specificity determinants for tyrosine phosphorylation by CK2 reveal that its specificity for tyrosine phosphorylation is distinct from its specificity for serine/threonine phosphorylation. Of particular note is the requirement for an aspartic acid immediately C-terminal to the phosphorylatable tyrosine residue. Collectively, these data provide conclusive evidence that CK2 catalyzes the phosphorylation of tyrosine residues in mammalian cells, a finding that adds a new level of complexity to the challenge of elucidating its cellular functions. Furthermore, these results raise the possibility that increased CK2 levels that frequently accompany transformation may contribute to the increased tyrosine phosphorylation that occurs in transformed cells.

The emerging CK2 interactome: insights into the regulation and functions of CK2

Protein kinase CK2 represents a small family of protein serine/threonine kinases implicated in a variety of biological processes including events relating to cell proliferation and survival. Notably, CK2 displays oncogenic activity in mice and exhibits altered expression in several types of cancer. Accordingly, a detailed understanding of the cellular functions of CK2 and elucidation of the mechanisms by which CK2 is regulated in cells is expected to contribute to understanding its role in tumorigenesis with the prospect of novel approaches to therapy. While CK2 has traditionally been viewed as a tetrameric complex composed of two catalytic and two regulatory subunits, mounting evidence suggests that its subunits may have functions independent of tetrameric CK2 complexes. In mammals, as is the case in the budding yeast Saccharomyces cerevisiae, there are two isozymic forms of CK2, adding additional heterogeneity to the CK2 family. Studies in yeast and in human cells demonstrate that the different forms of CK2 interact with a large number of cellular proteins. To reveal new insights regarding the regulation and functions of different forms of CK2, we have examined the emerging interactomes for each of the CK2 subunits. Analysis of these interactomes for both yeast and human CK2 reinforces the view that this family of enzymes participates in a broad spectrum of cellular events. Furthermore, while there is considerable overlap between the interactomes of the individual CK2 subunits, notable differences in each of the individual interactomes provides additional evidence for functional specialization for the individual forms of CK2.