Molecular cloning of casein kinase II alpha subunit from Dictyostelium discoideum and its expression in the life cycle

A Simple Retroelement Based Knock-Down System in Dictyostelium: Further Insights into RNA Interference Mechanisms

CHARACTERISTICS OF DIRS-1 MEDIATED KNOCK-DOWNS

We have previously shown that the most abundant Dictyostelium discoideum retroelement DIRS-1 is suppressed by RNAi mechanisms. Here we provide evidence that both inverted terminal repeats have strong promoter activity and that bidirectional expression apparently generates a substrate for Dicer. A cassette containing the inverted terminal repeats and a fragment of a gene of interest was sufficient to activate the RNAi response, resulting in the generation of ~21 nt siRNAs, a reduction of mRNA and protein expression of the respective endogene. Surprisingly, no transitivity was observed on the endogene. This was in contrast to previous observations, where endogenous siRNAs caused spreading on an artificial transgene. Knock-down was successful on seven target genes that we examined. In three cases a phenotypic analysis proved the efficiency of the approach. One of the target genes was apparently essential because no knock-out could be obtained; the RNAi mediated knock-down, however, resulted in a very slow growing culture indicating a still viable reduction of gene expression. ADVANTAGES OF THE DIRS-1–RNAI SYSTEM: The knock-down system required a short DNA fragment (~400 bp) of the target gene as an initial trigger. Further siRNAs were generated by RdRPs since we have shown some siRNAs with a 5'-triphosphate group. Extrachromosomal vectors facilitate the procedure and allowed for molecular and phenotypic analysis within one week. The system provides an efficient and rapid method to reduce protein levels including those of essential genes.

KSR1 is overexpressed in endometrial carcinoma and regulates proliferation and TRAIL-induced apoptosis by modulating FLIP levels

The Raf/MEK/extracellular signal-regulated kinase (ERK) pathway participates in many processes altered in development and progression of cancer in human beings such as proliferation, transformation, differentiation, and apoptosis. Kinase suppressor of Ras 1 (KSR1) can interact with various kinases of the Raf/MEK/extracellular signal-regulated kinase pathway to enhance its activation. The role of KSR1 in endometrial carcinogenesis was investigated. cDNA and tissue microarrays demonstrated that expression of KSR1 was up-regulated in endometrial carcinoma. Furthermore, inhibition of KSR1 expression by specific small hairpin RNA resulted in reduction of both proliferation and anchorage-independent cell growth properties of endometrial cancer cells. Because inhibition of apoptosis has a pivotal role in endometrial carcinogenesis, the effects of KSR1 in regulation of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis were investigated. KSR1 knock-down sensitized resistant endometrial cell lines to both TRAIL- and Fas-induced apoptosis. Sensitization to TRAIL and agonistic anti-Fas antibody was caused by down-regulation of FLIP (FLICE-inhibitory protein). Also investigated was the molecular mechanism by which KSR1 regulates FLIP protein levels. It was demonstrated that KSR1 small hairpin RNA did not affect FLIP transcription or degradation. Rather, FLIP down-regulation was caused by Fas-associated death domain protein-dependent inhibition of FLIP translation triggered after TRAIL stimulation in KSR1-silenced cells. Re-expression of heterologous KSR1 in cells with down-regulated endogenous KSR1 restored FLIP protein levels and TRAIL resistance. In conclusion, KSR1 regulates endometrial sensitivity to TRAIL by regulating FLIP levels.

Aloe-emodin, an anthraquinone, in vitro inhibits proliferation and induces apoptosis in human colon carcinoma cells

The present study aimed to investigate the anticancer effect of aloe-emodin, an anthraquinone compound present in the leaves of Aloe vera, on two human colon carcinoma cell lines, DLD-1 and WiDr. Colon carcinoma cells were treated with various concentrations of aloe-emodin for different durations. Cell viability was measured by sodium 3'-[1-(phenylamino-carbonyl)-3,4-tetrazolium]-bis(4-methoxy-6-nitro) benzene-sulfonic acid hydrate assay. DNA fragmentation was analyzed by agarose gel electrophoresis. Nuclear shrinkage was visualized by Hoechst 33258 staining. Western blotting was used to indicate the release of apoptosis-inducing factor and cytochrome c from mitochondria and the phosphorylation of Bid. Caspase-3 and casein kinase II activities were measured by the respective assays. Cell viability analyses showed that aloe-emodin induced cell death in a dose- and time-dependent manner. Notably, the WiDr cells were more sensitive to aloe-emodin than the DLD-1 cells. Aloe-emodin caused the release of apoptosis-inducing factor and cytochrome c from mitochondria, followed by activation of caspase-3 leading to DNA fragmentation, nuclear shrinkage and apoptosis. In addition, exposure of colon carcinoma cells to aloe-emodin suppressed the casein kinase II activity in a time-dependent manner and was accompanied by a reduced phosphorylation of Bid, a downstream substrate of casein kinase II and a pro-apoptotic molecule. These findings showed that the inhibition of casein kinase II activity, the release of apoptosis-inducing factor and cytochrome c, and the caspase-3 activation are involved in aloe-emodin-mediated apoptosis in colon carcinoma cells.

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.

Evidence for regulation of mitotic progression through temporal phosphorylation and dephosphorylation of CK2alpha

Proper mitotic progression is crucial for maintenance of genomic integrity in proliferating cells and is regulated through an intricate series of events, including protein phosphorylation governed by a complex network of protein kinases. One kinase family implicated in the regulation of mitotic progression is protein kinase CK2, a small family of enzymes that is overexpressed in cancer and induces transformation in mice and cultured fibroblasts. CK2alpha, one isoform of the catalytic subunits of CK2, is maximally phosphorylated at four sites in nocodazole-treated cells. To investigate the effects of CK2alpha phosphorylation on mitotic progression, we generated phosphospecific antibodies against its mitotic phosphorylation sites. In U2OS cells released from S-phase arrest, these antibodies reveal that CK2alpha is most highly phosphorylated in prophase and metaphase. Phosphorylation gradually decreases during anaphase and becomes undetectable during telophase and cytokinesis. Stable expression of phosphomimetic CK2alpha (CK2alpha-4D, CK2alpha-4E) results in aberrant centrosome amplification and chromosomal segregation defects and loss of mitotic cells through mitotic catastrophe. Conversely, cells expressing nonphosphorylatable CK2alpha (CK2alpha-4A) show a decreased ability to arrest in mitosis following nocodazole treatment, suggesting involvement in the spindle assembly checkpoint. Collectively, these studies indicate that reversible phosphorylation of CK2alpha requires precise regulation to allow proper mitotic progression.

Functional analysis of protein kinase CK2 of the human malaria parasite Plasmodium falciparum

Protein kinase CK2 (casein kinase 2) is a eukaryotic serine/threonine protein kinase with multiple substrates and roles in diverse cellular processes, including differentiation, proliferation, and translation. The mammalian holoenzyme consists of two catalytic alpha or alpha' subunits and two regulatory beta subunits. We report the identification and characterization of a Plasmodium falciparum CK2alpha orthologue, PfCK2alpha, and two PfCK2beta orthologues, PfCK2beta1 and PfCK2beta2. Recombinant PfCK2alpha possesses protein kinase activity, exhibits similar substrate and cosubstrate preferences to those of CK2alpha subunits from other organisms, and interacts with both of the PfCK2beta subunits in vitro. Gene disruption experiments show that the presence of PfCK2alpha is crucial to asexual blood stage parasites and thereby validate the enzyme as a possible drug target. PfCK2alpha is amenable to inhibitor screening, and we report differential susceptibility between the human and P. falciparum CK2alpha enzymes to a small molecule inhibitor. Taken together, our data identify PfCK2alpha as a potential target for antimalarial chemotherapeutic intervention.

Inhibition of casein kinase 2 enhances the death ligand- and natural kiler cell-induced hepatocellular carcinoma cell death

Recent studies have shown that the inhibition of casein kinase 2 (CK2) sensitizes many cancer cells to Fas ligand- and tumour necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis. However, it has not been demonstrated directly whether CK2 inhibition can also enhance the cytotoxicity of natural killer (NK) cells, which actually use the death ligands to kill cancer cells in vivo. To address whether NK cell-mediated cancer cell death is affected by the inhibition of CK2, we first checked whether the death ligand-induced apoptosis of hepatocellular carcinoma cells (HCCs) and HeLa were affected by CK2 inhibition. We then investigated the effect of CK2 inhibition on NK cytotoxicity against HCCs and HeLa cells and its mechanistic features. Inhibition of CK2 by emodin increased the apoptotic cell death of HepG2, Hep3B and HeLa when the cancer cell lines were treated with a soluble form of recombinant TRAIL or an agonistic antibody of Fas. This phenomenon appeared to be correlated with the expression level of death receptors on the cancer cell surface. More interestingly, the inhibition of CK2 also greatly increased the NK cell-mediated cancer cell killing. The NK cytotoxicity against the cancer cells increased about twofold when the target cells were pretreated with a specific CK2 inhibitor, emodin or 4,5,6,7-tetrabromobenzotriazole. Furthermore, the increase of the NK cytotoxicity against cancer cells by CK2 inhibition was granule-independent and mediated possibly by the death ligands on the NK cell surface. This suggests that CK2 inhibitors could be used to enhance the cytotoxicity of NK cells and consequently increase host tumour immunity.

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

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

The multiple personalities of the regulatory subunit of protein kinase CK2: CK2 dependent and CK2 independent roles reveal a secret identity for CK2beta

Protein kinase CK2 (formerly casein kinase II), an enzyme that participates in a wide variety of cellular processes, has traditionally been classified as a stable tetrameric complex consisting of two catalytic CK2α or CK2α' subunits and two regulatory CK2β subunits. While consideration of CK2 as a tetrameric complex remains relevant, significant evidence has emerged to challenge the view that its individual subunits exist exclusively within these complexes. This review will summarize biochemical and genetic evidence indicating that the regulatory CK2β subunit exists and performs functions independently of CK2 tetramers. For example, unbalanced expression of catalytic and regulatory CK2 subunits has been observed in a variety of tissues and tumors. Furthermore, localization studies including live cell imaging have demonstrated that while the catalytic and regulatory subunits of CK2 exhibit extensive co-localization, independent mobility of the individual CK2 subunits can also be observed within cells. Identification of proteins that interact with CK2β in the absence of catalytic CK2 subunits reinforces the notion that CK2β has functions distinct from CK2 and begins to offer insights into these CK2-independent functions. In this respect, the discovery that CK2β can interact with and modulate the activity of a number of other serine/threonine protein kinases including A-Raf, c-Mos and Chk1 is particularly striking. This review will discuss the interactions between CK2β and these protein kinases with special emphasis on the properties of CK2β that mediate these interactions and on the implications of these interactions in yielding new prospects for elucidation of the cellular functions of CK2β.

Dictyostelium discoideum protein phosphatase-1 catalytic subunit exhibits distinct biochemical properties

Protein phosphatase-1 (PP1) is expressed ubiquitously and is involved in many eukaryotic cellular functions, although PP1 enzyme activity could not be detected in the social amoeba Dictyostelium discoideum cell extracts. In the present paper, we show that D. discoideum has a single copy gene that codes for the catalytic subunit of PP1 (DdPP1c). DdPP1c is expressed throughout the D. discoideum life cycle with constant levels of mRNA, and its protein and amino acid sequence show a mean identity of 80% with other PP1c enzymes. However, it has a distinctive difference: the substitution of a phenylalanine residue (Phe(269) in the DdPP1c) for a highly conserved cysteine residue (Cys(273) in rabbit PP1c) in a region that was shown to have a critical role in the interaction of rabbit PP1c with toxin inhibitors. Wild-type DdPP1c and an engineered mutant form in which Phe(269) was replaced by a cysteine residue were expressed in Escherichia coli. Both recombinant activities were similarly inhibited by okadaic acid, tautomycin and microcystin. However, the Phe(269)-->Cys mutation resulted in a large increase in enzyme activity towards phosphorylase a and a higher sensitivity to calyculin A. These results, together with the molecular modelling of DdPP1c structure, indicate that the Phe(269) residue, which occurs naturally in D. discoideum, confers distinct biochemical properties on this enzyme.

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

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

Casein kinase II phosphorylates the fragile X mental retardation protein and modulates its biological properties

Fragile X syndrome is caused by loss of FMR1 protein expression. FMR1 binds RNA and associates with polysomes in the cytoplasm; thus, it has been proposed to function as a regulator of gene expression at the posttranscriptional level. Posttranslational modification of FMR1 had previously been suggested to regulate its activity, but no experimental support for this model has been reported to date. Here we report that FMR1 in Drosophila melanogaster (dFMR1) is phosphorylated in vivo and that the homomer formation and the RNA-binding activities of dFMR1 are modulated by phosphorylation in vitro. Identification of a protein phosphorylating dFMR1 showed it to be Drosophila casein kinase II (dCKII). dCKII directly interacts with and phosphorylates dFMR1 in vitro. The phosphorylation site in dFMR1 was identified as Ser406, which is highly conserved among FMR1 family members from several species. Using mass spectrometry, we established that Ser406 of dFMR1 is indeed phosphorylated in vivo. Furthermore, human FMR1 (hFMR1) is also phosphorylated in vivo, and alteration of the conserved Ser500 in hFMR1 abolishes phosphorylation by CKII in vitro. These studies support the model that the biological functions of FMR1, such as regulation of gene expression, are likely regulated by its phosphorylation.

Phosphorylation regulates the stability of the regulatory CK2beta subunit

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

Assembly of protein kinase CK2: investigation of complex formation between catalytic and regulatory subunits using a zinc-finger-deficient mutant of CK2beta

Protein kinase CK2 is a tetrameric enzyme comprised of two regulatory subunits (CK2beta) and two catalytic subunits (CK2alpha and/or CK2alpha'). The crystal structure of dimeric CK2beta demonstrated that a zinc finger mediates CK2beta dimerization, therefore we constructed a mutant in which cysteine residues 109 and 114 were mutated to serine. Our objectives were to examine the effects of disrupting the zinc finger of the regulatory CK2beta subunit on CK2 tetramer assembly. Examination of this zinc-finger-deficient mutant of CK2beta using a yeast two-hybrid assay demonstrates that the mutant fails to form CK2beta homodimers. In order to extend these studies, we co-transfected COS-7 cells with epitope-tagged constructs and performed co-immunoprecipitation assays. The results from these studies demonstrate that the mutant fails to form CK2beta homodimers and fails to interact with catalytic CK2 subunits. Furthermore, we demonstrate that the mutant CK2beta is not appreciably phosphorylated in cells. Using in vitro binding assays, we demonstrated that the mutant CK2beta protein fails to interact with glutathione S-transferase-CK2alpha'. Finally, we demonstrate that the mutant is translated at an equivalent rate to wild-type CK2beta, but is degraded much more rapidly. Overall, our results are consistent with the model that beta-beta dimerization precedes incorporation of catalytic subunits into tetrameric CK2 complexes, and that beta-beta dimerization is a prerequisite for the stable incorporation of catalytic subunits into CK2 complexes.

The SMRT corepressor is a target of phosphorylation by protein kinase CK2 (casein kinase II)

The Silencing-Mediator for Retinoid/Thyroid hormone receptors (SMRT) interacts with, and mediates transcriptional repression by, a variety of eukaryotic transcription factors, including the nuclear hormone receptors. The ability of SMRT to function as a transcriptional 'corepressor' is regulated by a variety of signal transduction pathways. We report here that SMRT is a phosphoprotein in vivo, and is also phosphorylated in vitro by unfractionated cell extracts. A major site of phosphorylation of SMRT is a protein kinase CK2 motif centered on serine 1492, and located within a C-terminal SMRT domain that mediates interaction of the corepressor with the nuclear hormone receptors. Phosphorylation of SMRT by CK2 stabilizes the ability of the SMRT protein to interact with nuclear hormone receptors. Our results indicate that SMRT is a member of an expanding family of transcriptional regulators that are modified, and potentially regulated, in response to protein kinase CK2.

Isolation and characterization of casein kinase I from Dictyostelium discoideum

In the present study, the molecular cloning and characterization of a 49-kDa form of casein kinase (CK)I from Dictyostelium discoideum is reported. The predicted amino acid sequence shares 70% identity with the catalytic domain of the mammalian delta and epsilon isoforms, Drosophila CKIepsilon and Schizosaccharomyces pombe Hhp1, and 63% identity with Hrr25, a 57-kDa form of yeast CK involved in DNA repair. D. discoideum CKI (DdCKI) was expressed in vegetative asynchronous cells as well as in differentiated cells, as detected by Northern-blot analysis. The level of DdCKI expression did not change during the cell cycle. Antibodies raised against a truncated version of the protein recognized a 49-kDa protein from D. discoideum extracts. Protein expression paralleled the pattern found for the RNA. The expression of DdCKI in Escherichia coli resulted in an active enzyme that autophosphorylated and phosphorylated casein. Immunofluorescence assays showed that DdCKI was localized in the cytoplasm and nuclei of Dictyostelium cells. The lack of disruptants of the CKI gene suggests that this protein is essential for the vegetative growth of D. discoideum. Overexpression of DdCKI resulted in cells with increased resistance to hydroxyurea, suggesting a potential role for this kinase in DNA repair.

Casein kinase II interacts with the bZIP domains of several transcription factors

Casein kinase II (CKII) is thought to regulate a broad range of transcription factors, but its mode of action is not well characterized. We previously showed that CKII is co-purified with the ATF family of transcription factors using DNA-affinity latex beads. Here we report a functional and physical interaction between CKII and transcription factors. We demonstrate that CKII binds through its catalytic alpha and alpha' subunits to the basic leucine zipper (bZIP) DNA-binding domains of many transcription factors, including ATF1. Kinetic analysis using a surface plasmon resonance sensor suggests that CKII loosely associates with ATF1 in vivo . Deletion of the bZIP domain of ATF1 markedly reduces its phosphorylation by CKII, suggesting that the bZIP recruits CKII to the vicinity of the target site. ATF1-CKII complex is also formed on DNA. Using CKIIalpha fusedto a heterologous DNA-binding domain, we also demonstrate that CKII, when bound to DNA, efficiently phosphorylates its substrate, which is bound to the same DNA molecule. Taken together, CKII may regulate transcription (and possibly other events) by phosphorylating proteins on DNA.

Proteolysis activated protein kinase in Dictyostelium discoideum

In the search for MBP phosphorylating activities in Dictyostelium discoideum, we have found a proteolysis-activated protein kinase. This activity which is distributed between the soluble and the particulate fractions of the cell, uses MBP and histone as substrate and has a molecular mass of 140 kDa as detected in an 'in situ' assay. This protein kinase has several features shared by the protein kinase C family, such as substrate specificity and sensitivity to proteolysis, but its molecular mass is much larger than that described for the known protein kinase C isoforms. To better characterize this activity we have studied its sensitivity to several protein kinase C inhibitors and activators. This protein kinase is activated neither by phorbol ester nor by phosphatidylserine or Ca2+. The activity is inhibited by staurosporine and PKC zeta pseudosubstrate, but is not affected by the specific protein kinase C inhibitor bisindolylmaleimide. These data lead us to propose that proteolytically activated Dictyostelium protein kinase belongs to the recently described protein kinase C-related family.

Copurification of casein kinase II with transcription factor ATF/E4TF3

We have developed a simple method to purify sequence-specific DNA-binding proteins directly from crude cell extracts by using DNA affinity latex beads. The method enabled us to purify not only DNA-binding proteins, but also their associated proteins. Using beads bearing the ATF/E4TF3 site from the adenovirus E4 gene promoter, a protein kinase activity was copurified with the ATF/E4TF3 family. We found that the kinase interacted with ATF1 in vitro efficiently. The kinase did not bind directly to DNA. The kinase mainly phosphorylated ATF1 on serine 36, which was one of target amino acids for casein kinase (CK) II. Biological features of the kinase were the same as those of CKII and an anti-CKII serum reacted with the kinase, indicating that the kinase was CKII. Moreover, it was clearly shown that one of CKII subunits, the CKII alpha protein bound to glutathione-S-transferase (GST) fusion ATF1 but not GST in vitro. It has been reported that a specific CKII inhibitor, 5,6-dichloro-1-beta-D-ribo-furanosylbenzimidazole (DRB) inhibits transcription by RNA polymerase II [Zandomeni et al., (1986) J. Biol. Chem. 261, 3414-3419]. Taken together, these results suggest that ATF/E4TF3 may recruit the CKII activity to a transcription initiation machinery and stimulate transcription.

The Schizosaccharomyces pombe casein kinase II alpha and beta subunits: evolutionary conservation and positive role of the beta subunit

Casein kinase II is a key regulatory enzyme involved in many cellular processes, including the control of growth and cell division. We report the molecular cloning and sequencing of cDNAs encoding the alpha and the beta subunits of casein kinase II of Schizosaccharomyces pombe. The deduced amino acid sequence of Cka1, the alpha catalytic subunit, shows high sequence similarity to alpha subunits identified in other species. The amino acid sequence of Ckb1, the S. pombe beta subunit, is 57% identical to that of the human beta subunit. Cka1 overexpression results in no detectable phenotype. In contrast, Ckb1 overexpression inhibits cell growth and cytokinesis, with formation of multiseptated cells. Disruption of the ckb1+ gene causes a cold-sensitive phenotype and abnormalities in cell shape. In these cells, the casein kinase II activity is reduced to undetectable levels, demonstrating that Ckb1 is required for enzyme activity in vivo. In agreement with this, the activity measured in a strain expressing high levels of Cka1 is enhanced only when the Ckb1 protein is coexpressed. Altogether, our data suggest that Ckb1 is a positive regulator of the enzyme activity, and that it plays a role in mediating the interaction of casein kinase II with downstream targets and/or with additional regulators.

The Schizosaccharomyces pombe casein kinase II alpha and beta subunits: evolutionary conservation and positive role of the beta subunit

Casein kinase II is a key regulatory enzyme involved in many cellular processes, including the control of growth and cell division. We report the molecular cloning and sequencing of cDNAs encoding the alpha and the beta subunits of casein kinase II of Schizosaccharomyces pombe. The deduced amino acid sequence of Cka1, the alpha catalytic subunit, shows high sequence similarity to alpha subunits identified in other species. The amino acid sequence of Ckb1, the S. pombe beta subunit, is 57% identical to that of the human beta subunit. Cka1 overexpression results in no detectable phenotype. In contrast, Ckb1 overexpression inhibits cell growth and cytokinesis, with formation of multiseptated cells. Disruption of the ckb1+ gene causes a cold-sensitive phenotype and abnormalities in cell shape. In these cells, the casein kinase II activity is reduced to undetectable levels, demonstrating that Ckb1 is required for enzyme activity in vivo. In agreement with this, the activity measured in a strain expressing high levels of Cka1 is enhanced only when the Ckb1 protein is coexpressed. Altogether, our data suggest that Ckb1 is a positive regulator of the enzyme activity, and that it plays a role in mediating the interaction of casein kinase II with downstream targets and/or with additional regulators.