Inducible expression of protein kinase CK2 in mammalian cells. Evidence for functional specialization of CK2 isoforms

C-terminal region of protein kinase CK2 alpha: How the structure can affect function and stability of the catalytic subunit

A novel mutant of the catalytic alpha subunit of human protein kinase CK2 (CK2 alpha) was designed in an attempt to clarify the role of the carboxylic-terminal segment characteristic of vertebrates, excluding fish. Starting from the sequence alignments, we constructed a phylogenetic tree of the primary structure of CK2 alpha. On this basis, we substituted two distal prolines with alanines (PA 382-384). Theoretical calculations and spectropolarimetry measurements, performed both on native and mutant subunits, indicate an increased content of alpha-helix after this double amino acidic substitution. In order to clarify the structure/function relationship of the C-terminal region, we verified if the structural change affects the catalytic activity of CK2 alpha. The mutant exhibits slightly increased phosphorylation efficiency, but reduced ability to transfer phosphate in comparison with the native subunit. At last, we compared the thermal stability of the mutant with respect to the native subunit and we tested the proteolytic degradability. The observation that the PA 382-384 mutant exhibits an increased thermal and proteolytic stability suggests that this mutant could be employed to solve the three-dimensional (3D) structure of human CK2 alpha and to overcome difficulties in crystallizing the native form.

Protein kinase CK2 is required for dorsal axis formation in Xenopus embryos

Dorsal axis formation in Xenopus embryos is dependent upon asymmetrical localization of beta-catenin, a transducer of the canonical Wnt signaling pathway. Recent biochemical experiments have implicated protein kinase CK2 as a regulator of members of the Wnt pathway including beta-catenin. Here, we have examined the role of CK2 in dorsal axis formation. CK2 was present in the developing embryo at an appropriate time and place to participate in dorsal axis formation. Overexpression of mRNA encoding CK2 in ventral blastomeres was sufficient to induce a complete ectopic axis, mimicking Wnt signaling. A kinase-inactive mutant of CK2alpha was able to block ectopic axis formation induced by XWnt8 and beta-catenin and was capable of suppressing endogenous axis formation when overexpressed dorsally. Taken together, these studies demonstrate that CK2 is a bona fide member of the Wnt pathway and has a critical role in the establishment of the dorsal embryonic axis.

The Pleckstrin Homology Domain of CK2 Interacting Protein-1 Is Required for Interactions and Recruitment of Protein Kinase CK2 to the Plasma Membrane

CKIP-1 is a recently identified interaction partner of protein kinase CK2 with a number of protein-protein interaction motifs, including an N-terminal pleckstrin homology domain. To test the hypothesis that CKIP-1 has a role in targeting CK2 to specific locations, we examined the effects of CKIP-1 on the localization of CK2. These studies demonstrated that CKIP-1 can recruit CK2 to the plasma membrane. Furthermore, the pleckstrin homology domain of CKIP-1 was found to be required for interactions with CK2 and for the recruitment of CK2 to the plasma membrane. In this regard, point mutations in this domain abolish membrane localization and compromise interactions with CK2. In addition, replacement of the pleckstrin homology domain with a myristoylation signal was insufficient to elicit any interaction with CK2. An investigation of the lipid binding of CKIP-1 reveals that it has broad specificity. A comparison with other pleckstrin homology domains revealed that the pleckstrin homology domain of CKIP-1 is distinct from other defined classes of pleckstrin homology domains. Finally, examination of CK2alpha for a region that mediates interactions with CKIP-1 revealed a putative HIKE domain, a complex motif found exclusively in proteins that bind pleckstrin homology domains. However, mutations within this motif were not able to abolish CKIP-1-CK2 interactions suggesting that this motif by itself may not be sufficient to mediate interactions. Overall, these results provide novel insights into how CK2, a predominantly nuclear enzyme, is targeted to the plasma membrane, and perhaps more importantly how it may be regulated.

Novel macrolide-adjustable bidirectional expression modules for coordinated expression of two different transgenes in mice

BACKGROUND

Precise control of transgene expression is essential for a variety of applications ranging from gene-function analysis, biopharmaceutical manufacturing to next-generation molecular interventions in gene therapy and tissue engineering. The regulation of gene expression is currently a key issue for clinical implementation of gene-therapy-based treatments since desired transgene expression may need to be maintained within a narrow therapeutic window for successful treatment of a particular human disease.

METHODS

We have designed a novel bidirectional expression module that enables adjustable coregulation of two different transgenes in response to clinical doses of macrolide antibiotics. A bidirectional macrolide-responsive promoter consisting of a central operator module (ETR) specific for the macrolide-dependent transactivator (ET1) is flanked by two minimal promoters (P(hCMVmin); P(hsp70min)) which drive expression of two divergently oriented transgenes. Macrolide antibiotics modulate the binding affinity of ET1 to ETR and adjust expression of both transgenes to desired levels.

RESULTS

Bidirectional expression configurations enabled excellent macrolide-adjustable coregulation profiles of two secreted reporter genes or one-vector-based autoregulated fine-tuning of a single transgene in various transgenic rodent and human cell lines. Following implantation of microencapsulated CHO-K1 cell derivatives transgenic for macrolide-controlled bidirectional expression of erythropoietin (EPO) and the human secreted alkaline phosphatase (SEAP) intraperitoneally into mice, serum EPO and SEAP levels could be coadjusted to desired levels by administration of different erythromycin doses.

CONCLUSIONS

Based on their in vivo compatibility, the versatile bidirectional and macrolide-responsive expression modules represent an important advancement on the way to implementing targeted and conditional molecular interventions into a clinical reality.

Proapoptotic function of protein kinase CK2alpha" is mediated by a JNK signaling cascade

Protein kinase CK2 (formerly casein kinase II) is a tetrameric enzyme constitutively expressed in all eurakyotic tissues that plays a significant role in the regulation of cell proliferation, malignant transformation, and apoptosis. The catalytic alpha-subunit of the enzyme is known to exist in three isoforms CK2alpha, CK2alpha' and CK2alpha". CK2alpha" is highly expressed in liver compared with other tissues and is required for the normal trafficking of several hepatocellular membrane proteins. Initial studies of dengue virus infection indicated that the CK2alpha"-deficient membrane trafficking mutant cell line (Trf1) was resistant to virus-induced cell death compared with the parental human hepatoma (HuH)-7 hepatoma line. Expression of recombinant CK2alpha" in Trf1 was capable of reverting this resistant phenotype. This study was extended to TNF-alpha in addition to other stimuli of cell death in an attempt to uncover common death pathways that might be modulated by CK2alpha". Evaluation of different pathways involved in death signaling suggest that the regulation of a critical proapoptotic step in HuH-7 cells by CK2alpha" is mediated by a JNK signaling cascade.

Spermatogenesis of mice lacking CK2alpha': failure of germ cell survival and characteristic modifications of the spermatid nucleus

Csnk2a2 encodes the CK2alpha'catalytic subunit of CK2 that is predominantly expressed in testis. Male mice in which Csnk2a2 has been disrupted were infertile and displayed oligozoospermia with an abnormal shape of the spermatid nucleus. In this study, Csnk2a2 null testes revealed extensive germ cell degenerative processes at all stages of spermatogenesis, including the first spermatogenesis wave. Nuclear envelope (NE) protrusions with loss of nuclear pores, swelling of the outer membrane, and disruption of the inner membrane were observed in cells ranging from spermatogonia to early spermatids. Most early round spermatids were depleted, and DNA-specific fluorescent dyes showed a large chromatin-free nuclear domain near the chromocenter. Spermatids that were not eliminated retained NE defects that could explain the acrosomal and nuclear abnormalities of Csnk2a2 null spermatozoa. Data suggest that CK2alpha' deficiency could impair the phosphorylation of nuclear proteins of male germ cells leading to a particular cell-death pathway characterized by NE protrusions and an unusual pattern of chromatin modifications in spermatids.

[Protein kinase CK2: an enzyme that likes to be different]

Protein kinase CK2 (formerly known as casein kinase 2) was among the first protein kinases to be identified and characterized. Surprisingly, in spite of intense efforts, the regulation and cellular functions of CK2 remain obscure. However, recent data on its molecular structure, its signal-mediated intracellular dynamic localization and its unexpected function in cell survival have raised new interest in this enzyme. These studies reveal unique features of CK2 and highlight its importance in the transduction of survival signals.

Genome-wide expression screens indicate a global role for protein kinase CK2 in chromatin remodeling

Protein kinase CK2, a vital, pleiotropic and highly conserved serine/threonine phosphotransferase is involved in transcription-directed signaling, gene control and cell cycle regulation and is suspected to play a role in global processes. Searching for these global roles, we analyzed the involvement of CK2 in gene expression at cell cycle entry by using genome-wide screens. Comparing expression profiles of Saccharomyces cerevisiae wild-type strains with strains with regulatory or catalytic subunits of CK2 deleted, we found significant alterations in the expression of genes at all cell cycle phases and often in a subunit- and isoform-specific manner. Roughly a quarter of the genes known to be regulated by the cell cycle are affected. Functionally, the genes are involved with cell cycle entry, progression and exit, including spindle pole body formation and dynamics. Strikingly, most CK2-affected genes exhibit no common transcriptional control features, and a considerable proportion of temporarily altered genes encodes proteins involved in chromatin remodeling and modification, including chromatin assembly, (anti-)silencing and histone (de-)acetylation. In addition, various metabolic pathway and nutritional supply genes are affected. Our data are compatible with the idea that CK2 acts at different levels of cellular organization and that CK2 has a global role in transcription-related chromatin remodeling.

Cell cycle dependent regulation of protein kinase CK2 signaling to the nuclear matrix

Protein kinase CK2 is a ubiquitous protein serine/threonine kinase that is involved in cell growth and proliferation as well as suppression of apoptosis. Several studies have suggested that the kinase plays a role in cell cycle progression; however, changes in enzyme activity during phases of cell cycle have not been detected. Nuclear matrix is a key locus for CK2 signaling in the nucleus. We therefore examined CK2 signaling to the nuclear matrix in distinct phases of cell cycle by employing synchronized ALVA-41 prostate cancer cells. Removal of serum from the culture medium resulted in G0/G1 arrest, and a reduction in the nuclear matrix-associated CK2 activity which was rapidly reversed on addition of serum. Arresting the cells in G(0)/G(1) phase with hydroxyurea and subsequent release to S phase by serum gave similar results. Cells arrested in the G(2)/M phase by treatment with nocodazole demonstrated an extensive reduction in the nuclear matrix-associated CK2 which was reversed rapidly on addition of serum. Changes in the immunoreactive CK2 protein were concordant with the activity data reflecting a dynamic trafficking of the kinase in distinct phases of cell cycle. Under the same conditions, CK2 activity in total cellular lysate remained essentially unaltered. These results provide the first direct evidence of discrete modulations of CK2 in the nuclear matrix during the cell cycle progression. Inducible overexpression of CK2 in CHO cells yielded only a modest increase in CK2 activity even though a significant increase in expression was apparent at the level of CK2 alpha-specific message. Stably transfected ALVA-41 cells, however, did not show a significant change in CK2 levels despite increased expression at the message level. Not surprisingly, both types of the stably transfected cells failed to show any alteration in cell cycle progression. Distribution of the CK2 activity in the cytosolic versus nuclear matrix fractions in normal cells appears to be different from that in the cancer cells such that the ratio of nuclear matrix to cytosolic activity is much higher in the latter. Considering that nuclear matrix is central to several nuclear functions, this pattern of intracellular distribution of CK2 may have implications for its role in the oncogenic process. Published 2003 Wiley-Liss, Inc.

Perturbation of protein kinase CK2 uncouples executive part of phosphate maintenance pathway from cyclin-CDK control

The budding yeast Saccharomyces cerevisiae encounters phosphate starvation by the transcription-regulated PHO pathway. We find that genetic perturbation of protein kinase CK2, a conserved tetrameric Ser/Thr phosphotransferase with links to cell cycle and transcription, affects expression of PHO pathway genes in a subunit- and isoform-specific manner. Remarkably, the genes encoding phosphate supplying phosphatases and transporters are significantly repressed, while the genes encoding components of the central pathway regulator complex, a cyclin-dependent kinase (CDK), a cyclin, and a CDK inhibitor, remain unaltered. Thus, perturbation of CK2 uncouples the executive part of the PHO pathway from its cyclin-CDK control complex.

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.

p53 serine 392 phosphorylation increases after UV through induction of the assembly of the CK2.hSPT16.SSRP1 complex

Previously, we purified a UV-responsive p53 serine 392 kinase from F9 and HeLa cells and found that its activity is attributed to a high molecular weight protein complex containing the protein kinase CK2, along with the chromatin-associated factors hSPT16 and SSRP1. Here we determine that these proteins interact in vitro and in cells via non-overlapping domains and provide evidence consistent with the idea that hSPT16 and SSRP1 change the conformation of CK2 upon binding such that it specifically targets p53 over other substrates. Also, UV irradiation apparently induces the association of the complex, thereby increasing the specificity of CK2 for p53 at the expense of other cellular CK2 substrates and leading to an overall increase in p53 serine 392 phosphorylation.

Translated Alu sequence determines nuclear localization of a novel catalytic subunit of casein kinase 2

Casein kinase 2 (CK2) is a tetrameric enzyme constitutively expressed in all eukaryotic tissues. The two known isoforms of the catalytic subunit, CK2alpha and CK2alpha', have been reported to have distinct tissue-dependent subcellular distributions. We recently described a third isoform of the catalytic subunit, designated CK2alpha", which is highly expressed in liver. Immunoblot analysis of HuH-7 human hepatoma cell fractions as well as immunofluorescent microscopy revealed that CK2alpha" was exclusively localized to the nucleus and preferentially associated with the nuclear matrix. CK2alpha and CK2alpha' were found in nuclear, membrane, and cytosolic compartments. Deletion of the carboxy-terminal 32 amino acids from the CK2alpha" sequence resulted in release of the truncated green fluorescent protein fusion protein from the nuclear matrix and redistribution to both the nucleus and the cytoplasm. Demonstration that the carboxy terminus is necessary but not sufficient for nuclear retention indicates that the underlying mechanism of CK2alpha" nuclear localization is dependent on the secondary structure of the holoenzyme directed by the carboxy-terminal sequence.

Interactions between protein kinase CK2 and Pin1. Evidence for phosphorylation-dependent interactions

The peptidyl-prolyl isomerase Pin1 interacts in a phosphorylation-dependent manner with several proteins involved in cell cycle events. In this study, we demonstrate that Pin1 interacts with protein kinase CK2, an enzyme that generally exists in tetrameric complexes composed of two catalytic CK2 alpha and/or CK2 alpha' subunits together with two regulatory CK2 beta subunits. Our results indicate that Pin1 can interact with CK2 complexes that contain CK2 alpha. Furthermore, Pin1 can interact directly with the C-terminal domain of CK2 alpha that contains residues that are phosphorylated in vitro by p34(Cdc2) and in mitotic cells. Substitution of the phosphorylation sites of CK2 alpha with alanines resulted in decreased interactions between Pin1 and CK2. The other catalytic isoform of CK2, designated CK2 alpha', is not phosphorylated in mitotic cells and does not interact with Pin1, but a chimeric protein consisting of CK2 alpha' with the C terminus of CK2 alpha was phosphorylated in mitotic cells and interacts with Pin1, further implicating the phosphorylation sites in the interaction. In vitro, Pin1 inhibits the phosphorylation of Thr-1342 on human topoisomerase II alpha by CK2. Topoisomerase II alpha also interacts with Pin1 suggesting that the effect of Pin1 on the phosphorylation of Thr-1342 could result from its interactions with CK2 and/or topoisomerase II alpha. As compared with wild-type Pin1, isomerase-deficient and WW domain-deficient mutants of Pin1 are impaired in their ability to interact with CK2 and to inhibit the CK2-catalyzed phosphorylation of topoisomerase II alpha. Collectively, these results indicate that Pin1 and CK2 alpha interact and suggest a possible role for Pin1 in the regulation of topoisomerase II alpha. Furthermore, these results provide new insights into the functional role of the mitotic phosphorylation of CK2 and provide a new mechanism for selectively regulating the ability of CK2 to phosphorylate one of its mitotic targets.

CK2 forms a stable complex with TFIIIB and activates RNA polymerase III transcription in human cells

CK2 is a highly conserved protein kinase with growth-promoting and oncogenic properties. It is known to activate RNA polymerase III (PolIII) transcription in Saccharomyces cerevisiae and is shown here to also exert a potent effect on PolIII in mammalian cells. Peptide and chemical inhibitors of CK2 block PolIII transcription in human cell extracts. Furthermore, PolIII transcription in mammalian fibroblasts is decreased significantly when CK2 activity is compromised by chemical inhibitors, antisense oligonucleotides, or kinase-inactive mutants. Coimmunoprecipitation and cofractionation show that endogenous human CK2 associates stably and specifically with the TATA-binding protein-containing factor TFIIIB, which brings PolIII to the initiation site of all class III genes. Serum stimulates TFIIIB phosphorylation in vivo, an effect that is diminished by inhibitors of CK2. Binding to TFIIIC2 recruits TFIIIB to most PolIII promoters; this interaction is compromised specifically by CK2 inhibitors. The data suggest that CK2 stimulates PolIII transcription by binding and phosphorylating TFIIIB and facilitating its recruitment by TFIIIC2. CK2 also activates PolI transcription in mammals and may therefore provide a mechanism to coregulate the output of PolI and PolIII. CK2 provides a rare example of an endogenous activity that operates on the PolIII system in both mammals and yeasts. Such evolutionary conservation suggests that this control may be of fundamental importance.

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.

Inducible expression of the regulatory protein kinase CK2beta subunit: incorporation into complexes with catalytic CK2 subunits and re-examination of the effects of CK2beta on cell proliferation

The regulatory subunit of protein kinase CK2, designated CK2beta, exists both free in cells and in complexes with the CK2 catalytic subunits. Growing evidence suggests that CK2beta has functions dependent and independent of the CK2 catalytic subunits. There have been indications that CK2beta has functions associated with DNA damage responses and in the control of cell proliferation. For example, transient and stable constitutive overexpression of CK2beta in mammalian cells was previously shown to perturb cell cycle progression and to attenuate proliferation. To systematically investigate the molecular mechanisms responsible for these effects of CK2beta on cell proliferation, we generated human osteosarcoma U2OS cell lines with tetracycline-regulated expression of CK2beta. Increased expression of CK2beta results in increases in total cellular CK2 activity, but no changes in cell cycle profiles or proliferation. Furthermore, following exposure to ultraviolet radiation, p53 induction was identical regardless of the levels of CK2beta in cells. Mouse 3T3-L1 cells stably transfected with CK2beta also showed no alterations in cell proliferation. The differences between these results and those previously reported emphasize the complex nature of CK2beta and its cellular functions. Furthermore, these results indicate that increased expression of CK2beta is not by itself sufficient to effect alterations in cell proliferation.

Functional conservation between the human, nematode, and yeast CK2 cell cycle genes

Protein kinase CK2 (formerly casein kinase II) is a highly conserved serine/threonine protein kinase ubiquitous in eukaryotic organisms. Previously, we have shown that CK2 is required for cell cycle progression and essential for the viability of the yeast Saccharomyces cerevisiae. We now report that either the human or the nematode Caenorhabditis elegans CK2alpha catalytic subunit can substitute for the yeast catalytic subunits. Additionally, expression of the human CK2 regulatory subunit (CK2beta) can suppress the temperature sensitivity of either of the two yeast CK2 mutant catalytic subunits. Taken together, these observations reinforce the view that the CK2 cell cycle progression genes have been highly conserved during evolution from yeast to humans, not only in structure but also in function.

Quantitative imaging of gene induction in living animals

Methods to repeatedly, non-invasively, and quantitatively image gene expression in living animals are rapidly emerging and should fundamentally change studies of gene expression in vivo. We previously developed assays utilizing positron emission tomography (PET) to image reporter gene expression. In this paper we: (1) describe a new bi-directional, tetracycline-inducible system that can be used to pharmacologically induce target gene expression and to quantitatively image induced expression by using a PET reporter gene; (2) demonstrate the potential of this system in transient and stable cell transfection assays; and (3) demonstrate the ability to repetitively and quantitatively image tetracycline and tetracycline analog induction of gene expression in living animals. We utilize the dopamine type-2 receptor (D(2)R) and the mutant herpes-simplex virus type 1 thymidine kinase (HSV1-sr39tk) reporter genes to validate this system. We utilize microPET technology to show that quantitative tomographic imaging of gene induction is possible. We find a high correlation (r(2) = 0.98) between 'target' and reporter gene expression. This work establishes a new technique for imaging time-dependent variation of gene expression both from vectors with inducible promoters and in transgenic animals in which pharmacologic induction of gene expression must be monitored. These techniques may be applied both in gene therapy and for the study of gene expression in transgenic animals.

A role for protein kinase CK2 in cell proliferation: evidence using a kinase-inactive mutant of CK2 catalytic subunit alpha

Protein kinase CK2 is an ubiquitous and pleiotropic Ser/Thr protein kinase composed of two catalytic (alpha and/or alpha') and two regulatory (beta) subunits generally combined to form alpha(2)beta(2), alphaalpha'beta(2), or alpha'(2)beta(2) heterotetramers. To gain more insight into the role of CK2 in the control of proliferation in mammalian cells, overexpression of isolated CK2 subunits alpha, alpha', or beta was carried out in two fibroblast cell lines: NIH3T3 and CCL39. To interfere with CK2 cellular functions, cells were also transfected with a kinase-inactive mutant of CK2alpha catalytic subunit: CK2alpha-K68A. In NIH3T3 cells, overexpression of either wild-type subunit (alpha, alpha' or beta) had no effect on cell proliferation. In contrast, overexpression of the CK2alpha kinase-deficient mutant induced a marked inhibition of cell proliferation. This resulted from a defect in G1/S progression as demonstrated in transient transfection experiments in both NIH3T3 and CCL39 cells using BrdU incorporation measurements and in CCL39 clones stably overexpressing the CK2alpha-K68A mutant by growth curve analysis. We demonstrated that the kinase-negative mutant has the capacity to integrate the endogenous CK2 subunit pool both as an isolated kinase-inactive alpha subunit and as associated to the beta subunit in a kinase-inactive tetramer. Finally we showed that expression of the kinase-inactive mutant interferes with phosphorylation of an endogenous CK2 substrate; we speculate that optimal phosphorylation of target proteins by CK2 is required to achieve optimal cell cycle progression.

Maize protein kinase CK2: regulation and functionality of three beta regulatory subunits

Biochemical and crystallographic data suggest that, in contrast with other organisms, the active maize protein kinase CK2 might be composed simply of a catalytic polypeptide (CK2alpha), thus lacking CK2beta regulatory subunits. To investigate the existence and functionality of CK2beta regulatory subunits in Zea mays, we have screened a maize cDNA library using different approaches and have isolated three full-length cDNAs encoding CK2beta regulatory subunits (CK2beta-1, CK2beta-2 and CK2beta-3) and a cDNA coding for a novel CK2alpha catalytic subunit, CK2alpha-3. The pattern of expression of all these alpha/beta subunits has been studied in different organs and developmental stages using specific probes for each isoform, and indicates that while CK2alpha subunits are constitutive, CK2beta subunits are expressed differentially during embryo development. The yeast two-hybrid system and pull-down assays have been used to study specific interactions between the different subunits. While CK2alpha subunits are unable to self-associate, preferential interactions between alpha/beta isoforms and beta/beta isoforms can be predicted. Furthermore, we show that maize CK2alpha/beta subunits assemble into a structural tetrameric complex which has very similar properties to those described in other organisms, and that expression of maize CK2beta subunits in yeast allows the rescue of the phenotypic defects associated to the lack of CK2 function, thus demonstrating the functionality of maize CK2beta regulatory subunits.

A novel casein kinase 2 alpha-subunit regulates membrane protein traffic in the human hepatoma cell line HuH-7

A previously isolated endocytic trafficking mutant (TRF1) isolated from HuH-7 cells is defective in the distribution of subpopulations of cell-surface receptors for asialoorosomucoid (asialoglycoprotein receptor (ASGR)), transferrin, and mannose-terminating glycoproteins. The pleiotropic phenotype of TRF1 also includes an increased sensitivity to Pseudomonas toxin and deficient assembly and function of gap junctions. HuH-7xTRF1 hybrids exhibited a normal subcellular distribution of ASGR, consistent with the TRF1 mutation being recessive. A cDNA expression library derived from HuH-7 mRNA was transfected into TRF1 cells, which were subsequently selected for resistance to Pseudomonas toxin. Sequence analysis of a recovered cDNA revealed a unique isoform of casein kinase 2 (CK2), CK2alpha". Western blot analysis of TRF1 proteins revealed a 60% reduction in total CK2alpha expression. Consistent with this finding, the hybrids HuH-7xHuH-7 and HuH-7xTRF1 expressed equivalent amounts of total CK2alpha. Immunoblots using antibodies against peptides unique to the previously described CK2 isoforms CK2alpha and CK2alpha' and the novel CK2alpha" isoform showed that, although TRF1 and parental HuH-7 cells expressed comparable amounts of CK2alpha and CK2alpha', the mutant did not express CK2alpha". Based on the genomic DNA sequence, RNA transcripts encoding CK2alpha" apparently originate from alternative splicing of a primary transcript. Protein overexpression following transfection of TRF1 cells with cDNAs encoding either CK2alpha or the newly cloned CK2alpha" restored the parental HuH-7 phenotype, including Pseudomonas toxin resistance, cell-surface ASGR binding activity, phosphorylation, and the assembly of gap junctions. This study suggests that HuH-7 cells express at least three CK2alpha isoforms and that the pleiotropic TRF1 phenotype is a consequence of a reduction in total CK2 expression.

The tumor suppressor PTEN is phosphorylated by the protein kinase CK2 at its C terminus. Implications for PTEN stability to proteasome-mediated degradation

The tumor suppressor phosphatase PTEN regulates cell migration, growth, and survival by dephosphorylating phosphatidylinositol second messengers and signaling phosphoproteins. PTEN possesses a C-terminal noncatalytic regulatory domain that contains multiple putative phosphorylation sites, which could play an important role in the control of its biological activity. The protein kinase CK2 phosphorylated, in a constitutive manner, a cluster of Ser/Thr residues located at the PTEN C terminus. PTEN-phosphorylated defective mutants showed decreased stability in comparison with wild type PTEN and were more rapidly degraded by the proteasome. Inhibition of PTEN phosphorylation by the CK2 inhibitor 5,6-dichloro-1-beta-d-ribofuranosyl-benzimidazole also diminished the PTEN protein content. Our results support the notion that proper phosphorylation of PTEN by CK2 is important for PTEN protein stability to proteasome-mediated degradation.

Tumor necrosis factor alpha-induced phosphorylation of RelA/p65 on Ser529 is controlled by casein kinase II

Nuclear factor kappaB (NF-kappaB)/Rel transcription factors are key regulators of a variety of genes involved in immune and inflammatory responses, growth, differentiation, apoptosis, and development. In unstimulated cells, NF-kappaB/Rel proteins are sequestered in the cytoplasm by IkappaB inhibitor proteins. Many extracellular stimuli, such as tumor necrosis factor alpha (TNFalpha), cause rapid phosphorylation of IkappaB at N-terminal serine residues leading to ubiquitination and degradation of the inhibitor. Subsequently, NF-kappaB proteins translocate to the nucleus and activate gene expression through kappaB response elements. TNFalpha, as well as certain other stimuli, also induces the phosphorylation of the NF-kappaB proteins. Previously, we have shown that TNFalpha induces RelA/p65 phosphorylation at serine 529 and that this inducible phosphorylation increases NF-kappaB transcriptional activity on an exogenously supplied reporter (). In this report, we demonstrate that casein kinase II (CKII) interacts with p65 in vivo and can phosphorylate p65 at serine 529 in vitro. A CKII inhibitor (PD144795) inhibited TNFalpha-induced p65 phosphorylation in vivo. Furthermore, our results indicate that the association between IkappaBalpha and p65 inhibits p65 phosphorylation by CKII and that degradation of IkappaBalpha allows CKII to phosphorylate p65 to increase NF-kappaB transactivation potential. These data may explain the ability of CKII to modulate cell growth and demonstrate a mechanism whereby CKII can function in an inducible manner.

Identification and characterization of CKIP-1, a novel pleckstrin homology domain-containing protein that interacts with protein kinase CK2

The catalytic subunits of protein kinase CK2, CK2alpha and CK2alpha', are closely related to each other but exhibit functional specialization. To test the hypothesis that specific functions of CK2alpha and CK2alpha' are mediated by specific interaction partners, we used the yeast two-hybrid system to identify CK2alpha- or CK2alpha'-binding proteins. We report the identification and characterization of a novel CK2-interacting protein, designated CKIP-1, that interacts with CK2alpha, but not CK2alpha', in the yeast two-hybrid system. CKIP-1 also interacts with CK2alpha in vitro and is co-immunoprecipitated from cell extracts with epitope-tagged CK2alpha and an enhanced green fluorescent protein fusion protein encoding CKIP-1 (i.e. EGFP-CKIP-1) when they are co-expressed. CK2 activity is detected in anti-CKIP-1 immunoprecipitates performed with extracts from non-transfected cells indicating that CKIP-1 and CK2 interact under physiological conditions. The CKIP-1 cDNA is broadly expressed and encodes a protein with a predicted molecular weight of 46,000. EGFP-CKIP-1 is localized within the nucleus and at the plasma membrane. The plasma membrane localization is dependent on the presence of an amino-terminal pleckstrin homology domain. We postulate that CKIP-1 is a non-enzymatic regulator of one isoform of CK2 (i.e. CK2alpha) with a potential role in targeting CK2alpha to a particular cellular location.

pCMB treatment reveals the essential role of cysteinyl residues in conferring functional competence to the regulatory subunit of protein kinase CK2

To assess the functional role of the four conserved cysteinyl residues in the regulatory beta-subunit of protein kinase CK2, the effect of pCMB and other reagents of sulfhydryl groups has been investigated. The pCMB-treated beta-subunit has lost its ability to form either homodimers or regular alpha(2)beta(2) heterotetramers with the catalytic subunit. It also fails to increase catalytic activity toward peptide substrates and to mediate the stimulatory effect of polylysine. The pCMB-treated beta-subunit, however, is still able to prevent calmodulin phosphorylation and to physically interact with the alpha-subunit to form inactive complexes whose sedimentation coefficient is lower than that of CK2 holoenzyme. These inactive complexes upon treatment with reducing agents like DTT are converted into a fully active heterotetrameric holoenzyme.