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

A Comparison of the Immunometabolic Effect of Antibiotics and Plant Extracts in a Chicken Macrophage-like Cell Line during a Salmonella Enteritidis Challenge

Immunometabolic modulation of macrophages can play an important role in the innate immune response of chickens triggered with a multiplicity of insults. In this study, the immunometabolic role of two antibiotics (oxytetracycline and gentamicin) and four plant extracts (thyme essential oil, grape seed extract, garlic oil, and capsicum oleoresin) were investigated on a chicken macrophage-like cell line (HD11) during a Salmonella Enteritidis infection. To study the effect of these substances, kinome peptide array analysis, Seahorse metabolic assay, and gene expression techniques were employed. Oxytetracycline, to which the bacterial strain was resistant, thyme essential oil, and capsicum oleoresin did not show any noteworthy immunometabolic effect. Garlic oil affected glycolysis, but this change was not detected by the kinome analysis. Gentamicin and grape seed extract showed the best immunometabolic profile among treatments, being able to both help the host with the activation of immune response pathways and with maintaining a less inflammatory status from a metabolic point of view.

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.

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

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

Evaluation of a Selective Chemical Probe Validates That CK2 Mediates Neuroinflammation in a Human Induced Pluripotent Stem Cell-Derived Microglial Model

Novel treatments for neurodegenerative disorders are in high demand. It is imperative that new protein targets be identified to address this need. Characterization and validation of nascent targets can be accomplished very effectively using highly specific and potent chemical probes. Human induced pluripotent stem cells (hiPSCs) provide a relevant platform for testing new compounds in disease relevant cell types. However, many recent studies utilizing this platform have focused on neuronal cells. In this study, we used hiPSC-derived microglia-like cells (MGLs) to perform side-by-side testing of a selective chemical probe, SGC-CK2-1, compared with an advanced clinical candidate, CX-4945, both targeting casein kinase 2 (CK2), one of the first kinases shown to be dysregulated in Alzheimer's disease (AD). CK2 can mediate neuroinflammation in AD, however, its role in microglia, the innate immune cells of the central nervous system (CNS), has not been defined. We analyzed available RNA-seq data to determine the microglial expression of kinases inhibited by SGC-CK2-1 and CX-4945 with a reported role in mediating inflammation in glial cells. As proof-of-concept for using hiPSC-MGLs as a potential screening platform, we used both wild-type (WT) MGLs and MGLs harboring a mutation in presenilin-1 (PSEN1), which is causative for early-onset, familial AD (FAD). We stimulated these MGLs with pro-inflammatory lipopolysaccharides (LPS) derived from E. coli and observed strong inhibition of the expression and secretion of proinflammatory cytokines by simultaneous treatment with SGC-CK2-1. A direct comparison shows that SGC-CK2-1 was more effective at suppression of proinflammatory cytokines than CX-4945. Together, these results validate a selective chemical probe, SGC-CK2-1, in human microglia as a tool to reduce neuroinflammation.

Modulation of transcriptional activity in brain lower grade glioma by alternative splicing

Proteins that modify the activity of transcription factors (TFs) are often called modulators and play a vital role in gene transcriptional regulation. Alternative splicing is a critical step of gene processing, and differentially spliced isoforms may have different functions. Alternative splicing can modulate gene function by adding or removing certain protein domains and thereby influence the activity of a protein. The objective of this study is to investigate the role of alternative splicing in modulating the transcriptional regulation in brain lower grade glioma (LGG), especially transcription factor ELK1, which is closely related to various disorders, including Alzheimer’s disease and Down syndrome. The results showed that changes in the exon inclusion ratio of proteins APP and STK16 are associated with changes in the expression correlation between ELK1 and its targets. In addition, the structural features of the two modulators are strongly associated with the pathological impact of exon inclusion. The results of our analysis suggest that alternatively spliced proteins have different functions in modifying transcription factors and can thereby induce the dysregulation of multiple genes.

Platelet-activating factor enhances tumour metastasis via the reactive oxygen species-dependent protein kinase casein kinase 2-mediated nuclear factor-κB activation

Platelet-activating factor (PAF) promotes tumour metastasis via activation of the transcription factor nuclear factor-κB (NF-κB). We here investigated the role of the protein kinase CK2 (formerly Casein Kinase 2 or II) in PAF-induced NF-κB activation and tumour metastasis, given that PAF has been reported to increase CK2 activity, and that CK2 plays a key role in NF-κB activation. PAF increased CK2 activity, phosphorylation and protein expression in vivo as well as in vitro. CK2 inhibitors inhibited the PAF-mediated NF-κB activation and expression of NF-κB-dependent pro-inflammatory cytokines and anti-apoptotic factors. Pre-treatment with the antioxidant N-Acetyl-L-Cysteine (NAC) resulted in a significant inhibition in PAF-induced enhancement of CK2 activity, phosphorylation and protein expression in vivo as well as in vitro. H2 O2 and known reactive oxygen species inducers, lipopolysaccharide (LPS) and tumour necrosis factor-α (TNF-α) enhanced CK2 activity, phosphorylation and protein expression, which was again inhibited by antioxidant. PAF, LPS and TNF-α induced increased CK2 activity, phosphorylationand protein expression, which were inhibited by p38 inhibitor. PAF, LPS or TNF-α increased pulmonary metastasis of B16F10, which was inhibited by antioxidants, CK2 inhibitor and p38 inhibitor. Our data suggest that (i) reactive oxygen species activate CK2 via p38, which, in turn, induces NF-κB activation, and (ii) PAF, LPS and TNF-α increase pulmonary tumour metastasis via the induction of the reactive oxygen species (ROS)/p38/CK2/NF-κB pathway.

Functional interaction of protein kinase CK2 and activating transcription factor 4 (ATF4), a key player in the cellular stress response

Protein kinase CK2 is a pleiotropic enzyme, which is implicated in the regulation of numerous biological processes. It seems to regulate the various functions by binding to other proteins and by phosphorylation of many different substrates. Here, we identified the activating transcription factor 4 (ATF4), an essential component of the ER stress signaling, as a new binding partner and a new substrate of CK2 in vitro and in vivo. Bifluorescence complementation analysis (BiFC) revealed that CK2α and ATF4 associate in the nucleus. By using mutants of ATF4 we identified serine 215 as the main CK2 phosphorylation site. The ATF4 S215A mutant turned out to be more stable than the wild-type form. We further noticed that an inhibition of CK2 caused an increased transcription of the ATF4 gene. Analyses of the transcription factor activity revealed an impaired activity of the CK2 phosphorylation mutant of ATF4. Thus, we show that (i) ATF4 is a binding partner of CK2α (ii) ATF4 is a substrate of CK2, (iii) the phosphorylation of ATF4 by CK2 influences the stability of ATF4, (iv) the transcription of ATF4 is regulated by CK2 and (v) the transcription factor activity of ATF4 is regulated by the CK2 phosphorylation of ATF4. Thus, CK2 plays an essential role in the regulation of the ER-stress induced signaling pathway.

GADD45beta enhances Col10a1 transcription via the MTK1/MKK3/6/p38 axis and activation of C/EBPbeta-TAD4 in terminally differentiating chondrocytes

GADD45beta (growth arrest- and DNA damage-inducible) interacts with upstream regulators of the JNK and p38 stress response kinases. Previously, we reported that the hypertrophic zone of the Gadd45beta(-/-) mouse embryonic growth plate is compressed, and expression of type X collagen (Col10a1) and matrix metalloproteinase 13 (Mmp13) genes is decreased. Herein, we report that GADD45beta enhances activity of the proximal Col10a1 promoter, which contains evolutionarily conserved AP-1, cAMP-response element, and C/EBP half-sites, in synergism with C/EBP family members, whereas the MMP13 promoter responds to GADD45beta together with AP-1, ATF, or C/EBP family members. C/EBPbeta expression also predominantly co-localizes with GADD45beta in the embryonic growth plate. Moreover, GADD45beta enhances C/EBPbeta activation via MTK1, MKK3, and MKK6, and dominant-negative p38alphaapf, but not JNKapf, disrupts the combined trans-activating effect of GADD45beta and C/EBPbeta on the Col10a1 promoter. Importantly, GADD45beta knockdown prevents p38 phosphorylation while decreasing Col10a1 mRNA levels but does not affect C/EBPbeta binding to the Col10a1 promoter in vivo, indicating that GADD45beta influences the transactivation function of DNA-bound C/EBPbeta. In support of this conclusion, we show that the evolutionarily conserved TAD4 domain of C/EBPbeta is the target of the GADD45beta-dependent signaling. Collectively, we have uncovered a novel molecular mechanism linking GADD45beta via the MTK1/MKK3/6/p38 axis to C/EBPbeta-TAD4 activation of Col10a1 transcription in terminally differentiating chondrocytes.

Protein kinase CK2, an important regulator of the inflammatory response?

Casein kinase 2 (CK2) is a highly conserved serine-threonine kinase that uses both adenosine triphosphate and guanosine triphosphate as phosphate donors. This constitutively active and ubiquitously expressed enzyme is often present as a tetrameric holoenzyme complex of two catalytic subunits (alpha and/or alpha') and two regulatory beta subunits. The enzyme is known to phosphorylate more than 300 substrates and controls a wide range of processes, including the regulation of cell cycle, apoptosis, transformation, and circadian rhythm. Several lines of recent evidence also suggest a potentially important role for CK2 in the control of the inflammatory response. This review will give an overview of CK2 and its regulation and describe the evidence implicating its role in inflammation.

Regulation of DeltaFosB stability by phosphorylation

The transcription factor DeltaFosB (also referred to as FosB2 or FosB[short form]) is an important mediator of the long-term plasticity induced in brain by chronic exposure to several types of psychoactive stimuli, including drugs of abuse, stress, and electroconvulsive seizures. A distinct feature of DeltaFosB is that, once induced, it persists in brain for relatively long periods of time in the absence of further stimulation. The mechanisms underlying this apparent stability, however, have remained unknown. Here, we demonstrate that DeltaFosB is a relatively stable transcription factor, with a half-life of approximately 10 h in cell culture. Furthermore, we show that DeltaFosB is a phosphoprotein in brain and that phosphorylation of a highly conserved serine residue (Ser27) in DeltaFosB protects it from proteasomal degradation. We provide several lines of evidence suggesting that this phosphorylation is mediated by casein kinase 2. These findings constitute the first evidence that DeltaFosB is phosphorylated and demonstrate that phosphorylation contributes to its stability, which is at the core of its ability to mediate long-lasting adaptations in brain.

Cross talk between protein kinase CK2 and eukaryotic translation initiation factor eIF2beta subunit

The beta-subunit of eukaryotic translation initiation factor eIF2 is a substrate and a partner for protein kinase CK2. Surface plasmon resonance analysis shows that the truncated form corresponding to residues 138-333 of eIF2beta (eIF2beta-CT) interacts with CK2beta as efficiently as full length eIF2beta, whereas the form corresponding to residues 1-137, which contains the CK2 phosphorylation sites, (eIF2beta-NT) does not bind. The use of different mutants and truncated forms of CK2alpha allowed us to map the basic segment K74-K83 at the beginning of helix alphaC and residues R191R195K198 in the p + 1 loop as the main determinants for the binding to eIF2beta-CT of either the isolated CK2alpha subunit or the CK2 holoenzyme. The presence of eIF2beta-CT stimulated the activity of CK2alpha towards the RRRAADSDDDDD peptide substrate; effect that was not observed with the CK2a K74-77A whose ability to bind to eIF2beta-CT is severely impaired. Gel filtration analysis confirmed the ability of CK2alpha to form complexes with eIF2beta-CT, and the contribution of the basic cluster in CK2alpha (K74-K77) in this association.

Phosphorylation of Epstein-Barr virus ZEBRA protein at its casein kinase 2 sites mediates its ability to repress activation of a viral lytic cycle late gene by Rta

ZEBRA, a member of the bZIP family, serves as a master switch between latent and lytic cycle Epstein-Barr virus (EBV) gene expression. ZEBRA influences the activity of another viral transactivator, Rta, in a gene-specific manner. Some early lytic cycle genes, such as BMRF1, are activated in synergy by ZEBRA and Rta. However, ZEBRA suppresses Rta's ability to activate a late gene, BLRF2. Here we show that this repressive activity is dependent on the phosphorylation state of ZEBRA. We find that two residues of ZEBRA, S167 and S173, that are phosphorylated by casein kinase 2 (CK2) in vitro are also phosphorylated in vivo. Inhibition of ZEBRA phosphorylation at the CK2 substrate motif, either by serine-to-alanine substitutions or by use of a specific inhibitor of CK2, abolished ZEBRA's capacity to repress Rta activation of the BLRF2 gene, but did not alter its ability to initiate the lytic cycle or to synergize with Rta in activation of the BMRF1 early-lytic-cycle gene. These studies illustrate how the phosphorylation state of a transcriptional activator can modulate its behavior as an activator or repressor of gene expression. Phosphorylation of ZEBRA at its CK2 sites is likely to play an essential role in proper temporal control of the EBV lytic life cycle.

Phosphorylation of conserved casein kinase sites regulates cAMP-response element-binding protein DNA binding in Drosophila

The Drosophila homolog of cAMP-response element-binding protein (CREB), dCREB2, exists with serine 231, equivalent to mammalian serine 133, in a predominantly phosphorylated state. Thus, unlike the mammalian protein, the primary regulation of dCREB2 may occur at a different step from serine 231 phosphorylation. Although bacterially expressed dCREB2 bound cAMP-response element sites, protein from Drosophila extracts was unable to do so unless treated with phosphatase. Phosphorylation of recombinant protein by casein kinase (CK) I or II, but not calcium-calmodulin kinase II or protein kinase A, inhibited DNA binding. Up to four conserved CK sites likely to be phosphorylated in vivo were responsible for this effect, and these sites were phosphorylated by a kinase present in Drosophila cell extracts that biochemically resembles CKII. We propose that the relative importance of different signaling pathways in regulating CREB activity may differ between Drosophila and mammals. In Drosophila, the dephosphorylation of CK sites appears to be the major regulatory step, while phosphorylation of serine 231 is necessary but secondary.

Eukaryotic translation-initiation factor eIF2beta binds to protein kinase CK2: effects on CK2alpha activity

eIF2 (eukaryotic translation-initiation factor 2) is a substrate and an interacting partner for CK2 (protein kinase CK2). Co-immuno-precipitation of CK2 with eIF2beta has now been observed in HeLa cells, overexpressing haemagglutinin-tagged human recombinant eIF2beta. A direct association between His6-tagged human recombinant forms of eIF2beta subunit and both the catalytic (CK2alpha) and the regulatory (CK2beta) subunits of CK2 has also been shown by using different techniques. Surface plasmon resonance analysis indicated a high affinity in the interaction between eIF2beta and CK2alpha, whereas the affinity for the association with CK2beta is much lower. Free CK2alpha is unable to phosphorylate eIF2beta, whereas up to 1.2 mol of phosphate/mol of eIF2beta was incorporated by the reconstituted CK2 holoenzyme. The N-terminal third part of eIF2beta is dispensable for binding to either CK2alpha or CK2beta, although it contains the phosphorylation sites for CK2. The remaining central/C-terminal part of eIF2beta is not phosphorylated by CK2, but is sufficient for binding to both CK2 subunits. The presence of eIF2beta inhibited CK2alpha activity on calmodulin and beta-casein, but it had a minor effect on that of the reconstituted CK2 holoenzyme. The truncated forms corresponding to the N-terminal or central/C-terminal regions of eIF2beta were much less inhibitory than the intact subunit. The results demonstrate that the ability to associate with CK2 subunits and to serve as a CK2 substrate are confined to different regions in eIF2beta and that it may act as an inhibitor on CK2alpha.

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.

One-thousand-and-one substrates of protein kinase CK2?

CK2 (formerly termed "casein kinase 2") is a ubiquitous, highly pleiotropic and constitutively active Ser/Thr protein kinase whose implication in neoplasia, cell survival, and virus infection is supported by an increasing number of arguments. Here an updated inventory of 307 CK2 protein substrates is presented. More than one-third of these are implicated in gene expression and protein synthesis as being either transcriptional factors (60) or effectors of DNA/RNA structure (50) or translational elements. Also numerous are signaling proteins and proteins of viral origin or essential to virus life cycle. In comparison, only a minority of CK2 targets (a dozen or so) are classical metabolic enzymes. An analysis of 308 sites phosphorylated by CK2 highlights the paramount relevance of negatively charged side chains that are (by far) predominant over any other residues at positions n+3 (the most crucial one), n+1, and n+2. Based on this signature, it is predictable that proteins phosphorylated by CK2 are much more numerous than those identified to date, and it is possible that CK2 alone contributes to the generation of the eukaryotic phosphoproteome more so than any other individual protein kinase. The possibility that CK2 phosphosites play some global role, e.g., by destabilizing alpha helices, counteracting caspase cleavage, and generating adhesive motifs, will be discussed.

DAP-like kinase interacts with the rat homolog of Schizosaccharomyces pombe CDC5 protein, a factor involved in pre-mRNA splicing and required for G2/M phase transition

DAP-like kinase (Dlk, also termed ZIP kinase) is a leucine zipper-containing serine/threonine-specific protein kinase with as yet unknown biological function(s). Interaction partners so far identified are either transcription factors or proteins that can support or counteract apoptosis. Thus, Dlk might be involved in regulating transcription or, more generally, survival or apoptosis. Here we report on a new interaction partner, the rat homolog of Schizosaccharomyces pombe CDC5 protein, a presumptive transcription and splicing factor involved in the G(2)/M transition. In vitro, rat CDC5 forms complexes with, but is not phosphorylated by, Dlk. Rather, it was phosphorylated by an associated kinase which was identified as CK2. The interaction domain of Dlk was mapped to the leucine zipper, while that of CDC5 was mapped to the C-terminal region between residues 500 and 802. In vivo, both proteins co-localize perfectly in distinct speckle-like structures in the nucleus, some of which overlap with promyelocytic leukemia protein. Interestingly, splicing factor SC35, which also resides in speckles, was partially displaced upon overexpression of either CDC5 or Dlk, perhaps due to phosphorylation by Dlk. Together with previous data, these results suggest that Dlk might play a role in coordinating specific transcription and splicing events.

Plant bZIP G-box binding factors. Modular structure and activation mechanisms

In this review we sum-up the knowledge about bZIP G-box binding factors (GBFs), which possess an N-terminal, proline-rich domain. The GBF has been one of the most extensively studied transcription factor family. Based on protein sequence homology with yeast and animal basic leucine-zipper (bZIP) transcription factors, bioinformatic studies have identified their main structural domains (proline-rich, basic and leucine-zipper), which have been further functionally characterized by in vitro and in vivo experiments. Recent reports have led to the discovery of other GBF-specific short amino-acid sequences that may take part in the regulation of gene expression by post-transcriptional modifications or interaction with other proteins such as bZIP enhancing factors or plant 14-3-3-like proteins. We identified a GBF region, called the 'multifunctional mosaic region', that may be implicated in cytoplasmic retention, translocation to the nucleus and regulation of transcription. We also identified many conserved protein motifs that suggest a modular structure for GBFs. At the whole plant level, GBFs have been shown to be involved in developmental and physiological processes in response to major cues such as light or hormones. Nevertheless, it remains difficult to assign a physiological role to a particular GBF protein modular structure. Finally, bringing together these different aspects of GBF studies we propose a model describing the puzzling transduction pathway involving GBFs from cytoplasmic events of signal transduction to the regulation of gene expression in the nucleus.

Visualization and molecular analysis of nuclear import of protein kinase CK2 subunits in living cells

We have generated fusion proteins between the subunits of CK2 and GFP and characterized their behaviour in living cells. The expressed fusion proteins were functional and interacted with endogenous CK2. Imaging of NIH3T3 cells expressing low level of GFP-CK2alpha or GFP-CK2beta showed that both proteins were mostly nuclear in interphase. Both CK2 subunits contain nuclear localization domains that target them independently to the nucleus. Once in the nucleus, both subunits diffused rapidly in the nucleoplasm. In mitotic cells, CK2 subunits were dispersed throughout the cytoplasm and were not associated to chromatin. Our data are compatible with the idea that each subunit can translocate individually to the nucleus to interact with each other or with important cellular partners. Understanding the molecular mechanisms which regulate the dynamic localization of CK2 subunits will be of central importance.

The carboxy-terminal domain of Grp94 binds to protein kinase CK2 alpha but not to CK2 holoenzyme

Surface plasmon resonance analysis shows that the carboxy-terminal domain of Grp94 (Grp94-CT, residues 518-803) physically interacts with the catalytic subunit of protein kinase CK2 (CK2 alpha) under non-stressed conditions. A K(D) of 4 x 10(-7) was determined for this binding. Heparin competed with Grp94-CT for binding to CK2 alpha. CK2 beta also inhibited the binding of Grp94-CT to CK2 alpha, and CK2 holoenzyme reconstituted in vitro was unable to bind Grp94-CT. The use of CK2 alpha mutants made it possible to map the Grp94-CT binding site to the four lysine stretch (residues 74-77) present in helix C of CK2 alpha. Grp94-CT stimulated the activity of CK2 alpha wild-type but was ineffective on the CK2 alpha K74-77A mutant.

The leucine zipper of NRL interacts with the CRX homeodomain. A possible mechanism of transcriptional synergy in rhodopsin regulation

Photoreceptor-specific expression of rhodopsin is mediated by multiple cis-acting elements in the proximal promoter region. NRL (neural retina leucine zipper) and CRX (cone rod homeobox) proteins bind to the adjacent NRE and Ret-4 sites, respectively, within this region. Although NRL and CRX are each individually able to induce rhodopsin promoter activity, when expressed together they exhibit transcriptional synergy in rhodopsin promoter activation. Using the yeast two-hybrid method and glutathione S-transferase pull-down assays, we demonstrate that the leucine zipper of NRL can physically interact with CRX. Deletion analysis revealed that the CRX homeodomain (CRX-HD) plays an important role in the interaction with the NRL leucine zipper. Although binding with the CRX-HD alone was weak, a strong interaction was detected when flanking regions including the glutamine-rich and the basic regions that follow the HD were included. A reciprocal deletion analysis showed that the leucine zipper of NRL is required for interaction with CRX-HD. Two disease-causing mutations in CRX-HD (R41W and R90W) that exhibit reduced DNA binding and transcriptional synergy also decrease its interaction with NRL. These studies suggest novel possibilities for protein-protein interaction between two conserved DNA-binding motifs and imply that cross-talk among distinct regulatory pathways contributes to the establishment and maintenance of photoreceptor function.

A novel transcriptional factor with Ser/Thr kinase activity involved in the transforming growth factor (TGF)-beta signalling pathway

Transforming growth factor-beta (TGF-beta) shows a variety of biological activities in various organs or cells. Recently some factors such as Smads (Sma and Mad proteins) and TGF-beta activating kinase 1 ('TAK1') have been characterized as signalling molecules downstream of TGF-beta. Several TGF-beta response elements have been identified such as cAMP response element, Smad binding element, and recognition sites for activating protein-1 and stimulating protein-1 in various gene promoters. We also reported a TGF-beta response element in the human C-type natriuretic peptide (CNP) gene promoter. In this paper, we report on a novel factor which regulates the TGF-beta response promoter. This factor, named TSF1 (TGF-beta stimulated factor 1), possessed DNA-binding ability and activated the TGF-beta responsive CNP promoter or vascular endothelial growth factor gene promoter which possesses a sequence element analogous to the TGF-beta responsive GC-rich element of the CNP promoter. TSF1 did not directly activate a Smads-dependent promoter from plasminogen activator inhibitor 1 gene, but it showed enhancement in co-operation with Smad3 and Smad4. Interestingly, this factor had the structural features of a Ser/Thr kinase and actually exhibited protein kinase activity. TSF1 mRNA as well as its protein level were stimulated by TGF-beta treatment. Thus, TSF1 is an unique factor with two biological functions, transcriptional regulation and protein phosphorylation, that may be involved in TGF-beta signals.

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.

Phosphorylation of the insect immunophilin FKBP46 by the Spodoptera frugiperda homolog of casein kinase II

Immunophilins are a family of conserved proteins found in both prokaryotes and eukaryotes, that exhibit peptidylprolyl isomerase (PPIase) activity. Members of this family bind to immunosuppressive drugs and on this basis are divided into two classes: FKBPs bind to FK506 and rapamycin, while cyclophilins bind to cyclosporin A. In this paper, we report on insect immunophilin FKBP46 and its associated kinase. The insect FKBP46 belongs to the high-molecular-weight immunophilins and shares many characteristic features with its mammalian counterparts, but its functional role remains unclear. Here, we show that FKBP46 is phosphorylated by a protein kinase present in the nucleus of both insect Spodoptera frugiperda (Sf9) and human Jurkat cells. This protein kinase is immunoreactive with polyclonal antiserum raised against Drosophila melanogaster casein kinase II (CKII). We have cloned, overexpressed and characterized a new member of the CKII family derived from Spodoptera frugiperda cells. Recombinant Sf9 CKII alpha subunit shares 75% identity to human, chicken and Drosophila melanogaster homologs, whereas the Sf9 CKII beta subunit is 77% identical to rat, chicken and human. Moreover, we demonstrate that the insect immunophilin FKBP46 can be phosphorylated by human and Sf9 casein kinase II. Finally, we show that FKBP46 interacts with DNA, and this interaction is not prevented by phosphorylation.

Interaction partners of Dlk/ZIP kinase: co-expression of Dlk/ZIP kinase and Par-4 results in cytoplasmic retention and apoptosis

Dlk/ZIP kinase is a newly discovered serine/threonine kinase which, due to its homology to DAP kinase, was named DAP like kinase, Dlk. This kinase is tightly associated with nuclear structures, it undergoes extensive autophosphorylation and phosphorylates myosin light chain and core histones H3, H2A and H4 in vitro. Moreover, it possesses a leucine zipper which mediates interaction with transcription factor ATF-4, therefore it was called ZIP kinase. We employed the yeast two-hybrid system to identify interaction partners of Dlk that might serve as regulators or targets. Besides ATF-4 and others we found Par-4, a modulator of transcription factor WT1 and mediator of apoptosis. Complex formation between Dlk and Par-4 was confirmed by GST pull-down experiments and kinase reactions in vitro and coexpression experiments in vivo. The interaction domain within Dlk was mapped to an arginine-rich region between residues 338 - 417, rather than to the leucine zipper. Strikingly, coexpression of Dlk and Par-4 lead to relocation of Dlk from the nucleus to the cytoplasm, particularly to actin filaments. These interactions provoked a dramatic reorganization of the cytoskeleton and morphological symptoms of apoptosis, thus suggesting a functional relationship between Dlk and Par-4 in the control of apoptosis.

The role of human MBF1 as a transcriptional coactivator

Multiprotein bridging factor 1 (MBF1) is a coactivator which mediates transcriptional activation by interconnecting the general transcription factor TATA element-binding protein and gene-specific activators such as the Drosophila nuclear receptor FTZ-F1 or the yeast basic leucine zipper protein GCN4. The human homolog of MBF1 (hMBF1) has been identified but its function, especially in transcription, remains unclear. Here we report the cDNA cloning and functional analysis of hMBF1. Two isoforms, which we term hMBF1alpha and hMBF1beta, have been identified. hMBF1alpha mRNA was detected in a number of tissues, whereas hMBF1beta exhibited tissue-specific expression. Both isoforms bound to TBP and Ad4BP/SF-1, a mammalian counterpart of FTZ-F1, and mediated Ad4BP/SF-1-dependent transcriptional activation. While hMBF1 was detected in the cytoplasm by immunostaining, coexpression of the nuclear protein Ad4BP/SF-1 with hMBF1 induced accumulation of hMBF1 in the nucleus, suggesting that hMBF1 is localized in the nucleus through its binding to Ad4BP/SF-1. hMBF1 also bound to ATF1, a member of the basic leucine zipper protein family, and mediated its activity as a transcriptional activator. These data establish that the coactivator MBF1 is functionally conserved in eukaryotes.

Casein kinase 2 binds and phosphorylates the nucleosome assembly protein-1 (NAP1) in Drosophila melanogaster

The nucleosome assembly protein-1 (NAP1) was originally identified in HeLa cells as a factor facilitating the in vitro assembly of nucleosomes. However, in yeast cells NAP1 is required in the control of mitotic events induced by the Clb2/p34(CDC28). Here, we show that Drosophila NAP1 is a phosphoprotein that is associated with a kinase able to phosphorylate NAP1. By using an in-gel kinase assay we found that this kinase displays a molecular mass of 38 kDa. Following purification and peptide microsequencing, we identified the kinase phosphorylating NAP1 as the alpha subunit of casein kinase 2 (CK2). With the help of a series of NAP1 segments and synthetic peptides, we assigned the CK2 phosphorylation sites to residues Ser118, Thr120, and Ser284. Interestingly, Ser118 and Thr120 are located within a PEST domain, while Ser284 is adjacent to the nuclear localization signal. Substitution of the identified phosphoresidues by alanine was found to reduce considerably the ability of CK2 to phosphorylate NAP1. The enhanced ability of CK2 to phosphorylate phosphatase-treated NAP1 extracted from Drosophila embryos and the similar tryptic phospho-peptide pattern of in vivo labelled NAP1 and in vitro labelled NAP1 with CK2 indicate that NAP1 is a natural substrate of CK2. Further analysis revealed that both CK2alpha and beta subunits are associated with NAP1 but we found that only the catalytic alpha subunit establishes direct contact with NAP1 on two distinct domains of this protein. The location of CK2 phosphorylation sites in NAP1 suggests that their phosphorylation can contribute to a PEST-mediated protein degradation of NAP1 and the translocation of NAP1 between cytoplasm and nucleus.

Survey of the 1998 optical biosensor literature

The utilization of optical biosensors to study molecular interactions continues to expand. In 1998, 384 articles relating to the use of commercial biosensors were published in 130 different journals. While significant strides in new applications and methodology were made, a majority of the biosensor literature is of rather poor quality. Basic information about experimental conditions is often not presented and many publications fail to display the experimental data, bringing into question the credibility of the results. This review provides suggestions on how to collect, analyze and report biosensor data.

The DNA-binding and transcriptional activities of MAZ, a myc-associated zinc finger protein, are regulated by casein kinase II

Myc-associated zinc finger protein (MAZ) is a transcription factor that contains proline-rich, alanine repeats and six C(2)H(2)-type zinc finger motifs, as well as five putative sites of phosphorylation by casein kinase II (CKII). Site-specific mutagenesis of MAZ revealed that the serine residue at position 480 was the major site of phosphorylation by CKII both in vitro and in vivo. Phosphorylation of MAZ by CKII at this serine residue was required for maximum binding of MAZ to the pyrimidine-rich DNA of the nuclease-hypersensitive element (NHE) in the 5'-end promoter region of the c-myc gene. Mutation of serine at position 480 to alanine eliminated the DNA-binding activity of MAZ to this element. Moreover, the mutated MAZ was unable to enhance the expression of luciferase encoded by a c-myc promoter/luciferase reporter gene in HeLa cells in the presence of CKII. These results suggest that phosphorylation of the serine residue at position 480 of MAZ by CKII can control the function of MAZ by altering its DNA-binding activity.

Hepatitis C virus NS5A protein is phosphorylated by casein kinase II

Hepatitis C virus (HCV) has a positive-strand RNA genome that encodes a polyprotein, which is post-translationally processed by cellular and viral proteinases into three structural and six non-structural (NS) proteins. The NS5A protein, expressed in mammalian cells, exists as two phosphorylated forms of 56 kDa and 58 kDa. In this study, we provide evidence for a stable association between NS5A and a protein kinase from rat-1 cells by affinity to immobilized glutathione-S-transferase (GST)-NS5A fusion protein. This protein kinase was associated through the N-terminus of NS5A and was not regulated by cell cycle. The GST-NS5A was also phosphorylated in vitro by the purified casein kinase II (CKII), a member of the CMCG kinase family. Since CKII and the NS5A-associated protein kinase have the same molecular size and property by In-gel kinase assay and an inhibitor treatment test, we conclude that HCV NS5A protein is phosphorylated by CKII.