Phosphorylation of transcriptional factors and cell-cycle-dependent proteins by casein kinase II

The Catalytic Subunit of Schizosaccharomyces pombe CK2 (Cka1) Negatively Regulates RNA Polymerase II Transcription through Phosphorylation of Positive Cofactor 4 (PC4)

Positive cofactor 4 (PC4) is a transcriptional coactivator that plays important roles in transcription and DNA replication. In mammals, PC4 is phosphorylated by CK2, and this event downregulates its RNA polymerase II (RNAPII) coactivator function. This work describes the effect of fission yeast PC4 phosphorylation on RNAPII transcription in a cell extract, which closely resembles the cellular context. We found that fission yeast PC4 is strongly phosphorylated by the catalytic subunit of CK2 (Cka1), while the regulatory subunit (Ckb1) downregulates the PC4 phosphorylation. The addition of Cka1 to an in vitro transcription assay can diminish the basal transcription from the Ad-MLP promoter; however, the addition of recombinant fission yeast PC4 or Ckb1 can stimulate the basal transcription in a cell extract. Fission yeast PC4 is phosphorylated in a domain which has consensus phosphorylation sites for CK2, and two serine residues were identified as critical for CK2 phosphorylation. Mutation of one of the serine residues in PC4 does not completely abolish the phosphorylation; however, when the two serine residues are mutated, CK2 is no longer able to phosphorylate PC4. The mutant which is not phosphorylated is able to stimulate transcription even though it is previously phosphorylated by Cka1, while the wild type and the point mutant are inactivated by Cka1 phosphorylation, and they cannot stimulate transcription by RNAPII in cell extracts. Those results demonstrate that CK2 can regulate the coactivator function of fission yeast PC4 and suggests that this event could be important in vivo as well.

Increasing β-catenin/Wnt3A activity levels drive mechanical strain-induced cell cycle progression through mitosis

Mechanical force and Wnt signaling activate β-catenin-mediated transcription to promote proliferation and tissue expansion. However, it is unknown whether mechanical force and Wnt signaling act independently or synergize to activate β-catenin signaling and cell division. We show that mechanical strain induced Src-dependent phosphorylation of Y654 β-catenin and increased β-catenin-mediated transcription in mammalian MDCK epithelial cells. Under these conditions, cells accumulated in S/G2 (independent of DNA damage) but did not divide. Activating β-catenin through Casein Kinase I inhibition or Wnt3A addition increased β-catenin-mediated transcription and strain-induced accumulation of cells in S/G2. Significantly, only the combination of mechanical strain and Wnt/β-catenin activation triggered cells in S/G2 to divide. These results indicate that strain-induced Src phosphorylation of β-catenin and Wnt-dependent β-catenin stabilization synergize to increase β-catenin-mediated transcription to levels required for mitosis. Thus, local Wnt signaling may fine-tune the effects of global mechanical strain to restrict cell divisions during tissue development and homeostasis.

DOI:http://dx.doi.org/10.7554/eLife.19799.001

Tissues and organs can both produce and respond to physical forces. For example, the lungs expand and contract; the heart pumps blood; and bones and muscles grow or shrink depending on how much they are used. These responses are possible because cells contain proteins that can respond to physical forces. One of the best studied of these is a protein called β-catenin, which increases the activity of genes that trigger cells to divide to promote the expansion of tissues. β-catenin is over-active in many types of cancer cells where it contributes to tumor growth. In addition to being switched on by mechanical force, β-catenin is also activated when cells detect a signal molecule called Wnt.

Cells cycle through a series of stages known as the cell cycle to ensure that they only divide when they are fully prepared to do so. Benham-Pyle et al. investigated if physical force and Wnt activate β-catenin in the same way or if they have different effects on cell division. The experiments were conducted on dog kidney cells that had left the cell cycle and had therefore temporarily stopped dividing. Physical forces, such as stretching, resulted in β-catenin being modified by an enzyme called SRC kinase, which allowed the cells to re-enter the cell cycle. On the other hand, Wnt stabilized β-catenin and temporarily increased the number of cell divisions.

When mechanical stretch and Wnt signaling were combined, the cells were more likely to re-enter the cell cycle and divide compared to either stimulus alone. These data suggest that physical force and Wnt signaling affect β-catenin differently and that they can therefore have a greater effect on cell or tissue growth when they act together than on their own.

The findings of Benham-Pyle et al. show that β-catenin is not simply switched on or off, but can have different levels of activity depending on the input the cells are receiving. Future experiments will test whether these mechanisms also exist in three-dimensional tissues, which will help us understand how organs develop.

DOI:http://dx.doi.org/10.7554/eLife.19799.002

Proteolytic processing by matrix metalloproteinases and phosphorylation by protein kinase CK2 of fetuin-A, the major globulin of fetal calf serum

Bovine fetuin-A is a member of a glycoprotein family with a wide spectrum of functions. Until now the bovine protein has been thought to be a single-chain protein. Recently we have shown that native bovine plasma fetuin-A partially exists as a disulfide-bridged two-chain protein with a heavy N-terminal and a lighter C-terminal chain similar to the structure of human fetuin-A homologue (alpha2HS glycoprotein), and also is partially phosphorylated at residues Ser120, Ser302, Ser305 and Ser306 (Wind et al., Anal. Biochem. 317 (2003) 26-33). Both fetuin-A modifications, the phosphorylation at the four sites as well as the proteolysis which causes longer or shorter light chains (termed lc-1 and lc-2, respectively), are probably brought about by targeted enzymatic activities which still need to be defined. In this study we show that authentic bovine fetuin-A disulfide-bridged two-chain forms, which include the original C-terminus, were liberated from the single-chain precursor by metalloproteinases MMP-3 (stromelysin-1) and MMP-7 (matrilysin), but not by elastase, cathepsin E and cathepsin G. Peptide sequencing suggested cleavage sites chiefly at the Pro277-Ser278 or Arg294-His295 peptide bonds. Fetuin-A radioactive phosphorylation in vitro by protein kinase CK2 caused (32)P incorporation into the fetuin-A light chain lc-1 but not lc-2 or the fetuin-A heavy chain, as revealed by MMP assisted proteolysis. Analysis by nanoESI-MS pinpointed phosphorylation at the native phospho-residues Ser302, Ser305 and Ser306 by increased relative abundance following in vitro phosphorylation. Moreover, CK2 phosphorylation of synthetic C-terminal fetuin-A peptides, used as effective controls to the native protein, strongly implies that CK2 is involved in the in vivo phosphorylation of fetuin-A. The phosphorylation of N-terminally truncated peptide homologs seemed highly dependent on the sequence context N-terminal of the phosphorylation sites, thus providing a likely explanation for the non-phosphorylation of the light chain lc-2 in native fetuin-A.

B23 is a downstream target of polyamine-modulated CK2

Our previous studies have shown that the overexpression of ornithine decarboxylase (ODC), the rate-limiting enzyme in polyamine biosynthesis, increases the enzymatic activity of the polyamine-responsive enzyme casein kinase 2 (CK2). Because CK2 is known to preferentially associate with the nuclear matrix in response to other trophic stimuli, we investigated the effects of ODC overexpression on CK2 localisation and on the CK2-mediated phosphorylation of a known CK2 substrate, the nucleolar phosphoprotein B23. Immunofluorescence analysis of CK2 and B23 in primary keratinocytes revealed that ODC overexpression resulted in the colocalisation of CK2 with B23 at the nucleolar borders. ODC overexpression also increased CK2 kinase activity 2-fold at the nuclear matrix, a response which could be abrogated by treatment of K6/ODC transgenic keratinocytes with the ODC inhibitor alpha-difluoromethylornithine (DFMO). Levels of B23 protein were also elevated in ODC-overexpressing cells compared to normal cells or transgenic cells treated with DFMO. This increase in protein level was neither due to an increase in steady-state mRNA levels, nor was it due to increased stability of B23 protein. Phosphorylation of B23 was also increased in ODC-overexpressing cells, and this increased phosphorylation could be blocked by treatment of the cells with the CK2 kinase inhibitors apigenin or 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB). These data suggest that B23 may be a downstream effector of polyamines via phosphorylation by the protein kinase CK2.

Exogenously Added Fibroblast Growth Factor 2 (FGF-2) to NIH3T3 CellsInteracts with Nuclear Ribosomal S6 Kinase 2 (RSK2) in a Cell Cycle-dependentManner

Fibroblast growth factor 2 (FGF-2) has been detected in the nuclei of many tissues and cell lines. Here we demonstrate that FGF-2 added exogenously to NIH3T3 cells enters the nucleus and interacts with the nuclear active 90-kDa ribosomal S6 kinase 2 (RSK2) in a cell cycle-dependent manner. By using purified proteins, FGF-2 is shown to directly interact through two separate domains with two RSK2 domains on both sides of the hydrophobic motif, namely the NH2-terminal kinase domain (residues 360-381) by amino acid Ser-117 and the COOH-terminal kinase domain (residues 388-400) by amino acids Leu-127 and Lys-128. Moreover, this interaction leads to maintenance of the sustained activation of RSK2 in G1 phase of the cell cycle. FGF-2 mutants (FGF-2 S117A, FGF-2 L127A, and FGF-2 K128A) that fail to interact in vitro with RSK2 fail to maintain a sustained RSK2 activity in vivo.

Protein kinase C-mediated phosphorylation of the leukemia-associated HOXA9 protein impairs its DNA binding ability and induces myeloid differentiation

HOXA9 expression is a common feature of acute myeloid leukemia, and high-level expression is correlated with poor prognosis. Moreover, HOXA9 overexpression immortalizes murine marrow progenitors that are arrested at a promyelocytic stage of differentiation when cultured and causes leukemia in recipient mice following transplantation of HOXA9 expressing bone marrow. The molecular mechanisms underlying the physiologic functions and transforming properties of HOXA9 are poorly understood. This study demonstrates that HOXA9 is phosphorylated by protein kinase C (PKC) and casein kinase II and that PKC mediates phosphorylation of purified HOXA9 on S204 as well as on T205, within a highly conserved consensus sequence, in the N-terminal region of the homeodomain. S204 in the endogenous HOXA9 protein was phosphorylated in PLB985 myeloid cells, as well as in HOXA9-immortalized murine marrow cells. This phosphorylation was enhanced by phorbol ester, a known inducer of PKC, and was inhibited by a specific PKC inhibitor. PKC-mediated phosphorylation of S204 decreased HOXA9 DNA binding affinity in vitro and the ability of the endogenous HOXA9 to form cooperative DNA binding complexes with PBX. PKC inhibition significantly reduced the phorbol-ester induced differentiation of the PLB985 hematopoietic cell line as well as HOXA9-immortalized murine bone marrow cells. These data suggest that phorbol ester-induced myeloid differentiation is in part due to PKC-mediated phosphorylation of HOXA9, which decreases the DNA binding of the homeoprotein.

Ser-123 of the large antigen of hepatitis delta virus modulates its cellular localization to the nucleolus, SC-35 speckles or the cytoplasm

Hepatitis delta virus (HDV) is a defective virus and requires hepatitis B virus (HBV) to supply envelope proteins (HBsAg) for maturation and secretion. It is known that two proteins produced by HDV, the small (SDAg) and large (LDAg) antigens, are located in the nucleolus, speckles and the cytoplasm and are involved in genome replication and virion packaging. However, little is known about how they are targeted to the specific sites where they act. A green fluorescence protein fused to LDAg (GFP–LD) has been shown previously to translocate from the nucleolus to SC-35 speckles in the presence of the casein kinase II inhibitor dichlororibofuranosyl benzimidazole. In this study, we determined which amino acids of GFP–LD were responsible for the translocation from the nucleolus to SC-35 speckles and created three GFP–LD derivatives, GFP–LDS2A, GFP–LDS123A and GFP–LDS2/123A. Fluorescence microscopy studies showed that Ser-123 mutants had a high tendency to target SC-35 speckles in both transfected HeLa and HuH-7 cells and suggested that Ser-123, but not Ser-2, plays a role in modulating LDAg translocation to the nucleolus or to SC-35 speckles. This study also demonstrated that HBsAg plays a role in facilitating the transportation of LDAg from the nucleus to cytoplasm. Compared with GFP–LD and GFP–LDS2A, mutants of Ser-123 were less efficiently transported to the cytoplasm and resulted in a lower level of secretion. In contrast, little or no isoprenylation mutant was observed in the cytoplasm of HuH-7 cells expressing HbsAg, suggesting that the isoprenylation of LDAg plays a role in export from the nucleus. Thus, the current study demonstrated that both cis and trans elements modulate HDAg translocation to various subcellular sites.

Compartment-specific phosphorylation of rat thyroid hormone receptor alpha1 regulates nuclear localization and retention

The thyroid hormone receptor alpha1 (TRalpha1) is a transcription factor, which can activate or repress gene expression in response to thyroid hormone. In addition, some of its actions, including DNA binding and transcriptional activation, are thought to be regulated by phosphorylation. Results presented here, using Xenopus oocyte microinjection assays, demonstrate that a phosphorylated form of rat TRalpha1 is present in the nucleus, whereas unphosphorylated TRalpha1 remains cytoplasmic. Changes in the phosphorylation state of TRalpha1 occur rapidly and point to the possibility that phosphorylation occurs in the nucleus. Furthermore, increasing the overall phosphorylation state of the cell leads to enhanced nuclear retention of TRalpha1, suggesting that compartment-specific phosphorylation regulates nuclear localization of TRalpha1. Enhanced nuclear retention of TRalpha1 is not dependent on phosphorylation of serine 12, a well-characterized casein kinase II site, nor is phosphorylation of this site necessary for import of TRalpha1 into the Xenopus oocyte nucleus. Similarly, mutational analysis in mammalian cells shows that nuclear localization and partitioning of TRalpha1 to the nuclear matrix are independent of serine 12 phosphorylation. Taken together, these studies suggest that phosphorylation of one or more sites in TRalpha1, excluding serine 12, enhances nuclear retention and/or inhibits nuclear export but is not directly involved in nuclear import.

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.

Role for Hes1-induced phosphorylation in Groucho-mediated transcriptional repression

Transcriptional corepressors of the Groucho/transducin-like Enhancer of split (Gro/TLE) family regulate a number of developmental pathways in both invertebrates and vertebrates. They form transcription repression complexes with members of several DNA-binding protein families and participate in the regulation of the expression of numerous genes. Despite their pleiotropic roles, little is known about the mechanisms that regulate the functions of Gro/TLE proteins. It is shown here that Gro/TLEs become hyperphosphorylated in response to neural cell differentiation and interaction with the DNA-binding cofactor Hairy/Enhancer of split 1 (Hes1). Hyperphosphorylation of Gro/TLEs is correlated with a tight association with the nuclear compartment through interaction with chromatin, suggesting that hyperphosphorylated Gro/TLEs may mediate transcriptional repression via chromatin remodeling mechanisms. Pharmacological inhibition of protein kinase CK2 reduces the Hes1-induced hyperphosphorylation of Gro/TLEs and causes a decrease in the chromatin association of the latter. Moreover, the transcription repression activity of Gro/TLEs is reduced by protein kinase CK2 inhibition. Consistent with these observations, Gro/TLEs are phosphorylated in vitro by purified protein kinase CK2. Taken together, these results implicate protein kinase CK2 in Gro/TLE functions. They suggest further that this kinase is involved in a hyperphosphorylation mechanism activated by Hes1 that promotes the transcription repression functions of Hes1-Gro/TLE protein complexes.

Identification of novel functional regions important for the activity of HOXB7 in mammalian cells

Members of the HOX family of homeobox transcription factors play a role in pattern formation in diverse developmental systems. The clearly documented role of HOX genes in the proliferation and differentiation of primary hematopoietic cells and cell lines provides a convenient system to pursue a biochemical analysis of HOX gene function in mammalian cells. To explore the role of HOXB7 in myeloid hematopoiesis, a number of mutations and deletions in the gene were constructed that targeted sequences with known functions or in regions that had not been examined previously. The wild-type and mutant B7 constructs were introduced into the murine myelomonocytic cell line, 32D, and assayed for their effects on G-CSF-induced myeloid differentiation. Wild-type HOXB7 inhibited the differentiation of 32D cells, whereas mutations in the Pbx-binding pentapeptide motif or the DNA-binding homeodomain, as well as internal deletions of the N-terminal unique region, blocked this effect. Interestingly, mutations eliminating two target sites for casein kinase II, the glutamate-rich C terminus, or the first 14 amino acids of HOXB7, led to enhanced 32D differentiation. A model proposing a role for these regions of HOXB7 is presented.

On the physiological role of casein kinase II in Saccharomyces cerevisiae

Casein kinase II (CKII) is a highly conserved serine/threonine protein kinase that is ubiquitous in eukaryotic organisms. This review summarizes available data on CKII of the budding yeast Saccharomyces cerevisiae, with a view toward defining the possible physiological role of the enzyme. Saccharomyces cerevisiae CKII is composed of two catalytic and two regulatory subunits encoded by the CKA1, CKA2, CKB1, and CKB2 genes, respectively. Analysis of null and conditional alleles of these genes identifies a requirement for CKII in at least four biological processes: flocculation (which may reflect an effect on gene expression), cell cycle progression, cell polarity, and ion homeostasis. Consistent with this, isolation of multicopy suppressors of conditional cka mutations has identified three genes that have a known or potential role in either the cell cycle or cell polarity: CDC37, which is required for cell cycle progression in both G1 and G2/M; ZDS1 and 2, which appear to have a function in cell polarity; and SUN2, which encodes a protein of the regulatory component of the 26S protease. The identity and properties of known CKII substrates in S. cerevisiae are also reviewed, and advantage is taken of the complete genomic sequence to predict globally the substrates of CKII in this organism. Although the combined data do not yield a definitive picture of the physiological role of CKII, it is proposed that CKII serves a signal transduction function in sensing and/or communicating information about the ionic status of the cell to the cell cycle machinery.

PTEN: life as a tumor suppressor

PTEN, a tumor suppressor located at chromosome 10q23, is mutated in a variety of sporadic cancers and in two autosomal dominant hamartoma syndromes. PTEN is a phosphatase which dephosphorylates phosphatidylinositol (3,4,5)-triphosphate (PtdIns-3,4,5-P3), an important intracellular second messenger, lowering its level within the cell. By dephosphorylating PtdIns-3,4,5-P3, PTEN acts in opposition to phosphatidylinositol 3-kinase (PI3K), which has a pivotal role in the creation of PtdIns-3,4,5-P3. PtdIns-3,4,5-P3 is necessary for the activation of Akt, a serine/threonine kinase involved in cell growth and survival. By blocking the activation of Akt, PTEN regulates cellular processes such as cell cycling, translation, and apoptosis. In this review, we will discuss the identification of PTEN, its mutational status in cancer, its role as a regulator of PI3K, and its domain structure.

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.

The E7 oncoprotein of human papillomavirus type 16 interacts with F-actin in vitro and in vivo

We report here that E7 oncoprotein of human papillomavirus type 16 (HPV-16) forms a complex in vivo and in vitro with actin, one of the components of the cellular cytoskeleton. The in vivo interaction was detected by immunofluorescent staining and confocal microscopic examination of normal human oral keratinocytes (NHOK) and CV-1 cells after transient expression of E7 employing the vaccinia virus-T7 RNA polymerase system and by coimmunoprecipitation from an immortalized, nontumorigenic cell line obtained after transfecting NHOK with the cloned HPV-16 DNA genome. The in vitro interaction was detected by cosedimentation of bacterially expressed E7 phosphorylated with rabbit reticulocyte lysate or purified casein kinase II (CKII) prior to incubation with F-actin. This interaction was inhibited if E7 phosphorylation by the rabbit reticulocyte lysate was prevented with heparin, a CKII inhibitor, or if the amino acids Ser-31 and Ser-32 in E7, which are phosphorylated by CKII, were replaced with amino acids that cannot be phosphorylated. Interestingly, a decrease in the amount of polymerized actin occurred in cells expressing E7.

Molecular characterization of a novel gene family (PHTF) conserved from Drosophila to mammals

PHTF1 (putative homeodomain transcriptional factor; HGMW-approved symbol PHTF1) is a putative homeobox gene located at band 1p11-p13 of the human genome. We report here the cloning and sequencing of its mouse and Drosophila orthologs. The conservation between mouse and human proteins extends over the entire protein and is localized at the putative homeodomain and at the N- and C-terminal regions of Drosophila protein sequence. Blast searches allowed us to identify another member of the PHTF family, PHTF2, located at 7q11.23-q21 of the human genome. The strongest homologies between human PHTF1 and PHTF2 are localized to the domains that we already described in Drosophila, i.e., the putative homeodomain and the N- and C-terminal regions. The human and mouse genes display 98% similarity to one another, 56% similarity with the Drosophila gene, and 67% similarity with PHTF2, suggesting that phtf might define a novel gene family of highly divergent homeobox genes. Finally, Northern blot analysis showed that while PHTF1 is expressed mainly in testis, PHTF2 is predominantly expressed in muscle, suggesting that these two genes may have acquired different functions after their duplication and divergence.

Casein kinase 2 as a potentially important enzyme in the nervous system

Protein kinase CK2 is a ubiquitous and pleiotropic seryl/threonyl protein kinase which is highly conserved in evolution indicating a vital cellular role for this kinase. The holoenzyme is generally composed of two catalytic (alpha and/or alpha') and two regulatory (beta) subunits, but the free alpha/alpha' subunits are catalytically active by themselves and can be present in cells under some circumstances. Special attention has been devoted to phosphorylation status and structure of these enzymic molecules, however, their regulation and roles remain intriguing. Until recently, CK2 was believed to represent a kinase especially required for cell cycle progression in non-neural cells. At present, with respect to recent findings, four essential features suggest potentially important roles for this enzyme in specific neural functions: (1) CK2 is much more abundant in brain than in any other tissue; (2) there appear to be a myriad of substrates for CK2 in both synaptic and nuclear compartments that have clear implications in development, neuritogenesis, synaptic transmission, synaptic plasticity, information storage and survival; (3) CK2 seems to be associated with mechanisms underlying long-term potentiation in hippocampus; and (4) neurotrophins stimulate activity of CK2 in hippocampus. In addition, some data are suggestive that CK2 might play a role in processes underlying progressive disorders due to Alzheimer's disease, ischemia, chronic alcohol exposure or immunodeficiency virus HIV. The present review focuses mainly on the latest data concerning the regulatory mechanisms and the possible neurophysiological functions of this enzyme.

The biology of the mammalian Krüppel-like family of transcription factors

Recent advances in molecular cloning have led to the identification of a large number of mammalian zinc finger-containing transcription factors that exhibit homology to the Drosophila melanogaster protein, Krüppel. Although the amino acid sequences in the zinc finger domains of these Krüppel-like factors (KLFs) are closely related to one another, the regions outside the zinc fingers of the proteins are usually unique. KLFs display seemingly different and broad biological properties with each functioning as an activator of transcription, a repressor or both. This review article provides a current phylogenetic classification of the identified KLFs to date. More importantly, the currently known biological activities of the KLFs in regulating transcription, cell proliferation, differentiation and development are summarized and compared. Further characterization of this interesting protein family should provide additional insights into the their respective regulatory role in various important biological processes.

Molecular cloning of a zinc finger autoantigen transiently associated with interphase nucleolus and mitotic centromeres and midbodies. Orthologous proteins with nine CXXC motifs highly conserved from nematodes to humans

We have cloned a novel human autoimmune antigen in a patient suffering from rheumatoid arthritis with high levels of antibodies to the nucleolus organizer regions. Initially the human autoimmune serum was used to select a cDNA of 317 amino acids from a hamster expression library. Using the hamster DNA as a probe, we isolated the human homologous cDNA of 320 amino acids. Human and hamster polypeptides share a 95% amino acid homology. The deduced 36-kDa protein contains a putative amino-terminal NLS signal, nine cysteine-X-X-cysteine motifs highly conserved, and a carboxyl-terminal poly acidic region. Several homologous expressed sequence tags have been identified in data bases suggesting that orthologous proteins are present throughout evolution from worms to humans. A Drosophila expressed sequence tag was further completely sequenced for a full-length protein with 60% amino acid identity to the human homologue. Northern blot analysis revealed that this novel protein is widely distributed in human tissues with significantly higher expression levels in heart and skeletal muscle. Specific antibodies to the recombinant protein and transfection experiments demonstrated by immunofluorescence the localization of the protein predominantly but not exclusively to the nucleolus of interphase mammalian cells. In actinomycin D-treated cells the protein remains associated with the nucleolus but is not segregated, like other ribosomal factors such as upstream binding factor. In mitosis the protein was found to be associated with centromeres and concentrated at the midbody in cytokinesis. Transient distribution of this evolutionarily conserved zinc finger nucleolar autoantigen to the mitotic centromeres may provide the means for several aspects of cell cycle control and transcriptional regulation.

Biochemical characterization of a casein kinase I-like actin kinase responsible for the actin-induced suppression of casein kinase II activity in vitro

By combination of column chromatographies (heparin-agarose, HiTrap heparin and HiTrap SP columns) and gel filtration on a Superdex 200-pg HPLC column, an actin kinase was partially purified from a 1. 5 M NaCl extract of porcine liver. The actin kinase was finally purified, by actin-Sepharose column chromatography (HPLC), as an actin-binding protein kinase. The biochemical properties, such as (1) requirements of divalent cations (10 mM Mg(2+) and 3 mM Mn(2+)) and effective phosphate acceptors (actin and alpha-casein), (2) phosphorylation of both Ser- and Thr-residues on these two phosphate acceptors, (3) autophosphorylation of the catalytic subunit (approximately 37 kDa), and (4) inhibition kinetics by CK-I-7 (a CK-I specific inhibitor), of the purified actin kinase were similar to those reported for CK-I purified from various mammalian cells, but it was distinguishable from three cellular actin kinases (A-kinase, C-kinase and actin-fragmin kinase (approximately 80 kDa)). The 37 kDa actin kinase-mediated phosphorylation of actin did not relate to its polymerizability. Inhibition of CK-II-mediated phosphorylation of functional cellular proteins, including calmodulin (CaM), by actin was significantly stimulated after its full phosphorylation by the purified 37 kDa actin kinase or rCK-I in vitro. These results suggest that: (1) the 37 kDa Ser/Thr actin-binding kinase may be classified as a member of the CK-I family; and (2) specific phosphorylation of actin by the actin kinase may be involved in the suppression mechanism of CK-II-mediated signal transduction at the cellular level.

Expression of the casein kinase 2 subunits in Chinese hamster ovary and 3T3 L1 cells provides information on the role of the enzyme in cell proliferation and the cell cycle

In order to investigate the in vivo functions of protein kinase CK2 (CK2), the expression of Myc-tagged versions of the subunits, Myc-CK2alpha and Myc-CK2beta, was carried out in Chinese hamster ovary cells (CHO cells) and in 3T3 L1 fibroblasts. Cell proliferation in these cells was examined. CHO cells that transiently overexpressed the Myc-CK2beta subunit exhibited a severe growth defect, as shown by a much lower value of [(3)H]thymidine incorporation than the vector controls, and a rounded shrunken morphology. In contrast, cells overexpressing Myc-tagged CK2alpha showed a slightly but consistently higher value of [(3)H]thymidine incorporation than the controls. The defect in cell growth and changes in morphology caused by Myc-CK2beta overexpression were partially rescued by coexpression of Myc-tagged CK2alpha. In parallel to the studies in CHO cells, the stable transfection of Myc-CK2alpha and Myc-CK2beta subunits was achieved in 3T3 L1 fibroblast cells. Similarly, the ectopic expression of Myc-CK2beta, but not Myc-CK2alpha, caused a growth defect. By measuring [(3)H]thymidine incorporation, it was found that expression of Myc-CK2beta prolonged the G(1) phase and inhibited up-regulation of cyclin D1 expression during G(1). In addition, a lower mitotic index and lower mitotic cyclin-dependent kinase activities were detected in Myc-CK2beta-expressing cells. Detailed analysis of stable cells that were synchronously released into the cell cycle revealed that the expression of Myc-CK2beta inhibited cells entering into mitosis and prevented the activation of mitotic cyclin-dependent kinases. Taken together, results from both transient and stable expression of CK2 subunits strongly suggest that CK2 may be involved in the control of cell growth and progression of the cell cycle.

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.

Erythroid Kruppel like factor: from fishing expedition to gourmet meal

Erythroid Kruppel like factor (EKLF) is the founding member of a family of transcription factors which are defined by the presence of three C-terminal C2H2-type zinc fingers. Since its discovery 6 years ago, the study of EKLF has been intense. In this review I will revisit the discovery of EKLF, and highlight recent advances in our understanding of how it interacts with other proteins to regulate erythroid gene transcription. The current knowledge of the biological role/s of EKLF in erythroid cell differentiation and globin gene switching are summarized.

Cell-cycle modulation of CK2 activity in tobacco BY-2 cells

Protein kinase CK2 is an ubiquitous Ser/Thr kinase essential for cell growth. We have used the highly synchronizable tobacco BY-2 cell line to investigate whether CK2 activity and expression are regulated in a cell cycle phase-dependent manner in higher plants. Specific cDNA probes for tobacco CK2alpha and beta subunits, respectively, and polyclonal antibodies recognising alpha and beta subunits separately, were obtained to determine mRNA and protein levels of both subunits. Our results show that CK2 activity oscillates throughout the cell cycle, peaking at G1/S and M phases, due to a post-translational regulation of the tetrameric enzyme. Additional levels of control of CK2 expression operate in relation to the proliferative state of the cells, including differential accumulation of alpha and beta transcripts and post-transcriptional regulation of protein levels (beta subunit). Moreover, in vivo inhibition of CK2 activity corroborates the requirement of the functional CK2 to progress through the cell division cycle, and suggests that CK2 might play an important role at the G2/M checkpoint.

Histone acetyltransferase and protein kinase activities copurify with a putative Xenopus RNA polymerase I holoenzyme self-sufficient for promoter-dependent transcription

Mounting evidence suggests that eukaryotic RNA polymerases preassociate with multiple transcription factors in the absence of DNA, forming RNA polymerase holoenzyme complexes. We have purified an apparent RNA polymerase I (Pol I) holoenzyme from Xenopus laevis cells by sequential chromatography on five columns: DEAE-Sepharose, Biorex 70, Sephacryl S300, Mono Q, and DNA-cellulose. Single fractions from every column programmed accurate promoter-dependent transcription. Upon gel filtration chromatography, the Pol I holoenzyme elutes at a position overlapping the peak of Blue Dextran, suggesting a molecular mass in the range of approximately 2 MDa. Consistent with its large mass, Coomassie blue-stained sodium dodecyl sulfate-polyacrylamide gels reveal approximately 55 proteins in fractions purified to near homogeneity. Western blotting shows that TATA-binding protein precisely copurifies with holoenzyme activity, whereas the abundant Pol I transactivator upstream binding factor does not. Also copurifying with the holoenzyme are casein kinase II and a histone acetyltransferase activity with a substrate preference for histone H3. These results extend to Pol I the suggestion that signal transduction and chromatin-modifying activities are associated with eukaryotic RNA polymerases.

The cell cycle: a review

The cell cycle is a complex process that involves numerous regulatory proteins that direct the cell through a specific sequence of events culminating in mitosis and the production of two daughter cells. Central to this process are the cyclin-dependent kinases (cdks), which complex with the cyclin proteins. These proteins regulate the cell's progression through the stages of the cell cycle and are in turn regulated by numerous proteins, including p53, p21, p16, and cdc25. Downstream targets of cyclin-cdk complexes include pRb and E2F. The cell cycle can be altered to the advantage of many viral agents, most notably polyomaviruses, papillomaviruses, and adenoviruses. The cell cycle often is dysregulated in neoplasia due to alterations either in oncogenes that indirectly affect the cell cycle or in tumor suppressor genes or oncogenes that directly impact cell cycle regulation, such as pRb, p53, p16, cyclin D1, or mdm-2. The cell cycle has become an intense subject of research in recent years. This research has led to the development of techniques useful for the determination of the effects of drugs and toxins on the cell cycle. Any drug or toxin with DNA damaging ability would be expected to alter cell cycle progression, and therefore, the cell cycle should be considered in the design of studies using such chemicals. With the appropriate techniques, cell cycle alterations may also be detected in tissue sections. Because of the ubiquitous nature of the cell cycle, it deserves consideration in the design and interpretation of studies in a wide variety of disciplines.

Regulation of erythroid Krüppel-like factor (EKLF) transcriptional activity by phosphorylation of a protein kinase casein kinase II site within its interaction domain

Erythroid Krüppel-like factor (EKLF) is a red cell-specific activator whose presence is crucial for establishing high levels of adult beta-globin expression in definitive cells during erythroid ontogeny. However, its simple presence within the erythroid lineage is not sufficient to activate the beta-globin promoter. One explanation that may account for this is that post-translational modification of EKLF differs within erythroid cell populations and regulates its activity. We have therefore addressed whether phosphorylation plays a role in modulating EKLF action. First, in vivo analyses implicate serine/threonine kinases as important players in the terminal differentiation of MEL cells, and demonstrate that EKLF is phosphorylated at serine and threonine residues within its transactivation region. Second, directed disruption of a protein kinase casein kinase (CK) II site, located within the EKLF interaction domain, abolishes EKLF transactivation and in vivo competition activity. Third, in vitro assays demonstrate that CKIIalpha interacts with EKLF, and that the EKLF interaction domain is phosphorylated by CKII only at Thr-41; however, the CKII-site mutant is not phosphorylated. Finally, the transactivation capability of EKLF is augmented by co-transfection of CKIIalpha. We conclude that EKLF is a phosphoprotein whose ability to transcriptionally activate an adjacent promoter is critically dependent on the phosphorylation status of a specific site located within the EKLF interaction domain, and that serine/threonine kinases play an important role in this process.

Protein kinase CK2alpha' is induced by serum as a delayed early gene and cooperates with Ha-ras in fibroblast transformation

Protein kinase CK2 is an ubiquitous and pleiotropic Ser/Thr protein kinase composed of two catalytic (alpha and/or alpha') and two noncatalytic (beta) subunits forming a heterotetrameric holoenzyme involved in cell growth and differentiation. Here we report the identification, cloning, and oncogenic activity of the murine CK2alpha' subunit. Serum treatment of quiescent mouse fibroblasts induces CK2alpha' mRNA expression, which peaks at 4 h. The kinetics of CK2alpha' expression correlate with increased kinase activity toward a specific CK2 holoenzyme peptide substrate. The ectopic expression of CK2alpha' (or CK2alpha) cooperates with Ha-ras in foci formation of rat primary embryo fibroblasts. Moreover, we observed that BALB/c 3T3 fibroblasts transformed with Ha-ras and CK2alpha' show a faster growth rate than cells transformed with Ha-ras alone. In these cells the higher growth rate correlates with an increase in calmodulin phosphorylation, a protein substrate specifically affected by isolated CK2 catalytic subunits but not by CK2 holoenzyme, suggesting that unbalanced expression of a CK2 catalytic subunit synergizes with Ha-ras in cell transformation.

Signals mediating nuclear targeting and their regulation: application in drug delivery

The recent progress with respect to understanding the signals mediating the transport of proteins in both directions through the NPC, and cellular proteins interacting with these signals to effect the transport process has made possible a number of advances in terms of the use of this information in a clinical setting. In particular, our knowledge of the mechanism of regulation of the process, and of how we may exploit the cellular transport machinery itself in a therapeutic situation, especially where there may be transport pathways specific to particular viruses, has advanced considerably. In this context, this review expounds current understanding of the signals conferring targeting to the nucleus, and their practical and potential use in delivering molecules of interest to the nucleus in a clinical context. It also deals with targeting signals conferring nuclear protein export/ shuttling between nuclear and cytoplasmic compartments as well as with those conferring nuclear or cytoplasmic retention, and with the specific mechanisms regulating the activity of these signals, and in particular those regulating signal-dependent nuclear protein import. Detailed understanding of the processes of signal-mediated nuclear protein import/export and its regulation enables the considered application and optimization of approaches to target molecules of interest, such as plasmid DNA or toxic molecules, efficiently to the nucleus according to need in a clinical or research context, and enhance the expression or efficiency of their action, respectively. The use of nuclear targeting signals in this context is reviewed, and future possibilities in terms of the application of our growing understanding of nuclear transport and its regulation are discussed.

Requirement for cellular cyclin-dependent kinases in herpes simplex virus replication and transcription

Several observations indicate that late-G1/S-phase-specific cellular functions may be required for herpes simplex virus (HSV) replication: (i) certain mutant HSV strains are replication impaired during infection of cells in the G0/G1 but not in the G1/S phase of the cell cycle, (ii) several late-G1/S-phase-specific cellular proteins and functions are induced during infection, and (iii) the activity of a cellular protein essential for expression of viral immediate-early (IE) genes, HCF, is normally required during the late G1/S phase of the cell cycle. To test the hypothesis that late-G1/S-phase-specific cellular functions are necessary for HSV replication, HEL or Vero cells were infected in the presence of the cell cycle inhibitors roscovitine (Rosco) and olomoucine (Olo). Both drugs inhibit cyclin-dependent kinase 1 (cdk-1) and cdk-2 (required for cell cycle progression into the late G1/S phase) and cdk-5 (inactive in cycling cells) but not cdk-4 or cdk-6 (active at early G1). We found that HSV replication was inhibited by Rosco and Olo but not by lovastatin (a cell cycle inhibitor that does not inhibit cdk activity), staurosporine (a broad-spectrum protein serine-threonine kinase inhibitor), PD98059 (an inhibitor specific for erk-1 and -2) or iso-Olo (a structural isomer of Olo that does not inhibit cdk activity). The concentrations of Rosco and Olo required to inhibit cell cycle progression and viral replication in both HEL and Vero cells were similar. Inhibition of viral replication was found not to be mediated by drug-induced cytotoxicity. Efforts to isolate Rosco- or Olo-resistant HSV mutants were unsuccessful, indicating that these drugs do not act by inhibiting a single viral target. Viral DNA replication and accumulation of IE and early viral RNAs were inhibited in the presence of cell cycle-inhibitory concentrations of Rosco or Olo. We therefore conclude that one or more cdks active from late G1 onward or inactive in nonneuronal cells are required for accumulation of HSV transcripts, viral DNA replication, and production of infectious virus.

Transcriptional factors for specific globin genes

Correct temporal control of the beta-like globin cluster is generated in part by the binding of tissue-restricted transcriptional regulators to their cognate sites. Erythroid Krüppel-like Factor (EKLF) is one of these red cell-specific activators that is particularly important for switching on adult beta-globin gene expression. However, its simple presence is not sufficient to activate the beta-globin promoter, as primitive erythroid cells and a number of erythroid cell lines express EKLF yet do not express adult beta-globin. One explanation that may account for these observations is that post-translational modification of EKLF differs within these cell populations. To address this issue, we are investigating whether phosphorylation plays a role in modulating EKLF activity. In vitro and in vivo approaches have been used to demonstrate that EKLF is a phosphoprotein whose ability to bind DNA and transcriptionally activate an adjacent promoter is critically dependent on its phosphorylation status. Of particular interest is a casein kinase II site within the EKLF minimal transactivation domain.

Multisite phosphorylation and the nuclear localization of phosphatase inhibitor 2-green fluorescent protein fusion protein during S phase of the cell growth cycle

Human phosphatase inhibitor 2 (Inh2) is a phosphoprotein that complexes with type 1 protein phosphatase, and its expression peaks during S phase and mitosis during the cell cycle. Localization of Inh2 was visualized in HS68 human fibroblasts by fusing Inh2 to green fluorescent protein (GFP). During G1 phase, Inh2-GFP was localized in the cytoplasm, and as cells progressed into S phase Inh2-GFP accumulated in the nucleus. Known phosphorylation sites of Inh2 at Thr-72, Ser-86, and Ser-120/121 were each replaced with alanine. None of the mutated Inh2-GFP proteins accumulated in the nucleus during S phase, indicating that all of these phosphorylation sites were required. Mutation of two lysine residues in a putative nuclear localization sequence in Inh2 also prevented the Inh2-GFP fusion protein from accumulating in the nucleus during S phase. Recombinant Inh2 was phosphorylated by kinases in cytosols prepared from G1 and S phase cells. The amount of Inh2 kinase attributed to casein kinase 2, based on inhibition by heparin, increased 2.6-fold from G1 to S phase. In addition, kinases in G1 versus S phase cytosols produced distinct Inh2 phosphopeptides. The results indicate that changes in phosphorylation of Inh2 are involved in intracellular redistribution of the protein during the cell cycle.

A role for CKII phosphorylation of the cactus PEST domain in dorsoventral patterning of the Drosophila embryo

Regulated proteolysis of Cactus, the cytoplasmic inhibitor of the Rel-related transcription factor Dorsal, is an essential step in patterning of the Drosophila embryo. Signal-induced Cactus degradation frees Dorsal for nuclear translocation on the ventral and lateral sides of the embryo, establishing zones of gene expression along the dorsoventral axis. Cactus stability is regulated by amino-terminal serine residues necessary for signal responsiveness, as well as by a carboxy-terminal PEST domain. We have identified Drosophila casein kinase II (CKII) as a Cactus kinase and shown that CKII specifically phosphorylates a set of serine residues within the Cactus PEST domain. These serines are phosphorylated in vivo and are required for wild-type Cactus activity. Conversion of these serines to alanine or glutamic acid residues differentially affects the levels and activity of Cactus in embryos, but does not inhibit the binding of Cactus to Dorsal. Taken together, these data indicate that wild-type axis formation requires CKII-catalyzed phosphorylation of the Cactus PEST domain.

Phosphorylation of the proteins of the extracellular matrix of mineralized tissues by casein kinase-like activity

The extracellular matrix of the connective tissue contains non-collagenous proteins (NCP) which are acidic in character. The NCP of mineralizing systems (bone, dentin) differ from those of the non-mineralizing systems (skin, tendon) in that the mineralized tissue NCP are frequently phosphorylated. The phosphorylated proteins have been implicated in various aspects of the mineralization process. Thus, it is of interest to consider the mechanism and regulation of phosphorylation of the major matrix NCP. The majority of the phosphorylation takes place at Ser or Thr residues embedded within acidic sequences, and therefore are targets for casein kinase I (CK1) or casein kinase II (CK2)-like kinases. CK1 and CK2 are distantly related members of the protein kinase family. They are ubiquitous, constitutively active, second-messenger-independent kinases. CK1 is found in a variety of isoforms, all homologous to the alpha-subunit of the protein kinase family. It acts as a monomer. The active form of CK2 is a tetrameric holoenzyme, with 2 alpha catalytic subunits and 2 beta regulatory subunits. The CK2 alpha has activity alone, but the holoenzyme is four- to five-fold that activity. CK2 can use either ATP or GTP as the phosphate donor, but CK1 can use only ATP. The CK2 activity which phosphorylates the mineralized tissue NCP appears to be localized to membrane-associated cell fractions, and is present in the endoplasmic reticulum and Golgi compartments in osteoblasts, where phosphorylation of the secreted proteins appears to take place as co- and post-translational processes. Data indicate that both alpha and beta subunits of the membrane-associated CK2 are isoforms of the cytosolic CK2 in the same cells. The CK1 has not been specifically localized. Studies of dephosphorylated NCP such as phosphophoryn (PP) have shown that CK1 will not phosphorylate dephosphorylated dPP unless prior phosphorylation with CK2 has been carried out. In turn, CK2 activity may be initiated only after an initial phosphorylation of one of the messenger-dependent kinases. Thus, the phosphorylation reactions in mineralized tissues may be a tightly regulated hierarchical or sequential cascade of intracellular phosphorylation events.

A single serine residue at position 375 of VP16 is critical for complex assembly with Oct-1 and HCF and is a target of phosphorylation by casein kinase II

We show that VP16 is phosphorylated by cellular kinases in vivo and in vitro and map the major sites of phosphorylation to be on serines towards the C-terminus, downstream of position 370 in both cases. Deletion of the acidic activation domain had no effect on phosphorylation, refining the sites to between position 370 and 411. Within VP16, the C-terminal boundary for complex formation with Oct-1 and HCF lies at position 388, and between 370 and 388 lies one serine, at position 375. This is a consensus casein kinase II (CKII) site and, using purified wild-type and mutant proteins, we show that it is the main CKII site in the body of the N-terminal complex-forming region. This site is also phosphorylated in nuclear extracts. Although other sites, mainly Ser411, are also phosphorylated by nuclear kinase(s), the single substitution of Ser375 to alanine abolishes CKII phosphorylation in vitro and virtually eliminates complex formation. This serine lies in a surface-exposed region of VP16 and, although complex formation is disrupted, other activities of the mutant are unaffected. Ser375 is also required in vivo where substitution to alanine abolishes transactivation, while replacement with threonine restores normal levels of activity.

Developmental alterations in casein kinase 2 activity during the morphogenesis of quail secondary palate

BACKGROUND

During the progression of avian secondary palate morphogenesis, the rate of cell proliferation declines, whereas the production and accumulation of extracellular matrices increases. To investigate the regulation of these events, we examined the quail secondary palate for the activity of casein kinase 2 (CK 2), a pleiotropic serine/threonine second messenger independent enzyme implicated in cell growth and differentiation.

METHODS

Quail palatal shelves were dissected between days 5 and 9 of incubation, which is the period of palate morphogenesis in quail, and prepared either for light microscopic observations or homogenized, cleared by ultracentrifugation, and then subjected to fractionation on a MonoQ column by fast protein liquid chromatography and Western immunoblotting.

RESULTS

Histological examination showed that the palatal shelves appeared on day 5 of incubation and approximated by day 8 of incubation. Fractionation of palate extract using a Mono-Q column revealed the presence of a major peak of phosvitin phosphotransferase activity which eluted with 0.5 M NaCl. This activity peak coincided with the presence of a 42 kDa subunit of CK 2 as determined by Western blotting with a CK 2 specific antibody. The CK 2 activity towards phosvitin was elevated on days 5 and 6 and then rapidly declined by day 9. The decrease in CK 2 activity did not correlate with a decrease in CK 2 protein during palate development indicating that the differential activity of the CK 2 enzyme observed during quail palate development may be due to post-translational modifications of the enzyme. A high positive correlation was found between the CK 2 phosphotransferase activity and both the proliferation index and DNA synthesis during palate development.

CONCLUSION

On the basis of literature analysis and the results of the present study, it was suggested that the activity of CK 2 may be regulated along with protein kinase A to coordinate cell proliferation and the synthesis of extracellular matrices during palate development in quail.

CADp44: a novel regulatory subunit of the 26S proteasome and the mammalian homolog of yeast Sug2p

We have identified a novel protein, CADp44, based on the analysis of cDNAs derived from the brainstem of the 13-lined ground squirrel, Spermophilus tridecemlineatus. CADp44 has an unmodified molecular mass of 44,178 Da and contains multiple functional domains, including a conserved ATPase domain (CAD) and a leucine zipper motif. We show that distinct regions of the CADp44 sequence are identical to a set of peptides prepared from a recently identified bovine protein, referred to as p42, which is found in the PA700 regulatory complex of the 26S proteasome (DeMartino et al., 1996). We also show that CADp44 is the functional homolog of the newly characterized Sug2 protein from the budding yeast, Saccharomyces cerevisiae (Russell et al., 1996). Consistent with its role as a component of the 26S proteasome, CADp44 mRNA is found in all ground squirrel tissues examined. Evolutionary relationships based on sequence analysis show that both CADp44 and yeast Sug2p are distinct from the other five CAD ATPases found in the PA700, and together comprise the sixth and newest CAD subunit of the regulatory complex of the 26S proteasome.

Threonine 1342 in human topoisomerase IIalpha is phosphorylated throughout the cell cycle

To investigate the relationship between the modulation of topoisomerase II activity and its phosphorylation state during the cell cycle, a monoclonal antibody against C-terminal peptide (residues 1335-1350) of topoisomerase IIalpha containing a consensus sequence of casein kinase II, TDDE and its phosphorylated threonine were prepared. In an enzyme-linked immunosorbent assay, the antibody, named PT1342, recognized the immunogenic phosphopeptide but not the non-phosphorylated form of the peptide. The PT1342 antibody reacted only with a 170-kDa protein from HeLa cells and recognized anti-topoisomerase IIalpha immunoprecipitants. Furthermore, the antibody did not react with the human topoisomerase IIalpha mutated at codon 1342 from threonine to alanine, showing that PT1342 was directed against the phosphorylated threonine 1342. To examine the level of phosphorylation of threonine 1342 of topoisomerase IIalpha through the cell cycle, HeLa cells were stained simultaneously for phosphorylated topoisomerase IIalpha and DNA and analyzed by flow cytometry. Cells in the G2-M phase contained about double the PT1341-reacted topoisomerase IIalpha than did cells in G1 or S phases. The antibody stained the nuclei in interphase and mitotic chromosomes and its periphery, as seen with anti-topoisomerase IIalpha antibody. Thus, threonine 1342 in topoisomerase IIalpha is phosphorylated throughout the cell cycle.

Casein kinase II and protein kinase C modulate hepatitis delta virus RNA replication but not empty viral particle assembly

Hepatitis delta virus (HDV) contains two virus-specific delta antigens (HDAgs), large and small forms, which are identical in sequence except that the large one contains 19 extra amino acids at the C terminus. HDAgs are nuclear phosphoproteins with distinct biological functions; the small form activates HDV RNA replication, whereas the large form suppresses this process but is required for viral particle assembly. In this study, we have characterized the phosphorylative property of HDAg in a human hepatoma cell line (HuH-7) and examined the role of phosphorylation in HDAg function. As demonstrated by in vivo labeling and kinase inhibitor experiments, the phosphorylation levels of both HDAgs were diminished by the inhibitor of casein kinase II (CKII). Nevertheless, phosphorylation of only the small form could be markedly reduced by the protein kinase C (PKC) inhibitor, suggesting different phosphorylation properties between the two HDAgs. When these two kinase inhibitors were added separately to the transient-expression system, HDV RNA replication was profoundly suppressed. In contrast, the inhibitors did not affect the assembly of empty HDAg particle from HDAgs and hepatitis B virus surface antigen. To further examine the role of phosphorylation in HDAg function, two conservative CKII recognition sites at Ser-2 and Ser-123 of both HDAgs and one potential PKC recognition site at Ser-210 of the large HDAg were altered to alanine by site-directed mutagenesis. Transfection experiments indicated that mutation at Ser-2, but not Ser-123, significantly impaired the activity of the small HDAg in assisting HDV RNA replication. This property is in accordance with our observation that Ser-2, not Ser-123, was the predominant CKII phosphorylation site in the small HDAg. Our studies also excluded the possibility that the phosphorylation of Ser-2, Ser-123, or Ser-210, had roles in the trans-suppression activity of the large HDAg, in the assembly of empty virus-like HDAg particle, and in the nuclear transport of HDAgs. In conclusion, our results indicate that both CKII and PKC positively modulate HDV RNA replication but not the assembly of empty HDAg particle. The role of CKII in HDV replication may at least in part be accounted for by the phosphorylation of Ser-2 in the small HDAg. The effect of PKC on HDV RNA replication is, however, not to mediate the phosphorylation of the conservative Ser-210 in the large HDAg but rather to act on as-yet-unidentified Ser or Thr residues in the small HDAg or cellular factors. These findings provide the first insight into the roles of phosphorylation of the two HDAgs in the HDV replication cycle.

Proteins in unexpected locations

Members of all classes of proteins--cytoskeletal components, secreted growth factors, glycolytic enzymes, kinases, transcription factors, chaperones, transmembrane proteins, and extracellular matrix proteins--have been identified in cellular compartments other than their conventional sites of action. Some of these proteins are expressed as distinct compartment-specific isoforms, have novel mechanisms for intercompartmental translocation, have distinct endogenous biological actions within each compartment, and are regulated in a compartment-specific manner as a function of physiologic state. The possibility that many, if not most, proteins have distinct roles in more than one cellular compartment has implications for the evolution of cell organization and may be important for understanding pathological conditions such as Alzheimer's disease and cancer.

Casein kinase 2 inhibits the renaturation of complementary DNA strands mediated by p53 protein

Considerable effort is currently being devoted to understand the functions of protein p53, a major regulator of cell proliferation. The protein p53 has been reported to catalyse the annealing of complementary DNA or RNA strands. We report that this activity is inhibited in the presence of the serine/threonine protein kinase CK2. It is shown that this inhibition can be explained by the occurrence of a high-affinity molecular association between p53 and CK2. The molecular complex involves an interaction between the C-terminal domain of p53 and the beta subunit of the oligomeric kinase. Accordingly, the isolated alpha subunit of the kinase was without effect. In addition, after phosphorylation by CK2, phosphorylated p53 lost its DNA annealing activity. Because the C-terminal domain of p53 is both involved in the association with CK2 and phosphorylated by it, our results suggest that either protein-protein interaction or phosphorylation of this domain might control the base pairing of complementary sequences promoted by p53 in processes related to DNA replication and repair.

Oct-1 [corrected] and Oct-2 DNA-binding site specificity is regulated in vitro by different kinases

The transcription factors Oct-1 and Oct-2 bind differentially to three octamer binding sequences corresponding to the octamer binding site from the H2B promoter [ATGCTAATAA], a simple TAATGARAT motif, found in herpes simplex virus IE4/5 genes [GCGGTAATGAGAT], and a perfect consensus overlapping octamer/TAATGARAT motif [ATGCTAATGAGAT]. By comparing the effects of protein kinase A, protein kinase C and casein kinase 2 in vitro on the binding of Oct-1 and Oct-2 to the three motifs, we show that the actions of these kinases regulate Oct-1 and Oct-2 DNA binding independently of each other in a binding-site-specific manner. Inhibition of cellular phosphatases also regulate Oct-1 and Oct-2 DNA binding in a binding-site-specific manner. Both kinase and phosphatase activity are important for regulating the DNA binding activity of Oct-1 and Oct-2 because, in the presence of phosphatase inhibitors, protein kinase A attenuates the binding of both Oct-1 and Oct-2 to the octamer binding site but enhances binding when phosphatase inhibitors are omitted. Thus the DNA specificity of Oct-1 and Oct-2 can be regulated in vitro by the action of different kinases.

Phosphorylation of IkappaBalpha in the C-terminal PEST domain by casein kinase II affects intrinsic protein stability

The NF-kappaB/Rel transcription factors participate in the activation of immune system regulatory genes and viral early genes including the human immunodeficiency virus type 1 long terminal repeat. NF-kappaB/Rel proteins are coupled to inhibitory molecules, collectively termed IkappaB, which are responsible for cytoplasmic retention of NF-kappaB. Cell activation leads to the phosphorylation and degradation of IkappaBalpha, permitting NG-kappaB/Rel translocation to the nucleus and target gene activation. To further characterize the signaling events that contribute to IkappaBalpha phosphorylation, a kinase activity was isolated from Jurkat T cells that specifically interacted with IkappaBalpha in an affinity chromatography step and phosphorylated IkappaBalpha with high specificity in vitro. By using an in-gel kinase assay with recombinant IkappaBalpha as substrate, two forms of the kinase (43 and 38 kDa) were identified. Biochemical criteria and immunological cross-reactivity identified the kinase activity as the alpha catalytic subunit of casein kinase II (CKII). Deletion mutants of IkappaBalpha delta1 to delta4) localized phosphorylation to the C-terminal PEST domain of IkappaBalpha. Point mutation of residues T-291, S-283, and T-299 dramatically reduced phosphorylation of IkappaBalpha by the kinase in vitro. NIH-3T3 cells that stably expressed wild-type IkappaBalpha (wtIkappaB), double-point-mutated IkappaBalpha (T291A, S283A), or triple-point-mutated IkappaBalpha (T291A, S283A, T299A) under the control of the tetracycline-responsive promoter were generated. Constitutive phosphorylation of the triple point mutant was eliminated in vivo, although tumor necrosis factor-inducible IkappaBalpha degradation was unaffected. In cell lines and in transiently transfected cells, mutation of the CKII sites in IkappaBalpha resulted in a protein with increased intrinsic stability. Together with results demonstrating a role for N-terminal sites in inducer-mediated phosphorylation and degradation of IkappaBalpha, these studies indicate that CKII sites in the C-terminal PEST domain are important for constitutive phosphorylation and intrinsic stability of IkappaBalpha.

The Surf-6 gene of the mouse surfeit locus encodes a novel nucleolar protein

The Surfeit locus contains the tightest cluster of mammalian genes so far described. The five Surfeit genes (Surf-1 to -5) that have been previously isolated and characterized do not share any DNA or amino acid sequence homology. These Surfeit genes appear to be housekeeping genes, with the Surf-3 gene encoding the 1.7a ribosomal protein and the Surf-4 gene encoding an integral membrane protein most likely associated with the endoplasmic reticulum. In this work, we have isolated the Surf-6 gene, a sixth member of the Surfeit locus. The Surf-6 gene contains four exons spanning a genomic region of 14 kb and specifies a mRNA of 2,571 bases. Surf-6 has features common to housekeeping genes because its transcript is present in every tissue tested, its 5' end is associated with a CpG-rich island, and its promoter does not contain a canonical TATA box. The Surf-6 long open reading frame encodes a novel highly basic polypeptide of 355 amino acids (28% Arg and Lys). By immunofluorescence and immunoblot analyses, the Surf-6 protein has been found to be located in the nucleolus and by immunocytochemical microscopy to be localized predominantly in the nucleolar granular component, a structure that is involved in ribosome maturation. These results indicate that the novel Surf-6 gene is involved in a nucleolar function.

Phosphorylation of p105 PEST sequence via a redox-insensitive pathway up-regulates processing of p50 NF-kappaB

The p105 Rel protein has dual functions; it is the precursor of the p5O subunit of NF-kappaB, and it acts as an IkappaB-like inhibitor to retain other Rel subunits in the cytoplasm. We have investigated the posttranslational regulation of p105 following activation of Jurkat T cells and find that a rapid and sustained phosphorylation of p105 is induced. The inducible phosphorylation occurs on multiple serines in the C-terminal-most 150 amino acids of the molecule, a region rich in Pro, Glu, Ser, and Thr residues. Phosphorylation of p105 in Jurkat cells treated with phorbol 12-myristate 13-acetate/ionomycin or with okadaic acid, another activator of NF-kappaB, is correlated with an increase in proteolytic processing to p5O. Intact PEST sequences are required for the phorbol 12-myristate 13-acetate/ionomycin-induced p105 processing, as a 68-amino acid C-terminal deletion abolishes the response to stimulation. When compounds that block Ikappa B alpha phosphorylation and degradation were tested, the serine protease inhibitors L-1-tosylamido-2-phenylethyl chloromethyl ketone and 1-chloro-3-tosyl-amido-7-amino-2-heptanone blocked inducible p105 phosphorylation, but the antioxidants pyrrolidine dithiocarbamate and butylated hydroxyanisol did not. Thus, while regulation of the p105 IkappaB resembles that of lkappaBa, involving inducible serine phosphorylation and proteolysis of the inhibitory ankyrin repeat domain, it depends on a different, redox-insensitive, signaling pathway.

Casein kinase II phosphorylates I kappa B alpha at S-283, S-289, S-293, and T-291 and is required for its degradation

The phosphoprotein I kappa B alpha exists in the cytoplasm of resting cells bound to the ubiquitous transcription factor NF-kappa B (p50-p65). In response to specific cellular stimulation, I kappa B alpha is further phosphorylated and subsequently degraded, allowing NF-kappa B to translocate to the nucleus and transactivate target genes. To identify the kinase(s) involved in I kappa B alpha phosphorylation, we first performed an I kappa B alpha in-gel kinase assay. Two kinase activities of 35 and 42 kDa were identified in cellular extracts from Jurkat T and U937 promonocytic cell lines. Specific inhibitors and immunodepletion studies identified the I kappa B alpha kinase activities as those of the alpha and alpha' subunits of casein kinase II (CKII). Immunoprecipitation studies demonstrated that CKII and I kappa B alpha physically associate in vivo. Moreover, phosphopeptide maps of I kappa B alpha phosphorylated in vitro by cellular extracts and in vivo in resting Jurkat T cells contained the same pattern of phosphopeptides as observed in maps of I kappa B alpha phosphorylated in vitro by purified CKII. Sequence analysis revealed that purified CKII and the kinase activity within cell extracts phosphorylated I kappa B alpha at its C terminus at S-283, S-288, S-293, and T-291. The functional role of CKII was tested in an in vitro I kappa B alpha degradation assay with extracts from uninfected and human immunodeficiency virus (HIV)-infected U937 cells. Immunodepletion of CKII from these extracts abrogated both the basal and enhanced HIV-induced degradation of I kappa B alpha. These studies provide new evidence that the protein kinase CKII physically associates with I kappa B alpha in vivo, induces multisite (serine/threonine) phosphorylation, and is required for the basal and HIV-induced degradation of I kappa B alpha in vitro.

Human papillomavirus type 6 E7 protein is a substrate in vitro of protein kinase C

The E7 proteins of 'low-risk' and 'high-risk' human papillomaviruses (HPV) are phosphorylated by casein kinase II. In this study, we report that the 'low-risk' HPV 6 E7 protein, but not the 'high-risk' HPV 16 E7 protein, can be phosphorylated in vitro on threonine at amino acid position 7 by protein kinase C. This is the first example of a qualitative biochemical difference between the HPV 6 E7 and HPV 16 E7 proteins.

The regulation of protein transport to the nucleus by phosphorylation

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Casein kinase II is required for cell cycle progression during G1 and G2/M in Saccharomyces cerevisiae

The catalytic subunit of Saccharomyces cerevisiae casein kinase II (Sc CKII) is encoded by the CKA1 and CKA2 genes, which together are essential for viability. Five independent temperature-sensitive alleles of the CKA2 gene were isolated and used to analyze the function of CKII during the cell cycle. Following a shift to the nonpermissive temperature, cka2ts strains arrested within a single cell cycle and exhibited a dual arrest phenotype consisting of 50% unbudded and 50% large-budded cells. The unbudded half of the arrested population contained a single nucleus and a single focus of microtubule staining, consistent with arrest in G1. Most of the large-budded fraction contained segregated chromatin and an extended spindle, indicative of arrest in anaphase, though a fraction contained an undivided nucleus with a short thick intranuclear spindle, indicative of arrest in G2 and/or metaphase. Flow cytometry of pheromone-synchronized cells confirmed that CKII is required in G1, at a point which must lie at or beyond Start but prior to DNA synthesis. Similar analysis of hydroxyurea-synchronized cells indicated that CKII is not required for completion of previously initiated DNA replication but confirmed that the enzyme is again required for cell cycle progression in G2 and/or mitosis. These results establish a role for CKII in regulation and/or execution of the eukaryotic cell cycle.