Casein kinase II phosphorylates DNA-polymerase-alpha--DNA-primase without affecting its basic enzymic properties

Phosphorylation and dephosphorylation of the B subunit of DNA polymerase alpha-primase complex in the early embryogenesis of Drosophila

In the early embryos of Drosophila, the B subunit of the DNA polymerase alpha-primase complex was found to migrate more slowly during the first 13 mitotic cycles than that from cycle 14 using SDS-polyacrylamide gel electrophoresis. Lambda phosphatase treatment showed that the reduced migration was caused by phosphorylation of the B subunit. Detailed analysis using the partially purified B subunit indicated that most of the B subunit until cycle 13 was a phosphorylated form while the B subunit of cycle 14 was a dephosphorylated form.

Characterization of the p125 subunit of human DNA polymerase delta and its deletion mutants. Interaction with cyclin-dependent kinase-cyclins

The catalytic subunit of human DNA polymerase (pol) delta was overexpressed in an active, soluble form by the use of a baculovirus system in insect cells. The recombinant enzyme was separated from endogenous DNA polymerases by phosphocellulose, Mono Q-Sepharose, and single-stranded DNA-cellulose chromatography. Recombinant DNA pol delta was also purified by immunoaffinity chromatography. The enzymatic properties of the purified catalytic subunit were characterized. The enzyme was active and possessed both DNA polymerase and associated 3' to 5' exonuclease activities. NH2-terminal deletion mutants retained polymerase activity, whereas the core and COOH-terminal deletion mutants were devoid of any measurable activities. Coinfection of Sf9 cells with recombinant baculovirus vectors for pol delta and cyclin-dependent kinase (cdk)-cyclins followed by metabolic labeling with 32Pi showed that the recombinant catalytic subunit of pol delta could be hyperphosphorylated by G1 phase-specific cdk-cyclins. When cdk2 was coexpressed with pol delta in Sf9 cells, pol delta was found to coimmunoprecipitate with antibodies against cdk2. Experiments with deletion mutants of pol delta showed that the NH2-terminal region was essential for this interaction. Coimmunoprecipitation and Western blot experiments in Molt 4 cells confirmed the interaction in vivo. Preliminary experiments showed that phosphorylation of the catalytic subunit of pol delta by cdk2-cyclins had little or no effect on the specific activity of the enzyme.

Modulation of DNA polymerases alpha, delta and epsilon by lactate dehydrogenase and 3-phosphoglycerate kinase

Literature documents that glycolytic enzymes (among them lactate dehydrogenase and 3-phosphoglycerate kinase) can reside in nuclei of mammalian cells and exert functions in DNA replication, transcription and DNA repair, in addition to their role as catalysts in the cytoplasm. Transfer of glycolytic enzymes to cell nuclei requires modification, for example phosphorylation. We studied the effects of phosphorylated lactate dehydrogenase and 3-phosphoglycerate kinase on (i) UV-induced DNA repair, using permeabilized human fibroblasts, and (ii) in vitro DNA synthesis catalyzed by purified DNA polymerases alpha, delta, and epsilon from proliferating rat liver. (i) Phosphorylated lactate dehydrogenase stimulated UV-induced DNA repair synthesis in normal fibroblasts in a dose-dependent manner; the unphosphorylated enzyme slightly inhibited. In repair-deficient xeroderma pigmentosum fibroblasts reparative synthesis was not enhanced whether lactate dehydrogenase was phosphorylated or not, indicating that reparative DNA synthesis must be possible in order to be stimulated. (ii) Activity of purified DNA polymerases alpha, delta, and epsilon was differentially stimulated or inhibited, according to the phosphorylation status of lactate dehydrogenase. DNA polymerases were also modulated by 3-phosphoglycerate kinase, depending on the primer-templates used which were gapped DNA (mimicking a repair mode of DNA synthesis) or single-stranded M13 DNA (representing the replicative mode of DNA synthesis). Since glycolytic enzymes in cell nuclei retain binding ability for their cofactors, cytoplasmic substrates and inhibitors, a regulatory linkage might exist between the energy state of a cell and its replicative and reparative functions.

Murine protein kinase CK2 alpha': cDNA and genomic cloning and chromosomal mapping

Protein kinase CK2 (casein kinase II) is a heterotetrameric enzyme implicated in many essential regulatory pathways in cells. We have determined the sequence of the murine CK2 alpha' cDNA that encodes a 350-amino-acid protein that would have 99 and 98% homology with the human and chicken proteins, respectively, and is also highly homologous to murine CK2 alpha. To clarify the sequence of the 5' end of the cDNA and to elucidate the structure and regulation of the gene, we obtained a bacterial artificial chromosome clone that contains the 35-kb CK2 alpha' gene. The gene consists of 12 small exons; the 5' end, including the first exon and intron, is extremely GC rich and contains a CpG island. The putative promoter contains potential binding sites for a variety of transcriptional factors but appears to lack CCAAT- or TATA-like elements. A polymorphic dinucleotide repeat in the fifth intron allowed us to map the CK2 alpha' gene to murine Chromosome 8.

Phosphorylation of tick-borne encephalitis virus NS5 protein

The largest tick-borne encephalitis virus (TBEV) non-structural protein NS5 (100 kDa) is believed to be involved in RNA replication. The protein is phosphorylated in infected cell extracts in the presence of [gamma-32P]ATP, as shown by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and Western blot analysis using monoclonal antibodies raised against TBEV NS5 protein. Radioactive labeling of NS5 in cellular extracts at an early stage post-infection is higher than at 24 h post-infection. Incubation of immunoprecipitates of NS5 protein with [gamma-32P]ATP in the presence of Mg2+ resulted in the phosphorylation of TBEV NS5 protein and of immunoglobulins. Phosphoamino acid analysis demonstrated that NS5 contains phosphoserine, but not phosphothreonine, or phosphotyrosine.

Preferential inhibition of DNA polymerases alpha, delta, and epsilon from Novikoff hepatoma cells by inhibitors of cell proliferation

DNA polymerases alpha, delta and epsilon from normal regenerating rat liver and Novikoff hepatoma cells were purified about 300-fold, characterized, and checked for sensitivity towards drugs known to inhibit cell proliferation. Characterization included (a) identification of associated proteins, (b) measurement of physiochemical constants (including sedimentation coefficients, diffusion coefficients, calculation of relative molecular masses), (c) quantification of catalytic activities using specific DNA primer templates (Km values) and specific inhibitors (Ki values), and (d) discrimination between DNA polymerases from normal cells and those from malignant cells using inhibitors of cell proliferation. (a) DNA primase associated with DNA polymerase alpha, and 3'-5' exonuclease accompanying DNA polymerases delta and epsilon had similar activities. (b) Comparison of physicochemical and catalytic properties of DNA polymerases from both sources revealed similarities but also some important differences. Sedimentation and diffusion coefficients of DNA polymerases alpha and epsilon from malignant cells differed significantly. (c) The DNA-binding domain of DNA polymerases alpha and epsilon from hepatoma cells was altered since Km values, determined with several specific DNA primer-templates, were higher. Furthermore, dNTP-binding sites of DNA polymerases from malignant cells, when probed with specific inhibitors (aphidicolin, butylphenyl-dGTP, carbonyldiphosphonate, and dideoxy-TTP) showed significantly lower Ki values, indicating lower affinity to deoxyribonucleoside 5'-triphosphates. (d) Sixteen drugs representative of various modes of interaction with DNA and protein were chosen. Dose/response experiments were performed and the concentration at which the polymerizing activity was reduced to 50% was calculated (K50 values). Preferential inhibition of DNA polymerases alpha, delta, and epsilon from Novikoff hepatoma cells was found for: the intercalating drugs doxorubicin, daunorubicin, amsacrine, mitoxantrone, quinacrine and ethidium bromide, the minor-groove binders distamycin and netropsin, the ATPase-blocking agents novobiocin and coumamycin, and the topoisomerase I inhibitors camptothecin and topotecan. When the sensitivity of polymerases delta and epsilon was measured using poly(dA.dT) as a primer-template, the preferential inhibition of the enzymes from malignant cells was even more pronounced. Drugs known to trap the DNA-topoisomerase-II complex, etoposide, nalidixic acid, teniposide, and merbarone did not affect DNA polymerases irrespective of the source. Since the majority of the inhibitors used, particularly intercalators and minor-groove binders, act by modification of the primer-template, inhibition of DNA synthesis must have occurred through weakening of non-covalent bonds between DNA and catalytic polypeptides. Consequently, preferential inhibition of DNA polymerases from malignant cells seems to be indicative of abnormally diminished binding of the enzymes to their primer-templates. This effect may be caused by conformational alterations in polymerases from malignant cells which affect the DNA binding domains. Similarly, changes in physicochemical and kinetic constants are indicative of alterations of dNTP-binding domains.

DNA polymerases alpha, delta, and epsilon of Novikoff hepatoma cells differ from those of normal rat liver in physicochemical and catalytic properties

To investigate whether DNA replication in malignant cells deviates from that of normal cells we compared DNA polymerases alpha, delta, and epsilon from normal rat liver to the enzymes from fast-growing (malignant) Novikoff hepatoma cells. DNA polymerases were purified 300-fold by three chromatographic steps. Characterization included measurement of physicochemical constants (including sedimentation coefficients, diffusion coefficients, calculation of relative molecular masses), quantitation of catalytic activities using specific DNA primer templates (Km values) and inhibitors (Ki values), and identification of polypeptides which are strongly associated with DNA polymerases. Comparison of physicochemical and catalytic properties of DNA polymerases from both sources revealed similarities but also some important differences. DNA primase associated with DNA polymerase alpha, and 3'-5' exonuclease accompanying DNA polymerases delta and epsilon had similar activities. In contrast, the DNA-binding domain of DNA polymerases alpha and epsilon from hepatoma cells was altered since Km values, determined with the specific primer templates gapped calf thymus DNA and poly(dA.dT), were higher. Furthermore, sedimentation and diffusion coefficients, Stokes' radii, and frictional coefficient ratios of DNA polymerases alpha and epsilon from malignant cells significantly deviated. In addition, when the dNTP-binding sites were probed with specific inhibitors (aphidicolin, butylphenyl-dGTP, carbonyldiphosphonate, and dideoxy-TTP), significantly lower Ki values were obtained for the polymerases from Novikoff cells indicating lower affinity of the dNTP binding site to deoxyribonucleoside 5'-triphosphates. Altered catalytic and molecular properties are possibly a consequence of malignant transformation. It is to be expected that similar changes occur in DNA polymerases of other tumors. In particular, diminished affinity to primer templates and weakened nucleotide binding leads to lowered specificity of nucleotide selection in the base-pairing process and is therefore likely to cause an enhanced mutation rate during malignant progression.

Casein kinases: pleiotropic mediators of cellular regulation

The present review on casein kinases focuses mainly on the possible metabolic role of CK-2, with special emphasis on its behavior in pathological tissues. From these data at least three ways to regulate CK-2 activity emerge: (i) CK-2 activity changes during embryogenesis, being high at certain stages of development and showing basal activity values at others; (ii) CK-2 activity can be enhanced in vitro by treatment of tissue culture cells with various growth factors and serum and (iii) CK-2 activity is constitutively enhanced in rapidly proliferating cells. The regulated CK-2 activity changes during embryogenesis cannot be explained as yet. In the case of the constitutive high expression of CK-2 in tumors, genetic changes may be responsible, e.g. through alterations of the regulatory genetic elements and/or regulation by specific transcription factors. In the case of serum induction, no genetic changes are necessarily involved; the observed changes may be entirely due to a signal transduction pathway where CK-2 could be phosphorylated by another kinase(s). CK-2 cDNAs from various organisms have been isolated and characterized. From the deduced amino acid sequence it turns out that CK-2 subunits are highly conserved during evolution. The relationship between CK-2 alpha from humans and plants is still 73%. Similar relationships are reported for the beta-subunit. Chromosomal assignment of CK-2 alpha shows two gene loci, one of which is a pseudogene. They are located on different chromosomes. Expression of the CK-2 subunits in Escherichia coli and the Baculo expression system is shown. The recombinant subunits can self-assemble to a functional holoenzyme in vitro. Biochemical and biophysical analysis of the recombinant beta-subunit suggests it to be trifunctional in association with the alpha-subunit affecting: (i) stability, (ii) enzyme specificity and (iii) enzyme activity. The question where CK-2 and its subunits are located throughout the cell cycle has also been addressed, mainly because of the large discrepancies that still exist between results obtained by different investigators. Tissue-specific expression of CK-2 at the mRNA and at the protein level has also been given attention. The fact that the enzyme activity is surprisingly high in brain and low in heart and lung may be indicative of involvement of CK-2 in processes other than proliferation.(ABSTRACT TRUNCATED AT 400 WORDS)

Immunocytochemical localization of casein kinase II during interphase and mitosis

We have developed specific antibodies to synthetic peptide antigens that react with the individual subunits of casein kinase II (CKII). Using these antibodies, we studied the localization of CKII in asynchronous HeLa cells by immunofluorescence and immunoelectron microscopy. Further studies were done on HeLa cells arrested at the G1/S transition by hydroxyurea treatment. Our results indicate that the CKII alpha and beta subunits are localized in the cytoplasm during interphase and are distributed throughout the cell during mitosis. Further electron microscopic investigation revealed that CKII alpha subunit is associated with spindle fibers during metaphase and anaphase. In contrast, the CKII alpha' subunit is localized in the nucleus during G1 and in the cytoplasm during S. Taken together, our results suggest that CKII may play significant roles in cell division control by shifting its localization between the cytoplasm and nucleus.