Casein kinase II in signal transduction and cell cycle regulation

Serine phosphorylation within a concise amino-terminal domain in nuclear respiratory factor 1 enhances DNA binding

Nuclear respiratory factor 1 (NRF-1) is a transcriptional activator that acts on a diverse set of nuclear genes required for mitochondrial respiratory function in mammalian cells. These genes encode respiratory proteins as well as components of the mitochondrial transcription, replication, and heme biosynthetic machinery. Here, we establish that NRF-1 is a phosphoprotein in vivo. Phosphorylation occurs on serine residues within a concise NH2-terminal domain with the major sites of phosphate incorporation at serines 39, 44, 46, 47, and 52. The in vivo phosphorylation pattern can be approximated in vitro by phosphorylating recombinant NRF-1 with purified casein kinase II. Phosphate incorporation at the sites utilized in vivo results in a marked stimulation of DNA binding activity which is not observed in mutated proteins lacking these sites. Pairwise expression of the wild-type protein with each of a series of truncated derivatives in transfected cells results in the formation of a dimer between wild-type and mutant forms demonstrating that a homodimer is the active binding species. Although NRF-1 can dimerize in the absence of DNA, phosphorylation does not enhance the formation of these dimers. These findings suggest that phosphorylation results in an intrinsic change in the NRF-1 dimer enhancing its ability to bind DNA.

Casein kinase II catalyzes tyrosine phosphorylation of the yeast nucleolar immunophilin Fpr3

In the yeast Saccharomyces cerevisiae, the nucleolar immunophilin, Fpr3, is phosphorylated at tyrosine and dephosphorylated by the phosphotyrosine-specific phosphoprotein phosphatase, Ptp1. In Ptp1-deficient cells, Fpr3 contains phospho-Tyr at a single site (Tyr184), but also contains phospho-Ser and phospho-Thr. Ser186 (adjacent to Tyr184) is situated within a canonical site for phosphorylation by casein kinase II (CKII). Yeast cell lysates contain an activity that binds to Fpr3 in vitro and phosphorylates Fpr3 at Ser, Thr, and Tyr; this activity was found to be dependent on expression of functional yeast CKII. Moreover, purified Fpr3 was phosphorylated on Tyr184 in vitro by either purified yeast CKII or purified, bacterially-expressed human CKII. Likewise, phosphorylation of Fpr3 at tyrosine in vivo was markedly enhanced in yeast cells overexpressing a heterologous (Drosophila) CKII, but was undetectable in yeast cells carrying only a temperature-sensitive allele of the endogenous CKII, even when the cells were grown at a permissive temperature. Phosphorylation of Fpr3 at Tyr184 by CKII in vitro lagged behind phosphorylation of Fpr3 at Ser, and was accelerated by pre-phosphorylation of Fpr3 at Ser using CKII. Furthermore, synthetic peptides corresponding to the sequence surrounding Tyr184 that contained P-Ser (or Glu) at position 186 were much more efficient substrates for CKII phosphorylation of Tyr184 than a synthetic peptide containing Ala at position 186. These findings indicate that CKII phosphorylates Fpr3 at tyrosine and serine both in vivo and in vitro and thus possesses dual specificity. These results also indicate that Tyr184 is phosphorylated by CKII via a two-step process, in which phosphorylation at the +2 position provides a negatively-charged specificity determinant that allows subsequent phosphorylation of Tyr184.

Ornithine decarboxylase expression leads to translocation and activation of protein kinase CK2 in vivo

Ornithine decarboxylase (ODC) is the key initial enzyme in the biosynthesis of polyamines. Since polyamines have been shown to enhance protein kinase CK2 activity in vitro, ODC was overexpressed to examine the role of polyamines in CK2 regulation in vivo. Infection of Balb/MK cells with an ODC retrovirus to elevate ODC and polyamine levels increased overall protein phosphorylation as well as CK2 protein levels and enzyme activity in mimosine- or nocodazole- arrested cells. Immunofluorescence microscopy and enzyme analyses of subcellular fractions from ODC-overexpressing cells demonstrated translocation of CK2 from the cytoplasm to the nucleus with no apparent loss of cytoplasmic CK2 activity, suggesting polyamine activation of the remaining cytoplasmic enzyme. Similarly, K6/ODC transgenic mice exhibited higher ODC and CK2 enzyme activities than their normal littermates. ODC-immunostained cells in the transgenic skin also stained intensely for CK2 protein. Primary cultures of K6/ODC keratinocytes also exhibited increased ODC and CK2 enzyme activities compared with those from normal littermates. However, the addition of difluoromethylornithine, a specific ODC inhibitor, to the transgenic keratinocytes reduced both intracellular polyamine levels and CK2 enzyme activity. These results suggest that polyamines regulate the CK2 enzyme by affecting its cellular distribution as well as its enzyme activity and levels.

Purification and characterization of protein kinase CK2 from Candida albicans: evidence for the presence of two distinct regulatory subunits beta and beta'

Protein kinase CK2 of Candida albicans has been purified to near homogeneity by a procedure which involves chromatography on DEAE-cellulose, phosphocellulose, Q-Sepharose, and heparin-agarose. The purified enzyme has the characteristic properties of animal and yeast CK2, i.e., it utilizes ATP as well as GTP as phosphate donor, phosphorylates serine and threonine residues on casein, is inhibited by low concentrations of heparin, and is stimulated by NaCl and polycationic compounds such as polylysine, spermine, and spermidine. The native form of the enzyme exhibits a molecular mass of 159 kDa, and SDS-PAGE analysis indicates that it is composed of four polypeptides with relative molecular masses of 44, 39, 37 and 36 kDa. The 39- and 37-kDa polypeptides were identified as distinct catalytic subunits alpha and alpha' on the basis of in situ phosphorylation assays and immunological recognition with heterologous antibodies. The purified kinase undergoes autophosphorylation on the 44- and 36-kDa polypeptides, a characteristic of the beta subunits from other species. Antibodies raised against the beta subunit of Drosophila melanogaster and human CK2 crossreact only with the 36-kDa polypeptide. The 44-kDa polypeptide was identified as an unusually large beta' subunit by Western blotting with an antibody raised against the beta' subunit of Saccharomyces cerevisiae. All these data suggest that C. albicans CK2 has an alpha alpha' beta beta' heterotetrameric composition similar to that found in S. cerevisiae.

Casein kinase II binds to and phosphorylates cytoplasmic dynein

We have isolated a 27-kDa protein that binds to cytoplasmic dynein. Microsequencing of a 17-amino acid peptide of this polypeptide yielded a sequence which completely matched the predicted sequence of the beta subunit of casein kinase II, a highly conserved serine/threonine kinase. Affinity chromatography using a dynein column indicates that both the alpha and beta subunits of casein kinase II are retained by the column from rat brain cytosol. Although dynactin is also bound to the column, casein kinase II is not a dynactin subunit. Casein kinase II does not co-immunoprecipitate with dynactin, and it binds to a dynein intermediate chain column which has been preblocked with excess p150(Glued), a treatment that inhibits the binding of dynactin from cytosol. Bacterially expressed and purified rat dynein intermediate chain can be phosphorylated by casein kinase II in vitro. Further, native cytoplasmic dynein purified from rat brain can also be phosphorylated by casein kinase II in vitro. We propose that CKII may be involved in the regulation of dynein function possibly by altering its cargo specificity or its ability to interact with dynactin.

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.

Fibroblast growth factor-2 binds to the regulatory beta subunit of CK2 and directly stimulates CK2 activity toward nucleolin

The presence of fibroblast growth factor-2 (FGF-2) in the nucleus has now been reported both in vitro and in vivo, but its nuclear functions are unknown. Here, we show that FGF-2 added to nuclear extract binds to protein kinase CK2 and nucleolin, a CK2 natural substrate. Added to baculovirus-infected cell extracts overexpressing CK2 or its isolated subunits, FGF-2 binds to the enzyme through its regulatory beta subunit. Using purified proteins, FGF-2 is shown to directly interact with CK2 and to stimulate CK2 activity toward nucleolin. Furthermore, a mitogenic-deficient FGF-2 mutant protein has an impaired ability to interact with CK2 and to stimulate CK2 activity using nucleolin as substrate. We propose that in growing cells, one function of nuclear FGF-2 is to modulate CK2 activity through binding to its regulatory beta subunit.

Developmental changes in transcription factors associated with the nuclear matrix of chicken erythrocytes

The nuclear matrix has roles in organizing nuclear DNA and in controlling transcription. Transcription factors are associated with the nuclear matrix, with the spectra of transcription factors differing from one cell type to another. In this study we identified the transcription factors and enzymes functioning in the regulation of gene expression that were associated with nuclear matrix and nonmatrix nuclear fractions in erythrocytes isolated from chick embryos at different stages of development, anemic and normal adult birds. We found that the primitive erythroid nuclear matrix had the greatest histone deacetylase activity and highest levels of several transcription factors, including GATA-1, CACCC-binding proteins, and NF1. These transcription factors have key roles in erythroid-specific gene expression. The levels of these transcription factors were lower in the nonmatrix and matrix fractions isolated from definitive erythrocytes. For primitive and definitive erythrocytes, the level of CACCC-binding proteins in the nuclear matrix fraction was greater than that of Sp1. The relative levels of these transcription factors were reversed in the nonmatrix fraction. Casein kinase II was not found in erythroid nuclear matrices. The observed erythroid lineage specific alterations in erythroid nuclear matrix transcription factor composition and abundance may be involved in erythroid-specific gene expression.

Phosphotyrosine specifies the phosphorylation by protein kinase CK2 of a peptide reproducing the activation loop of the insulin receptor protein tyrosine kinase

Protein kinase CK2 is a ubiquitous Ser/Thr-specific protein kinase responsible for the phosphorylation of many proteins implicated in signal transduction. It phosphorylates both threonyl and seryl residue(s) of the insulin receptor beta-subunit. In this study, a series of peptides, reproducing all the threonyl sites of the intracellular domain of the insulin receptor that display the consensus sequence for CK2, has been synthesized and used as substrate for purified rat liver CK2. The only peptide readily phosphorylated is the one reproducing the activation loop of the insulin receptor (EIYET1160DYYA), including three tyrosines (Y1158, Y1162 and Y1163) whose phosphorylation through an intermolecular autocatalytic process promotes the activation of the receptor kinase. The phosphorylation efficiency of T1160 is increased almost 20-fold if these three tyrosines are previously phosphorylated. By using variably phosphorylated peptides, the tyrosine mainly responsible for such a hierarchical phosphorylation process has been identified as Y1163. It can be concluded, from these data, that T1160 situated in the activation loop of the insulin receptor, represents an excellent target for CK2, its phosphorylation being triggered by the previous autophosphorylation of the three tyrosyl residues surrounding it, with special reference to Y1163. These data are consistent with the implication of CK2 in the regulation of the activation process of the insulin receptor protein tyrosine kinase.

Nuclear matrix as an anchor for protein kinase CK2 nuclear signalling

Nuclear matrix (NM) is not only the structural basis for nuclear shape but also is intimately involved in nuclear functional activities. Among the modulatory factors that may affect these diverse activities are the signals that may influence the state or composition of the NM proteins. One such mechanism for altering the functional activity of at least some NM proteins may be the extent of their phosphorylation. Protein kinase CK2 appears to associate with NM and to phosphorylate a number of NM-associated proteins. Chromatin- and NM-associated CK2 is rapidly modulated by mitogenic signals. We propose that NM serves as a physiological anchor for nuclear signalling of protein kinase CK2 which may influence functions of NM such as transcription of active genes and growth.

Analysis of interactions between the subunits of protein kinase CK2

Protein kinase CK2, which was formerly known as casein kinase II, is a highly conserved protein serine/threonine kinase implicated in the control of cell proliferation through its phosphorylation of regulatory nuclear proteins. The enzyme consists of catalytic (alpha and (or) alpha') subunits and beta subunits that modulate the activity of the catalytic subunits. These subunits are arranged in homotetrameric (i.e., alpha 2 beta 2 or alpha' 2 beta 2) or heterotetrameric (i.e., alpha alpha' beta 2) complexes. We previously demonstrated using the yeast two-hybrid system that alpha (or alpha') subunits can interact with beta subunits but not other alpha (or alpha') subunits. By comparison, beta subunits can interact with alpha (or alpha') and with beta subunits, suggesting that the protein kinase CK2 holoenzyme forms because of the ability of beta subunits to dimerize, bringing two heterodimers (alpha beta or alpha' beta) into a tetrameric complex. In the present study, we used the yeast two-hybrid system to examine the domains of interactions between the alpha and beta subunits of protein kinase CK2. These studies indicate that the ability of beta to interact with alpha resides within the carboxy-terminal domain of beta. By comparison, our studies suggest that individual domains of alpha are not sufficient for interactions with beta.

Phosphorylation of proteasomes in mammalian cells. Identification of two phosphorylated subunits and the effect of phosphorylation on activity

The proteasome, a multimeric protease, plays an important role in nonlysosomal pathways of intracellular protein degradation. This study was undertaken to determine which subunits of mammalian proteasomes are phosphorylated and to investigate the possible role of phosphorylation in regulating proteasome activity and the association with regulatory components. Rat-1 fibroblasts were grown in the presence of [32P]phosphate and proteasomes were immunoprecipitated from cell lysates with proteasome-specific polyclonal antibodies. Subsequent analysis by two-dimensional polyacrylamide gel electrophoresis showed two radiolabeled proteasome subunits which were identified using monoclonal antibodies as C8 and C9. Treatment of human embryonic lung cells (L-132), under identical conditions, also showed the same two phosphorylated subunits. Phosphoamino acid analysis revealed phosphoserine to be present in both C8 and C9. Examination of the sequence of C9 showed a potential cGMP-dependent phosphorylation site (-Arg3-Arg-Tyr-Asp-Ser-Arg8-), whilst C8 contains several potential casein kinase II phosphorylation sites. Following immunoprecipitation by a monoclonal antibody and dephosphorylation by acid phosphatase, proteasomes were observed to have significantly lower activities when compared to phosphorylated proteasomes, implying that phosphorylation may be an important mechanism of regulating proteasome function. Free proteasomes were separated by gel-filtration from those complexed with regulatory complexes to form the 26S proteinase. The ratio of phosphorylation of C8 and C9 was found to be very similar in the two complexes but the level of phosphorylation was higher in the 26S proteinase than in free proteasomes.

A new Saccharomyces cerevisiae ankyrin repeat-encoding gene required for a normal rate of cell proliferation

We have characterized a new ankyrin (ANK) repeat-containing Saccharomyces cerevisiae gene, YAR1, located between the HSP82 and SUI3 genes on chromosome XVI. YAR1 encodes a 200-amino-acid (aa) protein with two ANK repeat motifs and an acidic C terminus rich in PEST-like sequences. The Yar1 ANK repeats are most similar to the conserved ANK repeats in the yeast cell cycle transcription factor, Swi6. We show that YAR1 is transcribed as an 800-nucleotide (nt) poly(A)+ mRNA from a promoter lacking a consensus TATA sequence. YAR1 is transcribed in both haploid and diploid cells, and in haploid cells arrested in G1 with alpha-factor or in S phase with hydroxyurea. YAR1 shares an intergenic region with HSP82, and while HSP82 transcription is induced 15-fold by heat shock, transcription of YAR1 is transiently repressed by heat shock. We show that YAR1 is not an essential gene, but that haploid cells bearing a yar1 deletion grow significantly more slowly than do isogenic wild-type cells, especially at low temperature.

Cloning and characterization of cDNAs for novel proteins with glutamic acid-proline dipeptide tandem repeats

cDNAs with identical 3' sequences containing a hexanucleotide repeat -(GAGCCG)9- were isolated from rat pheochromocytoma and brain cDNA libraries. The cDNA with the longest open reading frame codes for a protein of 24.6 kDa containing a 16-fold -(Glu-Pro)- dipeptide repeat within a glutamate and proline rich region at its deduced C-terminus. cDNAs with the identical 3' sequence and a divergent 5' sequence were isolated from a rat skeletal muscle cDNA library. The latter are predicted to code for a protein of 15.5 kDa with a C-terminal repetitive domain identical to that in the pheochromocytoma and brain cDNAs. The cDNAs recognize a 1.8 kb mRNA species present in a variety of tissues, being particularly abundant in cardiac and skeletal muscle.

Casein kinase II is required for efficient transcription by RNA polymerase III

Casein kinase II (CKII) is a ubiquitous and highly conserved serine/threonine protein kinase found in the nucleus and cytoplasm of most cells. Using a combined biochemical and genetic approach in the yeast Saccharomyces cerevisiae, we assessed the role of CKII in specific transcription by RNA polymerases I, II, and III. CKII is not required for basal transcription by RNA polymerases I and II but is important for polymerase III transcription. Polymerase III transcription is high in extracts with normal CKII activity but low in extracts from a temperature-sensitive mutant that has decreased CKII activity due to a lesion in the enzyme's catalytic alpha' subunit. Polymerase III transcription of 5S rRNA and tRNA templates in the temperature-sensitive extract is rescued by purified, wild-type CKII. An inhibitor of CKII represses polymerase III transcription in wild-type extract, and this repression is partly overcome by supplementing reaction mixtures with active CKII. Finally, we show that polymerase III transcription in vivo is impaired when CKII is inactivated. Our results demonstrate that CKII, an oncogenic protein kinase previously implicated in cell cycle and growth control, is required for high-level transcription by RNA polymerase III.

Structure of protein kinase CK2: dimerization of the human beta-subunit

Protein kinase CK2 has been shown to be elevated in all so far investigated solid tumors and its catalytic subunit has been shown to serve as an oncogene product. CK2 is a heterotetrameric serine-threonine kinase composed of two catalytic (alpha and/or alpha') and two regulatory beta-subunits. Using the two-hybrid system we could show that the alpha- or alpha'-subunits of CK2 can interact with the beta-subunits of CK2, but not with other alpha- or alpha'-subunits. By comparison, the beta-subunit of CK2 can interact with another beta-subunit. Important amino acids for successful dimerization of the beta-subunit were localized between amino acid residues 156 and 165. Furthermore, we identified residues between amino acid 170 and 180 which antagonize the dimerization.

Enzymes in mineralizing systems: state of the art

The hallmark of biological mineralization is the precise regulation of mineral deposition in space and time. The cells which produce mineralized tissues are themselves controlled by developmental programs and hormonal signals which result in regulation of gene expression and modulation of protein function. These signals are transduced into changes in enzyme levels and/or activity. Upon activation, cellular enzymes then act to synthesize the organic matrix and process it extracellularly, utilize metabolic energy to transport ions from the blood to the matrix, and to initiate the mineralization cascade. The first enzyme activity described in mineralizing tissues was alkaline phosphatase and it is still the best characterized enzyme in the mineralization process. Yet, important questions about the role of this protein remain unanswered, and it continues to occupy a central focus in mineralized tissue investigation. Other phosphatases, including protein tyrosine phosphatases are important in regulating tyrosine kinase mediated signals. Investigators have now begun to look closely at several groups of kinases which are also important for proper mineralization. As peptide hormones are important modulators of mineralized tissues, protein kinase A has always been presumed to play a key role in phosphorylating intracellular proteins. There is also considerable interest in protein kinase C, as well as tyrosine kinases in mineralized tissue signal transduction. Another group of kinases important in mineralized tissues are the enzymes which phosphorylate the matrix phosphoproteins. Of these, casein kinase II appears to be involved in intracellular and extracellular protein phosphorylation. Several enzymes present in the premineralized matrix are thought to be significant in triggering mineralization. Alkaline phosphatase may act at this level, but new data also suggests that metalloproteases and gelatinases, by modifying or digesting matrix components, may be important in the initiation of calcification.

Casein kinase II phosphorylation site mutations in c-Myb affect DNA binding and transcriptional cooperativity with NF-M

Phosphorylation of c-Myb has been implicated in the regulation of the binding of c-Myb to DNA. We show that murine c-Myb is phosphorylated at Ser-11 and -12 in vivo and that these sites can be phosphorylated in vitro by casein kinase II (CKII), analogous to chicken c-Myb. An efficient method to study DNA binding properties of full-length c-Myb and Myb mutants under nondenaturing conditions was developed. It was found that a Myb mutant in which Ser-11 and -12 were replaced with Ala (Myb Ala-11/12), wild-type c-Myb, and Myb Asp-11/12 bound to the A site of the mim-1 promoter with decreasing affinities. In agreement with this finding, Myb Ala-11/12 transactivated better than wild-type c-Myb and Myb Asp-11/12 on the mim-1 promoter or a synthetic Myb-responsive promoter. Similar observations were made for the myeloid-specific neutrophil elastase promoter. The presence of NF-M or an NF-M-like activity abolished partially the differences seen with the Ser-11/12 mutants, suggesting that the reduced DNA binding due to negative charge at positions 11 and 12 can be compensated for by NF-M. Since no direct interaction of c-Myb and NF-M was observed, we propose that the cooperativity is mediated by a third factor. Our data offer two possibilities for how casein kinase II phosphorylation can influence c-Myb function: first, by reducing c-Myb DNA binding and thereby influencing transactivation, and second, by enhancing the apparent cooperativity between c-Myb and NF-M or an NF-M-like activity.

Phosphorylation of casein kinase II by p34cdc2. Identification of phosphorylation sites using phosphorylation site mutants in vitro

The alpha and beta subunits of casein kinase II are dramatically phosphorylated in cells that are arrested in mitosis (Litchfield, D. W., Lüscher, B., Lozeman, F. J., Eisenman, R. N., and Krebs, E.G. (1992) J. Biol. Chem. 267, 13943-13951). Comparative phosphopeptide mapping experiments indicated that the mitotic phosphorylation sites on the alpha subunit of casein kinase II can be phosphorylated in vitro by p34cdc2. In the present study, we have demonstrated that a glutathione S-transferase fusion protein encoding the C-terminal 126 amino acids of the alpha subunit is phosphorylated by p34cdc2 at the same sites as intact casein kinase II, indicating that the mitotic phosphorylation sites are localized within the C-terminal domain of alpha. Four residues within this domain, Thr-344, Thr-360, Ser-362, and Ser-370, conform to the minimal consensus sequence for p34cdc2 phosphorylation. Synthetic peptides corresponding to regions of alpha that contain each of these residues are phosphorylated by p34cdc2 at these sites. Furthermore, alterations in the phosphorylation of the glutathione S-transferase proteins encoding the C-terminal domain of alpha are observed when any of the four residues are mutated to alanine. When all four residues are mutated to alanine, the fusion protein is no longer phosphorylated by p34cdc2 at any of the sites that are phosphorylated in mitotic cells. These results indicate that Thr-344, Thr-360, Ser-362, and Ser-370 are the sites on the alpha subunit of casein kinase II that are phosphorylated in mitotic cells.

The protein kinase from mitotic human cells that phosphorylates Ser-209 on the casein kinase II beta-subunit is p34cdc2

Casein kinase II is a highly conserved enzyme that is essential for viability. In cells, the casein kinase II beta-subunit is phosphorylated at an autophosphorylation site and at a site (Ser-209) that is maximally phosphorylated in mitotic cells. To identify protein kinase activities that phosphorylate Ser-209, we fractionated extracts from mitosis-arrested human Burkitt lymphoma MANCA cells. A single Ser-209 kinase activity was detected following each fractionation step. The Ser-209 kinase was purified to a specific activity of approx. 250 nmol/min per mg and efficiently phosphorylated histone H1, a synthetic peptide containing Ser-209 (Ser-209 peptide), myelin basic protein and casein. Immunoblot analysis demonstrated that all fractions containing Ser-209 kinase activity contained p34cdc2. Furthermore, depletion of the Ser-209 kinase activity with p13suc1-Sepharose and anti-p34cdc2 antiserum demonstrated conclusively that the isolated Ser-209 kinase is p34cdc2. These studies provide strong biochemical evidence that p34cdc2 is the enzyme that phosphorylates Ser-209 on the beta-subunit of CKII in mitotic cells. In addition, these results indicate that the Ser-209 peptide can be utilized as a specific reagent for the assay of p34cdc2 activity in mitotic extracts, since no other Ser-209 peptide kinase activities were detected.

Interactions between the subunits of casein kinase II

Casein kinase II (CKII) is a protein serine/threonine kinase known to control the activity of a variety of regulatory nuclear proteins. This enzyme has a tetrameric structure composed of two catalytic (alpha and/or alpha ') subunits and two beta subunits. We have examined the subunit composition of tetrameric complexes of purified bovine CKII by immunoprecipitation using alpha, alpha ', or beta subunit-specific antibodies. These experiments indicate that the enzyme can exist as homotetramers (i.e., alpha 2 beta 2 or alpha 2' beta 2) as well as heterotetramers (i.e. alpha alpha ' beta 2). To further examine subunit interactions between the alpha, alpha ', or beta subunits of CKII, we have utilized the yeast two-hybrid system (Fields, S. and Song, O. (1989) Nature 340: 245-246). For these studies, each subunit of human CKII was expressed in yeast as a fusion with the DNA binding domain or with the transcriptional activation domain of the yeast GAL4 transcriptional activator. These studies demonstrate that the alpha or alpha ' subunits of CKII can interact with the beta subunits of CKII, but not with other alpha or alpha ' subunits. By comparison, the beta subunits of CKII can interact with alpha, alpha ', or beta subunits. These results indicate that the CKII holoenzyme forms because of the ability of beta subunits to dimerize, bringing two heterodimers (alpha beta or alpha ' beta) into a tetrameric complex.

Developmental changes in the activity and cellular localization of hepatic casein kinase II in the rat

The activity and cellular localization of hepatic casein kinase II (CKII) was examined during late fetal development in the rat. Cultured fetal hepatocytes displayed constitutive CKII activity which was not further activated by growth factor exposure. Similarly, fetal liver CKII showed approximately fivefold greater activity than adult liver. The fetal hepatic activity was, to a large degree, localized to a nuclear fraction. Postnuclear cytosol preparations from fetal and adult liver showed similar CKII activity. In all cases, FPLC ion exchange chromatography followed by Western immunoblotting showed that immunoreactive CKII coincided with kinase activity. However, parallel determinations of CKII activity and immunoreactive CKII levels showed a higher (five- to sixfold) CKII specific activity in nuclear extracts compared to cytosol. In summary, fetal hepatic CKII demonstrates coincident nuclear localization and activation. We hypothesize that the regulation of hepatic CKII is relevant to the mitogen-independent proliferation displayed by fetal rat hepatocytes.

Proteins of the Xenopus laevis zinc finger multigene family as targets for CK II phosphorylation

Zn finger proteins (ZFPs) of the C2/H2 type in Xenopus laevis are encoded by a multigene family comprising several hundred members. Based upon conserved sequence features outside the Zn finger region, ZFPs can be subdivided into distinct subfamilies. Two of such subfamilies are characterized by conserved, N-terminal amino acid sequences termed the FAX and the FAR Domain. Here we present data suggesting that the zinc finger proteins of the FAR-ZFP subfamily are targets for CK II mediated phosphorylation. Expression of these proteins during oogenesis coincides with CK II activity in unfertilized eggs. Additionally, we have found that XlcOF 7.1, a member of the FAX-ZFP subfamily, is also phosphorylated by CK II. The target sites for in vitro phosphorylation are localized within the conserved N-terminal domains but not within the Zn finger regions. However, amino acid sequence comparison revealed that individual phosphoacceptor sites are not generally conserved among all members of the respective ZFP subfamilies. The relevance of a potential CK II phosphorylation for the regulation of ZFP activity in vivo is discussed.

A novel mediator of class II gene transcription with homology to viral immediate-early transcriptional regulators

Our investigations of mammalian class II gene transcription resulted in identification, purification, and cloning of the corresponding cDNA of a cellular factor (p15) that mediates the effects of several distinct activators on transcription in vitro. Functional deletion analyses revealed a bipartite structure of p15 comprising an amino-terminal regulatory domain and a carboxy-terminal cryptic DNA-binding domain. We provide evidence that activity of p15 is controlled by protein kinases that target the regulatory domain. Structural and functional similarities, including sequence homology to domains essential for cofactor function, cofactor activity, promiscuity with respect to transcriptional activators, and interactions with components of the basal transcription machinery, relate this novel cellular cofactor to viral immediate-early transcriptional regulators.

Biosynthesis of casein kinase II in lymphoid cell lines

We have analyzed the biosynthesis of casein kinase II. In exponentially growing tissue culture cells, the beta subunit was synthesized in excess of the catalytic subunit (alpha). A substantial fraction of newly synthesized beta was degraded within the first hour. The remaining fraction of beta was incorporated into holoenzyme. In contrast, little degradation of newly synthesized alpha subunit was observed and most was quickly and efficiently incorporated into holoenzyme. The assembly of beta with alpha was paralleled by an increase in apparent molecular mass of beta due to phosphorylation. The subcellular distribution of newly synthesized [35S]Met-labelled casein kinase II and of enzyme labelled and chased in the presence of excess unlabelled methionine was very similar and compatible with a nuclear localization. The degradation of the excess beta subunit occurred through a non-lysosomal proteolytic system with a very low ATP requirement.