Serum-stimulated cell growth causes oscillations in casein kinase II activity

CK2-induced cooperation of HHEX with the YAP-TEAD4 complex promotes colorectal tumorigenesis

Dysregulation of Hippo pathway leads to hyperactivation of YAP-TEAD transcriptional complex in various cancers, including colorectal cancer (CRC). In this study, we observed that HHEX (Hematopoietically expressed homeobox) may enhance transcription activity of the YAP-TEAD complex. HHEX associates with and stabilizes the YAP-TEAD complex on the regulatory genomic loci to coregulate the expression of a group of YAP/TEAD target genes. Also, HHEX may indirectly regulate these target genes by controlling YAP/TAZ expression. Importantly, HHEX is required for the pro-tumorigenic effects of YAP during CRC progression. In response to serum stimulation, CK2 (Casein Kinase 2) phosphorylates HHEX and enhances its interaction with TEAD4. A CK2 inhibitor CX-4945 diminishes the interaction between HHEX and TEAD4, leading to decreased expression of YAP/TEAD target genes. CX-4945 synergizes the antitumor activity of YAP-TEAD inhibitors verteporfin and Super-TDU. Elevated expression of HHEX is correlated with hyperactivation of YAP/TEAD and associated with poor prognosis of CRC patients. Overall, our study identifies HHEX as a positive modulator of YAP/TEAD to promote colorectal tumorigenesis, providing a new therapeutic strategy for targeting YAP/TEAD in CRC.

Hippo signalling is often deregulated in cancers. Here the authors show that CK2 enhances the cooperation of HHEX with YAP-TEAD complex to promote colorectal tumorigenesis.

How can a traffic light properly work if it is always green? The paradox of CK2 signaling

CK2 is a constitutively active protein kinase that assuring a constant level of phosphorylation to its numerous substrates supports many of the most important biological functions. Nevertheless, its activity has to be controlled and adjusted in order to cope with the varying needs of a cell, and several examples of a fine-tune regulation of its activity have been described. More importantly, aberrant regulation of this enzyme may have pathological consequences, e.g. in cancer, chronic inflammation, neurodegeneration, and viral infection. Our review aims at summarizing our current knowledge about CK2 regulation. In the first part, we have considered the most important stimuli shown to affect protein kinase CK2 activity/expression. In the second part, we focus on the molecular mechanisms by which CK2 can be regulated, discussing controversial aspects and future perspectives.

The effect of ionizing radiation on the subcellular localization and kinase activity of protein kinase CK2 in human non-small cell lung cancer cells

Protein kinase CK2 is a ubiquitously expressed kinase in eukaryotes, which is known to phosphorylate many protein substrates. Because CK2 is involved in the regulation of various signaling pathways, we wondered whether CK2 participated in the regulation of ionizing radiation (IR) induced biological process. In this study, we investigated the effect of IR on the subcellular localization and kinase activity in human non-small cell lung cancer (NSCLC) cells. Immunofluorescent results showed that CK2 subunits shuttle into the nucleus mostly beginning 1 h after IR and lasting more than 6 h. We also conducted in vitro kinase assay and observed an increase in CK2 kinase activity at 6 h after IR. Furthermore, an increase in S phase was observed at 6 h after IR. Colony formation assay results demonstrated that CK2 inhibitor CX-4945 significantly enhanced the effect of irradiation in NSCLC cells. These results indicated that CK2 may be implicated in the regulation of IR-induced biological process.

CK2 is the regulator of SIRT1 substrate-binding affinity, deacetylase activity and cellular response to DNA-damage

SIRT1, an NAD(+) (nicotinamide adenine dinucleotide)-dependent deacetylase, protects cells from stress-induced apoptosis, and its orthologues delay aging in lower eukaryotes. SIRT1 increases survival in response to stress such as DNA damage by deacetylating a number of substrates including pro-apoptotic protein p53. The molecular mechanism by which DNA-damage activates SIRT1 is not known. By screening a kinase inhibitor library, we identified CK2 as a SIRT1 kinase. CK2 is a pleiotropic kinase with more than 300 substrates and well-known anti-apoptotic and pro-growth activities. We find that CK2 is recruited to SIRT1 after ionizing radiation (IR) and phosphorylates conserved residues Ser 154, 649, 651 and 683 in the N- and C-terminal domains of mouse SIRT1. Phosphorylation of SIRT1 increases its deacetylation rate but not if the four Ser residues are mutated. In addition, phosphorylation of SIRT1 increases its substrate-binding affinity. CK2-mediated phosphorylation increases the ability of SIRT1 to deacetylate p53 and protect cells from apoptosis after DNA damage. Based on these findings, we propose that CK2 protects against IR-induced apoptosis partly by phosphorylating and activating SIRT1. Thus, this work suggests that SIRT1 is a component of the expansive anti-apoptotic network controlled by CK2. Since expression of both CK2 and SIRT1 is upregulated with tumorigenesis and downregulated with senescence, the CK2-SIRT1 link sheds new light on how CK2 may regulate cancer development and aging.

Evaluating CK2 activity with the antibody specific for the CK2-phosphorylated form of a kinase-targeting cochaperone Cdc37

CK2-dependent phosphorylation of a kinase-specific Hsp90 co-chaperone Cdc37 on a conserved serine residue (Ser13) is essential for the function of Cdc37 [Bandhakavi S. et al. J. Biol. Chem. 278:2829-2836, 2003; Shao J. et al. J. Biol. Chem. 278:38117-38220, 2003; Miyata Y., & Nishida E. Mol. Cell. Biol. 24:4065-4074, 2004]. We have recently produced an anti-[pSer13]-Cdc37 antibody which specifically recognizes Cdc37 that is phosphorylated on Ser 13 [Miyata Y. & Nishida E. FEBS J. 274:5690-5703, 2007]. Here we investigated CK2 activity both in vitro and in cultured cells by using anti-[pSer13]-Cdc37 antibody. Immunoblotting with this antibody showed that heparin and 4,5,6,7-tetrabromobenzotriazole (TBB), known CK2 inhibitors, inhibited in vitro phosphorylation of Cdc37 on Ser13 by CK2 holoenzyme or CK2alpha, confirming the specificity of the antibody to detect CK2 activity. Treatment of cells with TBB resulted in the decrease in the phosphorylation level of endogenous Cdc37 on Ser13, as revealed by anti-[pSer13]-Cdc37, and overexpression of either CK2alpha or CK2beta subunit enhanced the Cdc37 phosphorylation level. While CK2 is suggested to be involved in cell proliferation, mitogenic stimulation of starved cells by fresh serum or insulin-like growth factor-I did not enhance phosphorylation of Cdc37 on Ser13. CK2 inhibitors are known to induce cell apoptosis, suggesting a reverse correlation between cell apoptosis and CK2 activity. However, cellular apoptotic stresses, such as anisomycin treatment and UV irradiation, were found to rather modestly increase phosphorylation of Cdc37 on Ser13. These results show that the anti-[pSer13]-Cdc37 antibody can be a promising new tool to evaluate in vivo CK2 activity.

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

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

Protein kinase CK2 activates the atypical Rio1p kinase and promotes its cell-cycle phase-dependent degradation in yeast

Using co-immunoprecipitation combined with MS analysis, we identified the alpha' subunit of casein kinase 2 (CK2) as an interaction partner of the atypical Rio1 protein kinase in yeast. Co-purification of Rio1p with CK2 from Deltacka1 or Deltacka2 mutant extracts shows that Rio1p preferentially interacts with Cka2p in vitro. The C-terminal domain of Rio1p is essential and sufficient for this interaction. Six C-terminally located clustered serines were identified as the only CK2 sites present in Rio1p. Replacement of all six serine residues by aspartate, mimicking constitutive phosphorylation, stimulates Rio1p kinase activity about twofold in vitro compared with wild-type or the corresponding (S > A)(6) mutant proteins. Both mutant alleles (S > A)(6) or (S > D)(6) complement in vivo, however, growth of the RIO1 (S > A)(6) mutant is greatly retarded and shows a cell-cycle phenotype, whereas the behaviour of the RIO1 (S > D)(6) mutant is indistinguishable from wild-type. This suggests that phosphorylation by protein kinase CK2 leads to moderate activation of Rio1p in vivo and promotes cell proliferation. Physiological studies indicate that phosphorylation by CK2 renders the Rio1 protein kinase susceptible to proteolytic degradation at the G(1)/S transition in the cell-division cycle, whereas the non-phosphorylated version is resistant.

Serine phosphorylation by casein kinase II controls endocytic L1 trafficking and axon growth

The cell adhesion molecule L1 plays crucial roles in axon tract development. In vitro, L1 presented as a culture substrate stimulates axon elongation by binding to L1 expressed on the growth cone. In migrating growth cones, L1 is endocytosed via the AP-2/clathrin-mediated pathway at the central domain, followed by anterograde vesicular transport and recycling to the plasma membrane of the leading front. It has previously been shown that this endocytic trafficking of L1 is critical for axon elongation (Kamiguchi and Yoshihara [2001] J. Neurosci. 21:9194-9203). Adjacent to the AP-2 recognition site, the L1 cytoplasmic domain has a cluster of acidic amino acids containing Ser1181 that can be phosphorylated by casein kinase II (CKII; Wong et al. [1996a] J. Neurochem. 66:779-786). In this paper, we demonstrate that Ser1181 phosphorylation by CKII is implicated in both normal endocytic trafficking of L1 and L1-stimulated axon growth. Whereas L1 is sorted into transferrin-positive endosomes after endocytosis, pharmacological inhibition of CKII caused some population of L1 to be internalized into transferrin-negative compartments. Single-amino-acid mutations at Ser1181, which either prevent or mimic phosphorylation by CKII, caused similar missorting of internalized L1. Furthermore, dorsal root ganglion neurons that had been treated with a CKII inhibitor or transfected with the L1 mutants showed impaired ability to extend axons on an L1 substrate but not on other control substrates. These results demonstrate the novel role of CKII in L1-mediated axon elongation and stress the importance of functional linkage between L1 phosphorylation and L1 trafficking in migrating growth cones.

In situ hybridization analysis of protein kinase CK2 expression during plant development

Protein kinase CK2 (EC 2.7.1.37) is a ubiquitous Ser/Thr kinase composed of two subunits, a subunit (catalytic) and a subunit (regulatory). Although CK2 is one of the components of the network that controls the eukaryotic cell cycle, very little is known about the developmental control of its expression, particularly in plants. Using in situ hybridization we have determined the pattern of expression of its two subunits in relation to plant morphogenesis. Our results show a cell-specific mRNA accumulation, equivalent for both subunits, which suggests a co-ordinate regulation during development. The spatial and temporal pattern of expression correlates with the appearance of the meristems, and high levels of transcripts are also present in differentiated tissues with high mitotic activity. These results confirm the transcriptional regulation of CK2 and establish a general correlation with cell proliferation in a developmental context. They also provide suggestive evidence for the requirement of CK2 expression for the initiation and organization of meristem activity.

Pyrrolidine dithiocarbamate-induced neuronal cell death is mediated by Akt, casein kinase 2, c-Jun N-terminal kinase, and IkappaB kinase in embryonic hippocampal progenitor cells

Pyrrolidine dithiocarbamate (PDTC) is known to induce cell death by the stimulation of intracellular zinc transport and subsequent modulation of nuclear factor-kappaB (NF-kappaB) activity. Zinc is a signaling messenger that is released by neuronal activity at many central excitatory synapses. Excessive synaptic release of zinc followed by entry into vulnerable neurons contributes to severe neuronal cell death. In the present study, we explored how PDTC modulates intracellular signal transduction pathways, leading to neuronal cell death. The exposure of immortalized embryonic hippocampal cells (H19-7) to PDTC within the range of 1-100 microM caused cell death in a dose-dependent manner. During the cell death, NF-kappaB activity increased in response to PDTC, and this activity corresponded well with the increase of intracellular free zinc levels, implying that the activation of NF-kappaB transmits the cell death signals of PDTC. Furthermore, PDTC caused the activation of IkappaB kinase (IKK), casein kinase 2 (CK2), phosphatidylinositol 3-kinase (PI-3K), and Akt, as well as mitogen-activated protein kinases (MAPKs), such as extracellular signal-regulated kinase (ERK) and c-Jun N-terminal kinase (JNK), but not p38 kinase. The blockade of PI-3K, JNK, and CK2 pathways resulted in a remarkable suppression of PDTC-induced cell death and also the activation of IKK, which subsequently led to a decrease of IkappaB phosphorylation. Although the overexpression of dominant-negative SEK in a transient manner did not inhibit the activation of Akt by PDTC, the transfection of kinase-inactive Akt mutants did cause a remarkable blockade of JNK activation, implying that Akt is present upstream of JNK in the PDTC-signaling pathways. Moreover, whereas selective CK2 inhibitors suppressed PDTC-induced JNK activation, the inhibition of JNK did not affect CK2 activity, suggesting that CK2 is directly related to the regulation of cell viability by PDTC and that the CK2-JNK pathway could be a downstream target of PDTC. Taken together, our results suggest that PDTC-mediated accumulation of intracellular zinc ions may affect cell viability by modulating several intracellular signaling pathways in neuronal hippocampal progenitor cells.

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.

Platelet-activating factor (PAF) activates casein kinase 2 in the protozoan parasite Herpetomonas muscarum muscarum

Herpetomonas muscarum muscarum is a flagellate parasite of the family Trypanosomatidae, whose cell differentiation can be triggered by the lipid mediator, PAF. In this study we demonstrate for the first time that PAF effect relies on the activation of casein kinase 2 (CK2). The classical antagonist of PAF receptor, WEB 2086, abrogated PAF-enhanced CK2 activity. CK2 activation by PAF was also inhibited when parasite extracts were assayed in the presence of modulators of PKC, MAPK, and both Ser/Thr and Tyr phosphatases. Finally, a cell permeable inhibitor of CK2 (DRB) suppressed PAF-induced cell differentiation in a dose-dependent manner.

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

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

Stress-induced activation of protein kinase CK2 by direct interaction with p38 mitogen-activated protein kinase

Protein kinase CK2 has been implicated in the regulation of a wide range of proteins that are important in cell proliferation and differentiation. Here we demonstrate that the stress signaling agents anisomycin, arsenite, and tumor necrosis factor-alpha stimulate the specific enzyme activity of CK2 in the human cervical carcinoma HeLa cells by up to 8-fold, and this could be blocked by the p38 MAP kinase inhibitor SB203580. We show that p38alpha MAP kinase, in a phosphorylation-dependent manner, can directly interact with the alpha and beta subunits of CK2 to activate the holoenzyme through what appears to be an allosteric mechanism. Furthermore, we demonstrate that anisomycin- and tumor necrosis factor-alpha-induced phosphorylation of p53 at Ser-392, which is important for the transcriptional activity of this growth suppressor protein, requires p38 MAP kinase and CK2 activities.

Sequential serum-dependent activation of CREB and NRF-1 leads to enhanced mitochondrial respiration through the induction of cytochrome c

Progression through the cell cycle requires ATP for protein synthesis, cytoskeletal rearrangement, chromatin remodeling, and protein degradation. The mechanisms by which mammalian cells increase respiratory capacity and ATP production in preparation for cell division are largely unexplored. Here, we demonstrate that serum induction of cytochrome c mRNA and processed protein in quiescent BALB/3T3 fibroblasts is associated with a marked increase in mitochondrial respiration. Cytochrome c was induced in the absence of any increase in citrate synthase activity or in subunit IV of the cytochrome c oxidase complex mRNA or protein, indicating that the enhanced respiratory rate did not require a general increase in mitochondrial biogenesis or respiratory chain expression. Transfections with a series of cytochrome c promoter mutants showed that both nuclear respiratory factor 1 (NRF-1) and cAMP-response element-binding protein (CREB) binding sites contributed equally to induced expression by serum. Moreover, CREB and NRF-1 were phosphorylated sequentially in response to serum, and the NRF-1 phosphorylation was accompanied by an increase in its ability to trans-activate target gene expression. The results demonstrate that the differential transcriptional expression of cytochrome c, through sequential transcription factor phosphorylations, leads to enhanced mitochondrial respiratory capacity upon serum-induced entry to the cell cycle.

PTP-S2, a nuclear tyrosine phosphatase, is phosphorylated and excluded from condensed chromosomes during mitosis

PTP-S2 is a tyrosine specific protein phosphatase that binds to DNA and is localized to the nucleus in association with chromatin. It plays a role in the regulation of cell proliferation. Here we show that the subcellular distribution of this protein changes during cell division. While PTP-S2 was localized exclusively to the nucleus in interphase cells, during metaphase and anaphase it was distributed throughout the cytoplasm and excluded from condensed chromosomes. At telophase PTP-S2 began to associate with chromosomes and at cytokinesis it was associated with chromatin in the newly formed nucleus. It was hyperphosphorylated and showed retarded mobility in cells arrested in metaphase. In vitro experiments showed that it was phosphorylated by CK2 resulting in mobility shift. Using a deletion mutant we found that CK2 phosphorylated PTP-S2 in the C-terminal non-catalytic domain. A heparin sensitive kinase from mitotic cell extracts phosphorylated PTP-S2 resulting in mobility shift. These results are consistent with the suggestion that during metaphase PTP-S2 is phosphorylated (possibly by CK2 or a CK2-like enzyme), resulting in its dissociation from chromatin.

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.

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.

All three IkappaB isoforms and most Rel family members are stably associated with the IkappaB kinase 1/2 complex

Nuclear factor kappa B (NF-kappaB) is an important transcription factor for the genes of many pro-inflammatory proteins and is strongly activated by the cytokines interleukin-1 and tumor necrosis factor (TNF)alpha under various pathological conditions. In nonstimulated cells, NF-kappaB is present in the cytosol where it is complexed to its inhibitor IkappaB. Activation of NF-kappaB depends on the signal-induced phosphorylation of IkappaB by specific IkappaB kinases which initiates the inhibitor's conjugation to ubiquitin and subsequent degradation by the proteasome. We used both TNF-stimulated and okadaic-acid-stimulated HeLa cells to purify three biochemically distinct kinase activities targeting one or both of the two serines (S32 and S36) in IkappaBalpha which induce its rapid degradation upon cytokine stimulation. All three activities correspond to known IkappaB kinases: the mitogen-activated 90 kDa ribosomal S6 kinase (p90rsk1), the IkappaB kinase 1/2 complex (IKK1/2) and casein kinase II (CK II). However, we found that only one of the activities, namely the IKK1/2 complex, exists as a pre-assembled kinase-substrate complex in which the IKKs are directly or indirectly associated with several NF-kappaB-related and IkappaB-related proteins: RelA, RelB, cRel, p100, p105, Ikappa Balpha, Ikappa Bbeta and Ikappa Bepsilon. The existence of stable kinase-substrate complexes, the presence of all three known IkappaB isoforms in these complexes and our observation that the IKK complex is capable of phosphorylating Ikappa Balpha-, Ikappa Bbeta- and Ikappa Bepsilon-derived peptides at the respective degradation-relevant serines suggests that the IKK complex exerts a broad regulatory role for the activation of different NF-kappaB species. In contrast to previous studies, which locate CK II phosphorylation sites exclusively to the C-terminal PEST sequence of Ikappa Balpha, we observed efficient phosphorylation of serine 32 in Ikappa Balpha by the purified endogenous CK II complex. Therefore, both p90rsk1 and CK II have the same preference for phosphorylating only one of the two serines which are relevant for inducible degradation.

Subcellular localization of the von Hippel-Lindau disease gene product is cell cycle-dependent

The von Hippel-Lindau gene product (pVHL) interacts with and inhibits the cellular transcription factor elongin. However, the subcellular localization of pVHL has remained uncertain. Naturally occurring pVHL mutants which fail to interact with elongin have been described in patients with VHL disease or sporadic renal cell carcinoma (RCC). Here, we have examined the cellular expression pattern of endogenous pVHL in different RCC cell lines by immunocytochemistry and confocal microscopy. Both anti-N-terminal and anti-C-terminal pVHL antibodies were able to recognize endogenous wild-type pVHL expressed by the RCC cells studied. A C-terminal truncated VHL mutant expressed by RCC cell line A498 was detected only by the N-terminal antibody but not by the C-terminal antibody as expected. The overall staining patterns of these cell lines are similar, with a predominant nuclear speckled pattern and a moderate cytoplasmic staining in subconfluent cell cultures. Interestingly, when cells reached confluency, more prominent nuclear staining with little or no cytoplasmic expression was observed. By using double labeling with anti-VHL and anti-bromodeoxyuridine (BrdU) antibodies and cell cycle analyses, we found that in the G1/G0-phase, pVHL was localized exclusively in the nucleus associated with distinctive large subnuclear structures, whereas the majority of the cells in S-phase of the cell cycle also showed a diffuse cytoplasmic staining. Our results indicate that subcellular localization of pVHL is regulated in a cell cycle-dependent manner.

Protein kinase CK2 ("casein kinase-2") and its implication in cell division and proliferation

Protein kinase CK2 (also termed casein kinase-2 or -II) is a ubiquitous Ser/Thr-specific protein kinase required for viability and for cell cycle progression. CK2 is especially elevated in proliferating tissues, either normal or transformed, and the expression of its catalytic subunit in transgenic mice is causative of lymphomas. CK2 is highly pleiotropic: more than 160 proteins phosphorylated by it at sites specified by multiple acidic residues are known. Despite its heterotetrameric structure generally composed by two catalytic (alpha and/or alpha') and two non catalytic beta-subunits, the regulation of CK2 is still enigmatic. A number of functional features of the beta-subunit which could cooperate to the modulation of CK2 targeting/activity will be discussed.

Nerve and epidermal growth factor induce protein synthesis and eIF2B activation in PC12 cells

The regulation of protein synthesis and of eukaryotic initiation factor eIF2B was studied in PC12 cells. An increase in protein synthesis was observed after nerve growth factor (NGF) and epidermal growth factor (EGF) treatment of PC12 cells, and this increase coincided with activation of eIF2B. Growth factor addition in the presence of the phosphatidylinositol-3'-OH kinase inhibitor wortmannin showed that both NGF- and EGF-induced protein synthesis and eIF2B activation were phosphatidylinositol-3'-OH kinase dependent. The EGF-induced stimulation of protein synthesis and activation of eIF2B was dependent upon FK506-binding protein-rapamycin-associated protein, as shown with the immunosuppressant rapamycin, whereas NGF induction was partially dependent upon FK506-binding protein-rapamycin-associated protein. The activities of two kinases that act on eIF2B, glycogen synthase kinase-3 and casein kinase II, were measured to assess their potential roles in the activation of eIF2B in PC12 cells. Inactivation of glycogen synthase kinase-3 was seen in response to both NGF and EGF and this coincided with activation of eIF2B. However, inactivation of glycogen synthase kinase-3 was not rapamycin sensitive, in contrast to the activation of eIF2B. This indicates the involvement of another protein kinase or regulatory mechanism in the eIF2B activation. Both growth factors activated casein kinase II. However, the time course of its activation and its insensitivity to wortmannin and rapamycin suggest that casein kinase II does not play a major regulatory role in eIF2B activation under these conditions.

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.

Two putative protein kinase CK2 phosphorylation sites are important for Myf-5 activity

Myf-5, a member of a family of muscle-specific transcription factors, is important for myogenic cell determination and differentiation. Here, we report that Myf-5 protein constitutes a substrate for phosphorylation in vitro by protein kinase CK2. We identified two potential phosphorylation sites at serine49 and serine133, both of which seem to be necessary for Myf-5 activity. Mutants which can no longer be phosphorylated fail to transactivate E-box-dependent reporter genes and act as trans-dominant repressors of wild-type Myf-5. Normal activity can be restored by replacing the serine residues with glutamate suggesting that a negative charge at these sites is obligatory for Myf-5 activity. Although serine133 is part of helix 2 which mediates dimerization, we find no evidence for impaired DNA-binding or heterodimerization of the Ser-Ala133 mutant. Some serine49 mutations exhibit reduced nuclear localization and/or protein stability. Our data suggest that CK2-mediated phosphorylation of Myf-5 is required for Myf-5 activity.

Modulation of the protein kinase CK2 activity by a synthetic peptide corresponding to the N-terminus of its beta regulatory subunit

Using a synthetic peptide corresponding to the sixteen amino-acid N-terminus of the beta subunit, the structure-activity relationship study of casein kinase 2 (CK2) was performed with regard to its previously reported property to polymerize and oligomerize in vitro. Velocity sedimentation experiments show that the peptide beta 1-16 prevents the thick filament formation and stabilizes the ring-like structure of the kinase. Furthermore, the peptide beta 1-16 stimulates the kinase activity by 3-fold toward exogenous substrates as well as the intrinsic autophosphorylation of the kinase. Such observations are in agreement with the proposed model of an activated state of CK2 when the ring-like structure is adopted. Comparison of the effects of spermine and peptide beta 1-16 on CK2 structure and activity suggests that these two activating molecules may function in a different way. Our study suggests that the N-terminal region of the beta subunit of CK2 could regulate the kinase activity by controlling the quaternary structure of the enzyme.

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.

The effect of prolactin on casein kinase II, MAP kinase and PKC in rabbit mammary cells and Nb2 rat lymphoid cells

Prolactin induces milk protein gene expression in rabbit primary mammary cells without any concomitant cell multiplication. Prolactin or other lactogenic hormones is the major inducer of cell division in the rat lymphoid Nb2 cells. In Nb2 cells, prolactin also rapidly induces the expression of the c-myc gene, and beta-actin and stathmin gene expression is induced more slowly. The possible involvement of casein kinase II (CKII), mitogen-activated protein kinase (MAPK) and protein kinase C (PKC) in these process is not well known. The present work was undertaken to evaluate the effect of prolactin on these protein kinases and to determine the possible involvement of these enzymes in the activity of several genes under the control of the hormone. In rabbit mammary cells, prolactin did not alter CKII activity but did transiently stimulate MAP kinase activity. Prolactin also stimulated Ca(2+)-independent PKC. This effect was visible after 10 min and was maintained for at least 24 hr. Staurosporine, an inhibitor of PKC and of several tyrosine kinases altered Ca(2+)-independent PKC only moderately. In contrast, GF 109203X, a potent and specific inhibitor of PKC, abrogated almost all PKC activity. Staurosporine, but not GF 109203X, prevented the induction of the casein gene by prolactin. In Nb2 cells, prolactin induced a slow stimulation of CKII activity. The hormone did not induce MAP kinase activity. Prolactin stimulated Ca(2+)-independent PKC over periods of 24 hr. GF 109203X, but not staurosporine, inhibited PKC activity, whereas staurosporine but not GF 109203X, inhibited the induction of Nb2 cell multiplication and the accumulation of c-myc, beta-actin and stathmin mRNAs. From these data, it can be concluded that (1) the stimulation of CKII by prolactin in Nb2 cells is concomitant with cell multiplication: (2) MAPK stimulation is not necessary for prolactin to induce Nb2 cell multiplication; and (3) PKC is stimulated in mammary and Nb2 cells, but this stimulation is not required for prolactin to stimulate casein, c-myc, beta-actin and stathmin gene expression and Nb2 cell division.

The physical association of casein kinase 2 with nucleolin

CK2 (formerly called casein kinase 2) is a ubiquitous messenger-independent serine/threonine protein kinase implicated in cell growth and proliferation. To investigate the regulation and functions of this enzyme, experiments were carried out to search for CK2-interacting proteins. The methods employed included an overlay technique, co-purification, co-immunoprecipitation, and the use of glutathione S-transferase (GST) CK2 fusion proteins. By the CK2 overlay technique, one protein of 110 kDa was found to bind to CK2 with very high affinity. The binding was inhibited by CK2 effectors such as heparin, polyarginine, and histone H1, but was not affected by the CK2 substrate, casein. Protein p110 was also detected by co-immunoprecipitation using anti-CK2 antiserum, suggesting an in vivo association of this protein with CK2. Co-purification of p110 with CK2 from Sf-9 cells that overexpressed CK2 was also observed through sequential chromatographic steps. Using GST fusion proteins of CK2, the CK2-p110 interaction was investigated further and was found to occur primarily through CK2 alpha or alpha' subunits, but not the beta subunit. Protein p110 was purified from 3T3 L1 mouse fibroblast cell lines using a GST-CK2 affinity resin. Amino acid sequence analysis of peptides obtained from the protein indicated that it is the nuclear protein, nucleolin. Furthermore, p110 was recognized by anti-nucleolin antiserum. At present, the physiological significance of the strong interaction between CK2 and nucleolin, an excellent substrate for the enzyme, is not clear. However, this association may be important for regulating rDNA transcription.

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.

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.

In vitro phosphorylation of the polyomavirus major capsid protein VP1 on serine 66 by casein kinase II

Phosphorylation of the polyomavirus major capsid protein VP1 plays a role in virus assembly and may function in virus-cell recognition. Previous mapping of the in vivo phosphorylation sites on VP1 identified phosphorylation of threonine residues Thr-63 and Thr-156 (Li, M., and Garcea, R. L. (1994) J. Virol. 68, 320-327). Phosphoserine was detected in a tryptic phosphopeptide encompassing residues 58-78. Because of consensus casein kinase II (CK II) sites in this peptide, we examined the in vitro phosphorylation of the purified recombinant VP1 protein by CK II. CK II phosphorylated VP1 on serine, and the resulting tryptic phosphopeptide eluted in a 30-31 min high performance liquid chromatography fraction corresponding to residues 58-78. The VP1 tryptic phosphopeptide also co-migrated in two-dimensional peptide analysis with one of the tryptic peptides obtained from VP1 isolated after in vivo 32P labeling of virus-infected cells. A site-directed mutant VP1 protein, Ser-66 to Ala, was phosphorylated poorly by CK II in vitro. As determined by electron microscopy, all of the mutant proteins were isolated in pentameric form similar to the wild-type protein, although the Ala-66 pentamers had a tendency to self-assemble in vitro into tubular as well as capsid-like structures. These findings identify Ser-66 as a site of VP1 phosphorylation in vitro, and suggest that VP1 may serve as a substrate for CK II in vivo.

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.

Direct identification of a polyamine binding domain on the regulatory subunit of the protein kinase casein kinase 2 by photoaffinity labeling

Phosphorylation of many protein substrates by the protein kinase casein kinase 2 (CK2) is stimulated severalfold in the presence of polyamines such as spermine. Previous experiments have shown that CK2 is a polyamine binding protein and that the regulatory beta subunit is required for this binding activity. To delineate the spermine binding site of CK2, we have applied a photoaffinity labeling method using a tritiated photoactivable analog of spermine, [3H]sperminediazonium. The photoaffinity labeled beta subunit was cleaved with cyanogen bromide, and two labeled peptides were separated by high performance liquid chromatography. The major one was the peptide T72EQAAEM78 and the minor one was a 22-amino acid peptide comprising residues Ile98 to Met119. Thr72 and His108 were identified as the labeled amino acids of the Thr72-Met78 and Ile98-Met119 peptides, respectively. In the same manner, we succeeded in determining the residue Leu220 as an alpha subunit residue covalently bound to the probe. The photoaffinity labeling method described here enabled the first elucidation, by direct microsequencing, of a polyamine binding site on CK2 for which we propose a provisional structural model. These observations suggest a possible mechanism for CK2 activation by polyamines at the molecular level.

Quaternary structure of casein kinase 2. Characterization of multiple oligomeric states and relation with its catalytic activity

The structure-activity relationship of casein kinase 2 (CK2) was examined with regard to its previously reported property to self-aggregate in vitro. Sedimentation velocity and electron microscopy studies showed that the purified kinase exhibited four major, different oligomeric forms in aqueous solution. This self-polymerization was a reproducible and fully reversible process, highly dependent upon the ionic strength of the medium, suggesting that electrostatic interactions are mostly involved. At high salt concentrations (e.g. 0.5 M NaCl), CK2 appears as spherical moieties with a 18.7 +/- 1.6 nm average diameter, roughly corresponding to the alpha 2 beta 2 protomer, as deduced by measurements of the Stokes radius and by light scattering studies. At lower ionic strength (e.g. 0.2 M NaCl), the protomers associate to form ring-like structures with a diameter (averaging 36.6 +/- 2.1 nm) and Stokes radius indicating that they are most likely made of four circularly associated alpha 2 beta 2 protomers. At 0.1 M NaCl, two additional polymeric structures were visualized: thin filaments (16.4 +/- 1.4 nm average), as long as 1 to 5 microns, and thick and shorter filaments (28.5 +/- 1.6 nm average). Examination of the molecular organization of CK2 under different catalytic conditions revealed that the ring-like structure is the favored conformation adopted by the enzyme in the presence of saturating concentrations of substrates and cofactors. During catalysis, well-known cofactors like MgCl2 or spermine are the main factors governing the stabilization of the active ring-like structure. On the other hand, inhibitory high salt concentrations promote the dissociation of the active ring-like structure into protomers. Such observations suggest a strong correlation between the ring-like conformation of the enzyme and optimal specific activity. Thus, CK2 may be considered as an associating-dissociating enzyme, and this remarkable property supports the hypothesis of a cooperative and allosteric regulation of the kinase in response to appropriate regulatory ligands possibly taking place in intact cells.

Identification and characterization of protein kinase CKII isoforms in HeLa cells. Isoform-specific differences in rates of assembly from catalytic and regulatory subunits

Protein kinase CKII (formerly casein kinase II) can be isolated as a heterotetramer, containing two catalytic (alpha or alpha') and two regulatory (beta) subunits. We have characterized the forms of CKII in HeLa cells using antibodies specific for the alpha or alpha' subunits. Following metabolic labeling with [35S]methionine, whole cell soluble extracts were analyzed by immunoprecipitation and gel electrophoresis. Both alpha and alpha' coprecipitate with beta and with each other. However, when extracts are depleted of alpha, a pool of CKII containing only alpha' and beta is identified. Similarly, depletion of alpha' revealed a pool exclusively of alpha and beta. Therefore, we propose that there are three distinct isoforms of CKII within HeLa cells with different catalytic subunit stoichiometries (alpha 2 beta 2, alpha alpha' beta 2, and alpha' 2 beta 2). With our immunodepletion procedure we have characterized the isoforms by activity analysis, turnover of pulse-labeled subunits, and by localization in subcellular fractions obtained from labeled cells. We have also analyzed complex formation between the catalytic and regulatory subunits by examining the differences in the rate of signal incorporation into subunits in immunoprecipitates obtained from continuously labeled and pulse-labeled cells. We have found that the alpha 2 beta 2 and alpha alpha' beta 2 isoforms assemble relatively slowly (12-16 h), whereas complex formation of the alpha' 2 beta 2 isoform occurs more rapidly (2-4 h). Analysis of isoform complex formation in subcellular fractions from pulse-labeled cells revealed that the majority of nuclear CKII is assembled in the nucleus from free catalytic and regulatory subunit polypeptides.

Casein kinase II alpha transgene-induced murine lymphoma: relation to theileriosis in cattle

Infection of cattle with the protozoan parasite Theileria parva results in a fatal lymphoproliferative syndrome that is associated with the overexpression of casein kinase II. The role of this enzyme in the pathogenesis of lymphoproliferative disorders was investigated by expressing the catalytic subunit in lymphocytes of transgenic mice. Adult transgenic mice displayed a stochastic propensity to develop lymphoma; co-expression of a c-myc transgene in addition to casein kinase II resulted in neonatal leukemia. Thus, the casein kinase II gene can serve as an oncogene, and its dysregulated expression is capable of transforming lymphocytes in a two-step pathway with c-myc.

A short core region of E-cadherin is essential for catenin binding and is highly phosphorylated

Classical cadherins associate with three cytoplasmic proteins, termed alpha, -beta- and gamma-catenin. This association mediates the attachment of cadherins to the microfilament network, which is believed to be of major importance for cadherin function. Deletion of the carboxyterminal 72-amino acid residues of E-cadherin had been previously shown to prevent catenin binding. Here we have analyzed additional mutants of E-cadherin with deletions within this region and identified a core region of 30 amino acids (E-cadherin pos. 832-862) essential for the interaction with catenins. Phosphorylation analysis of wild-type and mutant E-cadherin indicates that the catenin-binding domain is highly phosphorylated. In particular, the 30 amino acid region contains 8 serine residues which are well conserved among cadherins. To elucidate whether phosphorylation might be important for cadherin-catenin complex formation, site-directed mutagenesis experiments were performed. Partial substitutions of up to 5 of the 8 serine residues in the cluster had no influence on E-cadherin-catenin complex formation and E-cadherin mediated cell adhesion, although phosphorylation of E-cadherin was reduced. In contrast, substitution of the whole serine cluster completely abolished phosphorylation and affected complex formation with catenins. These results suggest that E-cadherin-catenin interaction may be regulated by phosphorylation of the catenin-binding domain, which might represent one molecular mechanism to regulate cadherin mediated cell adhesion.

Effects of autophosphorylation on casein kinase II activity: evidence from mutations in the beta subunit

Casein kinase II is a heterotetramer composed of two catalytic (alpha) and two regulatory (beta) subunits. To examine the effects of autophosphorylation of the beta subunit on enzyme activity, two mutants of the beta subunit from Drosophila were constructed in which either Ser4 or Ser2-4 was changed to alanine residues by oligonucleotide-directed mutagenesis and the proteins were expressed in Escherichia coli. The wild-type alpha and individual beta subunits present in inclusion bodies were renatured, and the biochemical properties of the reconstituted holoenzymes were examined. Analysis of autophosphorylation revealed that phosphate incorporation was about 0.8 mol/mol of beta subunit for the wild type and Ala4 mutant; Ser2 and Ser3 were the major sites of autophosphorylation with some phosphate in Ser4 as shown by Edman degradation. No autophosphorylation was observed with the Ala2-4 mutant. Substitution of alanine for serine residues at positions 4 or 2-4 of the beta subunits did not influence the reassociation of the alpha and beta subunits to form holoenzyme, or the function of the beta subunit in stimulating catalytic activity or in responding to basic compounds. To measure the effects of autophosphorylation on casein kinase II activity, the wild-type and mutant holoenzymes were preincubated in the presence and absence of ATP, and the rate of phosphorylation was measured with various substrates. In the absence of autophosphorylation, the wild-type, Ala4, and Ala2-4 forms of the holoenzyme displayed similar rates of phosphorylation of glycogen synthase. After preincubation with ATP, the rate of phosphorylation of glycogen synthase by the wild-type and Ala4 enzymes was inhibited by 30%.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Human casein kinase II: structures, genes, expression and requirement in cell growth stimulation

Casein kinase II (CKII) is an ubiquitous Ser/Thr protein phosphotransferase in control of a variety of crucial cellular functions including metabolism, signal transduction, transcription, translation and replication. CKII levels are consistently higher in neoplastic tissues. The human CKII is composed of subunits alpha, alpha', and beta with molecular masses of 43, 38 and 28 kDa, respectively, that form heterotetrameric holoenzymes (alpha 2 beta 2; alpha alpha' beta 2, alpha'2 beta 2) showing autophosphorylation particularly at subunit beta and hence suspected to play a regulatory role. The amino acid sequences of subunits indicate high evolutionary conservation. Employing the complete set of tissue-derived (placenta) and recombinant (expressed in E. coli) subunits and CKII holoenzymes, the catalytic function of alpha and alpha' and the several-fold stimulation by beta is shown to occur comparably in tissue-derived and recombinant CKII and the autophosphorylation of beta is shown by site-directed mutagenesis to be not decisive for the tuning of CKII activity. The human genome contains two genes encoding CKII alpha. First, there is a processed (pseudo)gene which is 99% homologous to the CKII alpha cDNA and which possesses a promoter-like region adjacently upstream with TATA and CAAT boxes so that transcription cannot be excluded. Second, there is an active gene of which we have characterized so far a 18.9 kb long central fragment which contains 8 exons comprising bases 102-824 of the CKII alpha coding region. The gene fragment contains repetitive elements, most prominently 16 Alu repeats. The genome further contains one as yet uncharacterized CKII alpha' gene and one gene encoding CKII beta. The CKII beta gene has been characterized as a 4.2 kb spanning gene composed of seven exons which possesses three transcription start sites and the translation start site in the second exon. The first intron harbors an Alu repeat also. The promoter region of the CKII beta gene contains elements such as multiple GC boxes, a CpG island, and nonstandard-positioned CAAT boxes but lacks a TATA box thus characterizing the gene as a housekeeping gene. The CKII genes are not clustered at a certain chromosome but rather are distributed over the whole human genome. Using the genomic clones as the probes for in situ hybridization, the active CKII alpha gene was mapped to chromosome 20p13, the processed CKII alpha (pseudo)gene to chromosome 11p15, and the CKII beta gene to chromosome 6p21. (The CKII alpha' gene has been localized on chromosome 16 with a cDNA probe.).(ABSTRACT TRUNCATED AT 400 WORDS)

Casein kinase II in signal transduction and cell cycle regulation

Casein kinase II is a protein serine/threonine kinase that is ubiquitously distributed in eukaryotes. Molecular cloning studies and protein sequence analysis of purified proteins have demonstrated the existence of two related, but distinct, isoenzymic forms of its catalytic subunit in mammals and birds. At present, the precise role of the individual casein kinase II isoforms in biological responses is poorly understood. However, a great deal of evidence indicates that casein kinase II is an important component of signalling pathways that control the growth and division of cells. In particular, casein kinase II is known to phosphorylate, and in several cases, regulate the activity of a variety of regulatory nuclear proteins including nuclear oncoproteins, transcription factors, and enzymes involved in other aspects of DNA metabolism. In this review, we will summarize evidence relating to the involvement of casein kinase II in signal transduction events that are relevant to cell proliferation.

C-terminal phosphorylation of the serum-response factor

The serum-response factor (SRF) is essential for the induction and repression of the protooncogene c-fos. Phosphorylation of SRF has been implicated to be involved in these processes and five phosphorylation sites have already been mapped within the N-terminal region. Here we show that in vivo additional phosphorylation of SRF does occur. This modification is located primarily within amino acids 206-289, which probably contain more than one phosphorylation site. Microsequencing allowed the identification of one phosphorylation site at Ser253, which is a potential target of casein kinase II. Mutational analysis revealed that, in contrast to N-terminal phosphorylation, Ser253 phosphorylation does not affect DNA-binding properties. In addition, phosphorylation at Ser253 does not seem to change transactivation activity of SRF but rather influences its contribution to transcriptional repression. Thus, C-terminal phosphorylation of SRF may modulate c-fos basal repression.