The tight association of protein kinase CK2 with plasma membranes is mediated by a specific domain of its regulatory beta-subunit

Distinctive expression and cellular localisation of zinc homeostasis-related proteins in breast and prostate cancer cells

BACKGROUND

Zinc transport proteins (ZIP and ZnT), metallothioneins (MT) and protein kinase CK2 are involved in dysregulation of zinc homeostasis in breast and prostate cancer cells. Following up our previous research, we targeted ZIP12, ZnT1, MT2A and CK2 in this study by investigating their expression levels and protein localisation.

METHODS

Quantitative reverse transcription polymerase chain reaction (qRT-PCR) and immunofluorescence confocal microscopy were employed to quantify the expression of ZIP12, ZnT1, MT2A and CK2 subunits in a panel of breast and prostate cell lines without or with extracellular zinc exposure. The cellular localisations of these target proteins were also examined by immunofluorescence confocal microscopy.

RESULTS

In response to the extracellular zinc exposure, the gene expression was elevated for SLC39A12 (ZIP12), SLC30A1 (ZnT1) and MT2A (MT2A) in normal prostate epithelial cells (RWPE-1) in contrast to their cancerous counterparts (PC3 and DU145), whilst the gene expression was higher for SLC39A12 (ZIP12) and SLC30A1 (ZnT1) in both normal (MCF10A) and basal breast cancer cells (MDA-MB-231) compared to luminal breast cancer cells (MCF-7). At the protein level, the expression for both ZIP12 and ZnT1 was trending lower in the time course for the breast cancer cells whilst their expression was remained constant in the normal breast epithelial cells. The expression of ZIP12 in prostate cancer cells was higher than the normal prostate cells. The protein expression for CK2 α/αꞌ and CK2β was markedly higher in prostate cancer cells than the normal prostate cells. Upon extracellular zinc exposure, ZIP12 was, for the first time, conspicuously localised in the plasma membrane of breast cancer cells but not in normal breast epithelial cells and prostate cells. ZnT1 is only localised in the plasma membrane of breast cancer cells. MT2A is distinctively seen close to the plasma membrane in breast cancer cells. CK2 is also for the first time shown to be localised in proximity to the plasma membrane of breast cancer cells.

CONCLUSION

The findings, particularly the localisation of ZIP12 and CK2, are novel and significant for our understanding of zinc homeostasis in breast and prostate cancer cells.

TTT (Tel2-Tti1-Tti2) Complex, the Co-Chaperone of PIKKs and a Potential Target for Cancer Chemotherapy

The heterotrimeric Tel2-Tti1-Tti2 or TTT complex is essential for cell viability and highly observed in eukaryotes. As the co-chaperone of ATR, ATM, DNA-PKcs, mTOR, SMG1, and TRRAP, the phosphatidylinositol 3-kinase-related kinases (PIKKs) and a group of large proteins of 300-500 kDa, the TTT plays crucial roles in genome stability, cell proliferation, telomere maintenance, and aging. Most of the protein kinases in the kinome are targeted by co-chaperone Cdc37 for proper folding and stability. Like Cdc37, accumulating evidence has established the mechanism by which the TTT interacts with chaperone Hsp90 via R2TP (Rvb1-Rvb2-Tah1-Pih1) complex or other proteins for co-translational maturation of the PIKKs. Recent structural studies have revealed the α-solenoid structure of the TTT and its interactions with the R2TP complex, which shed new light on the co-chaperone mechanism and provide new research opportunities. A series of mutations of the TTT have been identified that cause disease syndrome with neurodevelopmental defects, and misregulation of the TTT has been shown to contribute to myeloma, colorectal, and non-small-cell lung cancers. Surprisingly, Tel2 in the TTT complex has recently been found to be a target of ivermectin, an antiparasitic drug that has been used by millions of patients. This discovery provides mechanistic insight into the anti-cancer effect of ivermectin and thus promotes the repurposing of this Nobel-prize-winning medicine for cancer chemotherapy. Here, we briefly review the discovery of the TTT complex, discuss the recent studies, and describe the perspectives for future investigation.

Analyzing the interactome of human CK2β in prostate carcinoma cells reveals HSP70-1 and Rho guanin nucleotide exchange factor 12 as novel interaction partners

CK2β is the non-catalytic modulating part of the S/T-protein kinase CK2. However, the overall function of CK2β is poorly understood. Here, we report on the identification of 38 new interaction partners of the human CK2β from lysates of DU145 prostate cancer cells using photo-crosslinking and mass spectrometry, whereby HSP70-1 was identified with high abundance. The K_D value of its interaction with CK2β was determined as 0.57 μM by microscale thermophoresis, this being the first time, to our knowledge, that a K_D value of CK2β with another protein than CK2α or CK2α' was quantified. Phosphorylation studies excluded HSP70-1 as a substrate or activity modulator of CK2, suggesting a CK2 activity independent interaction of HSP70-1 with CK2β. Co-immunoprecipitation experiments in three different cancer cell lines confirmed the interaction of HSP70-1 with CK2β in vivo. A second identified CK2β interaction partner was Rho guanin nucleotide exchange factor 12, indicating an involvement of CK2β in the Rho-GTPase signal pathway, described here for the first time to our knowledge. This points to a role of CK2β in the interaction network affecting the organization of the cytoskeleton.

Minor Kinases with Major Roles in Cytokinesis Regulation

Cytokinesis, the conclusive act of cell division, allows cytoplasmic organelles and chromosomes to be faithfully partitioned between two daughter cells. In animal organisms, its accurate regulation is a fundamental task for normal development and for preventing aneuploidy. Cytokinesis failures produce genetically unstable tetraploid cells and ultimately result in chromosome instability, a hallmark of cancer cells. In animal cells, the assembly and constriction of an actomyosin ring drive cleavage furrow ingression, resulting in the formation of a cytoplasmic intercellular bridge, which is severed during abscission, the final event of cytokinesis. Kinase-mediated phosphorylation is a crucial process to orchestrate the spatio-temporal regulation of the different stages of cytokinesis. Several kinases have been described in the literature, such as cyclin-dependent kinase, polo-like kinase 1, and Aurora B, regulating both furrow ingression and/or abscission. However, others exist, with well-established roles in cell-cycle progression but whose specific role in cytokinesis has been poorly investigated, leading to considering these kinases as "minor" actors in this process. Yet, they deserve additional attention, as they might disclose unexpected routes of cell division regulation. Here, we summarize the role of multifunctional kinases in cytokinesis with a special focus on those with a still scarcely defined function during cell cleavage. Moreover, we discuss their implication in cancer.

CSNK2 in cancer: pathophysiology and translational applications

Protein kinase CSNK2 (CK2) is a pleiotropic serine/threonine kinase frequently dysregulated in solid and hematologic malignancies. To consolidate a wide range of biological and clinically oriented data from this unique kinase in cancer, this systematic review summarises existing knowledge from in vitro, in vivo and pre-clinical studies on CSNK2 across 24 different human cancer types. CSNK2 mRNA transcripts, protein levels and activity were found to be routinely upregulated in cancer, and commonly identified phosphotargets included AKT, STAT3, RELA, PTEN and TP53. Phenotypically, it frequently influenced evasion of apoptosis, enhancement of proliferation, cell invasion/metastasis and cell cycle control. Clinically, it held prognostic significance across 14 different cancers, and its inhibition in xenograft experiments resulted in a positive treatment response in 12. In conjunction with commentary on preliminary studies of CSNK2 inhibitors in humans, this review harmonises an extensive body of CSNK2 data in cancer and reinforces its emergence as an attractive target for cancer therapy. Continuing to investigate CSNK2 will be crucial to advancing our understanding of CSNK2 biology, and offers the promise of important new discoveries scientifically and clinically.

Ecto-protein kinase CK2, the neglected form of CK2

Ecto-protein kinases, including protein kinase CK2 (former name, casein kinase 2), have been the focus of research for more than 30 years. At the beginning of the ecto-kinase research their identification was performed with substrates and inhibitors whose specificity under the current knowledge was rather limited. Since all currently known ecto-kinases, including ecto-CK2, have intracellular counterparts, one has to exclude that an ecto-localization originates from intracellular counterparts after cell damage. Protein kinase CK2 is involved in cellular key processes such as cell cycle progression, inhibition of apoptosis, DNA damage repair, differentiation and many other processes. CK2 is composed of two catalytic CK2α or CK2α' subunits and two non-catalytic CK2β subunits. Progress in the ecto-kinase and in particular ecto-CK2 studies was made with the use of transfected tagged CK2 subunits, which allowed to follow their individual transport and localization on the cell surface after transfection. Furthermore, immunofluorescence studies with antibodies against CK2 subunits as well as affinity chromatography with a binding partner of CK2 subunits have improved ecto-kinase research. The use of new and more specific inhibitors as well as of substrates, which do not cross the plasma membrane, have further improved the specificity for ecto-CK2. From the various substrates of ecto-CK2, it can be concluded that ecto-CK2 plays a role in Alzheimer disease, cell adhesion, platelet aggregation, immune response and cellular signalling. New tools and techniques, to study ecto-CK2 activity, are required to identify new substrates and thereby new functional implications for ecto-CK2.

Crosstalk between PI(4,5)P₂and CK2 modulates actin polymerization during endocytic uptake

A transient burst of actin polymerization assists endocytic budding. How actin polymerization is controlled in this context is not understood. Here, we show that crosstalk between PI(4,5)P₂and the CK2 catalytic subunit Cka2 controls actin polymerization at endocytic sites. We find that phosphorylation of the myosin-I Myo5 by Cka2 downregulates Myo5-induced Arp2/3-dependent actin polymerization, whereas PI(4,5)P₂cooperatively relieves Myo5 autoinhibition and inhibits the catalytic activity of Cka2. Cka2 and the PI(4,5)P₂-5-phosphatases Sjl1 and Sjl2, the yeast synaptojanins, exhibit genetic interactions indicating functional redundancy. The ultrastructural analysis of plasma membrane invaginations in CK2 and synaptojanin mutants demonstrates that both cooperate to initiate constriction of the invagination neck, a process coupled to the remodeling of the endocytic actin network. Our data demonstrate a holoenzyme-independent function of CK2 in endocytic budding and establish a robust genetic, functional, and molecular link between PI(4,5)P₂and CK2, two masters of intracellular signaling.

CK2α is essential for embryonic morphogenesis

CK2 is a highly conserved serine-threonine kinase involved in biological processes such as embryonic development, circadian rhythms, inflammation, and cancer. Biochemical experiments have implicated CK2 in the control of several cellular processes and in the regulation of signal transduction pathways. Our laboratory is interested in characterizing the cellular, signaling, and molecular mechanisms regulated by CK2 during early embryonic development. For this purpose, animal models, including mice deficient in CK2 genes, are indispensable tools. Using CK2α gene-deficient mice, we have recently shown that CK2α is a critical regulator of mid-gestational morphogenetic processes, as CK2α deficiency results in defects in heart, brain, pharyngeal arch, tail bud, limb bud, and somite formation. Morphogenetic processes depend upon the precise coordination of essential cellular processes in which CK2 has been implicated, such as proliferation and survival. Here, we summarize the overall phenotype found in CK2α (-/- ) mice and describe our initial analysis aimed to identify the cellular processes affected in CK2α mutants.

The interaction of CK2alpha and CK2beta, the subunits of protein kinase CK2, requires CK2beta in a preformed conformation and is enthalpically driven

The protein kinase CK2 (former name: "casein kinase 2") predominantly occurs as a heterotetrameric holoenzyme composed of two catalytic chains (CK2alpha) and two noncatalytic subunits (CK2beta). The CK2beta subunits form a stable dimer to which the CK2alpha monomers are attached independently. In contrast to the cyclins in the case of the cyclin-dependent kinases CK2beta is no on-switch of CK2alpha; rather the formation of the CK2 holoenzyme is accompanied with an overall change of the enzyme's profile including a modulation of the substrate specificity, an increase of the thermostability, and an allocation of docking sites for membranes and other proteins. In this study we used C-terminal deletion variants of human CK2alpha and CK2beta that were enzymologically fully competent and in particular able to form a heterotetrameric holoenzyme. With differential scanning calorimetry (DSC) we confirmed the strong thermostabilization effect of CK2alpha on CK2beta with an upshift of the CK2alpha melting temperature of more than 9 degrees . Using isothermal titration calorimetry (ITC) we measured a dissociation constant of 12.6 nM. This high affinity between CK2alpha and CK2beta is mainly caused by enthalpic rather than entropic contributions. Finally, we determined a crystal structure of the CK2beta construct to 2.8 A resolution and revealed by structural comparisons with the CK2 holoenzyme structure that the CK2beta conformation is largely conserved upon association with CK2alpha, whereas the latter undergoes significant structural adaptations of its backbone.

RhoA GTPase and F-actin dynamically regulate the permeability of Cx43-made channels in rat cardiac myocytes

Gap junctions are clusters of transmembrane channels allowing a passive diffusion of ions and small molecules between adjacent cells. Connexin43, the main channel-forming protein expressed in ventricular myocytes, can associate with zonula occludens-1, a scaffolding protein linked to the actin cytoskeleton and to signal transduction molecules. The possible influence of Rho GTPases, major regulators of cellular junctions and of the actin cytoskeleton, in the modulation of gap junctional intercellular communication (GJIC) was examined. The activation of RhoA by cytoxic necrotizing factor 1 markedly enhanced GJIC, whereas its specific inhibition by the Clostridium botulinum C3 exoenzyme significantly reduced it. RhoA activity affects GJIC without major cellular redistribution of junctional plaques or changes in the Cx43 phosphorylation pattern. As these GTPases frequently act via the cortical cytoskeleton, the importance of F-actin in the modulation of GJIC was investigated by means of agents interfering with actin polymerization. Cytoskeleton stabilization by phalloidin slowed down the kinetics of channel rundown in the absence of ATP, whereas its disruption by cytochalasin D rapidly and markedly reduced GJIC despite ATP presence. Cytoskeleton stabilization by phalloidin markedly reduced the consequences of RhoA activation or inactivation. This mechanism appears to be the first described capable to both up- or down-regulate GJIC through RhoA activation or, conversely, inhibition. The inhibition of Rho downstream kinase effectors had no effect on GJIC. The present results provide further insight into the gating and regulation of junctional channels and identify a new downstream target for the small G-protein RhoA.

CK2alpha/CK1alpha chimeras are sensitive to regulation by the CK2beta subunit

The effect of CK2beta on the activity of CK2alpha and other protein kinases that can bind this regulatory subunit is not fully understood. In an attempt to improve our understanding of this effect, chimeras of CK2alpha and CK1alpha have been constructed. These chimeras contain different portions of the CK2alpha amino terminal region that are involved in the interaction with CK2beta to form CK2 tetramers. In the case of chimeras 1 and 2, the portions of CK2alpha replace the corresponding segments of CK1alpha. In the case of chimera 3, the fragment of CK2alpha is added to the whole CK1alpha molecule with the exception of the initial methionine. Chimera 3 has 8% of the activity of CK1alphaWT, while chimeras 1 and 2 are 3 orders of magnitude less active than CK1alphaWT. All three chimeras bind tightly to CK2beta, but only chimeras 1 and 2 are significantly stimulated in their capacity to phosphorylate casein and canonical peptide substrates by addition of the regulatory subunit. No stimulation was observed with phosvitin or non-canonical peptides derived from beta-catenin. CK2beta protects chimeras 1 and 2 from thermal inactivation. Chimera 2 can phosphorylate CK2beta and autophosphorylate; however, salt concentrations above 150 mM NaCl eliminate the phosphorylation of CK2beta but not the autophosphorylation of chimera 2. Similarly, high salt decrease the stimulatory effect of CK2beta on the phosphorylation of casein.

A role for CK2 upon interkinetic nuclear migration in the cell cycle of retinal progenitor cells

In developing retina, the nucleus of the elongated neuroepithelial cells undergoes interkinetic nuclear migration (INM), that is it migrates back and forth across the proliferative layer during the cell cycle. S-phase occurs at the basal side, while M-phase occurs at the apical margin of the retinal progenitors. G1 and G2-phases occur along the nuclear migration pathway. We tested whether this feature of the retinal cell cycle is controlled by CK2, which, among its many substrates, phosphorylates both molecular motors and cytoskeletal components. Double immunolabeling showed that CK2 is contained in BrdU-labeled retinal progenitors. INM was examined after pulse labeling the retina of newborn rats with BrdU, by plotting nuclear movement from basal to apical sides of the retinal progenitors during G2. The CK2 specific inhibitor 4,5,6,7-tetrabromobenzotriazole inhibited the activity of rat retinal CK2, and blocked nuclear movement proper in a dose-dependent way. No apoptosis was detected, and total numbers of BrdU-labeled nuclei remained constant following treatment. Immunohistochemistry showed that, following inhibition of CK2, the tubulin cytoskeleton is disorganized, with reduced acetylated and increased tyrosinated tubulin. This indicates a reduction in stable microtubules, with accumulation of free tubulin dimers. The results show that CK2 activity is required for INM in retinal progenitor cells.

Regulation of CK2 activity by phosphatidylinositol phosphates

The process of clathrin-coated vesicle (CCV) formation/disassembly involves numerous proteins that act cooperatively. Phosphorylation is an important regulatory mechanism governing protein interactions in CCVs, and many of the core and accessory proteins of the CCV machinery are reversibly phosphorylated in vivo. CK2 is highly enriched in CCVs and is capable of phosphorylating a number of peripheral membrane proteins involved in the process of clathrin-mediated endocytosis. At least some of these phosphorylation events have been shown to be inhibitory for CCV assembly, and CK2 has been shown to be inactive when associated with intact CCVs. Here we show that CCV membranes inhibit CK2 activity even after incubation in trypsin, indicating that a component of the lipid bilayer may be the inhibitory factor. Consistent with this, we showed that liposomes containing phosphatidylinositol phosphates inhibit the activity of CK2 and that CK2 binds to those liposomes. Notably, liposomes containing phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)), a component of CCVs, bind CK2 and inhibit its activity. Furthermore, we showed that the binding of CK2 to PtdIns(4,5)P(2)-containing liposomes is via the active site of CK2, thus providing a molecular explanation for the inhibition of CK2 activity when it is bound to PtdIns(4,5)P(2)-containing liposomes. Thus CK2 is inactive in CCVs because of the fact that it is bound to the CCV membrane via an interaction between PtdIns(4,5)P(2) in the CCV membrane and the active site in CK2.

Inhibition of human papillomavirus type 16 E7 phosphorylation by the S100 MRP-8/14 protein complex

The human papillomavirus type 16 (HPV16) E7 is a major viral oncoprotein that is phosphorylated by casein kinase II (CKII). Two S100 family calcium-binding proteins, macrophage inhibitory-related factor protein 8 (MRP-8) and MRP-14, form a protein complex, MRP-8/14, that inactivates CKII. The MRP-8/14 protein complex may inhibit CKII-mediated E7 phosphorylation and therefore may alter its interaction with cellular ligands and reduce E7 oncogenic activity. We examined the inhibitory effect of the MRP-8/14 complex on CKII activity and HPV16 E7 phosphorylation. We have shown that CKII activity and HPV16 E7 phosphorylation were inhibited by uptake of exogenous MRP-8/14 and activation of endogenous MRP-8/14. MRP-8/14-mediated inhibition of E7 phosphorylation occurred at the G1 phase of the cell cycle. Analysis of MRP expression in primary keratinocytes and in HPV16- and 18-transformed cervical and foreskin epithelial cell lines showed that expression of MRP-8, MRP-14, and the MRP-8/14 complex was detected only in primary untransformed keratinocytes and not in the HPV-infected immortalized epithelial cells. CKII activity in HPV-immortalized keratinocytes was approximately fourfold higher than in HPV-negative primary keratinocytes. Treatment of HPV-positive immortalized epithelial cells with exogenous MRP-8/14 resulted in E7 hypophosphorylation and complete inhibition of cell growth within 2 weeks, compared with HPV-negative primary and immortalized HPV-negative cervical epithelial cells, which showed 25 and 40% growth inhibition, respectively. Together these results suggests that the MRP-8/14 protein complex in HPV-infected epithelial cells may play an important role in regulation of CKII-mediated E7 phosphorylation and inhibition of its oncogenic activity.

Protein kinase CK2: structure, regulation and role in cellular decisions of life and death

Protein kinase CK2 ('casein kinase II') has traditionally been classified as a messenger-independent protein serine/threonine kinase that is typically found in tetrameric complexes consisting of two catalytic (alpha and/or alpha') subunits and two regulatory beta subunits. Accumulated biochemical and genetic evidence indicates that CK2 has a vast array of candidate physiological targets and participates in a complex series of cellular functions, including the maintenance of cell viability. This review summarizes current knowledge of the structural and enzymic features of CK2, and discusses advances that challenge traditional views of this enzyme. For example, the recent demonstrations that individual CK2 subunits exist outside tetrameric complexes and that CK2 displays dual-specificity kinase activity raises new prospects for the precise elucidation of its regulation and cellular functions. This review also discusses a number of the mechanisms that contribute to the regulation of CK2 in cells, and will highlight emerging insights into the role of CK2 in cellular decisions of life and death. In this latter respect, recent evidence suggests that CK2 can exert an anti-apoptotic role by protecting regulatory proteins from caspase-mediated degradation. The mechanistic basis of the observation that CK2 is essential for viability may reside in part in this ability to protect cellular proteins from caspase action. Furthermore, this anti-apoptotic function of CK2 may contribute to its ability to participate in transformation and tumorigenesis.

Regulation of protein kinase CKII by direct interaction with the C-terminal region of p47(phox)

Protein kinase CKII is a Ser/Thr kinase which is involved in many proliferation-related processes in the cell. p47(phox) is a component of the leukocyte NADPH oxidase, which is an important element of host defense against microbial infection. In this study, we demonstrate that a truncated form of the p47(phox) lacking its N-terminal region (p47(phox)/SH3-C), but not a truncated form of the p47(phox) lacking its C-terminal region (p47(phox)/N-SH3), undergoes better phosphorylation by CKII in the presence of arachidonic acid. The yeast two-hybrid test and the glutathione S-transferase (GST) pull-down assay showed that p47(phox) interacts specifically with the regulatory beta subunit (CKIIbeta), but not with the catalytic alpha subunit (CKIIalpha) of the tetrameric CKII holoenzyme. The binding of p47(phox) to CKIIbeta requires the C-terminal region of p47(phox) and is completely abolished by addition of spermine, indicating that a highly basic region in the C-terminal region of p47(phox) contributes to binding to CKIIbeta. In addition, p47(phox) stimulates the catalytic activity of CKII holoenzyme; this stimulation also requires the C-terminal region of p47(phox). Coimmunoprecipitation experiments showed that CKII holoenzyme interacts with p47(phox) in human neutrophils. Taken together, the present data indicate that the C-terminal region of p47(phox) plays a significant role in the arachidonic acid-dependent phosphorylation of p47(phox) by CKII and that the same region of p47(phox) associates directly with CKIIbeta and can modulate the catalytic activity of CKII holoenzyme.

The disruption of adherens junctions is associated with a decrease of E-cadherin phosphorylation by protein kinase CK2

The down-regulation of E-cadherin is a common event in carcinogenesis. Phosphorylation/dephosphorylation is one posttranscriptional process which may regulate intercellular junctions. Here we show that in okadaic acid-treated keratinocytes, E-cadherin expression is shifted from the membrane to the cytoplasm, preventing cells from forming aggregates. These changes of E-cadherin localization and function are associated with a decrease in its phosphorylation state. The decrease in E-cadherin phosphorylation was essentially detected in okadaic acid-treated cell lysates isolated from 0.5% Triton-soluble fraction and not in the Triton-insoluble fraction linked to the cytoskeleton, suggesting a role of E-cadherin phosphorylation in cell-cell interactions. E-cadherin was markedly phosphorylated by CK2, either the purified recombinant enzyme or the endogenous enzyme. Using specific CK2 inhibitors such as heparin and 5, 6-dichloro-1-beta-d-ribofuranosylbenzimidazole, endogenous CK2 was confirmed as the main enzyme phosphorylating E-cadherin. The decrease in E-cadherin phosphorylation by endogenous CK2 was not restored by the addition of purified CK2, confirming that it is not due to a defect in CK2 expression or to its reduced activity, but rather to the incapacity of CK2 to phosphorylate E-cadherin. The co-immunoprecipitation and colocalization of E-cadherin and CK2 suggests that CK2 may play a critical role in the maintenance of epidermis cohesion.

Identification and characterization of CKIP-1, a novel pleckstrin homology domain-containing protein that interacts with protein kinase CK2

The catalytic subunits of protein kinase CK2, CK2alpha and CK2alpha', are closely related to each other but exhibit functional specialization. To test the hypothesis that specific functions of CK2alpha and CK2alpha' are mediated by specific interaction partners, we used the yeast two-hybrid system to identify CK2alpha- or CK2alpha'-binding proteins. We report the identification and characterization of a novel CK2-interacting protein, designated CKIP-1, that interacts with CK2alpha, but not CK2alpha', in the yeast two-hybrid system. CKIP-1 also interacts with CK2alpha in vitro and is co-immunoprecipitated from cell extracts with epitope-tagged CK2alpha and an enhanced green fluorescent protein fusion protein encoding CKIP-1 (i.e. EGFP-CKIP-1) when they are co-expressed. CK2 activity is detected in anti-CKIP-1 immunoprecipitates performed with extracts from non-transfected cells indicating that CKIP-1 and CK2 interact under physiological conditions. The CKIP-1 cDNA is broadly expressed and encodes a protein with a predicted molecular weight of 46,000. EGFP-CKIP-1 is localized within the nucleus and at the plasma membrane. The plasma membrane localization is dependent on the presence of an amino-terminal pleckstrin homology domain. We postulate that CKIP-1 is a non-enzymatic regulator of one isoform of CK2 (i.e. CK2alpha) with a potential role in targeting CK2alpha to a particular cellular location.

Crystal structure of the human protein kinase CK2 regulatory subunit reveals its zinc finger-mediated dimerization

Protein kinase CK2 is a tetramer composed of two alpha catalytic subunits and two beta regulatory subunits. The structure of a C-terminal truncated form of the human beta subunit has been determined by X-ray crystallography to 1.7 A resolution. One dimer is observed in the asymmetric unit of the crystal. The most striking feature of the structure is the presence of a zinc finger mediating the dimerization. The monomer structure consists of two domains, one entirely alpha-helical and one including the zinc finger. The dimer has a crescent shape holding a highly acidic region at both ends. We propose that this acidic region is involved in the interactions with the polyamines and/or catalytic subunits. Interestingly, conserved amino acid residues among beta subunit sequences are clustered along one linear ridge that wraps around the entire dimer. This feature suggests that protein partners may interact with the dimer through a stretch of residues in an extended conformation.