Genetic dissection of intersubunit contacts within human protein kinase CK2

CK2 negatively regulates Galphas signaling

We present evidence, using biochemical and cellular approaches, that the kinase, CK2, negatively controls signaling via Galpha(s) (or Galpha(olf)) coupled to dopamine D1 and adenosine A2A receptors. Pharmacological inhibition of CK2 or CK2 knockdown by RNAi lead to elevated cAMP levels in dopamine D1 receptor-activated neuroblastoma cells. Phosphorylation levels of protein kinase A substrates were increased in the presence of CK2 inhibitors in mouse striatal slices. The effect of D1 receptor and A2A receptor agonists on the phosphorylation of protein kinase A sites was potentiated upon CK2 inhibition. Furthermore, in cell lines, we observed that reduction in CK2 activity, pharmacologically or genetically, reduced the amount of D1 receptor that was internalized in response to dopamine. Finally, the beta subunit of CK2 was found to interact specifically with the Galpha(s) subunit through protein interaction analyses. Thus CK2 can inhibit G protein-coupled receptor action by enabling faster receptor internalization, possibly through a direct association with Galpha(s).

Mass spectrometry analysis of a protein kinase CK2beta subunit interactome isolated from mouse brain by affinity chromatography

CK2, an acronym derived from the misnomer "casein kinase 2", denotes a ubiquitous and extremely pleiotropic Ser/Thr protein kinase, the holoenzyme of which is composed of two catalytic (alpha and/or alpha') and two noncatalytic beta subunits acting as a docking platform and the multifarious functions of which are still incompletely understood. By combining affinity chromatography and mass spectrometry, we have identified 144 mouse brain proteins that associate with immobilized CK2beta. A large proportion (60%) of the identified proteins had been previously reported to be functionally related to CK2, and a similar proportion have been classified as phosphoproteins with approximately half of these having the features of CK2 targets. A large number of the identified proteins ( approximately 40%) either are nuclear or shuttle between the nucleus and cytoplasm, and the biggest functional classes of CK2beta interactors are committed to protein synthesis and degradation (32 proteins) and RNA/DNA interaction (20 proteins). Also well represented are the categories of cytoskeletal/structural proteins (19), trafficking proteins (17), and signaling proteins (14). The identified proteins are examined in relation to their functions and potential as targets and/or regulators of CK2, disclosing in some cases unanticipated links between this kinase and a variety of biochemical events.

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.

A method of reversible biomolecular immobilization for the surface plasmon resonance quantitative analysis of interacting biological macromolecules

This article presents a new procedure for the immobilization of macromolecules on gold surfaces, with the purpose of studying macromolecular interactions by simple optical configurations rendering surface plasmon resonance. Gold surfaces were covered by a three-layer structure composed of poly-L-lysine irreversibly bound to gold, followed by a second layer of heparin and a third layer of polylysine. The three-layer structure of polylysine-heparin-polylysine remains irreversibly bound to gold, it prevents biomolecules from coming into direct contact with the metal surface, and it allows the irreversible binding of different proteins and polynucleotides. After binding of a macromolecule to the three-layer structure, the interaction with a second macromolecule can be studied, and then the complex formed by the two interacting macromolecules, together with the second heparin layer and the third polylysine layer, can be broken down just by treatment with an alkaline solution having a pH value above the pK value of the amino groups of polylysine. The first polylysine layer remains irreversibly bound to gold, ready to form a new three-layer structure and, therefore, to support a new macromolecular interaction on the same regenerated surface. Polynucleotide interactions, the proteolytic action of chymotrypsin, and the interaction between the component subunits of a heterotetrameric enzyme are described as examples of macromolecular interactions studied by using this system. The method may be especially suitable for developing of low-cost systems aimed to look for surface resonance signals, and it offers the advantage of allowing calculation of parameters related to the size and stoichiometry of the interacting macromolecules, in addition to the kinetic and equilibrium properties of the interaction.

Assembly of protein kinase CK2: investigation of complex formation between catalytic and regulatory subunits using a zinc-finger-deficient mutant of CK2beta

Protein kinase CK2 is a tetrameric enzyme comprised of two regulatory subunits (CK2beta) and two catalytic subunits (CK2alpha and/or CK2alpha'). The crystal structure of dimeric CK2beta demonstrated that a zinc finger mediates CK2beta dimerization, therefore we constructed a mutant in which cysteine residues 109 and 114 were mutated to serine. Our objectives were to examine the effects of disrupting the zinc finger of the regulatory CK2beta subunit on CK2 tetramer assembly. Examination of this zinc-finger-deficient mutant of CK2beta using a yeast two-hybrid assay demonstrates that the mutant fails to form CK2beta homodimers. In order to extend these studies, we co-transfected COS-7 cells with epitope-tagged constructs and performed co-immunoprecipitation assays. The results from these studies demonstrate that the mutant fails to form CK2beta homodimers and fails to interact with catalytic CK2 subunits. Furthermore, we demonstrate that the mutant CK2beta is not appreciably phosphorylated in cells. Using in vitro binding assays, we demonstrated that the mutant CK2beta protein fails to interact with glutathione S-transferase-CK2alpha'. Finally, we demonstrate that the mutant is translated at an equivalent rate to wild-type CK2beta, but is degraded much more rapidly. Overall, our results are consistent with the model that beta-beta dimerization precedes incorporation of catalytic subunits into tetrameric CK2 complexes, and that beta-beta dimerization is a prerequisite for the stable incorporation of catalytic subunits into CK2 complexes.

The crystal structure of the complex of Zea mays alpha subunit with a fragment of human beta subunit provides the clue to the architecture of protein kinase CK2 holoenzyme

The crystal structure of a complex between the catalytic alpha subunit of Zea mays CK2 and a 23-mer peptide corresponding the C-terminal sequence 181-203 of the human CK2 regulatory beta subunit has been determined at 3.16-A resolution. The complex, composed of two alpha chains and two peptides, presents a molecular twofold axis, with each peptide interacting with both alpha chains. In the derived model of the holoenzyme, the regulatory subunits are positioned on the opposite side with respect to the opening of the catalytic sites, that remain accessible to substrates and cosubstrates. The beta subunit can influence the catalytic activity both directly and by promoting the formation of the alpha2 dimer, in which each alpha chain interacts with the active site of the other. Furthermore, the two active sites are so close in space that they can simultaneously bind and phosphorylate two phosphoacceptor residues of the same substrate.

The regulatory beta subunit of protein kinase CK2 mediates formation of tetrameric CK2 complexes

Protein kinase CK2 is a tetrameric enzyme composed of two catalytic (alpha and/or alpha') subunits and two regulatory (beta) subunits. Because CK2beta is synthesized in excess of CK2alpha, we hypothesized that formation of CK2beta homodimers precedes the incorporation of the catalytic subunits of CK2 into complexes. To test this hypothesis, we cotransfected cells with two epitope-tagged variants of CK2beta. The results of these cotransfection studies demonstrate that interactions between two CK2beta subunits take place in the absence of CK2alpha. Together with results from previous biosynthetic labeling studies, these results suggest that formation of CK2beta homodimers occurs before incorporation of catalytic subunits of CK2 into CK2 complexes. We also cotransfected Cos-7 cells with a deletion fragment of CK2beta (i.e. Myc-beta1-166) together with full-length hemagglutinin (HA)-tagged CK2beta and/or CK2alpha'. Although complexes between Myc-beta1-166 and HA-beta were readily detected, we obtained no evidence of direct interactions between Myc-beta1-166 and HA-CK2alpha'. These results suggest that residues within the N-terminal 166 amino acids of CK2beta are sufficient for interactions between CK2beta subunits, whereas the C-terminal domain of CK2beta is required for complex formation with the catalytic subunits of CK2. Finally, we observed that expression of full-length HA-beta promotes phosphorylation of Myc-beta1-166 by HA-CK2alpha'.

pCMB treatment reveals the essential role of cysteinyl residues in conferring functional competence to the regulatory subunit of protein kinase CK2

To assess the functional role of the four conserved cysteinyl residues in the regulatory beta-subunit of protein kinase CK2, the effect of pCMB and other reagents of sulfhydryl groups has been investigated. The pCMB-treated beta-subunit has lost its ability to form either homodimers or regular alpha(2)beta(2) heterotetramers with the catalytic subunit. It also fails to increase catalytic activity toward peptide substrates and to mediate the stimulatory effect of polylysine. The pCMB-treated beta-subunit, however, is still able to prevent calmodulin phosphorylation and to physically interact with the alpha-subunit to form inactive complexes whose sedimentation coefficient is lower than that of CK2 holoenzyme. These inactive complexes upon treatment with reducing agents like DTT are converted into a fully active heterotetrameric holoenzyme.

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.

A gene located at 56F1-2 in Drosophila melanogaster encodes a novel metazoan beta-like subunit of casein kinase II

Drosophila melanogaster casein kinase II (DmCKII) is composed of catalytic alpha and regulatory beta subunits associated as an alpha2beta2 heterotetramer. Using the two-hybrid system, we have screened a Drosophila embryo cDNA library for proteins that interact with DmCKII alpha. One of the cDNAs encodes a novel beta-like polypeptide, which we designate beta'. In situ hybridization localizes the corresponding gene to 56F1-2, a site distinct from that of both the beta gene and the Stellate family of beta-like sequences. The predicted sequence of beta' is more closely related to the beta subunit of Drosophila and other metazoans than to the Stellate family of proteins, suggesting that it is a second regulatory subunit. In vitro reconstitution studies show that a GST-beta' fusion protein associates with the alpha subunit to generate a tetrameric complex with regulatory properties similar to those of the native alpha2beta2 holoenzyme. The data are consistent with the proposed role of the beta' subunit as an integral component of the holoenzyme.

Structural interpretation of site-directed mutagenesis and specificity of the catalytic subunit of protein kinase CK2 using comparative modelling

The catalytic subunit of protein kinase casein kinase 2 (CK2alpha), which has specificity for both ATP and GTP, shows significant amino acid sequence similarity to the cyclin-dependent kinase 2 (CDK2). We constructed site-directed mutants of CK2alpha and used a three-dimensional model to investigate the basis for the dual specificity. Introduction of Phe and Gly at positions 50 and 51, in order to restore the pattern of the glycine-rich motif, did not seriously affect the specificity for ATP or GTP. We show that the dual specificity probably originates from the loop situated around the position His115 to Asp120 (HVNNTD). The insertion of a residue in this loop in CK2 alpha subunits, compared with CDK2 and other kinases, might orient the backbone to interact with the base A and G; this insertion is conserved in all known CK2alpha. The mutant deltaN118, the design of which was based on the modelling, showed reduced affinity for GTP as predicted from the model. Other mutants were intended to probe the integrity of the catalytic loop, alter the polarity of a buried residue and explore the importance of the carboxy terminus. Introduction of Arg to replace Asn189, which is mapped on the activation loop, results in a mutant with decreased k(cat), possibly as a result of disruption of the interaction between this residue and basic residues in the vicinity. Truncation at position 331 eliminates the last 60 residues of the alpha subunit and this mutant has a reduced catalytic efficiency compared with the wild-type. Catalytic efficiency is restored in the truncation mutant by the replacement of a potentially buried Glu at position 252 by Lys, probably owing to a higher stability resulting from the formation of a salt bridge between Lys252 and Asp208.

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.

The beta subunit of CKII negatively regulates Xenopus oocyte maturation

CKII (formerly known as casein kinase II) is a ubiquitously expressed enzyme that plays an important role in regulating cell growth and differentiation. The beta subunit of CKII (CKIIbeta) is not catalytic but forms heterotetramers with the catalytic subunit alpha to generate an alpha2beta2 holoenzyme. In Xenopus oocytes, CKIIbeta also associates with another serine/threonine kinase, Mos. As a key regulator of meiosis, Mos is necessary and sufficient to initiate oocyte maturation. We have previously shown that the binding of CKIIbeta to Mos represses Mos-mediated mitogen-activated protein kinase (MAPK) activation and that the ectopic expression of CKIIbeta inhibits progesterone-induced Xenopus oocyte maturation. We have now used an antisense oligonucleotide technique to reduce the endogenous CKIIbeta protein level in Xenopus oocytes, and we find that oocytes with a reduced content of CKIIbeta are more sensitive to low doses of progesterone and show accelerated MAPK activation and germinal vesicle breakdown. Furthermore, ectopic expression of a Mos-binding fragment of CKIIbeta suppressed the effect of antisense oligonucleotide. These results suggest that the endogenous CKIIbeta normally sets a threshold level for Mos protein, which must be exceeded for Mos to activate the MAPK signaling pathway and induce oocyte maturation.

An inactive mutant of the alpha subunit of protein kinase CK2 that traps the regulatory CK2beta subunit

Protein kinase CK2 (casein kinase 2) is a ubiquitous Ser/Thr protein kinase involved in cell proliferation. Mutation of the alpha subunit of the Xenopus laevis CK2 to change aspartic acid 156 to alanine (CK2alphaA156) resulted in an inactive enzyme. The CK2alphaA156 mutant, however, binds the regulatory subunit as measured by retention of beta on a nickel chelating column mediated by (His)6-tagged CK2alphaA156. Addition of CK2alphaA156 also caused beta to shift sedimentation in a sucrose gradient from a beta2 dimer (52 kDa) to an alpha2beta2 tetramer (130,000 kDa). CK2alphaA156 can trap the beta subunit in an inactive complex reducing the stimulation of casein phosphorylation caused by addition of beta to wild-type alpha. This competitive effect depends on the ratio of alpha/alphaA156 and on the amount of beta available. Since beta inhibits the phosphorylation of calmodulin by CK2alpha, the addition of CK2alphaA156, in this case, increases calmodulin phosphorylation by the alpha and beta combination. These results suggest that CK2alphaA156 may be a useful dominant-negative mutant that can serve to explore the multiple functions of CK2beta.

Physical dissection of the structural elements responsible for regulatory properties and intersubunit interactions of protein kinase CK2 beta-subunit

The noncatalytic beta-subunit of protein kinase CK2 has been shown to display various and in some respects antagonistic effects on the catalytic alpha-subunit [Boldyreff et al. (1993) Biochemistry 32, 12672-12677; Meggio et al. (1994) Biochemistry 33, 4336-4342]. We have now examined the ability of peptides encompassing the N- and C-terminal regions of the beta-subunit (beta[1-77] and beta[155-215]) to mimic the functions of the whole-length beta-subunit. Peptide beta[155-215] possesses only the positive features of the beta-subunit in that it prevents thermal inactivation and stimulates basal activity of the alpha-subunit, while it does not inhibit but rather stimulates calmodulin phosphorylation. In sharp contrast, peptide beta[1-77] neither protects the alpha-subunit nor stimulates its basal activity, while acting as a powerful and specific inhibitor of calmodulin phosphorylation. Peptide beta[155-215], but not peptide beta[1-77], stably interacts with alpha-subunit and also displays remarkable self-associating properties. A shorter derivative of beta[155-215], beta[170-215], displaying weaker stimulatory properties fails to stably interact with the alpha-subunit and to give rise to dimeric/multimeric forms. These data show that the elements responsible for the negative regulation are concentrated in the N-terminal moiety of the beta-subunit, whereas the C-terminal region retains the beneficial properties of the beta-subunit and is capable of self-association and binding of the alpha-subunit. Residues between 155 and 170 are necessary for the latter functions.

The casein kinase II beta subunit binds to Mos and inhibits Mos activity

Mos is a germ cell-specific serine/threonine kinase and is required for Xenopus oocyte maturation. Active Mos stimulates a mitogen-activated protein kinase (MAPK) by directly phosphorylating and activating MAPK kinase (MKK). We report here that the Xenopus homolog of the beta subunit of casein kinase II (CKII beta) binds to and regulates Mos. The Mos-interacting region of CKII beta was mapped to the C terminus. Mos bound to CKII beta in somatic cells ectopically expressing Mos and CKII beta as well as in unfertilized Xenopus eggs. CKII beta inhibited Mos-mediated MAPK activation in rabbit reticulocyte lysates and repressed MKK activation by v-Mos in a coupled kinase assay. In addition, microinjection of CKII beta mRNA into Xenopus oocytes inhibited progesterone-induced meiotic maturation and MAPK activation, presumably by binding of CKII beta to Mos and thereby inhibiting MAPK activation. Moreover, this inhibitory phenotype could be rescued by another protein that binds to CKII beta, CKII alpha. The ability of ectopic CKII beta to inhibit meiotic maturation and the detection of a complex between endogenous Mos and CKII beta suggest that CKII beta may act as an inhibitor of Mos during oocyte maturation, perhaps setting a threshold beyond which Mos protein must accumulate before it can activate the MAPK pathway.

Protein kinase CK2

Protein kinase CK2 is a ubiquitous protein kinase responsible for the phosphorylation of Ser and Thr residues specified by acidic side chains in many proteins, including several key enzymes, growth factor receptors, transcription factors and cytoskeletal proteins. The holoenzyme is composed of two catalytic and two regulatory subunits, the latter having antagonistic roles. CK2 is constitutively active and its targeting seems to be modulated through association with a variety of cellular proteins (e.g. heat shock protein 90 and p53). CK2 is abnormally elevated in proliferating and neoplastic tissues and recent studies suggest that mice overexpressing CK2 develop leukemia. Specific inhibitors of CK2, currently being developed, may have therapeutic potential.

Specific interaction between casein kinase 2 and the nucleolar protein Nopp140

Casein kinase 2 (CK2) is a multifunctional second messenger-independent protein serine/threonine kinase that phosphorylates many different proteins. To understand the function and regulation of this enzyme, biochemical methods were used to search for CK2-interacting proteins. Using immobilized glutathione S-transferase fusion proteins of CK2, the nucleolar protein Nopp140 was identified as a CK2-associated protein. It was found that Nopp140 binds primarily to the CK2 regulatory subunit, beta. The possible in vivo association of Nopp140 with CK2 was also suggested from a coimmunoprecipitation experiment in which Nopp140 was detected in immunoprecipitates of CK2 prepared from cell extracts. Further studies using an overlay technique with radiolabeled CK2 as a probe revealed a direct CK2-Nopp140 interaction. Using deletion mutants of CK2beta subunits, the binding region of the CK2beta subunit to Nopp140 has been mapped. It was found that the NH2-terminal 20 amino acids of CK2beta are involved. Since Nopp140 has been identified as a nuclear localization sequence-binding protein and has been shown to shuttle between the cytoplasm and the nucleus, the finding of a CK2-Nopp140 interaction could shed light on our understanding of the function and regulation of CK2 and Nopp140.

Analysis of interactions between the subunits of protein kinase CK2

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