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 quality control of DYRK family protein kinases by the Hsp90-Cdc37 molecular chaperone

The DYRK (Dual-specificity tYrosine-phosphorylation Regulated protein Kinase) family consists of five related protein kinases (DYRK1A, DYRK1B, DYRK2, DYRK3, DYRK4). DYRKs show homology to Drosophila Minibrain, and DYRK1A in human chromosome 21 is responsible for various neuronal disorders including human Down syndrome. Here we report identification of cellular proteins that associate with specific members of DYRKs. Cellular proteins with molecular masses of 90, 70, and 50-kDa associated with DYRK1B and DYRK4. These proteins were identified as molecular chaperones Hsp90, Hsp70, and Cdc37, respectively. Microscopic analysis of GFP-DYRKs showed that DYRK1A and DYRK1B were nuclear, while DYRK2, DYRK3, and DYRK4 were mostly cytoplasmic in COS7 cells. Overexpression of DYRK1B induced nuclear re-localization of these chaperones with DYRK1B. Treatment of cells with specific Hsp90 inhibitors, geldanamycin and 17-AAG, abolished the association of Hsp90 and Cdc37 with DYRK1B and DYRK4, but not of Hsp70. Inhibition of Hsp90 chaperone activity affected intracellular dynamics of DYRK1B and DYRK4. DYRK1B and DYRK4 underwent rapid formation of cytoplasmic punctate dots after the geldanamycin treatment, suggesting that the chaperone function of Hsp90 is required for prevention of protein aggregation of the target kinases. Prolonged inhibition of Hsp90 by geldanamycin, 17-AAG, or ganetespib, decreased cellular levels of DYRK1B and DYRK4. Finally, DYRK1B and DYRK4 were ubiquitinated in cells, and ubiquitinated DYRK1B and DYRK4 further increased by Hsp90 inhibition with geldanamycin. Taken together, these results indicate that Hsp90 and Cdc37 discriminate specific members of the DYRK kinase family and play an important role in quality control of these client kinases in cells.     

Cdc37 promotes the stability of protein kinases Cdc28 and Cak1

In the budding yeast Saccharomyces cerevisiae, Cdc37 is required for the productive formation of Cdc28-cyclin complexes. The cdc37-1 mutant arrests at Start with low levels of Cdc28 protein, which is predominantly unphosphorylated at Thr169, fails to bind cyclin, and has little protein kinase activity. We show here that Cdc28 and not cyclin is specifically defective in the cdc37-1 mutant and that Cdc37 likely does not act as an assembly factor for Cdc28-cyclin complex formation. We have also found that the levels and activity of the protein kinase Cak1 are significantly reduced in the cdc37-1 mutant. Pulse-chase analysis indicates that Cdc28 and Cak1 proteins are both destabilized when Cdc37 function is absent during but not after translation. In addition, Cdc37 promotes the production of Cak1, but not that of Cdc28, when coexpressed in insect cells. We conclude that budding yeast Cdc37, like its higher eukaryotic homologs, promotes the physical integrity of multiple protein kinases, perhaps by virtue of a cotranslational role in protein folding.     

Phosphorylation of multifunctional galectins by protein kinases CK1, CK2, and PKA

Phosphorylation is known to have a strong impact on protein functions. We analyzed members of the lectin family of multifunctional galectins as targets of the protein kinases CK1, CK2, and PKA. Galectins are potent growth regulators able to bind both glycan and peptide motifs at intra- and extracellular sites. Performing in vitro kinase assays, galectin phosphorylation was detected by phosphoprotein staining and autoradiography. The insertion of phosphoryl groups varied to a large extent depending on the type of kinase applied and the respective galectin substrate. Sites of phosphorylation observed in the recombinant galectins were determined by a strategic combination of phosphopeptide enrichment and nano-ultra-performance liquid chromatography tandem mass spectrometry (nanoUPLC–MS/MS). By in silico modeling, phosphorylation sites were visualized three-dimensionally. Our results reveal galectin-type-specific Ser-/Thr-dependent phosphorylation beyond the known example of galectin-3. These data are the basis for functional studies and also illustrate the analytical sensitivity of the applied methods for further work on human lectins.     

A positive feedback loop between protein kinase CKII and Cdc37 promotes the activity of multiple protein kinases

We report here the identification of CDC37, which encodes a putative Hsp90 co-chaperone, as a multicopy suppressor of a temperature-sensitive allele (cka2-13(ts)) of the CKA2 gene encoding the alpha' catalytic subunit of protein kinase CKII. Unlike wild-type cells, cka2-13 cells were sensitive to the Hsp90-specific inhibitor geldanamycin, and this sensitivity was suppressed by overexpression of either Hsp90 or Cdc37. However, only CDC37 was capable of suppressing the temperature sensitivity of a cka2-13 strain, implying that Cdc37 is the limiting component. Immunoprecipitation of metabolically labeled Cdc37 from wild-type versus cka2-13 strains revealed that Cdc37 is a physiological substrate of CKII, and Ser-14 and/or Ser-17 were identified as the most likely sites of CKII phosphorylation in vivo. A cdc37-S14,17A strain lacking these phosphorylation sites exhibited severe growth and morphological defects that were partially reversed in a cdc37-S14,17E strain. Reduced CKII activity was observed in both cdc37-S14A and cdc37-S17A mutants at 37 degrees C, and cdc37-S14A or cdc37-S14,17A overexpression was incapable of protecting cka2-13 mutants on media containing geldanamycin. Additionally, CKII activity was elevated in cells arrested at the G(1) and G(2)/M phases of the cell cycle, the same phases during which Cdc37 function is essential. Collectively, these data define a positive feedback loop between CKII and Cdc37. Additional genetic assays demonstrate that this CKII/Cdc37 interaction positively regulates the activity of multiple protein kinases in addition to CKII.     

Alteration of the protein kinase binding domain enhances function of the Saccharomyces cerevisiae molecular chaperone Cdc37

Cdc37 is a molecular chaperone that has a general function in the biogenesis of protein kinases. We identified mutations within the putative "protein kinase binding domain" of Cdc37 that alleviate the conditional growth defect of a strain containing a temperature-sensitive allele, tpk2(Ts), of the cyclic AMP-dependent protein kinase (PKA). These dominant mutations alleviate the temperature-sensitive growth defect by elevating PKA activity, as judged by their effects on PKA-regulated processes, localization and phosphorylation of the PKA effector Msn2, as well as in vitro PKA activity. Although the tpk2(Ts) growth defect is also alleviated by Cdc37 overproduction, the CDC37 dominant mutants contain wild-type Cdc37 protein levels. In addition, Saccharomyces cerevisiae Ste11 protein kinase has an elevated physical interaction with the altered Cdc37 protein. These results implicate specific amino-terminal residues in the interaction between Cdc37 and client protein kinases and provide further genetic and biochemical support for a model in which Cdc37 functions as a molecular chaperone for protein kinases.     

Hsp90 and Cdc37 -- a chaperone cancer conspiracy

The Hsp90 molecular chaperone system is involved in the activation of an important set of cell regulatory proteins, including many whose disregulation drives cancer. Recruitment of protein kinases to the Hsp90 system is mediated by the co-chaperone adaptor Cdc37 -- an essential protein whose overexpression is itself, oncogenic. Current structural, biochemical and biological studies of Cdc37 are beginning to unravel the nature of its interactions with Hsp90 and protein kinase clients, and implicate it as a key permissive factor in cell transformation by disregulated protein kinases. The central role of the Hsp90-Cdc37 chaperone complex makes it an important target for future anti-cancer drug development.     

Hsp90-dependent activation of protein kinases is regulated by chaperone-targeted dephosphorylation of Cdc37

Activation of protein kinase clients by the Hsp90 system is mediated by the cochaperone protein Cdc37. Cdc37 requires phosphorylation at Ser13, but little is known about the regulation of this essential posttranslational modification. We show that Ser13 of uncomplexed Cdc37 is phosphorylated in vivo, as well as in binary complex with a kinase (C-K), or in ternary complex with Hsp90 and kinase (H-C-K). Whereas pSer13-Cdc37 in the H-C-K complex is resistant to nonspecific phosphatases, it is efficiently dephosphorylated by the chaperone-targeted protein phosphatase 5 (PP5/Ppt1), which does not affect isolated Cdc37. We show that Cdc37 and PP5/Ppt1 associate in Hsp90 complexes in yeast and in human tumor cells, and that PP5/Ppt1 regulates phosphorylation of Ser13-Cdc37 in vivo, directly affecting activation of protein kinase clients by Hsp90-Cdc37. These data reveal a cyclic regulatory mechanism for Cdc37, in which its constitutive phosphorylation is reversed by targeted dephosphorylation in Hsp90 complexes.     

Role of protein kinase CK2 phosphorylation in the molecular chaperone activity of nucleolar protein b23

Protein B23 is a multifunctional nucleolar protein whose molecular chaperone activity is proposed to play role in ribosome assembly. Previous studies (Szebeni, A., and Olson, M. O. J. (1999) Protein Sci. 8, 905-912) showed that protein B23 has several characteristics typical of molecular chaperones, including anti-aggregation activity, promoting the renaturation of denatured proteins, and preferential binding to denatured substrates. However, until now there has been no proposed mechanism for release of a bound substrate. Protein B23 can be phosphorylated by protein kinase CK2 (CK2) in a segment required for chaperone activity. The presence of bound substrate enhanced the rate of CK2 phosphorylation of protein B23 by 2-3-fold, and this enhancement was dependent on a nonpolar region in its N-terminal end. Formation of a complex between B23 and chaperone test substrates (rhodanese or citrate synthase) was inhibited by CK2 phosphorylation. Furthermore, CK2 phosphorylation of a previously formed B23-substrate complex promoted its dissociation. The dissociation of complexes between B23 and the human immunodeficiency virus-Rev protein required both CK2 phosphorylation and competition with a Rev nuclear localization signal peptide, suggesting that Rev binds B23 at two separate sites. These studies suggest that unlike many molecular chaperones, which directly hydrolyze ATP, substrate release by protein B23 is dependent on its phosphorylation by CK2.     

The molecular chaperone Cdc37 is required for Ste11 function and pheromone-induced cell cycle arrest

The molecular chaperone Cdc37 is thought to act in part as a targeting subunit of the heat-shock protein 90 (Hsp90) chaperone complex. We demonstrate here that Cdc37 is required for activity of the kinase Ste11 in budding yeast. A cdc37 mutant strain is defective in Ste11-mediated pheromone signaling and in accumulation and functional maturation of the constitutively active Ste11 version Ste11DeltaN. Moreover, Cdc37, Ste11DeltaN and Hsp90 coprecipitate pairwise. Thus, Hsp90 and Cdc37 may transiently associate with Ste11 to promote proper folding and/or association with additional regulatory factors. Our results establish Ste11 as the first endogenous Cdc37 client protein in yeast.     

Ras target protein canoe is a substrate for Cdc2 and Cdk5 kinases

Mutations in the canoe locus of Drosophila lead to failure in the dorsal closure of the embryonic epidermis and pattern formation defects in imaginal eyes and wings. In the wing, the canoe mutants develop extra veins when they are heterozygous for shaggy, a mutation in the locus encoding the glycogen synthase kinase 3 beta (Gsk3 beta), which has been known to phosphorylate the Armadillo protein. Although Canoe has a putative target sequence for phosphorylation by Gsk3 beta similar to that found in Armadillo, in vitro experiments indicate that Canoe is not phosphorylated by Gsk3 beta . Instead, Canoe is demonstrated to be a good substrate of Cdc2 and Cdk5 kinases. Thus, Cdc2 and Cdk5 kinases are the potential regulators of the function of Canoe in morphogenesis. Arch.     

Cdk2: a genuine protein kinase client of Hsp90 and Cdc37

Hsp90 and its cochaperone Cdc37 cooperate to provide requisite support to numerous protein kinases involved in cellular signal transduction. In this report, we studied the interactions of Hsp90 and Cdc37 with the cyclin-dependent kinase, Cdk2. Treatment of K562 cells with the Hsp90 inhibitor, geldanamycin, caused a 75% reduction in Cdk2 levels and reduced the levels of its activating kinase, Cdk7, by more than 60%, suggesting that both of these kinases may be Hsp90 clients. Using classical pull-down assays and the Hsp90 inhibitory agents geldanamycin and molybdate, Cdk2 is shown to be a genuine client of the Hsp90 chaperone complex. Subsequently, pull-down assays directed at helix alphaC of Cdk2 are shown to disrupt Hsp90 and Cdc37 binding and explain the initial difficulties in demonstrating these interactions. Mutant constructs containing deletions of secondary structural elements from the N- and C-termini of Cdk2 were prepared and assayed for their ability to coadsorb Hsp90 and Cdc37 in a salt-stable high-affinity manner with and without the addition of molybdate. Consistent with similar work done with the cyclin-dependent kinase relative Cdk4, the presence of the G-box motif of Cdk2 was shown to be critical for Cdc37 binding, whereas consistent with work done with the Src-family tyrosine kinase Lck, the presence of helix alphaC and the stabilization of helix alphaE were shown to be needed for Hsp90 binding.     

High phosphorylation of HBV core protein by two alpha-type CK2-activated cAMP-dependent protein kinases in vitro

Two alpha-type CK2-activated PKAs (CK2-aPKAIalpha and CK2-aPKAIIalpha) were biochemically characterized in vitro using GST-HBV core fusion protein (GST-Hcore) and GST-Hcore157B as phosphate acceptors. It was found that (i), in the absence of cAMP, these two CK2-aPKAs phosphorylated both Ser-170 and Ser-178 on GST-Hcore and Hcore157B; (ii) this phosphorylation was approx. 4-fold higher than their phosphorylation by cAMP-activated PKAs; and (iii) suramin effectively inhibited the phosphorylation of Hcore157B by CK2-aPKAIIalpha through its direct binding to Hcore157B in vitro. These results suggest that high phosphorylation of HBV-CP by two CK2-aPKAs, in the absence of cAMP, may be involved in the pregenomic RNA (pgRNA) encapsidation and DNA-replication in HBV-infected cells.     

The Schizosaccharomyces pombe Cdc7 protein kinase required for septum formation is a client protein of Cdc37

Cdc37 is an essential molecular chaperone found in fungi and metazoa whose main specificity is for certain protein kinases. Cdc37 can act as an Hsp90 cochaperone or alone; in yeasts, the interaction with Hsp90 is weak and appears not to be essential for Cdc37 function. Numerous genetic interactions between Cdc37 and likely client proteins have been observed in yeasts, but biochemical confirmation has been reported in only a few cases. We and others have generated and characterized temperature-sensitive cdc37 alleles in S. pombe and have used them to investigate the cellular roles of Cdc37: previous work has shown that mitotic Cdc2 is a major client. In this paper, we describe a screen for mutations synthetically lethal with a cdc37ts mutant with the aim of identifying genes encoding further client proteins of Cdc37. Ten such strains were isolated, and genomic libraries were screened for rescuing plasmids. In one case, a truncated cdc7 gene was identified. Further experiments showed that the mutation in this strain was indeed in cdc7. Cdc7 is a protein kinase required for septum initiation, and we show that its kinase activity is greatly reduced when Cdc37 function is impaired. Cdc7 normally locates to the spindle pole body during mitosis, and this appears to be unaffected in the cdc37ts mutant. Other evidence suggests that, in addition to mitosis and septum initiation, Cdc37 may also be required for septum cleavage.     

In mammalian skeletal muscle,phosphorylation of TOMM22 by protein kinase CSNK2/CK2 controls mitophagy

In yeast, Tom22, the central component of the TOMM (translocase of outer mitochondrial membrane) receptor complex, is responsible for the recognition and translocation of synthesized mitochondrial precursor proteins, and its protein kinase CK2-dependent phosphorylation is mandatory for TOMM complex biogenesis and proper mitochondrial protein import. In mammals, the biological function of protein kinase CSNK2/CK2 remains vastly elusive and it is unknown whether CSNK2-dependent phosphorylation of TOMM protein subunits has a similar role as that in yeast. To address this issue, we used a skeletal muscle-specific Csnk2b/Ck2β-conditional knockout (cKO) mouse model. Phenotypically, these skeletal muscle Csnk2b cKO mice showed reduced muscle strength and abnormal metabolic activity of mainly oxidative muscle fibers, which point towards mitochondrial dysfunction. Enzymatically, active muscle lysates from skeletal muscle Csnk2b cKO mice phosphorylate murine TOMM22, the mammalian ortholog of yeast Tom22, to a lower extent than lysates prepared from controls. Mechanistically, CSNK2-mediated phosphorylation of TOMM22 changes its binding affinity for mitochondrial precursor proteins. However, in contrast to yeast, mitochondrial protein import seems not to be affected in vitro using mitochondria isolated from muscles of skeletal muscle Csnk2b cKO mice. PINK1, a mitochondrial health sensor that undergoes constitutive import under physiological conditions, accumulates within skeletal muscle Csnk2b cKO fibers and labels abnormal mitochondria for removal by mitophagy as demonstrated by the appearance of mitochondria-containing autophagosomes through electron microscopy. Mitophagy can be normalized by either introduction of a phosphomimetic TOMM22 mutant in cultured myotubes, or by in vivo electroporation of phosphomimetic Tomm22 into muscles of mice. Importantly, transfection of the phosphomimetic Tomm22 mutant in muscle cells with ablated Csnk2b restored their oxygen consumption rate comparable to wild-type levels. In sum, our data show that mammalian CSNK2-dependent phosphorylation of TOMM22 is a critical switch for mitophagy and reveal CSNK2-dependent physiological implications on metabolism, muscle integrity and behavior.     

Regulation of the mitotic exit protein kinases Cdc15 and Dbf2

In budding yeast, the release of the protein phosphatase Cdc14 from its inhibitor Cfi1/Net1 in the nucleolus during anaphase triggers the inactivation of Clb CDKs that leads to exit from mitosis. The mitotic exit pathway controls the association between Cdc14 and Cfi1/Net1. It is comprised of the RAS-like GTP binding protein Tem1, the exchange factor Lte1, the GTPase activating protein complex Bub2-Bfa1/Byr4, and several protein kinases including Cdc15 and Dbf2. Here we investigate the regulation of the protein kinases Dbf2 and Cdc15. We find that Cdc15 is recruited to both spindle pole bodies (SPBs) during anaphase. This recruitment depends on TEM1 but not DBF2 or CDC14 and is inhibited by BUB2. Dbf2 also localizes to SPBs during anaphase, which coincides with activation of Dbf2 kinase activity. Both events depend on the mitotic exit pathway components TEM1 and CDC15. In cells lacking BUB2, Dbf2 localized to SPBs in cell cycle stages other than anaphase and telophase and Dbf2 kinase was prematurely active during metaphase. Our results suggest an order of function of mitotic exit pathway components with respect to SPB localization of Cdc15 and Dbf2 and activation of Dbf2 kinase. BUB2 negatively regulates all 3 events. Loading of Cdc15 on SPBs depends on TEM1, whereas loading of Dbf2 on SPBs and activation of Dbf2 kinase depend on TEM1 and CDC15.