title: Casein kinase I phosphorylates and destabilizes the beta-catenin degradation complex.
abstract: Wnt signaling plays a key role in cell proliferation and development. Recently, casein kinase I (CKI) and protein phosphatase 2A (PP2A) have emerged as positive and negative regulators of the Wnt pathway, respectively. However, it is not clear how these two enzymes with opposing functions regulate Wnt signaling. Here we show that both CKI delta and CKI epsilon interacted directly with Dvl-1, and that CKI phosphorylated multiple components of the Wnt-regulated beta-catenin degradation complex in vitro, including Dvl-1, adenomatous polyposis coli (APC), axin, and beta-catenin. Comparison of peptide maps from in vivo and in vitro phosphorylated beta-catenin and axin suggests that CKI phosphorylates these proteins in vivo as well. CKI abrogated beta-catenin degradation in Xenopus egg extracts. Notably, CKI decreased, whereas inhibition of CKI increased, the association of PP2A with the beta-catenin degradation complex in vitro. Additionally, inhibition of CKI in vivo stabilized the beta-catenin degradation complex, suggesting that CKI actively destabilizes the complex in vivo. The ability of CKI to induce secondary body axes in Xenopus embryos was reduced by the B56 regulatory subunit of PP2A, and kinase-dead CKI epsilon acted synergistically with B56 in inhibiting Wnt signaling. The data suggest that CKI phosphorylates and destabilizes the beta-catenin degradation complex, likely through the dissociation of PP2A, providing a mechanism by which CKI stabilizes beta-catenin and propagates the Wnt signal.
authors.affiliation: Department of Oncological Sciences, Huntsman Cancer Institute, 2000 East North Campus Drive, University of Utah, Salt Lake City, UT 84112-5550, USA.
authors.forename: Zhong-Hua
authors.initials: ZH
authors.lastname: Gao
authors.forename: Joni M
authors.initials: JM
authors.lastname: Seeling
authors.forename: Virginia
authors.initials: V
authors.lastname: Hill
authors.forename: April
authors.initials: A
authors.lastname: Yochum
authors.forename: David M
authors.initials: DM
authors.lastname: Virshup
mesh_terms: D000818:Animals; D060466:Axin Protein; D015820:Cadherins / Q000737:chemistry / Q000378:metabolism; D047388:Casein Kinases; D002460:Cell Line; D003001:Cloning, Molecular; D003598:Cytoskeletal Proteins / Q000737:chemistry / Q000378:metabolism*; D017931:DNA Primers; D005260:Female; D006801:Humans; D007700:Kinetics; D009865:Oocytes / Q000378:metabolism; D010449:Peptide Mapping; D010748:Phosphopeptides / Q000737:chemistry; D010749:Phosphoprotein Phosphatases / Q000378:metabolism; D010766:Phosphorylation; D014176:Protein Biosynthesis; D011494:Protein Kinases / Q000302:isolation & purification / Q000378:metabolism*; D054648:Protein Phosphatase 2; D011505:Protein-Tyrosine Kinases / Q000378:metabolism; D011506:Proteins / Q000737:chemistry / Q000378:metabolism; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D011994:Recombinant Proteins / Q000378:metabolism; D012097:Repressor Proteins; D012441:Saccharomyces cerevisiae / Q000235:genetics; D015534:Trans-Activators; D014158:Transcription, Genetic; D014162:Transfection; D051153:Wnt Proteins; D029867:Xenopus Proteins; D014982:Xenopus laevis; D029961:Zebrafish Proteins; D051176:beta Catenin
chemical_list: D060466:Axin Protein; C495266:CTNNB1 protein, Xenopus; C495270:CTNNB1 protein, human; D015820:Cadherins; D003598:Cytoskeletal Proteins; D017931:DNA Primers; D010748:Phosphopeptides; D011506:Proteins; D011518:Proto-Oncogene Proteins; D011994:Recombinant Proteins; D012097:Repressor Proteins; D015534:Trans-Activators; D051153:Wnt Proteins; D029867:Xenopus Proteins; D029961:Zebrafish Proteins; C555454:axin1 protein, Xenopus; D051176:beta Catenin; D011494:Protein Kinases; D011505:Protein-Tyrosine Kinases; D047388:Casein Kinases; D010749:Phosphoprotein Phosphatases; D054648:Protein Phosphatase 2