title: Interaction among GSK-3, GBP, axin, and APC in Xenopus axis specification.
abstract: Glycogen synthase kinase 3 (GSK-3) is a constitutively active kinase that negatively regulates its substrates, one of which is beta-catenin, a downstream effector of the Wnt signaling pathway that is required for dorsal-ventral axis specification in the Xenopus embryo. GSK-3 activity is regulated through the opposing activities of multiple proteins. Axin, GSK-3, and beta-catenin form a complex that promotes the GSK-3-mediated phosphorylation and subsequent degradation of beta-catenin. Adenomatous polyposis coli (APC) joins the complex and downregulates beta-catenin in mammalian cells, but its role in Xenopus is less clear. In contrast, GBP, which is required for axis formation in Xenopus, binds and inhibits GSK-3. We show here that GSK-3 binding protein (GBP) inhibits GSK-3, in part, by preventing Axin from binding GSK-3. Similarly, we present evidence that a dominant-negative GSK-3 mutant, which causes the same effects as GBP, keeps endogenous GSK-3 from binding to Axin. We show that GBP also functions by preventing the GSK-3-mediated phosphorylation of a protein substrate without eliminating its catalytic activity. Finally, we show that the previously demonstrated axis-inducing property of overexpressed APC is attributable to its ability to stabilize cytoplasmic beta-catenin levels, demonstrating that APC is impinging upon the canonical Wnt pathway in this model system. These results contribute to our growing understanding of how GSK-3 regulation in the early embryo leads to regional differences in beta-catenin levels and establishment of the dorsal axis.
authors.affiliation: Department of Biochemistry and Center for Developmental Biology, University of Washington, Seattle, Washington 98195-7350, USA.
authors.forename: G H
authors.initials: GH
authors.lastname: Farr
authors.forename: D M
authors.initials: DM
authors.lastname: Ferkey
authors.forename: C
authors.initials: C
authors.lastname: Yost
authors.forename: S B
authors.initials: SB
authors.lastname: Pierce
authors.forename: C
authors.initials: C
authors.lastname: Weaver
authors.forename: D
authors.initials: D
authors.lastname: Kimelman
mesh_terms: D025601:Adenomatous Polyposis Coli Protein; D000818:Animals; D060466:Axin Protein; D019521:Body Patterning; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000008:administration & dosage / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D002352:Carrier Proteins / Q000008:administration & dosage / Q000235:genetics / Q000378:metabolism*; D017362:Cyclic AMP Response Element-Binding Protein / Q000737:chemistry / Q000378:metabolism; D003598:Cytoskeletal Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D004789:Enzyme Activation; D005799:Genes, Dominant / Q000235:genetics; D038362:Glycogen Synthase Kinase 3; D038341:Glycogen Synthase Kinases; D047908:Intracellular Signaling Peptides and Proteins; D008845:Microinjections; D008954:Models, Biological; D009154:Mutation / Q000235:genetics; D010455:Peptides / Q000737:chemistry / Q000378:metabolism; D010766:Phosphorylation; D011233:Precipitin Tests; D011485:Protein Binding; D011506:Proteins / Q000008:administration & dosage / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D011518:Proto-Oncogene Proteins / Q000502:physiology; D051381:Rats; D011993:Recombinant Fusion Proteins / Q000008:administration & dosage / Q000378:metabolism; D012097:Repressor Proteins; D015398:Signal Transduction; D015534:Trans-Activators; D051153:Wnt Proteins; D029867:Xenopus Proteins; D014982:Xenopus laevis / Q000196:embryology / Q000235:genetics / Q000378:metabolism; D029961:Zebrafish Proteins; D051176:beta Catenin
chemical_list: D025601:Adenomatous Polyposis Coli Protein; D060466:Axin Protein; C495266:CTNNB1 protein, Xenopus; D002352:Carrier Proteins; C495268:Ctnnb1 protein, rat; D017362:Cyclic AMP Response Element-Binding Protein; D003598:Cytoskeletal Proteins; D047908:Intracellular Signaling Peptides and Proteins; C112892:PIAS1 protein, Xenopus; D010455:Peptides; D011506:Proteins; D011518:Proto-Oncogene Proteins; D011993:Recombinant Fusion Proteins; D012097:Repressor Proteins; D015534:Trans-Activators; D051153:Wnt Proteins; D029867:Xenopus Proteins; D029961:Zebrafish Proteins; C555454:axin1 protein, Xenopus; D051176:beta Catenin; D038341:Glycogen Synthase Kinases; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D038362:Glycogen Synthase Kinase 3