Functional domains of axin. Importance of the C terminus as an oligomerization domain.
To understand the mechanism of how Axin acts as an inhibitory molecule in the Wnt pathway, we generated a series of mutated forms of Axin. From the binding experiments, we defined the domains of Axin that bind glycogen synthase kinase-3beta (GSK-3beta) and beta-catenin. We also examined the ability of each Axin mutant to inhibit lymphoid enhancer factor-1 (Lef-1) reporter activity in a cell line expressing high levels of beta-catenin. Axin mutants that did not bind GSK-3beta or beta-catenin were ineffective in suppressing Lef-1 reporter activity. Binding GSK-3beta and beta-catenin was not sufficient for this inhibitory effect of Axin. Axin mutants with C-terminal truncations lacked the ability to inhibit Lef-1 reporter activity, even though they bound GSK-3beta and beta-catenin. The C-terminal region was required for binding to Axin itself. Substitution of the C-terminal region with an unrelated dimerizing molecule, the retinoid X receptor restored its inhibitory effect on Lef-1-dependent transcription. The oligomerization of Axin through its C terminus is important for its function in regulation of beta-catenin-mediated response.
D000818:Animals; D060466:Axin Protein; D001665:Binding Sites; D019556:COS Cells; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000378:metabolism; D003110:Colonic Neoplasms / Q000235:genetics / Q000378:metabolism; D003598:Cytoskeletal Proteins / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000378:metabolism; D038362:Glycogen Synthase Kinase 3; D038341:Glycogen Synthase Kinases; D006801:Humans; D051821:Lymphoid Enhancer-Binding Factor 1; D051379:Mice; D016297:Mutagenesis, Site-Directed; D011487:Protein Conformation; D011506:Proteins / Q000737:chemistry / Q000235:genetics / Q000502:physiology*; D012097:Repressor Proteins; D013329:Structure-Activity Relationship; D015534:Trans-Activators; D014157:Transcription Factors / Q000378:metabolism; D014158:Transcription, Genetic / Q000187:drug effects; D014407:Tumor Cells, Cultured; D051176:beta Catenin