Axil, a Member of the Axin Family, Interacts with Both Glycogen Synthase Kinase 3β and β-Catenin and Inhibits Axis Formation ofXenopus Embryos

Using a yeast two-hybrid method, we identified a novel protein which interacts with glycogen synthase kinase 3beta (GSK-3beta). This protein had 44% amino acid identity with Axin, a negative regulator of the Wnt signaling pathway. We designated this protein Axil for Axin like. Like Axin, Axil ventralized Xenopus embryos and inhibited Xwnt8-induced Xenopus axis duplication. Axil was phosphorylated by GSK-3beta. Axil bound not only to GSK-3beta but also to beta-catenin, and the GSK-3beta-binding site of Axil was distinct from the beta-catenin-binding site. Furthermore, Axil enhanced GSK-3beta-dependent phosphorylation of beta-catenin. These results indicate that Axil negatively regulates the Wnt signaling pathway by mediating GSK-3beta-dependent phosphorylation of beta-catenin, thereby inhibiting axis formation.