AXIL, A MEMBER OF THE AXIN FAMILY, INTERACTS WITH BOTH GLYCOGEN-SYNTHASE-KINASE-3-BETA AND BETA-CATENIN AND INHIBITS AXIS FORMATION OF XENOPUS EMBRYOS

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