AXIN, A NEGATIVE REGULATOR OF THE WNT SIGNALING PATHWAY, FORMS A COMPLEX WITH GSK-3-BETA AND BETA-CATENIN AND PROMOTES GSK-3-BETA-DEPENDENTPHOSPHORYLATION OF BETA-CATENIN

Glycogen synthase kinase-3 (GSK-3) mediates epidermal growth factor, i nsulin and Wnt signals to various downstream events such as glycogen m etabolism, gene expression, proliferation and differentiation. We have isolated here a GSK-3 beta-interacting protein from a rat brain cDNA library using a yeast two-hybrid method. This protein consists of 832 amino acids and possesses Regulators of G protein Signaling (RGS) and dishevelled (Dsh) homologous domains in its N- and C-terminal regions, respectively, The predicted amino acid sequence of this GSK-3 beta-in teracting protein shows 94% identity with mouse Axin, which recently h as been identified as a negative regulator of the Wnt signaling pathwa y; therefore, we termed this protein rAxin (rat Axin), rAxin interacte d directly with, and was phosphorylated by, GSK-3 beta, rAxin also int eracted directly with the armadillo repeats of beta-catenin, The bindi ng site of rAxin for GSK-3 beta was distinct from the beta-catenin-bin ding site, and these three proteins formed a ternary complex, Furtherm ore, rAxin promoted GSK-3 beta-dependent phosphorylation of beta-caten in, These results suggest that rAxin negatively regulates the Wnt sign aling pathway by interacting with GSK-3 beta and beta-catenin and medi ating the signal from GSK-3 beta to beta-catenin.