Upon binding of a Wnt to its receptor, GSK3beta is inhibited through an unknown mechanism involving Dishevelled (Dsh), resulting in the dephosphorylation and stabilization of beta-catenin, which translocates to the nucleus and interacts with Lef/Tcf transcription factors to activate target gene expression. Axin is a scaffold protein which binds beta-catenin and GSK3beta (as well as several other proteins) and thus promotes the phosphorylation of beta-catenin. Here we report that Axin is phosphorylated on Ser and Thr residues in several regions in vivo, while only one region (amino acids 600-672) is efficiently phosphorylated by GSK3beta in vitro. Site-directed mutagenesis, together with in vitro and in vivo phosphorylation assays, demonstrates that Axin residues T609 and S614 are physiological GSK3beta targets. Substitutions for one or more of these residues, which lie within a beta-catenin binding site, reduce the ability of Axin to modulate Wnt-induced signaling in a Lef/Tcf reporter assay. These amino acid substitutions also reduce the binding between Axin and beta-catenin. We propose a model in which inhibition of GSK3beta activity upon Wnt signaling leads to the dephosphorylation of GSK3beta sites in Axin, resulting in the release of beta-catenin from the phosphorylation complex.
Copyright 1999 Academic Press.