Axin, a Negative Regulator of the Wnt Signaling Pathway, Directly Interacts with Adenomatous Polyposis Coli and Regulates the Stabilization of β-Catenin*

The regulators of G protein signaling (RGS) domain of Axin, a negative regulator of the Wnt signaling pathway, made a complex with full-length adenomatous polyposis coli (APC) in COS, 293, and L cells but not with truncated APC in SW480 or DLD-1 cells. The RGS domain directly interacted with the region containing the 20-amino acid repeats but not with that containing the 15-amino acid repeats of APC, although both regions are known to bind to β-catenin. In the region containing seven 20-amino acid repeats, the region containing the latter five repeats bound to the RGS domain of Axin. Axin and β-catenin simultaneously interacted with APC. Furthermore, Axin stimulated the degradation of β-catenin in COS cells. Taken together with our recent observations that Axin directly interacts with glycogen synthase kinase-3β (GSK-3β) and β-catenin and that it promotes GSK-3β-dependent phosphorylation of β-catenin, these results suggest that Axin, APC, GSK-3β, and β-catenin make a tetrameric complex, resulting in the regulation of the stabilization of β-catenin.

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