Crystal structure of a β-catenin/Axin complex suggests a mechanism for the β-catenin destruction complex

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Abstract
The “β-catenin destruction complex” is central to canonical Wnt/β-catenin signaling. The scaffolding protein Axin and the tumor suppressor adenomatous polyposis coli protein (APC) are critical components of this complex, required for rapid β-catenin turnover. We determined the crystal structure of a complex between β-catenin and the β-catenin-binding domain of Axin (Axin-CBD). The Axin-CBD forms a helix that occupies the groove formed by the third and fourth armadillo repeats of β-catenin and thus precludes the simultaneous binding of other β-catenin partners in this region. Our biochemical studies demonstrate that, when phosphorylated, the 20-amino acid repeat region of APC competes with Axin for binding to β-catenin. We propose that a key function of APC in the β-catenin destruction complex is to remove phosphorylated β-catenin product from the active site.