Phosphorylation-Dependent Regulation of AXIN Modulates WNT Activity

In the absence of WNT ligand, β-catenin is phosphorylated and degraded via a destruction complex consisting of, among other proteins, the scaffold protein AXIN, glycogen synthase kinase 3 (GSK3), and adenomatous polyposis coli (APC), a well-known human tumor suppressor. WNT-induced phosphorylation of low-density lipoprotein receptor-related protein 6 (LRP6) triggers recruitment of AXIN to a signaling complex that promotes β-catenin stabilization. However, the mechanism by which WNT–LRP6 signaling stabilizes β-catenin is controversial, and the way in which the 2 AXIN complexes are regulated is unknown. Kim and colleagues found that protein phosphatase 1 (PP1) enhanced AXIN dephosphorylation in response to WNT stimulation and that this effect was correlated with reduced β-catenin phosphorylation. Suppression of PP1 activity impaired dephosphorylation of AXIN but not LRP6 and antagonized WNT/β-catenin signaling during Xenopus head development. Furthermore, the interaction between AXIN and β-catenin was decreased following treatment with WNT ligands but was partially rescued by PP1 inhibition. WNT stimulation also impaired AXIN binding to phosphorylated LRP6 in a PP1-dependent manner, whereas GSK3-mediated AXIN phosphorylation increased its association with activated LRP6 and β-catenin, suggesting that phosphorylation induces a conformational change in AXIN that modulates its binding partners. Consistent with this idea, the DIX domain of AXIN competed with β-catenin for binding to the AXIN β-catenin binding domain; this intramolecular interaction was blocked by GSK3-driven phosphorylation, indicating that dephosphorylated AXIN adopts a closed conformation that enables β-catenin stabilization. Mutations in AXIN that weakened its intramolecular interaction more potently inhibited WNT signaling, whereas mutations that strengthened this interaction were less effective in suppressing WNT activity. These findings delineate a phosphorylation-dependent mechanism that regulates AXIN scaffolding function and suggest that this intramolecular interaction may function as a feedback sensor to limit excessive signaling under high β-catenin levels.

Kim SE, Huang H, Zhao M, Zhang X, Zhang A, Semonov MV, et al. Wnt stabilization of β-catenin reveals principles for morphogen receptor-scaffold assemblies. Science 2013 Apr 11 [Epub ahead of print].