Axin-mediated CKI phosphorylation of β-catenin at Ser 45: a molecular switch for the Wnt pathway

  1. Sharon Amit1,
  2. Ada Hatzubai1,
  3. Yaara Birman1,
  4. Jens S. Andersen2,
  5. Etti Ben-Shushan1,
  6. Matthias Mann2,
  7. Yinon Ben-Neriah1,3, and
  8. Irit Alkalay1
  1. 1The Lautenberg Center for Immunology, The Hebrew University–Hadassah Medical School, Jerusalem 91120, Israel; 2Protein Interaction Laboratory, University of Southern Denmark, DK-5230 Odense M, Denmark

Abstract

The Wnt pathway controls numerous developmental processes via the β-catenin–TCF/LEF transcription complex. Deregulation of the pathway results in the aberrant accumulation of β-catenin in the nucleus, often leading to cancer. Normally, cytoplasmic β-catenin associates with APC and axin and is continuously phosphorylated by GSK-3β, marking it for proteasomal degradation. Wnt signaling is considered to prevent GSK-3β from phosphorylating β-catenin, thus causing its stabilization. However, the Wnt mechanism of action has not been resolved. Here we study the regulation of β-catenin phosphorylation and degradation by the Wnt pathway. Using mass spectrometry and phosphopeptide-specific antibodies, we show that a complex of axin and casein kinase I (CKI) induces β-catenin phosphorylation at a single site: serine 45 (S45). Immunopurified axin and recombinant CKI phosphorylate β-catenin in vitro at S45; CKI inhibition suppresses this phosphorylation in vivo. CKI phosphorylation creates a priming site for GSK-3β and is both necessary and sufficient to initiate the β-catenin phosphorylation–degradation cascade. Wnt3A signaling and Dvl overexpression suppress S45 phosphorylation, thereby precluding the initiation of the cascade. Thus, a single, CKI-dependent phosphorylation event serves as a molecular switch for the Wnt pathway.

Keywords

Footnotes

  • 3 Corresponding author.

  • E-MAIL yinon{at}cc.huji.ac.il; FAX 972-2-6758935.

  • Article and publication are at http://www.genesdev.org/cgi/doi/10.1101/gad.230302.

    • Received March 11, 2002.
    • Accepted March 28, 2002.
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