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

Genes Dev. 2002 May 1;16(9):1066-76. doi: 10.1101/gad.230302.

Abstract

The Wnt pathway controls numerous developmental processes via the beta-catenin-TCF/LEF transcription complex. Deregulation of the pathway results in the aberrant accumulation of beta-catenin in the nucleus, often leading to cancer. Normally, cytoplasmic beta-catenin associates with APC and axin and is continuously phosphorylated by GSK-3beta, marking it for proteasomal degradation. Wnt signaling is considered to prevent GSK-3beta from phosphorylating beta-catenin, thus causing its stabilization. However, the Wnt mechanism of action has not been resolved. Here we study the regulation of beta-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 beta-catenin phosphorylation at a single site: serine 45 (S45). Immunopurified axin and recombinant CKI phosphorylate beta-catenin in vitro at S45; CKI inhibition suppresses this phosphorylation in vivo. CKI phosphorylation creates a priming site for GSK-3beta and is both necessary and sufficient to initiate the beta-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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adaptor Proteins, Signal Transducing
  • Axin Protein
  • Calcium-Calmodulin-Dependent Protein Kinases / metabolism
  • Casein Kinases
  • Cells, Cultured
  • Cytoskeletal Proteins / genetics
  • Cytoskeletal Proteins / metabolism*
  • Dishevelled Proteins
  • Glycogen Synthase Kinase 3
  • Humans
  • Phosphoproteins / genetics
  • Phosphoproteins / metabolism
  • Phosphorylation
  • Protein Kinases / genetics
  • Protein Kinases / metabolism*
  • Proteins / immunology
  • Proteins / metabolism*
  • Repressor Proteins*
  • Serine / metabolism*
  • Signal Transduction
  • Trans-Activators*
  • Wnt Proteins
  • Wnt3 Protein
  • Wnt3A Protein
  • beta Catenin

Substances

  • Adaptor Proteins, Signal Transducing
  • Axin Protein
  • CTNNB1 protein, human
  • Cytoskeletal Proteins
  • Dishevelled Proteins
  • Phosphoproteins
  • Proteins
  • Repressor Proteins
  • Trans-Activators
  • WNT3A protein, human
  • Wnt Proteins
  • Wnt3 Protein
  • Wnt3A Protein
  • beta Catenin
  • Serine
  • Protein Kinases
  • Casein Kinases
  • Calcium-Calmodulin-Dependent Protein Kinases
  • Glycogen Synthase Kinase 3