Axin-mediated CKI phosphorylation of beta-catenin at Ser 45: a molecular switch for the Wnt pathway.
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.
Amit
Sharon
S
The Lautenberg Center for Immunology, The Hebrew University-Hadassah Medical School, Jerusalem 91120, Israel.
Hatzubai
Ada
A
Birman
Yaara
Y
Andersen
Jens S
JS
Ben-Shushan
Etti
E
Mann
Matthias
M
Ben-Neriah
Yinon
Y
Alkalay
Irit
I
D048868:Adaptor Proteins, Signal Transducing; D060466:Axin Protein; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000378:metabolism; D047388:Casein Kinases; D002478:Cells, Cultured; D003598:Cytoskeletal Proteins / Q000235:genetics / Q000378:metabolism*; D000072261:Dishevelled Proteins; D038362:Glycogen Synthase Kinase 3; D006801:Humans; D010750:Phosphoproteins / Q000235:genetics / Q000378:metabolism; D010766:Phosphorylation; D011494:Protein Kinases / Q000235:genetics / Q000378:metabolism*; D011506:Proteins / Q000276:immunology / Q000378:metabolism*; D012097:Repressor Proteins; D012694:Serine / Q000378:metabolism*; D015398:Signal Transduction; D015534:Trans-Activators; D051153:Wnt Proteins; D060508:Wnt3 Protein; D060509:Wnt3A Protein; D051176:beta Catenin
D048868:Adaptor Proteins, Signal Transducing; D060466:Axin Protein; C495270:CTNNB1 protein, human; D003598:Cytoskeletal Proteins; D000072261:Dishevelled Proteins; D010750:Phosphoproteins; D011506:Proteins; D012097:Repressor Proteins; D015534:Trans-Activators; C555510:WNT3A protein, human; D051153:Wnt Proteins; D060508:Wnt3 Protein; D060509:Wnt3A Protein; D051176:beta Catenin; D012694:Serine; D011494:Protein Kinases; D047388:Casein Kinases; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D038362:Glycogen Synthase Kinase 3