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

Abstract

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

PAWS 1 controls Wnt signalling through association with casein kinase 1α

“…It has long been known that CK1 isoforms play crucial roles in the Wnt/β‐catenin signalling 28, 29, 30, 34, 35, 43, 44, 45, 46. In fact, early observations that overexpression of CK1 isoforms in Xenopus embryos caused axis duplication suggested a positive role for CK1 in Wnt signalling 47.…”

Wnt signaling in disease and in development

“…1). In the absence of Wnt signaling, β-catenin is phosphorylated by the serine/threonine kinases, Casein Kinase [10][11][12] and GSK-3 [13]. The interaction between these kinases and β-catenin is facilitated by the scaffolding proteins, Axin and APC [14,15].…”

The ubiquitin-like modifier FAT10 in cancer development

“…4). Upon phosphorylation of ␤-catenin by the serine/threonine glycogen synthase kinase 3␤ (GSK3␤) and by casein kinase I, ␤-catenin gets ubiquitylated by the E3 ligase ␤-TRCP and degraded by the proteasome (Amit et al, 2002;Polakis, 2000Polakis, , 2012. Mutations in ␤-catenin which prevent its phosphorylation and subsequent ubiquitylation, and which result in a constitutive active transcription factor are found with significant frequency in HCC and it was shown that ␤-catenin plays a key role in the metastasis of human hepatocellular carcinoma (de La Coste et al, 1998;Lai et al, 2011).…”