Threonine 41 in β-Catenin Serves as a Key Phosphorylation Relay Residue in β-Catenin Degradation.

β-Catenin phosphorylation at serine 45 (Ser45), threonine 41 (Thr41), Ser37, and Ser33 is critical for β-catenin degradation, and regulation of β-catenin phosphorylation is a central part of the canonical Wnt signaling pathway. β-Catenin mutations at Ser45, Thr41, Ser37, and Ser33 perturb β-catenin degradation and are frequently found in cancers. It is established that Ser45 phosphorylation by casein kinase I (CKI) initiates phosphorylation at Thr41, Ser37, and Ser33 by glycogen synthase kinase 3 (GSK3) and that phosphorylated Ser37 and Ser33 are recognized by the F-box protein β-TrCP, a component of a ubiquitin ligase complex that mediates β-catenin degradation. While the roles of Set45, Ser37, and Ser33 are well documented, the function of Thr41 remains less defined. Here we show that Thr41 strictly acts as a phosphorylation relay residue and that the Ser-X-X-X-Ser (X is any amino acid) motif is obligatory for β-catenin phosphorylation by GSK3. β-Catenin phosphorylation/degradation and its regulation by Wnt can occur normally in the absence of Thr41 as long as the Ser-X-X-X-Ser motif/spacing is preserved. These results suggest that Thr41 functions to bridge sequential phosphorylation from Ser45 to Ser37 and provide further insights into the discrete steps and logic in β-catenin phosphorylation-degradation. [ABSTRACT FROM AUTHOR]