Phosphorylation and regulation of β-catenin by casein kinase Iε

beta-Catenin transduces cytosolic signals to the nucleus in the Wnt pathway. The Wnt ligand stabilizes cytosolic beta-catenin protein, preventing its phosphorylation by inhibiting glycogen synthase kinase 3 (GSK3). Serine-33 and -37 of beta-catenin are GSK3 phosphorylation sites that serve as recognition sites for the beta-TRCP-ubiquitin ligase complex, which ultimately triggers beta-catenin degradation. Mutations at those two sites, as well as in Ser-45, stabilize beta-catenin. Recently, casein kinase Iepsilon (CKIepsilon) has been shown to be a positive regulator of the Wnt pathway. Its action mechanism, however, remains unknown. Here I show that Ser-45 is phosphorylated not by GSK3 but by CKIepsilon. Axin, a scaffold protein that binds CKIepsilon and beta-catenin, enhances this CKIepsilon-mediated phosphorylation. Overexpression of CKIepsilon in cells increases the amount of beta-catenin phosphorylated at Ser-45. Ser-45 phosphorylated beta-catenin is a better substrate for GSK3, which suggests that CKIepsilon and GSK3 may co-operate in destabilizing beta-catenin. In spite of the fact that CKIepsilon was found as a positive regulator of the Wnt pathway, mutational analysis suggests that mutation of Ser-45 regulates beta-catenin stability by inhibiting the ability of GSK3 to phosphorylate Ser-33 and -37, thereby disrupting the interaction between beta-catenin, beta-TRCP and Axin. I propose that phosphorylation of Ser-45 by CKIepsilon plays an important role in regulating beta-catenin stability.