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Multiple phosphorylation of β-catenin by glycogen synthase kinase 3 (GSK3) in the Wnt pathway is primed by CK1 through phosphorylation of Ser-45, which lacks a typical CK1 canonical sequence. Synthetic peptides encompassing amino acids 38–64 of β-catenin are phosphorylated by CK1 on Ser-45 with low affinity (K_m ≈1 mM), whereas intact β-catenin is phosphorylated at Ser-45 with very high affinity (K_m ≈200 nM). Peptides extended to include a putative CK1 docking motif (FXXXF) at 70–74 positions or a F74AA mutation in full-length β-catenin had no significant effect on CK1 phosphorylation efficiency. β-Catenin C-terminal deletion mutants up to residue 181 maintained their high affinity, whereas removal of the 131–181 fragment, corresponding to the first armadillo repeat, was deleterious, resulting in a 50-fold increase in K_m value. Implication of the first armadillo repeat in β-catenin targeting by CK1 is …