The terminal region of β-catenin promotes stability by shielding the armadillo repeats from the axin-scaffold destruction complex

Abstract

Post-translational stabilization of β-catenin is a key step in Wnt signaling, but the features of β-catenin required for stabilization are incompletely understood. We show that forms of β-catenin lacking the unstructured C-terminal domain (CTD) show faster turnover than full-length or minimally truncated β-catenins. Mutants that exhibit faster turnover show enhanced association with axin in co-transfected cells, and excess CTD polypeptide can compete binding of the β-catenin armadillo (arm) repeat domain to axin in vitro, indicating that the CTD may restrict β-catenin binding to the axin-scaffold complex. Fluorescent resonance energy transmission (FRET) analysis of cyan fluorescent protein (CFP)-arm-CTD-yellow fluorescent proteinβ-catenin reveals that the CTD of β-catenin can become spatially close to the N-terminal arm repeat region of β-catenin. FRET activity is strongly diminished by the coexpression of β-catenin binding partners, indicating that an unliganded groove is absolutely required for an orientation that allows FRET. Amino acids 733-759 are critical for β-catenin FRET activity and stability. These data indicate that an N-terminal orientation of the CTD is required for β-catenin stabilization and suggest a model where the CTD extends toward the N-terminal arm repeats, shielding these repeats from the β-catenin destruction complex. © 2009 by The American Society for Biochemistry and Molecular Biology, Inc.