Thermodynamics of β-Catenin-Ligand Interactions

Abstract

β-Catenin is a structural component of adherens junctions, where it binds to the cytoplasmic domain of cadherin cell adhesion molecules. β-Catenin is also a transcriptional coactivator in the Wnt signaling pathway, where it binds to Tcf/Lef family transcription factors. In the absence of a Wnt signal, nonjunctional β-catenin is present in a multiprotein complex containing the proteins axin and adenomatous polyposis coli (APC), both of which bind directly to β-catenin. The thermodynamics of β-catenin binding to E-cadherin, Lef-1, APC, axin, and the transcriptional inhibitor ICAT have been determined by isothermal titration calorimetry. Most of the interactions showed large, unfavorable entropy changes, consistent with these ligands being natively unstructured in the absence of β-catenin. Phosphorylation of serine residues present in a sequence motif common to cadherins and APC increased the affinity for β-catenin 300-700-fold, and surface plasmon resonance measurements revealed that phosphorylation of E-cadherin both enhanced its on rate and decreased its off rate. The effects of the N- and C-terminal "tails" that flank the β-catenin armadillo repeat domain on ligand binding have also been investigated using constructs lacking one or both tails. Contrary to earlier studies that employed less direct binding assays, the tails did not affect the affinity of β-catenin for tight ligands such as E-cadherin, Lef-1, and phosphorylated APC. However, the β-catenin C-terminal tail was found to decrease the affinity for the weaker ligands APC and axin, suggesting that this region may have a regulatory role in β-catenin degradation. © 2006 by The American Society for Biochemistry and Molecular Biology, Inc.