title: The third 20 amino acid repeat is the tightest binding site of APC for beta-catenin.
abstract: Adenomatous polyposis coli (APC) plays a critical role in the Wnt signaling pathway by tightly regulating beta-catenin turnover and localization. The central region of APC is responsible for APC-beta-catenin interactions through its seven 20 amino acid (20aa) repeats and three 15 amino acid (15aa) repeats. Using isothermal titration calorimetry, we have determined the binding affinities of beta-catenin with an APC 15aa repeat fragment and each of the seven 20aa repeats in both phosphorylated and unphosphorylated states. Despite sequence homology, different beta-catenin binding repeats of APC have dramatically different binding affinities with beta-catenin and thus may play different biological roles. The third 20aa repeat is by far the tightest binding site for beta-catenin among all the repeats. The fact that most APC mutations associated with colon cancers have lost the third 20aa repeat underlines the importance of APC-beta-catenin interaction in Wnt signaling and human diseases. For every 20aa repeat, phosphorylation dramatically increases its binding affinity for beta-catenin, suggesting phosphorylation has a critical regulatory role in APC function. In addition, our CD and NMR studies demonstrate that the central region of APC is unstructured in the absence of beta-catenin and Axin, and suggest that beta-catenin may interact with each of the APC 15aa and 20aa repeats independently.
authors.affiliation: Department of Biological Structure, University of Washington, Seattle, WA 98195, USA.
authors.forename: Jing
authors.initials: J
authors.lastname: Liu
authors.forename: Yi
authors.initials: Y
authors.lastname: Xing
authors.forename: Thomas R
authors.initials: TR
authors.lastname: Hinds
authors.forename: Jie
authors.initials: J
authors.lastname: Zheng
authors.forename: Wenqing
authors.initials: W
authors.lastname: Xu
mesh_terms: D011125:Adenomatous Polyposis Coli / Q000378:metabolism*; D000595:Amino Acid Sequence; D001665:Binding Sites; D004252:DNA Mutational Analysis; D006801:Humans; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010766:Phosphorylation; D011485:Protein Binding; D015398:Signal Transduction; D013816:Thermodynamics; D051176:beta Catenin / Q000737:chemistry* / Q000378:metabolism
chemical_list: D051176:beta Catenin