The 15-Amino Acid Repeat Region of Adenomatous Polyposis Coli Is Intrinsically Disordered and Retains Conformational Flexibility upon Binding beta-Catenin

The tumor suppressor Adenomatous polyposis coli (APC) is a large, multidomain protein with many identified cellular functions. The best characterized role of APC is to scaffold a protein complex that negatively regulates Wnt signaling via beta-catenin destruction. This destruction is mediated by beta-catenin binding to centrally located 15- and 20-amino acid repeat regions of APC. More than 80% of cancers of the colon and rectum present with an APC mutation. Most carcinomas with mutant APC express a truncated APC protein that retains the similar to 200-amino acid long' 15-amino acid repeat region'. This study demonstrates that the 15-amino acid repeat region of APC is intrinsically disordered. We investigated the backbone dynamics in the presence of beta-catenin and predicted residues that may contribute to transient secondary features. This study reveals that the 15-amino acid region of APC retains flexibility upon binding beta-catenin and that APC does not have a single, observable "highest-affinity" binding site for beta-catenin. This flexibility potentially allows beta-catenin to be more readily captured by APC and then remain accessible to other elements of the destruction complex for subsequent processing.