Testing Models of the APC Tumor Suppressor/β-Catenin Interaction Reshapes Our View of the Destruction Complex in Wnt Signaling

The Wnt pathway is a conserved signal transduction pathway that contributes to normal development and adult homeostasis, but is also misregulated in human diseases such as cancer. The tumor suppressor adenomatous polyposis coli (APC) is an essential negative regulator of Wnt signaling inactivated in >80% of colorectal cancers. APC participates in a multiprotein destruction complex that targets the proto-oncogene beta-catenin for ubiquitin-mediated proteolysis; however, the mechanistic role of APC in the destruction complex remains unknown. Several models of APC function have recently been proposed, many of which have emphasized the importance of phosphorylation of high-affinity beta-catenin-binding sites [20-amino-acid repeats (20Rs)] on APC. Here we test these models by generating a Drosophila APC2 mutant lacking all beta-catenin-binding 20Rs and performing functional studies in human colon cancer cell lines and Drosophila embryos. Our results are inconsistent with current models, as we find that beta-catenin binding to the 20Rs of APC is not required for destruction complex activity. In addition, we generate an APC2 mutant lacking all beta-catenin-binding sites (including the 15Rs) and find that a direct beta-catenin/APC interaction is also not essential for beta-catenin destruction, although it increases destruction complex efficiency in certain developmental contexts. Overall, our findings support a model whereby beta-catenin-binding sites on APC do not provide a critical mechanistic function per se, but rather dock beta-catenin in the destruction complex to increase the efficiency of beta-catenin destruction. Furthermore, in Drosophila embryos expressing some APC2 mutant transgenes we observe a separation of beta-catenin destruction and Wg/Wnt signaling outputs and suggest that cytoplasmic retention of beta-catenin likely accounts for this difference.