Exploring the mechanistic roles of APC in the Armadillo/β-catenin destruction complex
Authors: Katherine Gerber; Julia Kiefer; Emily Errickson; Sonia Hafiz; Matthew Krause; Carmen Navia; Hannah Salvucci; David Roberts
Affiliation: Franklin & Marshall College, Lancaster, PA
Keywords: b. Wingless; h. tumorigenesis
The tumor suppressor, Adenomatous Polyposis Coli (APC), is an important negative regulator of the Wg/Wnt signaling pathway and is inactivated in nearly 80% of all colon cancer cases. APC participates in a multi-protein “destruction complex” that phosphorylates the proto-oncogene, Armadillo/β-catenin (Arm/β-cat), thereby targeting Arm/β-cat for ubiquitin-mediated proteolysis. Despite nearly 30 years of research on APC, its precise mechanistic role in the destruction complex remains unknown. APC contains several Arm/β-cat binding sites, and prior research from several groups has suggested that these sites play important mechanistic roles in the destruction complex, suggesting specific models of how APC contributes to the destruction complex. We have been testing proposed models using a structure/function approach using Drosophila APC2 as a model system. Surprisingly, our previous research demonstrated that Arm/β-cat binding sites in APC2 are dispensable for Arm/β-cat destruction in cells of the embryonic epidermis that do not receive Wg/Wnt signaling, but are required in cells that receive Wg/Wnt signaling. These findings suggest that Arm/β-cat binding sites on APC and Axin could be redundant in cells that do not receive Wg/Wnt signaling, but that APC’s Arm/β-cat binding sites may play a unique mechanistic role(s) in cells that receive Wg/Wnt signal. To test the redundancy hypothesis, we have generated APC2 and Axin transgenes either containing or lacking Arm/β-cat binding sites. Additionally, to explore that possibility that APC’s Arm/β-cat binding sites have a unique mechanistic role, we have generated APC2 transgenes that replace the Arm/β-cat bindings sites with Arm/β-cat binding sites from other Arm/β-cat-interacting proteins such as α-catenin, TCF, and Axin. To date, our results indicate that APC’s Arm/β-cat binding sites are partially replaceable by Arm/β-cat binding sites from TCF and Axin, but not the Arm/β-cat binding site from α-catenin. Collectively, these findings suggest that APC’s Arm/β-cat binding sites do not function simply to recruit Arm/β-cat into the destruction complex, but rather they also likely play a more complex mechanistic role(s). Furthermore, it suggests that specific contacts to Arm/β-cat are required for efficient Arm/β-cat destruction.