title: Testing models of the APC tumor suppressor/β-catenin interaction reshapes our view of the destruction complex in Wnt signaling.
abstract: 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 β-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 β-catenin-binding sites [20-amino-acid repeats (20Rs)] on APC. Here we test these models by generating a Drosophila APC2 mutant lacking all β-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 β-catenin binding to the 20Rs of APC is not required for destruction complex activity. In addition, we generate an APC2 mutant lacking all β-catenin-binding sites (including the 15Rs) and find that a direct β-catenin/APC interaction is also not essential for β-catenin destruction, although it increases destruction complex efficiency in certain developmental contexts. Overall, our findings support a model whereby β-catenin-binding sites on APC do not provide a critical mechanistic function per se, but rather dock β-catenin in the destruction complex to increase the efficiency of β-catenin destruction. Furthermore, in Drosophila embryos expressing some APC2 mutant transgenes we observe a separation of β-catenin destruction and Wg/Wnt signaling outputs and suggest that cytoplasmic retention of β-catenin likely accounts for this difference.
authors.affiliation: Department of Biology, Franklin & Marshall College, Lancaster, Pennsylvania 17604.
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Yamulla
authors.affiliation: Department of Biology, Franklin & Marshall College, Lancaster, Pennsylvania 17604.
authors.forename: Eric G
authors.initials: EG
authors.lastname: Kane
authors.affiliation: Department of Biology, Franklin & Marshall College, Lancaster, Pennsylvania 17604.
authors.forename: Alexandra E
authors.initials: AE
authors.lastname: Moody
authors.affiliation: Department of Biology, Franklin & Marshall College, Lancaster, Pennsylvania 17604.
authors.forename: Kristin A
authors.initials: KA
authors.lastname: Politi
authors.affiliation: Department of Biology, Franklin & Marshall College, Lancaster, Pennsylvania 17604.
authors.forename: Nicole E
authors.initials: NE
authors.lastname: Lock
authors.affiliation: Department of Biology, Franklin & Marshall College, Lancaster, Pennsylvania 17604.
authors.forename: Andrew V A
authors.initials: AV
authors.lastname: Foley
authors.affiliation: Department of Biology, Franklin & Marshall College, Lancaster, Pennsylvania 17604 david.roberts@fandm.edu.
authors.forename: David M
authors.initials: DM
authors.lastname: Roberts
mesh_terms: D011125:Adenomatous Polyposis Coli / Q000235:genetics / Q000378:metabolism*; D000818:Animals; D045744:Cell Line, Tumor; D004330:Drosophila / Q000196:embryology / Q000235:genetics*; D029721:Drosophila Proteins / Q000235:genetics / Q000378:metabolism*; D005260:Female; D017353:Gene Deletion; D015972:Gene Expression Regulation, Neoplastic; D006801:Humans; D008297:Male; D008958:Models, Molecular; D010766:Phosphorylation; D000090063:Proto-Oncogene Mas; D025521:Tumor Suppressor Proteins / Q000235:genetics / Q000378:metabolism*; D060449:Wnt Signaling Pathway; D051176:beta Catenin / Q000235:genetics / Q000378:metabolism*
chemical_list: C467323:APC2 protein, Drosophila; D029721:Drosophila Proteins; C000715470:MAS1 protein, human; D000090063:Proto-Oncogene Mas; D025521:Tumor Suppressor Proteins; D051176:beta Catenin
keywords: Wnt signaling; adenomatous polyposis coli (APC); axin destruction complex; colon cancer; β-catenin