α-Catenin interacts with APC to regulate β-catenin proteolysis and transcriptional repression of Wnt target genes

Mutation of the adenomatous polyposis coli (APC) tumor suppressor stabilizes beta-catenin and aberrantly reactivates Wnt/beta-catenin target genes in colon cancer. APC mutants in cancer frequently lack the conserved catenin inhibitory domain (CID), which is essential for beta-catenin proteolysis. Here we show that the APC CID interacts with alpha-catenin, a Hippo signaling regulator and heterodimeric partner of beta-catenin at cell: cell adherens junctions. Importantly, alpha-catenin promotes beta-catenin ubiquitylation and proteolysis by stabilizing its association with APC and protecting the phosphodegron. Moreover, beta-catenin ubiquitylation requires binding to alpha-catenin. Multidimensional protein identification technology (MudPIT) proteomics of multiple Wnt regulatory complexes reveals that alpha-catenin binds with beta-catenin to LEF-1/TCF DNA-binding proteins in Wnt3a signaling cells and recruits APC in a complex with the CtBP:CoREST:LSD1 histone H3K4 demethylase to regulate transcription and beta-catenin occupancy at Wnt target genes. Interestingly, tyrosine phosphorylation of alpha-catenin at Y177 disrupts binding to APC but not beta-catenin and prevents repression of Wnt target genes in transformed cells. Chromatin immunoprecipitation studies further show that alpha-catenin and APC are recruited with beta-catenin to Wnt response elements in human embryonic stem cells (hESCs). Knockdown of alpha-catenin in hESCs prevents the switch-off of Wnt/beta-catenin transcription and promotes endodermal differentiation. Our findings indicate a role for alpha-catenin in the APC destruction complex and at Wnt target genes.