HectD1 E3 Ligase Modifies Adenomatous Polyposis Coli (APC) with Polyubiquitin to Promote the APC-Axin Interaction

The adenomatous polyposis coli (APC) protein functions as a negative regulator of the Wnt signaling pathway. In this capacity, APC forms a "destruction complex" with Axin, CK1α, and GSK3β to foster phosphorylation of the Wnt effector β-catenin earmarking it for Lys-48-linked polyubiquitylation and proteasomal degradation. APC is conjugated with Lys-63-linked ubiquitin chains when it is bound to Axin, but it is unclear whether this modification promotes the APC-Axin interaction or confers upon APC an alternative function in the destruction complex. Here we identify HectD1 as a candidate E3 ubiquitin ligase that modifies APC with Lys-63 polyubiquitin. Knockdown of HectD1 diminished APC ubiquitylation, disrupted the APC-Axin interaction, and augmented Wnt3a-induced β-catenin stabilization and signaling. These results indicate that HectD1 promotes the APC-Axin interaction to negatively regulate Wnt signaling. Background: APC is modified with Lys-63-linked polyubiquitin when bound to Axin in an assembled β-catenin destruction complex. Results: HectD1 E3 ligase modifies APC with Lys-63-linked ubiquitin chains to facilitate the APC-Axin interaction. Conclusion: HectD1 is a candidate E3 ligase for APC. Significance: The identification of HectD1 could lead to a better understanding of APC function.