Two Fingers on the Pulse of Wnt: Molecular Mechanism of Wnt Pathway Regulation by RING‐finger E3 Ligases ZNRF3/RNF43

Abstract

Wnt signaling pathways play critical roles in embryonic development and tissue homeostasis. The homologous transmembrane E3 ubiquitin ligases, ZNRF3 and RNF43, are critical negative regulators of Wnt pathway by mediating the ubiquitination and degradation of Wnt co‐receptors Frizzled (FZD) and LRP6, and this activity is counteracted by secreted Wnt agonists R‐spondin (RSPO) proteins. Despite increasing interest in these E3 ligases, the mechanism by which ZNRF3/RNF43 recognizes the Wnt receptors remains unclear. Here we identified ZRBP1 functions as an adaptor that recruits ZNRF3/RNF43 to Wnt receptors. ZRBP1 knockout phenocopies ZNRF3/RNF43 knockout, leading to increased cell surface levels of FZD and LRP6. Mechanistically, ZRBP1 is required for ZNRF3/RNF43‐mediated ubiquitination and degradation of FZD. The intracellular domain of ZNRF3/RNF43 physically interacts with ZRBP1, and this interaction is crucial for the Wnt inhibitory activity of the E3 ligases. ZRBP1 also interacts with FZD, and this binding is required for down‐regulation of FZD. Our study reveals ZRBP1 acts as a critical adaptor to recruit negative regulators ZNRF3/RNF43 to Wnt receptors to ensure proper control of pathway activity, and presents new opportunities to modulate Wnt pathway activity.