The E3 ubiquitin ligase ZNRF3 restricts WNT receptor complex activity by stimulating the selective degradation of WNT-engaged FZD

Ligands of the WNT family induce formation of the WNT receptor signalosome and promote stabilization of the transcriptional coactivator β-catenin. The homologous transmembrane E3 ubiquitin ligases ZNRF3 and RNF43 inhibit WNT-dependent stabilization of β-catenin by stimulating the degradation of the WNT receptor FZD, whereas the secreted R-spondin proteins promote the stabilization of FZD by inducing the degradation of ZNRF3 and RNF43. Here, we report that the R-spondin-induced stabilization of β-catenin in HEK293 cells was not mimicked by FZD overexpression, highlighting a gap in our understanding of this important regulatory mechanism. Contrary to the conventional view that ZNRF3 constitutively mediates the ubiquitylation and degradation of FZD, we found that ZNRF3-induced FZD degradation depended on endogenous WNT and that ZNRF3 selectively degraded WNT-engaged FZD. WNT enhanced the association between FZD and the intracellular adaptor protein DVL, and DVL subsequently recruited ZNRF3 to FZD to promote FZD degradation. Our data suggest that WNT signaling actively restricts itself through ZNRF3-dependent degradation of WNT-engaged FZD and that R-spondin enhances WNT signaling by prolonging the action of the WNT-engaged FZD complex, rather than by simply increasing the abundance of FZD on the cell surface.