E3 ligases RNF43 and ZNRF3 display differential specificity for endocytosis of Frizzled receptors

Abstract

The transmembrane E3 ligases RNF43 and ZNRF3 perform key tumour suppressor roles by inducing endocytosis of members of the Frizzled (FZD) family, the primary receptors for WNT. Loss-of-function mutations inRNF43andZNRF3mediate FZD stabilisation and a WNT-hypersensitive growth state in various cancer types. Strikingly,RNF43andZNRF3mutations are differentially distributed across cancer types, raising questions about their functional redundancy. Here, we compare the efficacy of RNF43 and ZNRF3 of targeting different FZDs for endocytosis. We find that RNF43 preferentially down-regulates FZD1/FZD5/FZD7, whereas ZNRF3 displays a preference towards FZD6. We show that the RNF43 transmembrane domain (TMD) is a key molecular determinant for inducing FZD5 endocytosis. Furthermore, a TMD swap between RNF43 and ZNRF3 re-directs their preference for FZD5 down-regulation. We conclude that RNF43 and ZNRF3 preferentially down-regulate specific FZDs, in part by a TMD-dependent mechanism. In accordance, tissue-specific expression patterns of FZD homologues correlate with the incidence ofRNF43orZNRF3cancer mutations in those tissues. Consequently, our data point to druggable vulnerabilities of specific FZD receptors inRNF43- orZNRF3-mutant human cancers.