Interaction with both ZNRF3 and LGR4 is required for the signalling activity of R-spondin

R‐spondin proteins sensitize cells to Wnt signalling and act as potent stem cell growth factors. Various membrane proteins have been proposed as potential receptors of R‐spondin, including LGR4/5, membrane E3 ubiquitin ligases ZNRF3/RNF43 and several others proteins. Here, we show that R‐spondin interacts with ZNRF3/RNF43 and LGR4 through distinct motifs. Both LGR4 and ZNRF3 binding motifs are required for R‐spondin‐induced LGR4/ZNRF3 interaction, membrane clearance of ZNRF3 and activation of Wnt signalling. Importantly, Wnt‐inhibitory activity of ZNRF3, but not of a ZNRF3 mutant with reduced affinity to R‐spondin, can be strongly suppressed by R‐spondin, suggesting that R‐spondin primarily functions by binding and inhibiting ZNRF3. Together, our results support a dual receptor model of R‐spondin action, where LGR4/5 serve as the engagement receptor whereas ZNRF3/RNF43 function as the effector receptor. This study shows that both ZNRF3‐ and LGR4‐binding motifs of R‐spondin are required for its Wnt‐promoting activity. These results support a dual receptor model of R‐spondin signalling, where LGR4 serves as the engagement receptor while ZNRF3 functions as the effector receptor.