Structural basis of Wnt signalosome extracellular complex assembly

Recognition of Wnt proteins by Frizzled (Fzd) receptors and the low-density lipoprotein receptor-related protein 5/6 (LRP5/6) co-receptor is essential for canonical Wnt signaling. It remains enigmatic how Wnt simultaneously interacts with Fzd and LRP5/6 and activates intracellular Wnt/β-catenin signaling. Here, we report cryo-electron microscopy (cryo-EM) structures of Wnt3a/Fzd8/LRP6 extracellular complexes captured in a 2:4:2 stoichiometry, consisting of a Wnt3a-Wnt3a homodimer, whereby each Wnt3a monomer binds to two Fzd8 receptors and one LRP6 co-receptor. This implies that Wnt3a induces Fzd cystine-rich domain (Fzd-CRD) tetramerization, which in turn could promote recruitment of oligomeric Disheveled (Dvl) to Fzd on the cytoplasmic side. Indeed, mutations of key Wnt3a-Wnt3a interface residues abolish Fzd-LRP clustering and downstream signaling, supporting a critical role of Wnt3a-Wnt3a dimerization in Wnt signalosome assembly and signaling. Our structures also show how the Wnt3a N-helical domain recognizes the LRP6 extracellular domain (LRP6-ECD) E3 β-propeller, while the Wnt3a N-C hairpin interacts with the valley between LRP6-E3 and -E4 propellers, underpinning the development of targeted Wnt therapeutics.