A Wnt-Frizzled structure suggests a role for WNT lipidation and a basis for polyspecific interactions.
Major finding: A Wnt-Frizzled structure suggests a role for WNT lipidation and a basis for polyspecific interactions.
Approach: Xenopus Wnt8 was crystallized in complex with the murine Frizzled-8 cysteine-rich domain.
Impact: An understanding of WNT binding and ligand specificity can inform the development of WNT-based drugs.
A structural understanding of how WNT ligands and Frizzled (Fz) receptors interact would be valuable for the rational development of WNT pathway-targeted anticancer drugs. However, the posttranslational addition of lipids to WNT amino acid residues has hindered attempts to express, purify, and crystallize WNT proteins for structural analyses. Janda and colleagues screened vertebrate and invertebrate WNT ligands for their ability to be recombinantly expressed in vitro and bind the extracellular cysteine-rich domains (CRD) of Frizzled proteins that interact with WNTs. The authors found that Xenopus Wnt8 (XWnt8), which had previously been shown to bind and activate mammalian Frizzled receptors, could be expressed at high levels in vitro and copurified with the murine Frizzled-8 (Fz8) CRD. Notably, an XWnt8-Fz8-CRD complex could be purified and crystallized in the absence of detergent, whereas XWnt8 alone could not, suggesting that CRD binding shields WNT lipids from aqueous solvent. Indeed, XWnt8 adopts an unusual two-domain, hand-like structure with a lipid “thumb” that fits into a groove on the Fz8-CRD. This lipid-dependent interaction was enabled by amino acids that are conserved in all 19 human WNTs and 8 of 10 Frizzleds, thus providing a potential reason for the previous observation that WNT lipidation is required for activation of WNT signaling. A peptide “index finger” contacts the Fz8-CRD at a second distinct site that seems to mediate WNT ligand discrimination by Frizzled receptors although a significant amount of cross-reactivity appears to exist. These structural data provide important insight into WNT function and may serve as a framework for the development of WNT-based anticancer compounds.