Modular Mechanism of Wnt Signalling Inhibition by Wnt Inhibitory Factor 1

Abstract

Wnt morphogens control embryonic development and adult tissue homeostasis. In vertebrates the N-terminal WIF domain (WIF-1WD) of six-domain Wnt inhibitory factor 1 (WIF-1) binds Wnts and inhibits signal transduction. Our human WIF-1WD crystal structure reveals a novel binding site for phospholipid; two acyl chains extend deep into the domain while the head group is surface exposed. Biophysical and cellular assays, combined with structure-guided mutagenesis, indicate a WIF-1WD Wnt-binding surface proximal to the lipid head group, but also implicate the five epidermal growth factor (EGF)-like domains (EGFs I-V) in Wnt binding. The crystal structure of six-domain WIF-1 reveals EGFs I-V wrapped-back to interface with WIF-1WD at EGF III. Binding studies locate a heparan sulfate proteoglycan (HSPG)-binding site in EGFs II-V, consistent with highly conserved positively charged residues on EGF IV. This combination of HSPG- and Wnt-binding properties suggests a modular model for WIF-1 localization, and signal inhibition, within morphogen gradients.