Structural Basis of Wnt Recognition by Frizzled
Howard Hughes Medical Institute, Stanford University School of Medicine, Stanford, CA 94305, USA. Science
(Impact Factor: 33.61).
05/2012; 337(6090):59-64. DOI: 10.1126/science.1222879
Wnts are lipid-modified morphogens that play critical roles in development principally through engagement of Frizzled receptors. The 3.25 angstrom structure of Xenopus Wnt8 (XWnt8) in complex with mouse Frizzled-8 (Fz8) cysteine-rich domain (CRD) reveals an unusual two-domain Wnt structure, not obviously related to known protein folds, resembling a "hand" with "thumb" and "index" fingers extended to grasp the Fz8-CRD at two distinct binding sites. One site is dominated by a palmitoleic acid lipid group projecting from serine 187 at the tip of Wnt's thumb into a deep groove in the Fz8-CRD. In the second binding site, the conserved tip of Wnt's "index finger" forms hydrophobic amino acid contacts with a depression on the opposite side of the Fz8-CRD. The conservation of amino acids in both interfaces appears to facilitate ligand-receptor cross-reactivity, which has important implications for understanding Wnt's functional pleiotropy and for developing Wnt-based drugs for cancer and regenerative medicine.
Available from: László Patthy
- "In the XWnt8-Frizzled-8 complex, the two domain Wnt structure resembles a hand, where the N-terminal and C-terminal domains provide the ''thumb " and ''index " fingers that grasp the Frizzled receptor at two distinct binding sites . The lipid group at the tip of the N-terminal thumb is inserted into a deep groove in the Wnt-binding domain of the receptor, whereas the tip of the C-terminal 'index finger' interacts with residues on the opposite side of the receptor . Although three dimensional structures of ternary Wnt-receptor complexes have not yet been solved, mutational analysis of mouse Wnt3a revealed that the linker connecting the N-and C-terminal domains participates in LRP6 binding . "
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ABSTRACT: Wnts have a structure resembling a hand with "thumb" and "index" fingers that grasp the cysteine rich domains of Frizzled receptors at two distinct binding sites. In the present work we show that the WIF domain of Wnt Inhibitory Factor 1 is also bound by Wnts at two sites. Using C-terminal domains of Wnt5a and Wnt7a and arginine-scanning mutagenesis of the WIF domain we demonstrate that, whereas the N-terminal, lipid-modified "thumb" of Wnts interacts with the alkyl-binding site of the WIF domain, the C-terminal domain of Wnts (Wnt-CTD) binds to a surface on the opposite side of the WIF domain.
- "The seven-transmembrane Frizzled receptors (Fzds) were first demonstrated to function as Wnt receptors through direct binding and activation of downstream signaling (Bhanot et al., 1996; Janda et al., 2012). Fzd proteins, of which there are 10 family members in mammals, bind to Wnts through their large extracellular cysteine-rich domain (CRD), and have been shown to mediate both " canonical " and " noncanonical " Wnt signaling (Schulte, 2010; Mac- Donald and He, 2012; Niehrs, 2012). "
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ABSTRACT: The morphology of the dendritic tree is critical to neuronal function and neural circuit wiring. Several Wnt family members have been demonstrated to play important roles in dendrite development. However, the Wnt receptors responsible for mediating this process remain largely elusive. Using primary hippocampal neuronal cultures as a model system, we report that Frizzled4 (Fzd4), a member of the Fzd family of Wnt receptors, specifically signals downstream of Wnt5a to promote dendrite branching and growth. Interestingly, the less conserved distal PDZ binding motif of Fzd4, and not its conserved proximal Dvl-interacting PDZ motif, is required for mediating this effect. We further showed that Dvl signaled parallel to and independent of Fzd4 in promoting dendrite growth. Unlike most previously described pathways, Wnt5a/Fzd4 signaling promoted dendrite development in an activity-independent and autocrine fashion. Together, these results provide the first identification of a Wnt receptor for regulating dendrite development in the mammalian system, and demonstrate a novel function of the distal PDZ motif of Fzd4 in dendrite morphogenesis, thereby expanding our knowledge of the complex roles of Wnt signaling in neural development. This article is protected by copyright. All rights reserved.
Available from: Suat Özbek
- "It was shown previously that a mini-Wnt comprising the Xenopus Wnt8 CTD shows autonomous binding to Fz . In addition, the inhibitory effects of Wnt NTDs have been reported in various studies [11-15]. "
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Wnt proteins are a family of secreted signaling molecules that regulate key developmental processes in metazoans. The molecular basis of Wnt binding to Frizzled and LRP5/6 co-receptors has long been unknown due to the lack of structural data on Wnt ligands. Only recently, the crystal structure of the Wnt8-Frizzled8-cysteine-rich-domain (CRD) complex was solved, but the significance of interaction sites that influence Wnt signaling has not been assessed.
Here, we present an extensive structure-function analysis of mouse Wnt3a in vitro and in vivo. We provide evidence for the essential role of serine 209, glycine 210 (site 1) and tryptophan 333 (site 2) in Fz binding. Importantly, we discovered that valine 337 in the site 2 binding loop is critical for signaling without contributing to binding. Mutations in the presumptive second CRD binding site (site 3) partly abolished Wnt binding. Intriguingly, most site 3 mutations increased Wnt signaling, probably by inhibiting Wnt-CRD oligomerization. In accordance, increasing amounts of soluble Frizzled8-CRD protein modulated Wnt3a signaling in a biphasic manner.
We propose a concentration-dependent switch in Wnt-CRD complex formation from an inactive aggregation state to an activated high mobility state as a possible modulatory mechanism in Wnt signaling gradients.
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