Department of Molecular and Cellular Physiology, Stanford University School of Medicine, Stanford, United States; Department of Structural Biology, Stanford University School of Medicine, Stanford, United States; Howard Hughes Medical Institute, Stanford University School of Medicine, Stanford, United States
Department of Molecular and Cellular Physiology, Stanford University School of Medicine, Stanford, United States; Department of Structural Biology, Stanford University School of Medicine, Stanford, United States; Princess Máxima Center for Pediatric Oncology, Utrecht, Netherlands
Department of Molecular and Cellular Physiology, Stanford University School of Medicine, Stanford, United States; Department of Structural Biology, Stanford University School of Medicine, Stanford, United States; Howard Hughes Medical Institute, Stanford University School of Medicine, Stanford, United States
Department of Molecular and Cellular Physiology, Stanford University School of Medicine, Stanford, United States; Department of Structural Biology, Stanford University School of Medicine, Stanford, United States
John S Burg
Department of Molecular and Cellular Physiology, Stanford University School of Medicine, Stanford, United States; Department of Structural Biology, Stanford University School of Medicine, Stanford, United States
Department of Structural Biology, Stanford University School of Medicine, Stanford, United States; SLAC National Accelerator Laboratory, Bioscience Division, Menlo Park, United States
Anthony A Kossiakoff
Department of Biochemistry and Molecular Biology, The University of Chicago, Chicago, United States
Department of Structural Biology, Stanford University School of Medicine, Stanford, United States; SLAC National Accelerator Laboratory, Bioscience Division, Menlo Park, United States
Department of Molecular and Cellular Physiology, Stanford University School of Medicine, Stanford, United States; Department of Structural Biology, Stanford University School of Medicine, Stanford, United States; Howard Hughes Medical Institute, Stanford University School of Medicine, Stanford, United States
Frizzleds (Fzd) are the primary receptors for Wnt morphogens, which are essential regulators of stem cell biology, yet the structural basis of Wnt signaling through Fzd remains poorly understood. Here we report the structure of an unliganded human Fzd5 determined by single-particle cryo-EM at 3.7 Å resolution, with the aid of an antibody chaperone acting as a fiducial marker. We also analyzed the topology of low-resolution XWnt8/Fzd5 complex particles, which revealed extreme flexibility between the Wnt/Fzd-CRD and the Fzd-TM regions. Analysis of Wnt/β-catenin signaling in response to Wnt3a versus a ‘surrogate agonist’ that cross-links Fzd to LRP6, revealed identical structure-activity relationships. Thus, canonical Wnt/β-catenin signaling appears to be principally reliant on ligand-induced Fzd/LRP6 heterodimerization, versus the allosteric mechanisms seen in structurally analogous class A G protein-coupled receptors, and Smoothened. These findings deepen our mechanistic understanding of Wnt signal transduction, and have implications for harnessing Wnt agonism in regenerative medicine.
