Frizzled Proteins are bona fide G Protein-Coupled Receptors

Abstract Receptors of the Frizzled family initiate Wnt ligand-dependent signaling controllingmultiple steps in organism development and highly conserved in evolution.Misactivation of the Wnt/Frizzled signaling is cancerogenic. Frizzled receptorslaunch several signaling cascades: the canonical pathway regulating beta-catenin-dependent transcription; the planar cell polarity pathway polarizing thecytoskeleton within the epithelial plane; and the calcium pathway. Frizzledreceptors possess seven transmembrane domains and their signaling depends ontrimeric G proteins in various organisms. However, Frizzleds constitute adistinct group within the G protein-coupled receptors (GPCR) superfamily, andFrizzled signaling can be G protein-independent in some experimental setups, which led to concerns about the GPCR nature of Frizzled. Here we demonstratethat human Frizzled receptors can directly bind the trimeric Go protein in apertussis toxin-sensitive manner. Furthermore, addition of Wnt ligands elicitsFrizzled-dependent guanine nucleotide exchange on Go. An excess of secretedFrizzled-related protein (a Wnt antagonist) prevents Go activation, as doespretreatment of Go with pertussis toxin. These experiments provide a biochemicalproof of the GPCR activities of Frizzled receptors and establish an in vitro assay tomonitor Frizzled activation by Wnt ligands, applicable for the high-throughputagonist/antagonist screening.