Receptors of the Frizzled family initiate Wnt ligand-dependent signaling controlling
multiple steps in organism development and highly conserved in evolution.
Misactivation of the Wnt/Frizzled signaling is cancerogenic. Frizzled receptors
launch several signaling cascades: the canonical pathway regulating beta-catenin-dependent transcription; the planar cell polarity pathway polarizing the
cytoskeleton within the epithelial plane; and the calcium pathway. Frizzled
receptors possess seven transmembrane domains and their signaling depends on
trimeric G proteins in various organisms. However, Frizzleds constitute a
distinct group within the G protein-coupled receptors (GPCR) superfamily, and
Frizzled signaling can be G protein-independent in some experimental setups, which led to concerns about the GPCR nature of Frizzled. Here we demonstrate
that human Frizzled receptors can directly bind the trimeric Go protein in a
pertussis toxin-sensitive manner. Furthermore, addition of Wnt ligands elicits
Frizzled-dependent guanine nucleotide exchange on Go. An excess of secreted
Frizzled-related protein (a Wnt antagonist) prevents Go activation, as does
pretreatment of Go with pertussis toxin. These experiments provide a biochemical
proof of the GPCR activities of Frizzled receptors and establish an in vitro assay to
monitor Frizzled activation by Wnt ligands, applicable for the high-throughput
agonist/antagonist screening
