Structure-function analysis of Frizzleds

Frizzleds, cell surface receptors that mediate the actions of Wnt ligands on early development, are heptahelical (based upon hydropathy analysis) and couple to heterotrimeric G proteins. The primary structure of all ten mammalian Frizzleds display many landmarks observed in virtually all G protein-coupled receptors, including an exofacial N-terminus that is N-glycosylated, the presence of seven hydrophobic transmembrane segments predicted to form α-helixes, and three intracellular loops as well as a cytoplasmic, C-terminal tail that harbor suspected sites for protein phosphorylation. Prediction of the G proteins to which Frizzleds mediate signaling based upon a bioinformatic analysis of the primary sequence of the intracellular domains are in good agreement with functional screens in Drosophila, zebrafish, and mouse models of development, e.g., predicting Frizzled-1 to interact with members of the Gi/Go protein family. Likewise various Wnt signaling pathways are sensitive to treatment with pertussis toxin and knock-down of specific G protein α-subunits. Homology among the sequences encoding the cytoplasmic domains of human Frizzleds is high and the various Frizzleds can be segregated into subsets predicted to share some common downstream signaling elements. Among different species, homologies can reveal conservation of signaling to cognate G protein partners. Additionally, cytoplasmic domains of the prototypic β2-adrenergic receptor can be substituted with those from either Frizzled-1 or Frizzled-2 to create chimeric receptors that are activated by β-adrenergic agonists, yet signal with high fidelity to the Wnt/β-catenin and Wnt/Ca2+, cyclic GMP pathways, respectively, regulating key aspects of early development. The nature of Frizzled-based signaling complexes, their temporal assembly, and spatial distribution via scaffold protein remains to be elucidated, as does whether or not these Wnt receptors display agonist-induced desensitization, internalization, and re-cycling to the cell membrane.