Frizzled Proteins are bona fide G Protein-Coupled Receptors

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

Reggie-1/flotillin-2 promotes secretion of the long-range signalling forms of Wingless and Hedgehog in Drosophila

The lipid-modified morphogens Wnt and Hedgehog diffuse poorly in isolation yet can spread over long distances in vivo, predicting existence of two distinct forms of these mophogens. The first is poorly mobile and activates short-range target genes. The second is specifically packed for efficient spreading to induce long-range targets. Subcellular mechanisms involved in the discriminative secretion of these two forms remain elusive. Wnt and Hedgehog can associate with membrane microdomains, but the function of this association was unknown. Here we show that a major protein component of membrane microdomains, reggie-1/flotillin-2, plays important roles in secretion and spreading of Wnt and Hedgehog in Drosophila. Reggie-1 loss-of-function results in reduced spreading of the morphogens, while its overexpression stimulates secretion of Wnt and Hedgehog and expands their diffusion. The resulting changes in the morphogen gradients differently affect the short- and long-range targets. In its action reggie-1 appears specific for Wnt and Hedgehog. These data suggest that reggie-1 is an important component of the Wnt and Hedgehog secretion pathway dedicated to formation of the mobile pool of these morphogens