Reception and transduction of Wnt signals involves binding of Wnt receptors to members of two distinct families of cell surface receptors, members of the Frizzled gene family and members of the LDL-receptor-related protein (LRP) family. Wnt receptors are members of growth factor receptors. The canonical Frizzled receptor has an aminoterminal cysteine-rich domain (CRD) that binds Wnt, seven transmembrane domains and a short cytoplasmic tail containing a consensus PDZ domain binding motif at the carboxyl terminus. The CRD forms a novel protein fold with a conserved dimerization interface that may be important for Wnt binding. Frizzled receptors have been identified in vertebrates and invertebrates; there are ten known members in humans and mice, four in flies, and three in worms. The general structure of Frizzled receptors resembles that of seven-transmembrane G-protein-coupled receptors, suggesting that Frizzled proteins may use heterotrimeric G proteins to transduce Wnt signals. Several recent studies provide evidence consistent with this idea, showing that a subgroup of Frizzled receptors can signal through the pertussis-toxin-sensitive subclass of heterotrimeric G proteins to stimulate an increase in intracellular Ca2+ and activate protein kinase C (PKC). Heterotrimeric G proteins do not appear to be involved in transducing Wnt/Frizzled signals that regulate the cytoskeleton associated protein.
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