Transducing properties of Drosophila Frizzled proteins.

In Drosophila, two closely related serpentine receptors, Frizzled (Fz) and D-Frizzled2 (Fz2) are able to act as receptors for the secreted Wnt peptide, Wingless (Wg). In addition to transducing the Wg signal, Fz (but not Fz2) is able to transduce a second, unidentified signal that mediates planar polarity. Much attention has been focused on the structure of the N-termini of the Fz-class receptors and their role in ligand binding. Experiments using techniques of high-level expression have suggested a role for the C-termini in specifying which of the two second messenger systems the receptors are able to activate (M. Boutros, J. Mihaly, T. Bouwmeeste and M. Mlodzik (2000). Science 288, 1825-1828). We argue here that experiments involving high level expression of the receptors cannot be adequately interpreted and we have tested the ability of the receptors and chimeric forms when driven at moderate levels to rescue loss of function of the fz and fz2 genes. Under these conditions we find that all receptors tested will function as Wg receptors, but only a subset show the ability to rescue the polarity pathway. The presence of this subset implies that the N terminus is necessary but not sufficient and suggests that the ability to transduce the polarity signal is widely distributed throughout the protein.