During Drosophila development, the Wingless (Wg) morphogen modulates the activity of the transcriptional activator Armadillo(Arm/β-catenin) by inactivating a destruction complex that targets it for proteolysis. In the prevailing model of Wg signal transduction, Axin levels limit the activity of this destruction complex (consisting of Axin, Apc and the GSK3 homolog Zeste-white 3), whereas Apc is present in vast excess. Now,on , Benchabane and co-workers propose that Apc activity is also present in limiting amounts in the developing Drosophila retina, to ensure accurate concentration-dependent responses to Wg. When the researchers tested the prevailing model in developing retinas, they unexpectedly discovered that the loss of Apc2 (which reduces total Apc activity by less than twofold)caused ectopic Wg signalling and aberrant cell fate specification in regions of low Wg concentration. The researchers conclude that within the retinal Wg gradient, both Axin and Apc are present near the threshold levels required for Arm destruction, and together ensure accurately graded responses to Wg.
