Summary of Hh signaling in the wing disc.

A) The diffusible signaling protein Hh exerts its function by binding to its receptor Ptc. This relieves the transmembrane protein Smo from inhibition by Ptc. Smo then signals to the cytoplasm, where the signal is eventually relayed to the transcription factor Cubitus interruptus (Ci). Target genes under the control of Ci thus respond to Hh signaling. B) Expression domains of genes important in the process. Engrailed (en) and ci mark the two compartments. Hh is produced under the direction of en, and then diffuses freely. Smo is expressed throughout the disc. Ci is needed to activate or repress target genes in response to Hh signaling. Ci is not present in the posterior half, hence Hh target genes cannot be activated there. Immediately at the boundary, target genes exhibit the largest difference in expression. C) When small groups of cells are made mutant for the smo gene, they can no longer activate target genes. When such clones are located within the target gene stripe, they are known to round off and to tend to minimize contact with neighboring cells.