Fig. 1

Schematic of Hh signal transduction in Drosophila and vertebrates. a and b In Drosophila, without Hh ligand, the phosphorylated transcription factor Ci undergoes proteolytic cleavage to produce a truncated form as a repressor for target gene transcription (a). With Hh, the pathway activator Smo is relieved from receptor Ptc inhibition to get phosphorylation and plasma membrane accumulation, which triggers a series of phosphorylation events on Cos2 and Fu. Eventually, full-length Ci is stabilized to enter nucleus activating target gene expression (b). c and d In vertebrates, the principal rule of Hh signal transduction is conserved. However, several differences are indeed existing: signaling activation takes place at the primary cilia instead of plasma membrane, Sufu plays a more critical role to transduce signal rather than Fu, and Cos2 homolog Kif7 obtains phosphorylation at basal condition but not during Hh signal activation