The Hedgehog Signalling Pathway

In the absence of Hh, smo activity is inhibited by Ptc. A tetrameric complex is formed with Cos2, Fu and Su(Fu) with full length Ci. Through Cos2, the complex binds to microtubules. Ci is targeted to the proteosome and cleaved, releasing Ci-75 (the transcriptional repressor). Ci-155 is phosphorylated and promotes cleavage.

Presence of Hh

Hh signalling initiates in the plasma membrane. When Hh binds to its receptor Ptc it relieves the suppression Ptc has on Smo. Activation of Smo leads to dissociation of the trimeric Fu-Cos2-Ci complex from the microtubules by an unknown mechanism. Ci cleavage is inhibited, and Ci-155 (the active form) translocates to the nucleus, binding with the co-activator Creb Binding Protein (CBP), where it activates transcription of the target genes ptc, wg and dpp.