Figure 2.

Schematic illustration of the HH signaling pathway. HH is a secreted morphogen which acts in an autocrine and paracrine fashion. In the absence of HH morphogen, Patched1 (Ptc1), a transmembrane protein, inhibits Smoothened (Smo) activity. Inhibition of Smo activity by Ptc1 has been hypothesized to involve a second messenger mediated mechanism. In the absence of Smo activity, protein kinase A (PKA) phosphorylates Gli proteins (a downstream target of HH signaling) leading to the generation of repressor Gli (Gli-rep), thereby resulting in inactive HH signaling. The binding of the HH morphogen results in loss of Ptc1 activity and subsequent activation of Smo activity. Activated Smo then transduces the signal, resulting in the activation of Gli2A (Gli-Act) and transcription of downstream targets.