Figure 8. Summary of graded processing, activation, and proteolysis of Ci-155 and underlying mechanisms at the AP border.

At the AP border, Hh (emanating from posterior, mustard yellow, territory) inhibits Ci-155 processing stimulated by Cos2, PKA, CK1, and GSK3, and activates Fu protein kinase activity to activate Ci-155, overcoming inhibition by Su(fu) and other factors. Hh also promotes a reduction of Ci-155 levels, most likely by promoting full Ci-155 proteolysis through induction of Rdx/Hib or reducing Su(fu) association. Here we used processing-resistant Ci variants to show that PKA and Cos2 (through binding the CORD domain on Ci-155) limit Ci-155 activity in anterior cells (left) and (right) to deduce the spatial profiles of Hh-stimulated Ci-155 reduction (‘proteolysis’, upward arrows) and inhibition of Ci-155 processing (downward arrows, blue triangle) at the AP border that underlie steady-state Ci-155 levels (brown). Graded target gene (En, Ptc, Dpp) activation is normally elicited by a combination of activated Ci-155 and Ci-75 repressor but was still observed when there was no regulation of Ci-155 processing, indicating that Ci-155 activation must be graded. Although graded Hh signaling was observed when Ci-155 processing is not regulated, Ci-75 repressor must be present in anterior cells to prevent ectopic dpp expression and inhibition of processing was shown to be important for activated Fu to induce high levels of ptc expression. Thus, Hh normally elicits graded inhibition of Ci-155 processing and graded activation of full-length Ci-155 but the activation gradient can suffice provided there is some repressor in anterior cells and Ci-155 is substantially spared from processing at the AP border.