Protein kinase A antagonizes Hedgehog signaling by regulating both the activator and repressor forms of Cubitus interruptus

The Hedgehog (Hh) family of secreted proteins controls many aspects of anim al development. In Drosophila, Hh transduces its signal via Cubitus interru ptus (Ci), a transcription factor present in two forms: a full-length activ ator and a carboxy-terminally truncated repressor that is derived from the full-length form by proteolytic processing. The proteolytic processing of C i is promoted by the activities of protein kinase A (PKA) and Slimb, wherea s it is inhibited by Hh. Here we show that PKA inhibits the activity of the full-length Ci in addition to its role in regulating Ci proteolysis. Where as Ci processing is blocked in both PKA and slimb mutant cells the accumula ted full-length Ci becomes activated only in PKA but not in slimb mutant ce lls. Moreover, PKA inhibits an uncleavable activator form of Ci. These obse rvations suggest that PKA regulates the activity of the full-length Ci inde pendent of its proteolytic processing. We also provide evidence that PKA re gulates both the proteolytic processing and transcriptional activity of Ci by directly phosphorylating Ci. We propose that phosphorylation of Ci by PK A has two separable roles: (1) It blocks the transcription activity of the full-length activator form of Ci, and (2) it targets Ci for Slimb-mediated proteolytic processing to generate the truncated form that functions as a r epressor.