Drosophila Smoothened phosphorylation sites essential for Hedgehog signal transduction

Abstract

The Hedgehog (Hh) signalling pathway is crucial for animal development and is aberrantly activated in several types of cancer¹. In Drosophila melanogaster, Hh signalling regulates target gene expression through the transcription factor Cubitus interruptus (Ci). Together, Protein Kinase A, Casein Kinase 1 and Glycogen Synthase Kinase 3 silence the pathway in the absence of ligand by phosphorylating Ci at a defined cluster of sites, thereby promoting its proteolytic conversion to a transcriptional repressor (Ci-75)^2,3. In the presence of Hh, Ci-155 is no longer converted to Ci-75 and its ability to activate transcription is potentiated. All Hh responses require the seven transmembrane domain protein Smoothened^1,4, which itself becomes hyperphosphorylated during Hh signalling⁵. Here we show that a cluster of protein kinase A and protein kinase A-primed casein kinase 1 phosphorylation sites in Smoothened, similarly distributed to those regulating Ci, are essential for Smoothened to transduce a Hh signal and for normal regulation of Smoothened protein levels.