Drosophila Hedgehog can act as a morphogen in the absence of regulated Ci processing

Abstract

SummaryExtracellular Hedgehog (Hh) proteins induce transcriptional changes in target cells by inhibiting the proteolytic processing of full-length Drosophila Ci or mammalian Gli proteins to nuclear transcriptional repressors and by activating the full-length proteins, which are otherwise held inactive by cytoplasmic binding partners and subject to accelerated degradation following activation. We used Ci variants expressed at physiological levels to investigate the contributions of these mechanisms to dose-dependent Hh signaling at the anteroposterior (AP) border of Drosophila wing imaginal discs. Ci variants that cannot be processed supported a normal pattern of graded target gene activation and the development of adults with normal wing morphology when supplemented by constitutive Ci repressor, showing that Hh can signal normally in the absence of regulated processing. The full-length Ci-155 protein profile of these variants revealed a linear gradient of Hh-stimulated degradation, allowing derivation of a spatial profile of inhibition of processing of normal C-155 by Hh. The processing-resistant Ci variants were also significantly activated in the absence of Hh by elimination of Cos2, acting through association with the CORD domain of Ci, or PKA, revealing separate inhibitory roles of these two components in addition to their well-established roles in promoting Ci-155 processing.