Hedgehog Signaling Downregulates Su(Fu) Through The HIB/SPOP-Crn Axis In Drosophila

Hedgehog (Hh) signaling plays a vital role in embryonic development and adult tissue homeostasis in species ranging from insects to human. Dysregulation of Hh signaling is implicated in many human disorders including birth defects and several different types of human cancer.Suppressor of Fused (Su(fu)) as a negative regulator in Hh signaling is conserved between invertebrate and vertebrate, but how it is regulated remains poorly understood. In this study, we find in Drosophila Hh signaling promotes downregulation of Su(fu) through its target protein HIB (for Hh-induced BTB protein). Interestingly, although HIB-mediated downregulation of Su(fu) depends on the E3 ubiquitin ligase Cul3, HIB does not regulate Su(fu) protein stability. In addition, HIB also does not affect Su(fu) mRNA level and mRNA translocation between nucleus and cytoplasm. Instead, we demonstrate that HIB may downregulate Su(fu) through regulating its translation. Subsequently, through an RNAi-based candidate gene screen, we identify the spliceosome factor Crooked neck (Cm) as a regulator of Su(fu) level. Epistasis analysis indicates that HIB downregulates Su(fu) through Cm. Furthermore, we provide evidence that HIB retains Crn in the nucleus, leading to reduced Su(fu) protein level. Finally, Speckle-type POZ protein (SPOP), the mammalian homologue of HIB, can substitute HIB to downregulate Su(fu) level in Drosophila.Our study suggests that Hh regulates both Cubitus interruptus (Ci) and Su(fu) levels through its target HIB, thus uncovering a novel feedback mechanism that regulates Hh signal transduction. The dual function of HIB may provide a buffering mechanism to fine-tune Hh pathway activity.