A positive role for Patched in Hedgehog signaling revealed by the intracellular trafficking of Sex-lethal, the Drosophila sex determination master switch

The sex determination master switch, Sex-lethal (Sxl), controls sexual development as a splicing and translational regulator. Hedgehog (Hh) is a secreted protein that specifies cell fate during development. We show that Sxl is in a complex that contains all of the known Hh cytoplasmic components, including Cubitus interruptus (Ci) the only known target of Hh signaling. Hh promotes the entry of Sxl into the nucleus in the wing disc. In the anterior compartment, the Hh receptor Patched (Ptc) is required for this effect, revealing Ptc as a positive effector of Hh. Some of the downstream components of the Hh signaling pathway also alter the rate of Sxl nuclear entry. Mutations in Suppressor of Fused or Fused with altered ability to anchor Ci are also impaired in anchoring Sxl in the cytoplasm. The levels, and consequently, the ability of Sxl to translationally repress downstream targets in the sex determination pathway, can also be adversely affected by mutations in Hh signaling genes. Conversely, overexpression of Sxl in the domain that Hh patterns negatively affects wing patterning. These data suggest that the Hh pathway impacts on the sex determination process and vice versa and that the pathway may serve more functions than the regulation of Ci.     

Casein Kinase 2 Promotes Hedgehog Signaling by Regulating both Smoothened and Cubitus Interruptus

Casein kinase 2 (CK2) is a typical serine/threonine kinase consisting of α and β subunits and has been implicated in many cellular and developmental processes. In this study, we demonstrate that CK2 is a positive regulator of the Hedgehog (Hh) signal transduction pathway. We found that inactivation of CK2 by CK2β RNAi enhances the loss-of-Hh wing phenotype induced by a dominant negative form of Smoothened (Smo). CK2β RNAi attenuates Hh-induced Smo accumulation and down-regulates Hh target gene expression, whereas increasing CK2 activity by coexpressing CK2α and CK2β increases Smo accumulation and induces ectopic Hh target gene expression. We identified the serine residues in Smo that can be phosphorylated by CK2 in vitro. Mutating these serine residues attenuates the ability of Smo to transduce high level Hh signaling activity in vivo. Furthermore, we found that CK2 plays an additional positive role downstream of Smo by regulating the stability of full-length Cubitus interruptus (Ci). CK2β RNAi promotes Ci degradation whereas coexpressing CK2α and CK2β increases the half-life of Ci. We showed that CK2 prevents Ci ubiquitination and degradation by the proteasome. Thus, CK2 promotes Hh signaling activity by regulating multiple pathway components.