Tales of the weary regulated scaffold, Costal2
Hedgehog (Hh) deficient flies, mice and humans reveal a conserved developmental requirement for Hh signaling, while aberrant Hh signaling causes basal cell carcinoma, medulloblastoma, and is implicated in other tumors including breast, lung and pancreas. We address the poorly understood mechanism of Hh signal transduction from the plasma membrane to the nucleus, taking advantage of the extensive genetic characterization of the Hh pathway in Drosophila melanogaster. The secreted protein Hh binds its receptor Patched (Ptc), relieving Ptc inhibition of the GPCR-like protein, Smoothened (Smo). Downstream of Smo is the microtubule associated Hedgehog Signaling Complex (HSC). The HSC consists of the Kinesin related protein (KRP) Costal2 (Cos2), the serine/threonine protein kinase Fused (Fu) and Cubitus interruptus (Ci), the zinc finger transcription factor that regulates Hh target genes. Cos2 and Fu are required to produce transcriptional repressor and activator forms of Ci. Drosophila genetics suggest that fu, cos2, ci and Suppressor of fused [Su(fu)], which suppresses fu phenotypes, interact in a non linear fashion, and phenotypes of the Drosophila wing suggest two complexes could be involved. However, it is not known how Smo signals to the HSC, nor how this complex that tethers Ci from the nucleus, also serves to activate Ci. We demonstrate that Su(fu) associates with the HSC, however, Su(fu) does not appear to bind the microtubule associated complex, and the HSC is stable in the absence of Su(fu). While Ci accumulates in the nucleus of cells exposed to Hh, we show that other HSC members and Su(fu) do not. Cos2 tethers the HSC to vesicular membranes, and releases membranes in response to Hh. Cos2 also binds Smo, although with lower affinity than membranes and with no Hh sensitivity. Additionally, the bulk of Cos2 membrane association is Smo independent. We have thus provided biochemical evidence for two HSC complexes, targeted by Cos2 to Smo or membranes. Differential regulation of these two complexes can explain observations that Ci repressor and Ci activator formation occur with different kinetics. We conclude that Cos2 is a regulated and versatile scaffold that functions by providing the localization required for both types of Ci processing.