Interactions with Costal2 and Suppressor of fused regulate nuclear translocation and activity of Cubitus interruptus

Abstract

The Hedgehog (Hh) family of secreted proteins controls many aspects of growth and patterning in animal development. In Drosophila, Hh acts by preventing the formation of a truncated repressor form of Cubitus interruptus (Ci) and stimulating the transcriptional activity of full-length Ci. Here, we provide evidence that Costal2 (Cos2) and Suppressor of Fused [Su(fu)] inhibit Ci by tethering it in the cytoplasm, whereas Hh induces nuclear translocaltion of Ci through Fused (Fu). We have identified a 125 amino acid domain in the C-terminal part of Ci that mediates response to Cos2 inhibition. We show that Cos2 binds Ci, prevents its nuclear import, and inhibits its activity via this domain. We also provide evidence that Su(fu) regulates Ci through two distinct mechanisms: (1) Su(fu) blocks Ci nuclear import through the N-terminal region of Ci , and (2) it inhibits the activity of Ci through a mechanism independent of Ci nuclear translocation. Finally, we show that Cos2 is required for transducing high levels of Hh signaling activity, and it does so by alleviating the blockage of Ci activity imposed by Su(fu).

Regulation of Hedgehog signaling: a complex story

“…Clones of cells over-expressing Su(fu) demonstrate decreased nuclear accumulation of full-length Ci activator(s) [87]. Conversely, cells lacking Su(fu) demonstrate increased nuclear Ci, consistent with Su(fu) acting in a partially redundant fashion with Cos2 to block Ci nuclear entry [66,71,87]. Studies involving mammalian Su(fu) support this hypothesis.…”

“…CBP binds the carboxyl-terminal activation domain of Ci, presumably to facilitate modifications of target gene chromatin structure. Coincidentally, the CBP binding site overlaps one of the two known Cos2 interaction domains [71], suggesting that Cos2 may regulate Ci function by occluding the Ci activation domain. Cos2 is also involved in regulating the nuclear localization of Ci: Cos2 has been demonstrated to block Ci nuclear entry [71].…”

“…Coincidentally, the CBP binding site overlaps one of the two known Cos2 interaction domains [71], suggesting that Cos2 may regulate Ci function by occluding the Ci activation domain. Cos2 is also involved in regulating the nuclear localization of Ci: Cos2 has been demonstrated to block Ci nuclear entry [71]. Ci that does gain access to the nucleus, does so without other members of the HSC [30], implying that Cos2, Fu and Su(fu) regulation of Ci occurs in the cytoplasm.…”

“…Clones of cells in far anterior regions of the A compartment of the wing imaginal disc that lack cos2 trigger anterior overgrowth phenotypes, and activation of low level Hh target genes, suggesting a negative regulatory role [23,66,67,71]. However, cos2 clones near the A/P compartment boundary do not express high level Hh-target genes, suggesting a role for Cos2 in activation of Ci [48,66,71]. These studies demonstrate that Cos2 plays a role in establishing both activated and repressed Hh states.…”

“…Drosophila with Su(fu) mutation(s) are phenotypically normal, suggesting that it is not required for functional Hh signaling [26,31,80]. However, since Su(fu) has been demonstrated to interact directly with both Fu and Ci, but not Cos2, it has been suggested that Su(fu) function may be partially redundant with that of Cos2 [30,71]. This is supported by genetic studies demonstrating that loss of cos2 suppresses fu mutations [80].…”