Hairless-mediated repression of Notch target genes requires the combined activity of Groucho and CtBP corepressors

Notch signal transduction centers on a conserved DNA-binding protein called Suppressor of Hairless [Su(H)] in Drosophila species. In the absence of Notch activation, target genes are repressed by Su(H) acting in conjunction with a partner, Hairless, which contains binding motifs for two global corepressors, CtBP and Groucho (Gro). Usually these corepressors are thought to act via different mechanisms; complexed with other transcriptional regulators, they function independently and/or redundantly. Here we have investigated the requirement for Gro and CtBP in Hairless-mediated repression. Unexpectedly, we find that mutations inactivating one or the other binding motif can have detrimental effects on Hairless similar to those of mutations that inactivate both motifs. These results argue that recruitment of one or the other corepressor is not sufficient to confer repression in the context of the Hairless-Su(H) complex; Gro and CtBP need to function in combination. In addition, we demonstrate that Hairless has a second mode of repression that antagonizes Notch intracellular domain and is independent of Gro or CtBP binding.