Analysis of Groucho-histone interactions suggests mechanistic similaritiesbetween Groucho- and Tup1-mediated repression

The Drosophila Groucho (Gro) protein is the defining member of a family of metazoan corepressors that have roles in many aspects of development, inclu ding segmentation, dorsal/ventral pattern formation, Notch signaling, and W nt/Wg signaling. Previous speculation has suggested that Gro may be ortholo gous to the yeast corepressor Tup1. In support of this idea, a detailed ali gnment between the C-terminal WD-repeat domains of these two proteins shows that each Gro WD repeat is most similar to the Tup1 WD repeat occupying th e corresponding position in that protein. Our analysis of Gro-histone inter actions provides further support for a close evolutionary relationship betw een Gro and Tup1. In particular, we show that, as with the N-terminal regio n of Tup1, the N-terminal region of Gro is necessary and sufficient for dir ect binding to histones. The highest affinity interaction is with histone H 3 and binding is primarily observed with hypoacetylated histones. Using tra nsient transfection assays, we show that a Gal4-Gro fusion protein containi ng the histone-binding domain is able to repress transcription. Deletions t hat weaken histone binding also weaken repression. These findings, along wi th our recent report that Gro interacts with the histone deacetylase Rpd3, suggest a mechanism for Gro-mediated repression.