Groucho-mediated repression

     We have recently become very interested in a corepressor protein called Groucho because of its roles in both dorsal/ventral and terminal pattern formation. As a corepressor, Groucho does not bind to DNA directly, but rather, it is recruited to DNA by protein:protein interactions with DNA bound repressor proteins. Once recruited to the template in this way, Groucho serves to repress transcription by an unknown mechanism.

     Our recent structure/function analysis of Groucho is beginning to shed light on this mechanism. In addition to a WD-repeat domain at its C-terminus, Groucho contains an evolutionarily conserved region at its N-terminus. We have found that this conserved N-terminal region mediates the formation of high order Groucho oligomers. Single point mutations within this domain that disrupt oligomerization also disrupt Groucho-mediated transcriptional repression (13).

	Primary structure of Groucho. Groucho contains a highly conserved WD-repeat domain and Glutamine-rich (Q) domain, and weakly conserved Glycine/Proline-rich (GP), Serine/Proline-rich (SP), and CcN domains. The CcN domain contains putative phosphorylation sites as well as a nuclear localization signal.

Thus, the ability of Groucho to repress transcription appears to depend upon the ability of this factor to form high order oligomers. Since Groucho can repress transcription from long distances, this leads us to speculate that Groucho mediated repression could involve the oligomerization of Groucho along the chromosome perhaps to induce long-distance changes in chromatin structure. Support for the idea that Groucho could oligomerize along the chromatin fiber comes from our finding that Groucho binds to histones in vitro. We have succeeded in producing large quantities of the Groucho oligomerization domain in E. coli and have recently initiated efforts to determine the atomic resolution structure of this important and interesting domain.

     In addition to the conserved WD repeat domain and the glutamine-rich tetramerization domain, Groucho contains another critical domain termed the GP domain (so-named because of its high content of glycine and proline residues). We have recently discovered that this region of Groucho mediates a direct protein:protein interaction with the histone deacetylase Rpd3 (16). Biochemical assays demonstrate that this interaction as well as Rpd3 histone deacetylase activity are critical for the ability of Groucho to repress transcription. Furthermore, we have discovered a genetic interaction between the genes encoding Groucho and Rpd3 indicating that the interaction discovered through biochemical means is required for normal Drosophila embryogenesis. Thus, it appear that Groucho represses transcription by recruiting chromatin remodeling enzymes such as histone deacetylases to the template. These enzymes may then modify chromatin structure altering the accessibility of the template to the transcriptional machinery.