The SAGA complex is one of the most studied transcriptional co-activator complexes. To facilitate transcription by RNA Polymerase II, SAGA presents a modular organization and harbours two enzymatic activities. In human cells, these two enzymes are called GCN5 and USP22 and they can respectivelly acetylate histones H3 and deubiquitinate histones H2B. During my PhD thesis, I have worked on the regulation of SAGA deubiquitination activity. In the SAGA complex, USP22 interacts strongly with three other subunits to form a structural and functionnal module, named deubiquitination module (DUBm). We have shown that the free recombinant USP22 enzyme is not active, but that the formation of a stable DUBm triggers a strong stimulation of USP22 catalytic activity. Secondly, in human cells, two subunits of the DUBm, ATXN7 and ATXN7L3, contain a domain, called SCA7, that is not found in any other protein. Our results show that the new structural fold adopted by these two domains is specific to these zinc-fingers. These two SCA7 domains share a common structural heart, but their atomic structures reveal also differences, especially in the spatial organization of secondary structure elements. Indeed, we have shown that ATXN7 SCA7 domain can interact in vitro with a nucleosome which is not the case of ATXN7L3 SCA7 domain. Finally, I could show that in vivo the SCA7 domain of Sgf73, the ortholog of ATXN7 has a role in fine tunning SAGA deubiquitination activity. We hypothesize that the interaction between a nucleosome and the SCA7 domain of ATXN7 or Sgf73 would regulate SAGA deubiquitination activity by an optimal positionning of the module to its substrate.
