Bases moléculaires de la régulation de l'activité de Deubiquitination des histones – HISTONEDUB

Post-translational modifications of core histone proteins are crucial for transcriptional regulation in eukaryotes. A strong emerging picture is that defects in these cellular mechanisms lead to various diseases, including cancer. Different histone modifications are observed that influence both chromatin structure and transcriptional effector recruitment. Among those, H2A and H2B ubiquitination and deubiquitination play critical roles in regulating many nuclear processes, including gene silencing and DNA repair. The transcriptional effector SAGA is a large multi-protein complex involved in many nuclear mechanisms, ranging from transcription activation to mRNA export. SAGA has been shown to acetylate histones through its GCN5 submodule (composed of the Gcn5, Ada2, Ada3 and Sgf29 proteins) and, more recently, to deubiquitinate histone H2B through its DUB submodule (composed of the Usp22, Ataxin-7, Ataxin-7-L3 and Eny2 proteins). The DUB submodule is currently attracting considerable attention as it appears at the cross-roads of transcription (due to its deubiquitination activity) and mRNA export through its Eny2 subunit. Strikingly, only the full DUB subcomplex reproduces the deubiquitination activity of SAGA, indicating a positive regulatory effect on the activity of the Usp22 catalytic subunit by the other subunits. More importantly, Usp22 has been linked to cancer, while polyglutamine expansion at the N-terminus of Ataxin-7 causes spinocerebellar ataxia type 7 (SCA7) that leads to retinal degeneration. The HISTONEDUB project focuses on the characterization of the molecular mechanisms of deubiquitination performed by the DUB subcomplex of the human and yeast SAGA transcriptional complexes. This should, in turn, further our understanding of the molecular bases of the diseases involving this module. We will investigate the architecture and function of the SAGA DUB subcomplex using an integrated structural, molecular and cellular biology approach. X-ray crystallography and NMR spectroscopy will be closely associated, providing information on both the structure and the dynamics of this complex. This combination of complementary biophysical approaches will enable a better comprehension not only of the assembly of the complex, but also of the regulatory mechanisms within the complex. This knowledge will drive molecular and cell biology experiments aimed at understanding the function of the DUB subcomplex and its implications in diseases, notably the effect of the polyglutamine expansion on the deubiquitination process. As such, the project will not only tackle an important biological subject, but will also provide a proof of concept for future structure/function studies of macromolecular complexes.