DS0401 - Etude des systèmes biologiques, de leur dynamique, des interactions et inter-conversions au niveau moléculaire

Structure, Activity and Genome-wide Analysis of the SAGA co-activator – SAGA2

Submission summary

Our objective is to decipher the structure and the function of the transcriptional coactivator SAGA in regulated gene expression by combining cryo-electron microscopy, genome wide transcriptional profiling and in vivo mutational analysis. Regulation of gene expression at the transcriptional level is a vital mechanism that determines cell fate and influences all physiological and pathophysiological aspects of life. Transcription of protein coding genes by RNA polymerase II is tightly controlled and requires the coordinated action of a large number of proteins that trigger the specific assembly of the Pre-Initiation Complex (PIC) at the promoter of the transcribed genes, leading to the correct positioning of RNA polymerase II at the transcription start site. Sequence-specific transcriptional activators and post-translational modifications of nucleosomal histones contribute to the recruitment of multi-subunit coactivator complexes acting as bridging factors between the transcriptional activators and the PIC. Co-activators function to modify the chromatin structure around the promoter region and coordinates PIC assembly with epigenetic chromatin modifications and with activator-mediated signaling events.
Our goal is to understand the functional role, the structural organization and the mode of action of the transcriptional co-activator SAGA; a multi protein complex containing 19 different subunits with for a total molecular weight of 1.9 MDa. SAGA contains two enzymatic activities to acetylate or deubiquinate nucleosomal histone tails by the GCN5 and USP22 subunits, respectively. Impairment of either of SAGA’s enzymes, GCN5 or USP22, leads to embryonic lethality and mutations have been described in human genes encoding these subunits that cause neurodegeneration or contribute to tumorigenesis. A dedicated module of SAGA is responsible for its interaction with the TATA-box binding protein (TBP), a key component of the PIC. The large 400 kDa Tra1 subunits is involved in transcriptional activator binding thus making the link with the cellular signaling pathways.
Our groups played key roles in the characterization of human and yeast SAGA, in the determination of the first structural models and in identifying mutated subunits in human diseases. However, the impact of each individual SAGA activity on gene expression is presently poorly understood and one objective of this proposal is to uncouple the effects of chromatin recognition, catalytic activities, activator recognition and TBP binding on the efficiency of gene transcription. The mechanism by which activator binding to Tra1 triggers PIC assembly and increases mRNA production is currently unknown, and the activator-dependant recruitment of SAGA to chromatin likely involves coordinated conformational changes yet to be characterized. An important question in the field is to understand how SAGA can regulate transcription initiation both positively and negatively. A striking example of this dual role of SAGA is found in the fission yeast S. pombe, where cells stop proliferation in response to nutrient starvation and differentiate into quiescent spores. Distinct SAGA subunits play opposing roles at the promoters of differentiation genes, depending on nutrient availability. In rich conditions, SAGA represses these promoters, whereas upon nutrient starvation, it activates the transcription of the same genes. We aim at uncovering the molecular mechanism by which SAGA integrates environmental signals to regulate gene expression either positively or negatively.

Project coordination


The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.


CRBM Centre de Recherche de Biochimie Macromoléculaire

Help of the ANR 465,000 euros
Beginning and duration of the scientific project: September 2015 - 36 Months

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