Abstract
Spt‐Ada‐Gcn5‐Acetyltransferase (SAGA) is a conserved histone acetyltransferase complex that regulates transcription through acetylation and deubiquitination of nucleosomal histones. The 19 subunits constituting SAGA are thought to organize into four modules. To further elucidate the relationship between these modules, we developed an improved purification method that enhances the stability of SAGA and used single‐particle electron microscopy to examine the overall architecture and organization of this complex. Our two‐ and three‐dimensional analyses revealed that SAGA is flexible and adopts three major conformations involving large‐scale rearrangements of its subunits. Subunit deletion and labeling showed that the acetyltransferase module is located in the most mobile region of the complex, and that the loss of the deubiquitination module reduces SAGA’s ability to adopt one specific conformation. These results lead to a working model in which the different modules of SAGA coordinate to mediate structural remodeling, which in turn accommodates binding of various substrates and transcriptional activators.
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