Structural and mechanistic insights into the SAGA coactivator complex (361.1)

Abstract

SAGA is a transcriptional coactivator complex that is conserved across eukaryotes and carries out multiple functions during transcriptional activation and elongation, including deubiquitination of histone H2B and histone acetylation. The deubiquitinating activity of SAGA resides in a distinct subcomplex called the deubiquitinating module (DUBm), which contains the ubiquitin‐specific protease, Ubp8, bound to Sgf11, Sus1 and Sgf73. Although Ubp8 contains a canonical USP isopeptidase domain, all four DUB module proteins are required for activity. Our structural studies of the DUB module revealed an unusual arrangement of non‐globular proteins bound to Ubp8 that give rise to a highly interconnected complex, in which each protein contacts the other three. These studies shed light on the contributions of Sgf11 and Sgf73 in regulating Ubp8 activity. One of the key features of the complex is the zinc finger domain of Sgf11, which binds near the catalytic site of the USP domain of Ubp8. Mutations and deletions of the region affect both enzymatic activity and gene activation, suggesting a dual role in activating Ubp8 and chromatin targeting. Structural and solution studies of the DUB module lacking the Sgf11 zinc finger reveal a surprising role for this domain in stabilizing the overall organization and proper folding of the entire DUB module, whereas point mutations in this domain point to its role in governing interactions with the nucleosome as well with polyubiquitin chains. We will describe the implications of these finding for understanding the overall interdependence of SAGA subunits and how they are targeted to the appropriate chromatin target during transcription.