Structural basis for activation of SAGA histone acetyltransferase Gcn5 by partner subunit Ada2

Abstract

The Gcn5 histone acetyltransferase (HAT) subunit of the SAGA transcriptional coactivator complex catalyzes acetylation of histone H3 and H2B N-terminal tails, posttranslational modifications associated with gene activation. Binding of the SAGA subunit partner Ada2 to Gcn5 activates Gcn5's intrinsically weak HAT activity on histone proteins, but the mechanism for this activation by the Ada2 SANT domain has remained elusive. We have employed Fab antibody fragments as crystallization chaperones to determine crystal structures of a yeast Ada2/Gcn5 complex. Our structural and biochemical results indicate that the Ada2 SANT domain does not activate Gcn5's activity by directly affecting histone peptide binding as previously proposed. Instead, the Ada2 SANT domain enhances Gcn5 binding of the enzymatic cosubstrate acetyl-CoA. This finding suggests a mechanism for regulating chromatin modification enzyme activity: controlling binding of the modification cosubstrate instead of the histone substrate.

A synthetic non-histone substrate to study substrate targeting by the Gcn5 HAT and sirtuin HDACs

“…We favor a model wherein Ada2 and Ada3 make distinct and relatively direct contributions to Gcn5's activity even toward ideal target sequences such as that present in our 3X consensus substrate. This interpretation is consistent with recent structural work that provides clear evidence for Ada2's role in acetyl-CoA binding (51). We also find that overall levels of Ada2-GFP were reduced in ada3⌬ cells, which may suggest a role for Ada3 in promoting Ada2 stability and explain at least part of the defects observed in ada3⌬ mutant cells.…”

Structure of the transcription coactivator SAGA

“…SAGA contains four modules 7 – 9 , the activator-binding Tra1 module, the core module, the histone acetyltransferase (HAT) module, and the histone deubiquitination (DUB) module. Previous works provided partial structures 10 – 14 , but the structure of the central core module is unknown. Here we present the cryo-electron microscopy structure of SAGA from the yeast Saccharomyces cerevisiae and resolve the core module at 3.3 Å resolution.…”

The ZZ domain as a new epigenetic reader and a degradation signal sensor

“…The ZZ domain of HERC2 was defined as a SUMO binding module that promotes protein interactions and SUMOylation (Danielsen et al 2012). While in Mib1 the ZZ domain plays a scaffolding role, the ZZ domain of yeast transcriptional adapter 2 (Ada2) associates with the N-terminal extension of the GCN5 HAT domain, and the ZZ domain of CPEB1 was suggested to interact with proteins and/or RNA (Afroz et al 2014; McMillan et al 2015; Sun et al 2018). In addition to their roles in cellular processes occurred in the nucleus, some ZZ domains are essential in cytoplasmic signaling.…”