Adam Ben-Shem

Transcriptional co-activator complexes play essential roles in regulating gene transcription by integrating signals from transcription factors and chromatin. Among these, the SAGA (Spt-Ada-Gcn5 acetyltransferase) and ATAC (Ada Two-A Containing) complexes represent related histone acetyltransferase assemblies that contain both shared and specific subunits. SAGA and ATAC possess similar and divergent enzymatic activities that shape their contributions to transcriptional control. Here, we review emerging evidence that highlights differential interactions of transcriptional activators with SAGA or ATAC and their differential chromatin-reading capabilities through specific recognition of histone modifications. Distinct requirements for SAGA and for ATAC have been observed in gene regulation important for maintaining cellular homeostasis and for oncogenic transformation, indicating that dysregulation of these complexes can contribute to cancer progression. The development of small-molecule inhibitors targeting key components of SAGA and/or ATAC offers promising strategies to disrupt their recruitment and transcriptional activity in cancer cells. Our review dissects the unique and overlapping functions of SAGA and ATAC, which are crucial for understanding regulation by these complexes in order to exploit them as targets in precision oncology.

Human SAGA is a 20-subunit transcriptional coactivator. Compared with yeast, metazoan SAGA uniquely incorporates a 150-kDa splicing-factor module (SPL), also present in U2 small nuclear ribonucleoprotein (U2snRNP). Metazoan gene duplication further specialized shared TFIID/SAGA subunits into SAGA-specific paralogs (TAF5L and TAF6L), but the functional consequences of this divergence are unknown. We report the structure of endogenous human SAGA purified via an affinity ligand from cells that were not disturbed by any genomic engineering tools. Our work reveals the high-resolution structure of SPL and the TAF6L HEAT repeat domain that provides the SPL with a docking surface. We elucidate how SPL and the HEAT repeats are incorporated into SAGA. We identify major structural differences between TAF6L/TAF5L and their canonical paralogs that enable SPL accommodation. SPL engages SAGA through a substantially smaller interface than in U2snRNP, despite sharing a deeply inserted helical motif. The seemingly weaker interaction of SPL with SAGA raises the possibility that SAGA relays this module to the splicing machinery.

Sirtuins are a class of NAD-dependent histone deacetylases that regulate important biological pathways in prokaryotes and eukaryotes. This enzyme family comprises seven members, named SIRT1 to SIRT7. Among them, Sirtuin 6 (SIRT6) is a human sirtuin that deacetylates histones and plays a key role in DNA repair, telomere maintenance, carbohydrate and lipid metabolism, and lifespan. SIRT6’s structure consists of a zinc finger domain, a Rossmann fold domain containing the NAD+ binding site, and disordered N-terminal and C-terminal (CTD) extensions. The specific role of the CTD on SIRT6 interaction with nucleosomes for histone deacetylation remains unclear. Here, we resort to extended molecular dynamics simulations to uncover the dynamical behavior of the full-length SIRT6 bound to a nucleosome core particle. Our simulations reveal that the CTD preferentially interacts with DNA at the entry/exit near the enzyme’s docking site, exhibiting a variety of different binding modes. In specific cases, the CTD participates to the promotion of DNA unwrapping and promotes H3K27 accessibility to SIRT6’s active site, suggesting a pivotal role of this domain for H3K27 deacetylation. This work provides new structural insights into the binding process of the full-length SIRT6 to a nucleosome core particle, shedding light on the conformational behavior and functional role of its CTD. It constitutes an important step towards the understanding of SIRT6 deacetylation mechanisms and specificity.

Chromatin structure is a key regulator of DNA transcription, replication, and repair¹. In humans, the TIP60/EP400 complex (TIP60-C) is a 20-subunit assembly that impacts chromatin structure via two enzymatic activities: ATP-dependent exchange of histone H2A/H2B for H2A.Z/H2B and histone acetylation, which in yeast are carried out by two independent complexes, SWR1 and NuA4, respectively^2,3. How these activities are merged in humans into one super-complex and what this association entails for their structure, mechanism and recruitment to chromatin is unknown. Here we describe the 2.4-3.3 Å resolution structure of the endogenous human TIP60-C. We find a three lobed architecture composed of SWR1-like (SWR1L) and NuA4-like (NuA4L) parts, that associate with a TRRAP activator-binding module. The huge EP400 subunit harbors the ATPase motor, traverses twice the junction between SWR1L and NuA4L, and constitutes the scaffold of the three-lobed architecture. NuA4L is completely re-arranged compared to its yeast counterpart. TRRAP is flexibly tethered to NuA4L, in stark contrast to its robust connection to the complete opposite side of yeast NuA4^4-7. A modeled nucleosome bound to SWR1L, supported by activity tests, suggests that some aspects of the histone exchange mechanism diverge from the yeast example^8,9. Furthermore, a fixed actin module, as opposed to the mobile actin subcomplex in SWR1⁸, the flexibility of TRRAP and the weak effect of extra-nucleosomal DNA on exchange activity, lead to a different, activator-based, mode of enlisting TIP60-C to chromatin.

Sirtuin 6 (SIRT6) is an NAD⁺-dependent histone H3 deacetylase that is prominently found associated with chromatin, attenuates transcriptionally active promoters and regulates DNA repair, metabolic homeostasis and lifespan. Unlike other sirtuins, it has low affinity to free histone tails but demonstrates strong binding to nucleosomes. It is poorly understood how SIRT6 docking on nucleosomes stimulates its histone deacetylation activity. Here, we present the structure of human SIRT6 bound to a nucleosome determined by cryogenic electron microscopy. The zinc finger domain of SIRT6 associates tightly with the acidic patch of the nucleosome through multiple arginine anchors. The Rossmann fold domain binds to the terminus of the looser DNA half of the nucleosome, detaching two turns of the DNA from the histone octamer and placing the NAD⁺ binding pocket close to the DNA exit site. This domain shows flexibility with respect to the fixed zinc finger and moves with, but also relative to, the unwrapped DNA terminus. We apply molecular dynamics simulations of the histone tails in the nucleosome to show that in this mode of interaction, the active site of SIRT6 is perfectly poised to catalyze deacetylation of the H3 histone tail and that the partial unwrapping of the DNA allows even lysines close to the H3 core to reach the enzyme.

Sirtuin 6 (SIRT6) is an NAD+ dependent histone H3 deacetylase that is prominently found associated with chromatin, attenuates transcriptionally active promoters and regulates DNA repair, metabolic homeostasis and lifespan. Unlike other sirtuins, it has low affinity to free histone tails but demonstrates strong binding to nucleosomes. It is poorly understood how SIRT6 docking on nucleosomes stimulates its histone deacetylation activity. Here we present the structure of human SIRT6 bound to a nucleosome determined by cryogenic electron microscopy. The zinc finger domain of SIRT6 associates tightly with the acidic patch of the nucleosome through multiple arginine anchors. The Rossmann fold domain binds to the terminus of the looser DNA half of the nucleosome, detaching two turns of the DNA from the histone octamer and placing the NAD+ binding pocket close to the DNA exit site. This domain shows flexibility with respect to the fixed zinc finger and moves with, but also relative to, the unwrapped DNA terminus. We apply molecular dynamics simulations of the histone tails in the nucleosome to show that in this mode of interaction, the active site of SIRT6 is perfectly poised to catalyze deacetylation of the H3 histone tail and that the partial unwrapping of the DNA allows even lysines close to the H3 core to reach the enzyme.

Histone acetylation regulates most DNA transactions and is dynamically controlled by highly conserved enzymes. The only essential histone acetyltransferase (HAT) in yeast, Esa1, is part of the 1-MDa NuA4 complex, which plays pivotal roles in both transcription and DNA-damage repair. NuA4 has the unique capacity to acetylate histone targets located several nucleosomes away from its recruitment site. Neither the molecular mechanism of this activity nor its physiological importance are known. Here we report the structure of the Pichia pastoris NuA4 complex, with its core resolved at 3.4-Å resolution. Three subunits, Epl1, Eaf1 and Swc4, intertwine to form a stable platform that coordinates all other modules. The HAT module is firmly anchored into the core while retaining the ability to stretch out over a long distance. We provide structural, biochemical and genetic evidence that an unfolded linker region of the Epl1 subunit is critical for this long-range activity. Specifically, shortening the Epl1 linker causes severe growth defects and reduced H4 acetylation levels over broad chromatin regions in fission yeast. Our work lays the foundations for a mechanistic understanding of NuA4’s regulatory role and elucidates how its essential long-range activity is attained.

Sirtuin 6 (SIRT6) is an NAD ⁺ dependent histone H3 deacetylase that is prominently found associated with chromatin, attenuates transcriptionally active promoters and regulates DNA repair, metabolic homeostasis and lifespan. Unlike other sirtuins, it has low affinity to free histone tails but demonstrates strong binding to nucleosomes. It is poorly understood how SIRT6 docking on nucleosomes stimulates its histone deacetylation activity. Here we present the structure of human SIRT6 bound to a nucleosome determined by cryogenic electron microscopy. The zinc finger domain of SIRT6 associates tightly with the acidic patch of the nucleosome through multiple arginine anchors. The Rossmann fold domain binds to the terminus of the looser DNA half of the nucleosome, detaching two turns of the DNA from the histone octamer and placing the NAD ⁺ binding pocket close to the DNA exit site. This domain shows flexibility with respect to the fixed zinc finger and moves with, but also relative to, the unwrapped DNA terminus. We apply molecular dynamics simulations of the histone tails in the nucleosome to show that in this mode of interaction, the active site of SIRT6 is perfectly poised to catalyze deacetylation of the H3 histone tail and that the partial unwrapping of the DNA allows even lysines close to the H3 core to reach the enzyme.

In eukaryotes, transcription of protein encoding genes is initiated by the controlled deposition of the TATA‐box binding protein TBP onto gene promoters, followed by the ordered assembly of a pre‐initiation complex. The SAGA co‐activator is a 19‐subunit complex that stimulates transcription by the action of two chromatin‐modifying enzymatic modules, a transcription activator binding module, and by delivering TBP. Recent cryo electron microscopy structures of yeast SAGA with bound nucleosome or TBP reveal the architecture of the different functional domains of the co‐activator. An octamer of histone fold domains is found at the core of SAGA. This octamer, which deviates considerably from the symmetrical analogue forming the nucleosome, establishes a peripheral site for TBP binding where steric hindrance represses interaction with spurious DNA. The structures point to a mechanism for TBP delivery and release from SAGA that requires TFIIA and whose efficiency correlates with the affinity of DNA to TBP. These results provide a structural basis for understanding specific TBP delivery onto gene promoters and the role played by SAGA in regulating gene expression. The properties of the TBP delivery machine harboured by SAGA are compared with the TBP loading device present in the TFIID complex and show multiple similitudes.

SAGA (Spt–Ada–Gcn5–acetyltransferase) is a 19-subunit complex that stimulates transcription via two chromatin-modifying enzymatic modules and by delivering the TATA box binding protein (TBP) to nucleate the pre-initiation complex on DNA, a pivotal event in the expression of protein-encoding genes¹. Here we present the structure of yeast SAGA with bound TBP. The core of the complex is resolved at 3.5 Å resolution (0.143 Fourier shell correlation). The structure reveals the intricate network of interactions that coordinate the different functional domains of SAGA and resolves an octamer of histone-fold domains at the core of SAGA. This deformed octamer deviates considerably from the symmetrical analogue in the nucleosome and is precisely tuned to establish a peripheral site for TBP, where steric hindrance represses binding of spurious DNA. Complementary biochemical analysis points to a mechanism for TBP delivery and release from SAGA that requires transcription factor IIA and whose efficiency correlates with the affinity of DNA to TBP. We provide the foundations for understanding the specific delivery of TBP to gene promoters and the multiple roles of SAGA in regulating gene expression.