Patrick Schultz

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.

Deep learning methods using convolutional neural networks are very effective for automatic image segmentation tasks with no exception for cellular electron micrographs. However, the lack of dedicated easy‐to‐use tools largely reduces the widespread use of these techniques. Here we present DeepSCEM, a straightforward tool for fast and efficient segmentation of cellular electron microscopy images using deep learning with a special focus on efficient and user‐friendly generation and training of models for organelle segmentation.

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.

Advances in cryo‐electron microscopy (EM) enable imaging of protein assemblies within mammalian cells in a near native state when samples are preserved by cryogenic vitrification. To accompany this progress, specialized EM labelling protocols must be developed. Gold nanoparticles (AuNPs) of 2 nm are synthesized and functionalized to bind selected intracellular targets inside living human cells and to be detected in vitreous sections. As a proof of concept, thioaminobenzoate‐, thionitrobenzoate‐coordinated gold nanoparticles are functionalized on their surface with SV40 Nuclear Localization Signal (NLS)‐containing peptides and 2 kDa polyethyleneglycols (PEG) by thiolate exchange to target the importin‐mediated nuclear machinery and facilitate cytosolic diffusion by shielding the AuNP surface from non‐specific binding to cell components, respectively. After delivery by electroporation into the cytoplasm of living human cells, the PEG‐coated AuNPs diffuse freely in the cytoplasm but do not enter the nucleus. Incorporation of NLS within the PEG coverage promotes a quick nuclear import of the nanoparticles in relation to the density of NLS onto the AuNPs. Cryo‐EM of vitreous cell sections demonstrate the presence of 2 nm AuNPs as single entities in the nucleus. Biofunctionalized AuNPs combined with live‐cell electroporation procedures are thus potent labeling tools for the identification of macromolecules in cellular cryo‐EM.

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.