Sharing the SAGA

De novo variants in ATXN7L3 lead to developmental delay, hypotonia and distinctive facial features

Deubiquitination is crucial for the proper functioning of numerous biological pathways, such as DNA repair, cell cycle progression, transcription, signal transduction and autophagy. Accordingly, pathogenic variants in deubiquitinating enzymes (DUBs) have been implicated in neurodevelopmental disorders and congenital abnormalities. ATXN7L3 is a component of the DUB module of the Spt-Ada-Gcn5 acetyltransferase (SAGA) complex and two other related DUB modules, and it serves as an obligate adaptor protein of three ubiquitin-specific proteases (USP22, USP27X or USP51). Through exome sequencing and by using GeneMatcher, we identified nine individuals with heterozygous variants in ATXN7L3. The core phenotype included global motor and language developmental delay, hypotonia and distinctive facial characteristics, including hypertelorism, epicanthal folds, blepharoptosis, a small nose and mouth, and low-set, posteriorly rotated ears. To assess pathogenicity, we investigated the effects of a recurrent nonsense variant [c.340C>T; p.(Arg114Ter)] in fibroblasts of an affected individual. ATXN7L3 protein levels were reduced, and deubiquitylation was impaired, as indicated by an increase in histone H2Bub1 levels. This is consistent with the previous observation of increased H2Bub1 levels in Atxn7l3-null mouse embryos, which have developmental delay and embryonic lethality. In conclusion, we present clinical information and biochemical characterization supporting ATXN7L3 variants in the pathogenesis of a rare syndromic neurodevelopmental disorder.

The KAT module of the SAGA complex maintains the oncogenic gene expression program in MYCN-amplified neuroblastoma

Pediatric cancers are frequently driven by genomic alterations that result in aberrant transcription factor activity. Here, we used functional genomic screens to identify multiple genes within the transcriptional coactivator Spt-Ada-Gcn5-acetyltransferase (SAGA) complex as selective dependencies for MYCN-amplified neuroblastoma, a disease of dysregulated development driven by an aberrant oncogenic transcriptional program. We characterized the DNA recruitment sites of the SAGA complex in neuroblastoma and the consequences of loss of SAGA complex lysine acetyltransferase (KAT) activity on histone acetylation and gene expression. We demonstrate that loss of SAGA complex KAT activity is associated with reduced MYCN binding on chromatin, suppression of MYC/MYCN gene expression programs, and impaired cell cycle progression. Further, we showed that the SAGA complex is pharmacologically targetable in vitro and in vivo with a KAT2A/KAT2B proteolysis targeting chimeric. Our findings expand our understanding of the histone-modifying complexes that maintain the oncogenic transcriptional state in this disease and suggest therapeutic potential for inhibitors of SAGA KAT activity in MYCN-amplified neuroblastoma.

Differing SAGA module requirements for NCR-sensitive gene transcription in yeast

Nitrogen catabolite repression (NCR) is a means for yeast to adapt its transcriptome to changing nitrogen sources in its environment. In conditions of derepression (under poor nitrogen conditions, upon rapamycin treatment, or when glutamine production is inhibited), two transcriptional activators of the GATA family are recruited to NCR-sensitive promoters and activate transcription of NCR-sensitive genes. Earlier observations have involved the Spt-Ada-Gcn5 acetyltransferase (SAGA) chromatin remodeling complex in these transcriptional regulations. In this report, we provide an illustration of the varying NCR-sensitive responses and question whether differing SAGA recruitment could explain this diversity of responses.

Regulation of Disease-Resistance Genes against CWMV Infection by NbHAG1-Mediated H3K36ac

Post-translational modification of proteins plays a critical role in plant-pathogen interactions. Here, we demonstrate in Nicotiana benthamiana that knockout of NbHAG1 promotes Chinese wheat mosaic virus (CWMV) infection, whereas NbHAG1 overexpression inhibits infection. Transcriptome sequencing indicated that a series of disease resistance-related genes were up-regulated after overexpression of NbHAG1. In addition, cleavage under targets and tagmentation (Cut&Tag)-qPCR results demonstrated that NbHAG1 may activate the transcription of its downstream disease-resistance genes by facilitating the acetylation level of H3K36ac. Therefore, we suggest that NbHAG1 is an important positive regulator of resistance to CWMV infestation.

Co-translational Assembly Pathways of Nuclear Multiprotein Complexes Involved in the Regulation of Gene Transcription

Most factors that regulate gene transcription in eukaryotic cells are multimeric, often large, protein complexes. The understanding of the biogenesis pathways of such large and heterogeneous protein assemblies, as well as the dimerization partner choice among transcription factors, is crucial to interpret and control gene expression programs and consequent cell fate decisions. Co-translational assembly (Co-TA) is thought to play key roles in the biogenesis of protein complexes by directing complex formation during protein synthesis. In this review we discuss the principles of Co-TA with a special focus for the assembly of transcription regulatory complexes. We outline the expected molecular advantages of establishing co-translational interactions, pointing at the available, or missing, evidence for each of them. We hypothesize different molecular mechanisms based on Co-TA to explain the allocation "dilemma" of paralog proteins and subunits shared by different transcription complexes. By taking as a paradigm the different assembly pathways employed by three related transcription regulatory complexes (TFIID, SAGA and ATAC), we discuss alternative Co-TA strategies for nuclear multiprotein complexes and the widespread - yet specific - use of Co-TA for the formation of nuclear complexes involved in gene transcription. Ultimately, we outlined a series of open questions which demand well-defined lines of research to investigate the principles of gene regulation that rely on the coordinated assembly of protein complexes.

Histone acetylation and deacetylation - Mechanistic insights from structural biology

Histones are subject to a diverse array of post-translational modifications. Among them, lysine acetylation is not only the most pervasive and dynamic modification but also highly consequential for regulating gene transcription. Although enzymes responsible for the addition and removal of acetyl groups were discovered almost 30 years ago, high-resolution structures of the enzymes in the context of their native complexes are only now beginning to become available, thanks to revolutionary technologies in protein structure determination and prediction. Here, we will review our current understanding of the molecular mechanisms of acetylation and deacetylation engendered by chromatin-modifying complexes, compare and contrast shared features, and discuss some of the pressing questions for future studies.

SAGA Complex Subunit Hfi1 Is Important in the Stress Response and Pathogenesis of Cryptococcus neoformans

The Spt-Ada-Gcn Acetyltransferase (SAGA) complex is a highly conserved co-activator found across eukaryotes. It is composed of a number of modules which can vary between species, but all contain the core module. Hfi1 (known as TADA1 in Homo sapiens) is one of the proteins that forms the core module, and has been shown to play an important role in maintaining complex structural integrity in both brewer's yeast and humans. In this study we successfully identified the gene encoding this protein in the important fungal pathogen, Cryptococcus neoformans, and named it HFI1. The hfi1Δ mutant is highly pleiotropic in vitro, influencing phenotypes, ranging from temperature sensitivity and melanin production to caffeine resistance and titan cell morphogenesis. In the absence of Hfi1, the transcription of several other SAGA genes is impacted, as is the acetylation and deubiquination of several histone residues. Importantly, loss of the gene significantly impacts virulence in a murine inhalation model of cryptococcosis. In summary, we have established that Hfi1 modulates multiple pathways that directly affect virulence and survival in C. neoformans, and provided deeper insight into the importance of the non-enzymatic components of the SAGA complex.

Functional implications of paralog genes in polyglutamine spinocerebellar ataxias

Polyglutamine (polyQ) spinocerebellar ataxias (SCAs) comprise a group of autosomal dominant neurodegenerative disorders caused by (CAG/CAA)n expansions. The elongated stretches of adjacent glutamines alter the conformation of the native proteins inducing neurotoxicity, and subsequent motor and neurological symptoms. Although the etiology and neuropathology of most polyQ SCAs have been extensively studied, only a limited selection of therapies is available. Previous studies on SCA1 demonstrated that ATXN1L, a human duplicated gene of the disease-associated ATXN1, alleviated neuropathology in mice models. Other SCA-associated genes have paralogs (i.e., copies at different chromosomal locations derived from duplication of the parental gene), but their functional relevance and potential role in disease pathogenesis remain unexplored. Here, we review the protein homology, expression pattern, and molecular functions of paralogs in seven polyQ dominant ataxias-SCA1, SCA2, MJD/SCA3, SCA6, SCA7, SCA17, and DRPLA. Besides ATXN1L, we highlight ATXN2L, ATXN3L, CACNA1B, ATXN7L1, ATXN7L2, TBPL2, and RERE as promising functional candidates to play a role in the neuropathology of the respective SCA, along with the parental gene. Although most of these duplicates lack the (CAG/CAA)n region, if functionally redundant, they may compensate for a partial loss-of-function or dysfunction of the wild-type genes in SCAs. We aim to draw attention to the hypothesis that paralogs of disease-associated genes may underlie the complex neuropathology of dominant ataxias and potentiate new therapeutic strategies.

RNA polymerase II transcription with partially assembled TFIID complexes

The recognition of core promoter sequences by the general transcription factor TFIID is the first step in the process of RNA polymerase II (Pol II) transcription initiation. Metazoan holo-TFIID is composed of the TATA binding protein (TBP) and of 13 TBP associated factors (TAFs). Inducible Taf7 knock out (KO) results in the formation of a Taf7-less TFIID complex, while Taf10 KO leads to serious defects within the TFIID assembly pathway. Either TAF7 or TAF10 depletions correlate with the detected TAF occupancy changes at promoters, and with the distinct phenotype severities observed in mouse embryonic stem cells or mouse embryos. Surprisingly however, under either Taf7 or Taf10 deletion conditions, TBP is still associated to the chromatin, and no major changes are observed in nascent Pol II transcription. Thus, partially assembled TFIID complexes can sustain Pol II transcription initiation, but cannot replace holo-TFIID over several cell divisions and/or development.

SGF29 nuclear condensates reinforce cellular aging

Phase separation, a biophysical segregation of subcellular milieus referred as condensates, is known to regulate transcription, but its impacts on physiological processes are less clear. Here, we demonstrate the formation of liquid-like nuclear condensates by SGF29, a component of the SAGA transcriptional coactivator complex, during cellular senescence in human mesenchymal progenitor cells (hMPCs) and fibroblasts. The Arg 207 within the intrinsically disordered region is identified as the key amino acid residue for SGF29 to form phase separation. Through epigenomic and transcriptomic analysis, our data indicated that both condensate formation and H3K4me3 binding of SGF29 are essential for establishing its precise chromatin location, recruiting transcriptional factors and co-activators to target specific genomic loci, and initiating the expression of genes associated with senescence, such as CDKN1A. The formation of SGF29 condensates alone, however, may not be sufficient to drive H3K4me3 binding or achieve transactivation functions. Our study establishes a link between phase separation and aging regulation, highlighting nuclear condensates as a functional unit that facilitate shaping transcriptional landscapes in aging.

ATAC and SAGA co-activator complexes utilize co-translational assembly, but their cellular localization properties and functions are distinct

To understand the function of multisubunit complexes, it is of key importance to uncover the precise mechanisms that guide their assembly. Nascent proteins can find and bind their interaction partners during their translation, leading to co-translational assembly. Here, we demonstrate that the core modules of ATAC (ADA-two-A-containing) and SAGA (Spt-Ada-Gcn5-acetyltransferase), two lysine acetyl transferase-containing transcription co-activator complexes, assemble co-translationally in the cytoplasm of mammalian cells. In addition, a SAGA complex containing all of its modules forms in the cytoplasm and acetylates non-histone proteins. In contrast, ATAC complex subunits cannot be detected in the cytoplasm of mammalian cells. However, an endogenous ATAC complex containing two functional modules forms and functions in the nucleus. Thus, the two related co-activators, ATAC and SAGA, assemble using co-translational pathways, but their subcellular localization, cytoplasmic abundance, and functions are distinct.

ATAC and SAGA coactivator complexes utilize co-translational assembly, but their cellular localization properties and functions are distinct

To understand the function of multisubunit complexes it is of key importance to uncover the precise mechanisms that guide their assembly. Nascent proteins can find and bind their interaction partners during their translation, leading to co-translational assembly. Here we demonstrate that the core modules of ATAC (ADA-Two-A-Containing) and SAGA (Spt-Ada-Gcn5-acetyltransferase), two lysine acetyl transferase-containing transcription coactivator complexes, assemble co-translationally in the cytoplasm of mammalian cells. In addition, SAGA complex containing all of its modules forms in the cytoplasm and acetylates non-histones proteins. In contrast, fully assembled ATAC complex cannot be detected in the cytoplasm of mammalian cells. However, endogenous ATAC complex containing two functional modules forms and functions in the nucleus. Thus, the two related coactivators, ATAC and SAGA, assemble by using co-translational pathways, but their subcellular localization, cytoplasmic abundance and functions are distinct.

Hierarchical TAF1-dependent co-translational assembly of the basal transcription factor TFIID

Large heteromeric multiprotein complexes play pivotal roles at every step of gene expression in eukaryotic cells. Among them, the 20-subunit basal transcription factor TFIID nucleates the RNA polymerase II preinitiation complex at gene promoters. Here, by combining systematic RNA-immunoprecipitation (RIP) experiments, single-molecule imaging, proteomics and structure-function analyses, we show that human TFIID biogenesis occurs co-translationally. We discovered that all protein heterodimerization steps happen during protein synthesis. We identify TAF1-the largest protein in the complex-as a critical factor for TFIID assembly. TAF1 acts as a flexible scaffold that drives the co-translational recruitment of TFIID submodules preassembled in the cytoplasm. Altogether, our data suggest a multistep hierarchical model for TFIID biogenesis that culminates with the co-translational assembly of the complex onto the nascent TAF1 polypeptide. We envision that this assembly strategy could be shared with other large heteromeric protein complexes.

Chromatin remodeling by Pol II primes efficient Pol III transcription

The packaging of the genetic material into chromatin imposes the remodeling of this barrier to allow efficient transcription. RNA polymerase II activity is coupled with several histone modification complexes that enforce remodeling. How RNA polymerase III (Pol III) counteracts the inhibitory effect of chromatin is unknown. We report here a mechanism where RNA Polymerase II (Pol II) transcription is required to prime and maintain nucleosome depletion at Pol III loci and contributes to efficient Pol III recruitment upon re-initiation of growth from stationary phase in Fission yeast. The Pcr1 transcription factor participates in the recruitment of Pol II, which affects local histone occupancy through the associated SAGA complex and a Pol II phospho-S2 CTD / Mst2 pathway. These data expand the central role of Pol II in gene expression beyond mRNA synthesis.

Re-recognition of innate immune memory as an integrated multidimensional concept

In the past decade, the concept of immunological memory, which has long been considered a phenomenon observed in the adaptive immunity of vertebrates, has been extended to the innate immune system of various organisms. This de novo immunological memory is mainly called "innate immune memory", "immune priming", or "trained immunity" and has received increased attention because of its potential for clinical and agricultural applications. However, research on different species, especially invertebrates and vertebrates, has caused controversy regarding this concept. Here we discuss the current studies focusing on this immunological memory and summarize several mechanisms underlying it. We propose "innate immune memory" as a multidimensional concept as an integration between the seemingly different immunological phenomena.

Targeting ATM and ATR for cancer therapeutics: inhibitors in clinic

The DNA Damage and Response (DDR) pathway ensures accurate information transfer from one generation to the next. Alterations in DDR functions have been connected to cancer predisposition, progression, and response to therapy. DNA double-strand break (DSB) is one of the most detrimental DNA defects, causing major chromosomal abnormalities such as translocations and deletions. ATR and ATM kinases recognize this damage and activate proteins involved in cell cycle checkpoint, DNA repair, and apoptosis. Cancer cells have a high DSB burden, and therefore rely on DSB repair for survival. Therefore, targeting DSB repair can sensitize cancer cells to DNA-damaging agents. This review focuses on ATM and ATR, their roles in DNA damage and repair pathways, challenges in targeting them, and inhibitors that are in current clinical trials.

Chemical Genetics Screen of EVP4593 Sensitivity in Budding Yeast Identifies Effects on Mitochondrial Structure and Function

Mitochondria are essential eukaryotic organelles. Mitochondrial dysfunction can lead to mitochondrial myopathies and may contribute to neurodegenerative diseases, cancer, and diabetes. EVP4593, a 6-aminoquinazoline derivative with therapeutic potential, has been shown to inhibit NADH-ubiquinone oxidoreductase (Complex I) of the mitochondrial electron transport chain, causing the release of reactive oxygen species (ROS) and a reduction in ATP synthesis. In isolated mitochondria, EVP4593 inhibits respiration in the nanomolar range (IC 50 = 14-25 nM). However, other EVP4593-specific effects on biological processes have also been described. Consistent with an effect on mitochondrial function in budding yeast, we find that EVP4593 [>25µM] induces a pronounced growth defect when wildtype cells are grown on a non-fermentable carbon source. This sensitivity to EVP4593 is exacerbated by deletion of PDR5 , an ABC transporter that confers multidrug resistance. To better understand the cellular pathways and processes affected by EVP4593, we conducted a genome-wide chemical genetics screen of the yeast knockout collection. The objective was to identify yeast gene deletion strains that exhibit growth defects when subjected to a sublethal concentration of EVP4593 [15µM]. Our screen identified 21 yeast genes that are required for resistance to 15µM EVP4593 in glycerol-containing media. The genes identified in our screen are functionally involved in several distinct categories including mitochondrial structure and function, translational regulation and nutritional sensing, cellular stress response and detoxification. Additionally, we identified cellular phenotypes associated with the exposure to EVP4593, including changes in mitochondrial structure. In conclusion, our study represents the first genome-wide screen in yeast to identify the genetic pathways and cell-protective mechanisms involved in EVP4593 resistance and reveals that this small molecule inhibitor affects both mitochondrial structure and function.

Epigenetic moonlighting: Catalytic-independent functions of histone modifiers in regulating transcription

The past three decades have yielded a wealth of information regarding the chromatin regulatory mechanisms that control transcription. The "histone code" hypothesis-which posits that distinct combinations of posttranslational histone modifications are "read" by downstream effector proteins to regulate gene expression-has guided chromatin research to uncover fundamental mechanisms relevant to many aspects of biology. However, recent molecular and genetic studies revealed that the function of many histone-modifying enzymes extends independently and beyond their catalytic activities. In this review, we highlight original and recent advances in the understanding of noncatalytic functions of histone modifiers. Many of the histone modifications deposited by these enzymes-previously considered to be required for transcriptional activation-have been demonstrated to be dispensable for gene expression in living organisms. This perspective aims to prompt further examination of these enigmatic chromatin modifications by inspiring studies to define the noncatalytic "epigenetic moonlighting" functions of chromatin-modifying enzymes.

Recessive embryonic lethal mutations uncovered in heterozygous condition in silkworm semiconsomic strains

In this study, we found two embryonic lethal mutations, t04 lethal (l-t04) and m04 lethal (l-m04), in semiconsomic strains T04 and M04, respectively. In these semiconsomic strains, the entire diploid genome, except for one chromosome 4 of the wild silkworm Bombyx mandarina, is substituted with chromosomes of the domesticated silkworm B. mori, and l-t04 and l-m04 mutations are located on B. mandarina-derived chromosome 4. To clarify the cause of the lethalities and the genes responsible for these mutations, positional cloning and CRISPR/Cas9 mediated knockout screening were performed. Finally, genetic complementation tests identified the mutations responsible for the l-t04 and l-m04 as the Bombyx homolog of imaginal discs arrested (Bmida) and TATA box binding protein-associated factor 5 (BmTaf5), respectively. Lethal stages of each knockout mutant indicated the importance of these genes in B. mori late embryogenesis. The lethal mutations responsible for l-t04 and l-m04 were not found in parental strains or wild B. mandarina collected from 39 distinct locations in Japan, indicating that both mutations were independently introduced during or after the development of the semiconsomic strains. We conclude that the recessive embryonic lethality in the T04 and M04 strains is due to deleterious mutations produced in B. mandarina-derived chromosome 4.

Conserved and plant-specific histone acetyltransferase complexes cooperate to regulate gene transcription and plant development

Although a conserved SAGA complex containing the histone acetyltransferase GCN5 is known to mediate histone acetylation and transcriptional activation in eukaryotes, how to maintain different levels of histone acetylation and transcription at the whole-genome level remains to be determined. Here we identify and characterize a plant-specific GCN5-containing complex, which we term PAGA, in Arabidopsis thaliana and Oryza sativa. In Arabidopsis, the PAGA complex consists of two conserved subunits (GCN5 and ADA2A) and four plant-specific subunits (SPC, ING1, SDRL and EAF6). We find that PAGA and SAGA can independently mediate moderate and high levels of histone acetylation, respectively, thereby promoting transcriptional activation. Moreover, PAGA and SAGA can also repress gene transcription via the antagonistic effect between PAGA and SAGA. Unlike SAGA, which regulates multiple biological processes, PAGA is specifically involved in plant height and branch growth by regulating the transcription of hormone biosynthesis and response related genes. These results reveal how PAGA and SAGA cooperate to regulate histone acetylation, transcription and development. Given that the PAGA mutants show semi-dwarf and increased branching phenotypes without reduction in seed yield, the PAGA mutations could potentially be used for crop improvement.

The SAGA histone acetyltransferase module targets SMC5/6 to specific genes

BACKGROUND

Structural Maintenance of Chromosomes (SMC) complexes are molecular machines driving chromatin organization at higher levels. In eukaryotes, three SMC complexes (cohesin, condensin and SMC5/6) play key roles in cohesion, condensation, replication, transcription and DNA repair. Their physical binding to DNA requires accessible chromatin.

RESULTS

We performed a genetic screen in fission yeast to identify novel factors required for SMC5/6 binding to DNA. We identified 79 genes of which histone acetyltransferases (HATs) were the most represented. Genetic and phenotypic analyses suggested a particularly strong functional relationship between the SMC5/6 and SAGA complexes. Furthermore, several SMC5/6 subunits physically interacted with SAGA HAT module components Gcn5 and Ada2. As Gcn5-dependent acetylation facilitates the accessibility of chromatin to DNA-repair proteins, we first analysed the formation of DNA-damage-induced SMC5/6 foci in the Δgcn5 mutant. The SMC5/6 foci formed normally in Δgcn5, suggesting SAGA-independent SMC5/6 localization to DNA-damaged sites. Next, we used Nse4-FLAG chromatin-immunoprecipitation (ChIP-seq) analysis in unchallenged cells to assess SMC5/6 distribution. A significant portion of SMC5/6 accumulated within gene regions in wild-type cells, which was reduced in Δgcn5 and Δada2 mutants. The drop in SMC5/6 levels was also observed in gcn5-E191Q acetyltransferase-dead mutant.

CONCLUSION

Our data show genetic and physical interactions between SMC5/6 and SAGA complexes. The ChIP-seq analysis suggests that SAGA HAT module targets SMC5/6 to specific gene regions and facilitates their accessibility for SMC5/6 loading.

The Gcn5-Ada2-Ada3 histone acetyltransferase module has divergent roles in pathogenesis of Candida glabrata

Candida glabrata is an opportunistic fungal pathogen and the second most prevalent species isolated from candidiasis patients. C. glabrata has intrinsic tolerance to antifungal drugs and oxidative stresses and the ability to adhere to mucocutaneous surfaces. However, knowledge about the regulation of its virulence traits is limited. The Spt-Ada-Gcn5 acetyltransferase (SAGA) complex modulates gene transcription by histone acetylation through the histone acetyltransferase (HAT) module comprised of Gcn5-Ada2-Ada3. Previously, we showed that the ada2 mutant was hypervirulent but displayed decreased tolerance to antifungal drugs and cell wall perturbing agents. In this study, we further characterized the functions of Ada3 and Gcn5 in C. glabrata. We found that single, double, or triple deletions of the HAT module, as expected, resulted in a decreased level of acetylation on histone H3 lysine 9 (H3K9) and defective growth. These mutants were more susceptible to antifungal drugs, oxidative stresses, and cell wall perturbing agents compared with the wild-type. In addition, HAT module mutants exhibited enhanced agar invasion and upregulation of adhesin and proteases encoding genes, whereas the biofilm formation of those mutants was impaired. Interestingly, HAT module mutants exhibited enhanced induction of catalases (CTA1) expression upon treatment with H2O2 compared with the wild-type. Lastly, although ada3 and gcn5 exhibited marginal hypervirulence, the HAT double and triple mutants were hypervirulent in a murine model of candidiasis. In conclusion, the HAT module of the SAGA complex plays unique roles in H3K9 acetylation, drug tolerance, oxidative stress response, adherence, and virulence in C. glabrata.

Perturbed fatty-acid metabolism is linked to localized chromatin hyperacetylation, increased stress-response gene expression and resistance to oxidative stress

Oxidative stress is associated with cardiovascular and neurodegenerative diseases, diabetes, cancer, psychiatric disorders and aging. In order to counteract, eliminate and/or adapt to the sources of stress, cells possess elaborate stress-response mechanisms, which also operate at the level of regulating transcription. Interestingly, it is becoming apparent that the metabolic state of the cell and certain metabolites can directly control the epigenetic information and gene expression. In the fission yeast Schizosaccharomyces pombe, the conserved Sty1 stress-activated protein kinase cascade is the main pathway responding to most types of stresses, and regulates the transcription of hundreds of genes via the Atf1 transcription factor. Here we report that fission yeast cells defective in fatty acid synthesis (cbf11, mga2 and ACC/cut6 mutants; FAS inhibition) show increased expression of a subset of stress-response genes. This altered gene expression depends on Sty1-Atf1, the Pap1 transcription factor, and the Gcn5 and Mst1 histone acetyltransferases, is associated with increased acetylation of histone H3 at lysine 9 in the corresponding gene promoters, and results in increased cellular resistance to oxidative stress. We propose that changes in lipid metabolism can regulate the chromatin and transcription of specific stress-response genes, which in turn might help cells to maintain redox homeostasis.

Polyglutamine-expanded ATXN7 alters a specific epigenetic signature underlying photoreceptor identity gene expression in SCA7 mouse retinopathy

BACKGROUND

Spinocerebellar ataxia type 7 (SCA7) is a neurodegenerative disorder that primarily affects the cerebellum and retina. SCA7 is caused by a polyglutamine expansion in the ATXN7 protein, a subunit of the transcriptional coactivator SAGA that acetylates histone H3 to deposit narrow H3K9ac mark at DNA regulatory elements of active genes. Defective histone acetylation has been presented as a possible cause for gene deregulation in SCA7 mouse models. However, the topography of acetylation defects at the whole genome level and its relationship to changes in gene expression remain to be determined.

METHODS

We performed deep RNA-sequencing and chromatin immunoprecipitation coupled to high-throughput sequencing to examine the genome-wide correlation between gene deregulation and alteration of the active transcription marks, e.g. SAGA-related H3K9ac, CBP-related H3K27ac and RNA polymerase II (RNAPII), in a SCA7 mouse retinopathy model.

RESULTS

Our analyses revealed that active transcription marks are reduced at most gene promoters in SCA7 retina, while a limited number of genes show changes in expression. We found that SCA7 retinopathy is caused by preferential downregulation of hundreds of highly expressed genes that define morphological and physiological identities of mature photoreceptors. We further uncovered that these photoreceptor genes harbor unusually broad H3K9ac profiles spanning the entire gene bodies and have a low RNAPII pausing. This broad H3K9ac signature co-occurs with other features that delineate superenhancers, including broad H3K27ac, binding sites for photoreceptor specific transcription factors and expression of enhancer-related non-coding RNAs (eRNAs). In SCA7 retina, downregulated photoreceptor genes show decreased H3K9 and H3K27 acetylation and eRNA expression as well as increased RNAPII pausing, suggesting that superenhancer-related features are altered.

CONCLUSIONS

Our study thus provides evidence that distinctive epigenetic configurations underlying high expression of cell-type specific genes are preferentially impaired in SCA7, resulting in a defect in the maintenance of identity features of mature photoreceptors. Our results also suggest that continuous SAGA-driven acetylation plays a role in preserving post-mitotic neuronal identity.

Structure of the NuA4 histone acetyltransferase complex

Nucleosome acetyltransferase of H4 (NuA4), one of two major histone acetyltransferase complexes in Saccharomyces cerevisiae specifically acetylates histone H2A and H4, resulting in increased transcriptional activity. Here we present a 3.8-4.0 Å resolution structure of the NuA4 complex from cryoelectron microscopy and associated biochemical studies. The determined structure comprises six subunits and appropriately 5,000 amino acids, with a backbone formed by subunits Eaf1 and Eaf2 spanning from an Actin-Arp4 module to a platform subunit Tra1. Seven subunits are missing from the cryo-EM map. The locations of missing components, Yaf9, and three subunits of the Piccolo module Esa1, Yng2, and Eaf6 were determined. Biochemical studies showed that the Piccolo module and the complete NuA4 exhibit comparable histone acetyltransferase activities, but the Piccolo module binds to nucleosomes, whereas the complete NuA4 does not. The interaction lifetime of NuA4 and nucleosome is evidently short, possibly because of subunits of the NuA4 complex that diminish the affinity of the Piccolo module for the nucleosome, enabling rapid movement from nucleosome to nucleosome.

Nuclear pore complex acetylation regulates mRNA export and cell cycle commitment in budding yeast

Nuclear pore complexes (NPCs) mediate communication between the nucleus and the cytoplasm, and regulate gene expression by interacting with transcription and mRNA export factors. Lysine acetyltransferases (KATs) promote transcription through acetylation of chromatin-associated proteins. We find that Esa1, the KAT subunit of the yeast NuA4 complex, also acetylates the nuclear pore basket component Nup60 to promote mRNA export. Acetylation of Nup60 recruits the mRNA export factor Sac3, the scaffolding subunit of the Transcription and Export 2 (TREX-2) complex, to the nuclear basket. The Esa1-mediated nuclear export of mRNAs in turn promotes entry into S phase, which is inhibited by the Hos3 deacetylase in G1 daughter cells to restrain their premature commitment to a new cell division cycle. This mechanism is not only limited to G1/S-expressed genes but also inhibits the expression of the nutrient-regulated GAL1 gene specifically in daughter cells. Overall, these results reveal how acetylation can contribute to the functional plasticity of NPCs in mother and daughter yeast cells. In addition, our work demonstrates dual gene expression regulation by the evolutionarily conserved NuA4 complex, at the level of transcription and at the stage of mRNA export by modifying the nucleoplasmic entrance to nuclear pores.

Regulation of jasmonate signaling by reversible acetylation of TOPLESS in Arabidopsis

The plant hormone jasmonate (JA) regulates plant immunity and adaptive growth by orchestrating a genome-wide transcriptional program. Key regulators of JA-responsive gene expression include the master transcription factor MYC2, which is repressed by the conserved Groucho/Tup1-like corepressor TOPLESS (TPL) in the resting state. However, the mechanisms underlying TPL-mediated transcriptional repression of MYC2 activity and hormone-dependent switching between repression and de-repression remain enigmatic. Here, we report the regulation of TPL activity and JA signaling by reversible acetylation of TPL. We found that the histone acetyltransferase GCN5 could mediate TPL acetylation, which enhances its interaction with the NOVEL-INTERACTOR-OF-JAZ (NINJA) adaptor and promotes its recruitment to MYC2 target promoters, facilitating transcriptional repression. Conversely, TPL deacetylation by the histone deacetylase HDA6 weakens TPL-NINJA interaction and inhibits TPL recruitment to MYC2 target promoters, facilitating transcriptional activation. In the resting state, the opposing activities of GCN5 and HDA6 maintain TPL acetylation homeostasis, promoting transcriptional repression activity of TPL. In response to JA elicitation, HDA6 expression is transiently induced, resulted in decreased TPL acetylation and repressor activity, thereby transcriptional activation of MYC2 target genes. Thus, the GCN5-TPL-HDA6 module maintains the homeostasis of acetylated TPL, thereby determining the transcriptional state of JA-responsive genes. Our findings uncovered a mechanism by which the TPL corepressor activity in JA signaling is actively tuned in a rapid and reversible manner.

KAT2B Gene Polymorphisms Are Associated with Body Measure Traits in Four Chinese Cattle Breeds

Simple Summary

Genetic improvement is one of the most important keys to overcoming the shortcomings of beef production. Identifying molecular markers related to growth characteristics and meat quality is significant in improving beef cattle breeds. Studies have shown that KAT2B, a transcriptional co-activator regulating the acetylation modification of histones, may be involved in the development and metabolism of muscle and adipose. However, there are no reports on investigating KAT2B genetic variation in Chinese native cattle. Firstly, this manuscript reports the initial bioinformatics analysis of KAT2B, finding that KAT2B protein is highly conserved among ruminants. The KAT2B gene expression profile in Qinchuan cattle was characterized, showing the spatiotemporal specificity of KAT2B gene expression in tissues such as adipose and liver during fetal and adult periods. Then, the investigation of KAT2B gene polymorphisms was carried out. Three SNPs of the KAT2B gene were identified and were found to be correlated with multiple body measurements in Fu, Qinchuan, Yak, and Chaidam cattle. These findings suggest that these three SNPs of KAT2B can serve as the molecular markers to select individuals for beef cattle breed improvement.

Abstract

Identifying molecular markers related to growth characteristics or meat quality is significant for improving beef cattle breeds. K(lysine) acetyltransferase 2B (KAT2B) is a transcriptional co-activator regulating the acetylation modification of histones, which may be involved in the development and metabolism of muscle and adipose. However, investigations of KAT2B genetic variations in Chinese native cattle are still limited. This study aimed to identify crucial single nucleotide polymorphisms (SNPs) influencing the body measurements of Chinese native cattle. Biological evolution and conservation analysis showed that KAT2B was highly conserved among the ruminants. By qPCR assay, KAT2B gene expression was found to be spatiotemporally specific in bovine tissues such as adipose and liver. By the RFLP-PCR method, three SNPs of KAT2B (g.T61908C, g.T62131C, and g.C73406T) were identified in 827 individuals of four Chinese cattle breeds, including Qinchuan (n = 658), Fu (n = 52), Yak (n = 48), and Chaidam (n = 69) cattle. Association analysis between these KAT2B polymorphisms and the body measurements of Chinese native cattle revealed significant observations. The genetic effects of g.T61908C, g.T62131C, and g.C73406T on the associated phenotypes were illustrated in each breed. In Qinchuan cattle, g.T62131C was significantly associated with better body height, chest width, hip width, and withers height, for which TC and/or TT were the advantageous genotype. In Fu cattle, TT genotype of g.T61908C was associated to better body length, while individuals with TT or CC of g.T62131C showed higher circumference of cannon bone than those with TC genotype. In Yak, individuals with TT genotype of g.C73406T had heavier body weight. In Chaidam cattle, TC genotype of g.C73406T was associated to superior body weight, while CC genotype of g.C73406T was associated to superior chest girth and circumference of cannon bone. These findings suggest that KAT2B gene polymorphisms can be used as the molecular markers for the early molecular marker-assisted selection in beef cattle breeding programs.

SUPT3H-less SAGA coactivator can assemble and function without significantly perturbing RNA polymerase II transcription in mammalian cells

Coactivator complexes regulate chromatin accessibility and transcription. SAGA (Spt-Ada-Gcn5 Acetyltransferase) is an evolutionary conserved coactivator complex. The core module scaffolds the entire SAGA complex and adopts a histone octamer-like structure, which consists of six histone-fold domain (HFD)-containing proteins forming three histone-fold (HF) pairs, to which the double HFD-containing SUPT3H adds one HF pair. Spt3, the yeast ortholog of SUPT3H, interacts genetically and biochemically with the TATA binding protein (TBP) and contributes to global RNA polymerase II (Pol II) transcription. Here we demonstrate that (i) SAGA purified from human U2OS or mouse embryonic stem cells (mESC) can assemble without SUPT3H, (ii) SUPT3H is not essential for mESC survival, but required for their growth and self-renewal, and (iii) the loss of SUPT3H from mammalian cells affects the transcription of only a specific subset of genes. Accordingly, in the absence of SUPT3H no major change in TBP accumulation at gene promoters was observed. Thus, SUPT3H is not required for the assembly of SAGA, TBP recruitment, or overall Pol II transcription, but plays a role in mESC growth and self-renewal. Our data further suggest that yeast and mammalian SAGA complexes contribute to transcription regulation by distinct mechanisms.

Genome-wide screening identifies Polycomb repressive complex 1.3 as an essential regulator of human naïve pluripotent cell reprogramming

Uncovering the mechanisms that establish naïve pluripotency in humans is crucial for the future applications of pluripotent stem cells including the production of human blastoids. However, the regulatory pathways that control the establishment of naïve pluripotency by reprogramming are largely unknown. Here, we use genome-wide screening to identify essential regulators as well as major impediments of human primed to naïve pluripotent stem cell reprogramming. We discover that factors essential for cell state change do not typically undergo changes at the level of gene expression but rather are repurposed with new functions. Mechanistically, we establish that the variant Polycomb complex PRC1.3 and PRDM14 jointly repress developmental and gene regulatory factors to ensure naïve cell reprogramming. In addition, small-molecule inhibitors of reprogramming impediments improve naïve cell reprogramming beyond current methods. Collectively, this work defines the principles controlling the establishment of human naïve pluripotency and also provides new insights into mechanisms that destabilize and reconfigure cell identity during cell state transitions.

The Verticillium dahliae Spt-Ada-Gcn5 Acetyltransferase Complex Subunit Ada1 Is Essential for Conidia and Microsclerotia Production and Contributes to Virulence

Verticillium dahliae is a destructive soil-borne pathogen of many economically important dicots. The genetics of pathogenesis in V. dahliae has been extensively studied. Spt-Ada-Gcn5 acetyltransferase complex (SAGA) is an ATP-independent multifunctional chromatin remodeling complex that contributes to diverse transcriptional regulatory functions. As members of the core module in the SAGA complex in Saccharomyces cerevisiae, Ada1, together with Spt7 and Spt20, play an important role in maintaining the integrity of the complex. In this study, we identified homologs of the SAGA complex in V. dahliae and found that deletion of the Ada1 subunit (VdAda1) causes severe defects in the formation of conidia and microsclerotia, and in melanin biosynthesis and virulence. The effect of VdAda1 on histone acetylation in V. dahliae was confirmed by western blot analysis. The deletion of VdAda1 resulted in genome-wide alteration of the V. dahliae transcriptome, including genes encoding transcription factors and secreted proteins, suggesting its prominent role in the regulation of transcription and virulence. Overall, we demonstrated that VdAda1, a member of the SAGA complex, modulates multiple physiological processes by regulating global gene expression that impinge on virulence and survival in V. dahliae.

The TRRAP transcription cofactor represses interferon-stimulated genes in colorectal cancer cells

Transcription is essential for cells to respond to signaling cues and involves factors with multiple distinct activities. One such factor, TRRAP, functions as part of two large complexes, SAGA and TIP60, which have crucial roles during transcription activation. Structurally, TRRAP belongs to the phosphoinositide 3 kinase-related kinases (PIKK) family but is the only member classified as a pseudokinase. Recent studies established that a dedicated HSP90 co-chaperone, the triple T (TTT) complex, is essential for PIKK stabilization and activity. Here, using endogenous auxin-inducible degron alleles, we show that the TTT subunit TELO2 promotes TRRAP assembly into SAGA and TIP60 in human colorectal cancer cells (CRCs). Transcriptomic analysis revealed that TELO2 contributes to TRRAP regulatory roles in CRC cells, most notably of MYC target genes. Surprisingly, TELO2 and TRRAP depletion also induced the expression of type I interferon genes. Using a combination of nascent RNA, antibody-targeted chromatin profiling (CUT&RUN), ChIP, and kinetic analyses, we propose a model by which TRRAP directly represses the transcription of IRF9, which encodes a master regulator of interferon-stimulated genes. We have therefore uncovered an unexpected transcriptional repressor role for TRRAP, which we propose contributes to its tumorigenic activity.

Acetylation-dependent SAGA complex dimerization promotes nucleosome acetylation and gene transcription

Cells reprogram their transcriptomes to adapt to external conditions. The SAGA (Spt-Ada-Gcn5 acetyltransferase) complex is a highly conserved transcriptional coactivator that plays essential roles in cell growth and development, in part by acetylating histones. Here, we uncover an autoregulatory mechanism of the Saccharomyces cerevisiae SAGA complex in response to environmental changes. Specifically, the SAGA complex acetylates its Ada3 subunit at three sites (lysines 8, 14 and 182) that are dynamically deacetylated by Rpd3. The acetylated Ada3 lysine residues are bound by bromodomains within SAGA subunits Gcn5 and Spt7 that synergistically facilitate formation of SAGA homo-dimers. Ada3-mediated dimerization is enhanced when cells are grown under sucrose or under phosphate-starvation conditions. Once dimerized, SAGA efficiently acetylates nucleosomes, promotes gene transcription and enhances cell resistance to stress. Collectively, our work reveals a mechanism for regulation of SAGA structure and activity and provides insights into how cells adapt to environmental conditions.

Now open: Evolving insights to the roles of lysine acetylation in chromatin organization and function

Protein acetylation is conserved across phylogeny and has been recognized as one of the most prominent post-translational modifications since its discovery nearly 60 years ago. Histone acetylation is an active mark characteristic of open chromatin, but acetylation on specific lysine residues and histone variants occurs in different biological contexts and can confer various outcomes. The significance of acetylation events is indicated by the associations of lysine acetyltransferases, deacetylases, and acetyl-lysine readers with developmental disorders and pathologies. Recent advances have uncovered new roles of acetylation regulators in chromatin-centric events, which emphasize the complexity of these functional networks. In this review, we discuss mechanisms and dynamics of acetylation in chromatin organization and DNA-templated processes, including gene transcription and DNA repair and replication.

A pan-CRISPR analysis of mammalian cell specificity identifies ultra-compact sgRNA subsets for genome-scale experiments

A genetic knockout can be lethal to one human cell type while increasing growth rate in another. This context specificity confounds genetic analysis and prevents reproducible genome engineering. Genome-wide CRISPR compendia across most common human cell lines offer the largest opportunity to understand the biology of cell specificity. The prevailing viewpoint, synthetic lethality, occurs when a genetic alteration creates a unique CRISPR dependency. Here, we use machine learning for an unbiased investigation of cell type specificity. Quantifying model accuracy, we find that most cell type specific phenotypes are predicted by the function of related genes of wild-type sequence, not synthetic lethal relationships. These models then identify unexpected sets of 100-300 genes where reduced CRISPR measurements can produce genome-scale loss-of-function predictions across >18,000 genes. Thus, it is possible to reduce in vitro CRISPR libraries by orders of magnitude—with some information loss—when we remove redundant genes and not redundant sgRNAs.

Context specificity confounds genetic analysis and prevents reproducible genome engineering. Here, the authors report a pan-CRISPR analysis of specificity in mammalian cells and identify ultra-compact sgRNA subsets for genome-scale screens.

TOR complex 2 contributes to regulation of gene expression via inhibiting Gcn5 recruitment to subtelomeric and DNA replication stress genes

The fission yeast TOR complex 2 (TORC2) is required for gene silencing at subtelomeric regions and for the induction of gene transcription in response to DNA replication stress. Thus, TORC2 affects transcription regulation both negatively and positively. Whether these two TORC2-dependent functions share a common molecular mechanism is currently unknown. Here, we show that Gad8 physically interacts with proteins that regulate transcription, including subunits of the Spt-Ada-Gcn5-acetyltransferase (SAGA) complex and the BET bromodomain protein Bdf2. We demonstrate that in the absence of TORC2, Gcn5, the histone acetyltransferase subunit of SAGA, accumulates at subtelomeric genes and at non-induced promoters of DNA replication genes. Remarkably, the loss of Gcn5 in TORC2 mutant cells restores gene silencing as well as transcriptional induction in response to DNA replication stress. Loss of Bdf2 alleviates excess of Gcn5 binding in TORC2 mutant cells and also rescues the aberrant regulation of transcription in these cells. Furthermore, the loss of either SAGA or Bdf2 suppresses the sensitivity of TORC2 mutant cells to a variety of stresses, including DNA replication, DNA damage, temperature and nutrient stresses. We suggest a role of TORC2 in transcriptional regulation that is critical for gene silencing and gene induction in response to stress and involves the binding of Gcn5 to the chromatin.

Secondary Metabolite Gene Regulation in Mycotoxigenic Fusarium Species: A Focus on Chromatin

Fusarium is a species-rich group of mycotoxigenic plant pathogens that ranks as one of the most economically important fungal genera in the world. During growth and infection, they are able to produce a vast spectrum of low-molecular-weight compounds, so-called secondary metabolites (SMs). SMs often comprise toxic compounds (i.e., mycotoxins) that contaminate precious food and feed sources and cause adverse health effects in humans and livestock. In this context, understanding the regulation of their biosynthesis is crucial for the development of cropping strategies that aim at minimizing mycotoxin contamination in the field. Nevertheless, currently, only a fraction of SMs have been identified, and even fewer are considered for regular monitoring by regulatory authorities. Limitations to exploit their full chemical potential arise from the fact that the genes involved in their biosynthesis are often silent under standard laboratory conditions and only induced upon specific stimuli mimicking natural conditions in which biosynthesis of the respective SM becomes advantageous for the producer. This implies a complex regulatory network. Several components of these gene networks have been studied in the past, thereby greatly advancing the understanding of SM gene regulation and mycotoxin biosynthesis in general. This review aims at summarizing the latest advances in SM research in these notorious plant pathogens with a focus on chromatin structure.

High-speed fluorescence image-enabled cell sorting

Fast and selective isolation of single cells with unique spatial and morphological traits remains a technical challenge. Here, we address this by establishing high-speed image-enabled cell sorting (ICS), which records multicolor fluorescence images and sorts cells based on measurements from image data at speeds up to 15,000 events per second. We show that ICS quantifies cell morphology and localization of labeled proteins and increases the resolution of cell cycle analyses by separating mitotic stages. We combine ICS with CRISPR-pooled screens to identify regulators of the nuclear factor κB (NF-κB) pathway, enabling the completion of genome-wide image-based screens in about 9 hours of run time. By assessing complex cellular phenotypes, ICS substantially expands the phenotypic space accessible to cell-sorting applications and pooled genetic screening.

KAT2A complexes ATAC and SAGA play unique roles in cell maintenance and identity in hematopoiesis and leukemia

Epigenetic histone modifiers are key regulators of cell fate decisions in normal and malignant hematopoiesis. Their enzymatic activities are of particular significance as putative therapeutic targets in leukemia. In contrast, less is known about the contextual role in which those enzymatic activities are exercised and specifically how different macromolecular complexes configure the same enzymatic activity with distinct molecular and cellular consequences. We focus on KAT2A, a lysine acetyltransferase responsible for histone H3 lysine 9 acetylation, which we recently identified as a dependence in acute myeloid leukemia stem cells and that participates in 2 distinct macromolecular complexes: Ada two-A-containing (ATAC) and Spt-Ada-Gcn5-Acetyltransferase (SAGA). Through analysis of human cord blood hematopoietic stem cells and progenitors, and of myeloid leukemia cells, we identify unique respective contributions of the ATAC complex to regulation of biosynthetic activity in undifferentiated self-renewing cells and of the SAGA complex to stabilization or correct progression of cell type-specific programs with putative preservation of cell identity. Cell type and stage-specific dependencies on ATAC and SAGA-regulated programs explain multilevel KAT2A requirements in leukemia and in erythroid lineage specification and development. Importantly, they set a paradigm against which lineage specification and identity can be explored across developmental stem cell systems.

P300/CBP-associated factor (PCAF)-mediated acetylation of Fascin at lysine 471 inhibits its actin-bundling activity and tumor metastasis in esophageal cancer

BACKGROUND

Fascin is crucial for cancer cell filopodium formation and tumor metastasis, and is functionally regulated by post-translational modifications. However, whether and how Fascin is regulated by acetylation remains unclear. This study explored the regulation of Fascin acetylation and its corresponding roles in filopodium formation and tumor metastasis.

METHODS

Immunoprecipitation and glutathione-S-transferase pull-down assays were performed to examine the interaction between Fascin and acetyltransferase P300/CBP-associated factor (PCAF), and immunofluorescence was used to investigate their colocalization. An in vitro acetylation assay was performed to identify Fascin acetylation sites by using mass spectrometry. A specific antibody against acetylated Fascin was generated and used to detect the PCAF-mediated Fascin acetylation in esophageal squamous cell carcinoma (ESCC) cells using Western blotting by overexpressing and knocking down PCAF expression. An in vitro cell migration assay was performed, and a xenograft model was established to study in vivo tumor metastasis. Live-cell imaging and fluorescence recovery after photobleaching were used to evaluate the function and dynamics of acetylated Fascin in filopodium formation. The clinical significance of acetylated Fascin and PCAF in ESCC was evaluated using immunohistochemistry.

RESULTS

Fascin directly interacted and colocalized with PCAF in the cytoplasm and was acetylated at lysine 471 (K471) by PCAF. Using the specific anti-AcK471-Fascin antibody, Fascin was found to be acetylated in ESCC cells, and the acetylation level was consequently increased after PCAF overexpression and decreased after PCAF knockdown. Functionally, Fascin-K471 acetylation markedly suppressed in vitro ESCC cell migration and in vivo tumor metastasis, whereas Fascin-K471 deacetylation exhibited a potent oncogenic function. Moreover, Fascin-K471 acetylation reduced filopodial length and density, and lifespan of ESCC cells, while its deacetylation produced the opposite effect. In the filipodium shaft, K471-acetylated Fascin displayed rapid dynamic exchange, suggesting that it remained in its monomeric form owing to its weakened actin-bundling activity. Clinically, high levels of AcK471-Fascin in ESCC tissues were strongly associated with prolonged overall survival and disease-free survival of ESCC patients.

CONCLUSIONS

Fascin interacts directly with PCAF and is acetylated at lysine 471 in ESCC cells. Fascin-K471 acetylation suppressed ESCC cell migration and tumor metastasis by reducing filopodium formation through the impairment of its actin-bundling activity.

Structure of the human SAGA coactivator complex

The SAGA complex is a regulatory hub involved in gene regulation, chromatin modification, DNA damage repair and signaling. While structures of yeast SAGA (ySAGA) have been reported, there are noteworthy functional and compositional differences for this complex in metazoans. Here we present the cryogenic-electron microscopy (cryo-EM) structure of human SAGA (hSAGA) and show how the arrangement of distinct structural elements results in a globally divergent organization from that of yeast, with a different interface tethering the core module to the TRRAP subunit, resulting in a dramatically altered geometry of functional elements and with the integration of a metazoan-specific splicing module. Our hSAGA structure reveals the presence of an inositol hexakisphosphate (InsP6) binding site in TRRAP and an unusual property of its pseudo-(Ψ)PIKK. Finally, we map human disease mutations, thus providing the needed framework for structure-guided drug design of this important therapeutic target for human developmental diseases and cancer.

Genome-wide CRISPR-Cas9 screens identify mechanisms of BET bromodomain inhibitor sensitivity

BET bromodomain inhibitors hold promise as therapeutic agents in diverse indications, but their clinical progression has been challenging and none have received regulatory approval. Early clinical trials in cancer have shown heterogeneous clinical responses, development of resistance, and adverse events. Increased understanding of their mechanism(s) of action and identification of biomarkers are needed to identify appropriate indication(s) and achieve efficacious dosing. Using genome-wide CRISPR-Cas9 screens at different concentrations, we report molecular mechanisms defining cellular responses to BET inhibitors, some of which appear specific to a single compound concentration. We identify multiple transcriptional regulators and mTOR pathway members as key determinants of JQ1 sensitivity and two Ca²⁺/Mn²⁺ transporters, ATP2C1 and TMEM165, as key determinants of JQ1 resistance. Our study reveals new molecular mediators of BET bromodomain inhibitor effects, suggests the involvement of manganese, and provides a rich resource for discovery of biomarkers and targets for combination therapies.

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CRISPR screens identify genes regulating sensitivity to BET bromodomain inhibitors

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Sensitivity and resistance hit lists are concentration-dependent

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mTOR pathway mediates sensitivity to BET bromodomain inhibitors

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Manganese regulates sensitivity to BET bromodomain inhibitors

Functional characterization of the developmental genes asm2, asm3, and spt3 required for fruiting body formation in the filamentous ascomycete Sordaria macrospora

The formation of fruiting bodies is one of the most complex developmental processes in filamentous ascomycetes. It requires the development of sexual structures that give rise to meiosporangia (asci) and meiotic spores (ascospores) as well as surrounding structures for protection and dispersal of the spores. Previous studies have shown that these developmental processes are accompanied by significant changes of the transcriptome, and comparative transcriptomics of different fungi as well as the analysis of transcriptome changes in developmental mutants have aided in the identification of differentially regulated genes that are themselves involved in regulating fruiting body development. In previous analyses, we used transcriptomics to identify the genes asm2 and spt3, which result in developmental phenotypes when deleted in Sordaria macrospora. In this study, we identified another gene, asm3, required for fruiting body formation, and performed transcriptomics analyses of Δasm2, Δasm3, and Δspt3. Deletion of spt3, which encodes a subunit of the SAGA complex, results in a block at an early stage of development and drastic changes in the transcriptome. Deletion mutants of asm2 and asm3 are able to form fruiting bodies, but have defects in ascospore maturation. Transcriptomics analysis of fruiting bodies revealed a large overlap in differentially regulated genes in Δasm2 and Δasm3 compared to the wild type. Analysis of nuclear distribution during ascus development showed that both mutants undergo meiosis and postmeiotic divisions, suggesting that the transcriptomic and morphological changes might be related to defects in the morphogenesis of structural features of the developing asci and ascospores.

ZFTA Translocations Constitute Ependymoma Chromatin Remodeling and Transcription Factors

ZFTA (C11orf95)-a gene of unknown function-partners with a variety of transcriptional coactivators in translocations that drive supratentorial ependymoma, a frequently lethal brain tumor. Understanding the function of ZFTA is key to developing therapies that inhibit these fusion proteins. Here, using a combination of transcriptomics, chromatin immunoprecipitation sequencing, and proteomics, we interrogated a series of deletion-mutant genes to identify a tripartite transformation mechanism of ZFTA-containing fusions, including: spontaneous nuclear translocation, extensive chromatin binding, and SWI/SNF, SAGA, and NuA4/Tip60 HAT chromatin modifier complex recruitment. Thereby, ZFTA tethers fusion proteins across the genome, modifying chromatin to an active state and enabling its partner transcriptional coactivators to promote promiscuous expression of a transforming transcriptome. Using mouse models, we validate further those elements of ZFTA-fusion proteins that are critical for transformation-including ZFTA zinc fingers and partner gene transactivation domains-thereby unmasking vulnerabilities for therapeutic targeting. SIGNIFICANCE: Ependymomas are hard-to-treat brain tumors driven by translocations between ZFTA and a variety of transcriptional coactivators. We dissect the transforming mechanism of these fusion proteins and identify protein domains indispensable for tumorigenesis, thereby providing insights into the molecular basis of ependymoma tumorigenesis and vulnerabilities for therapeutic targeting.This article is highlighted in the In This Issue feature, p. 2113.

The related coactivator complexes SAGA and ATAC control embryonic stem cell self-renewal through acetyltransferase-independent mechanisms

SAGA (Spt-Ada-Gcn5 acetyltransferase) and ATAC (Ada-two-A-containing) are two related coactivator complexes, sharing the same histone acetyltransferase (HAT) subunit. The HAT activities of SAGA and ATAC are required for metazoan development, but the role of these complexes in RNA polymerase II transcription is less understood. To determine whether SAGA and ATAC have redundant or specific functions, we compare the effects of HAT inactivation in each complex with that of inactivation of either SAGA or ATAC core subunits in mouse embryonic stem cells (ESCs). We show that core subunits of SAGA or ATAC are required for complex assembly and mouse ESC growth and self-renewal. Surprisingly, depletion of HAT module subunits causes a global decrease in histone H3K9 acetylation, but does not result in significant phenotypic or transcriptional defects. Thus, our results indicate that SAGA and ATAC are differentially required for self-renewal of mouse ESCs by regulating transcription through different pathways in a HAT-independent manner.

A unique GCN5 histone acetyltransferase complex controls erythrocyte invasion and virulence in the malaria parasite Plasmodium falciparum

The histone acetyltransferase GCN5-associated SAGA complex is evolutionarily conserved from yeast to human and functions as a general transcription co-activator in global gene regulation. In this study, we identified a divergent GCN5 complex in Plasmodium falciparum, which contains two plant homeodomain (PHD) proteins (PfPHD1 and PfPHD2) and a plant apetela2 (AP2)-domain transcription factor (PfAP2-LT). To dissect the functions of the PfGCN5 complex, we generated parasite lines with either the bromodomain in PfGCN5 or the PHD domain in PfPHD1 deleted. The two deletion mutants closely phenocopied each other, exhibiting significantly reduced merozoite invasion of erythrocytes and elevated sexual conversion. These domain deletions caused dramatic decreases not only in histone H3K9 acetylation but also in H3K4 trimethylation, indicating synergistic crosstalk between the two euchromatin marks. Domain deletion in either PfGCN5 or PfPHD1 profoundly disturbed the global transcription pattern, causing altered expression of more than 60% of the genes. At the schizont stage, these domain deletions were linked to specific down-regulation of merozoite genes involved in erythrocyte invasion, many of which contain the AP2-LT binding motif and are also regulated by AP2-I and BDP1, suggesting targeted recruitment of the PfGCN5 complex to the invasion genes by these specific factors. Conversely, at the ring stage, PfGCN5 or PfPHD1 domain deletions disrupted the mutually exclusive expression pattern of the entire var gene family, which encodes the virulent factor PfEMP1. Correlation analysis between the chromatin state and alteration of gene expression demonstrated that up- and down-regulated genes in these mutants are highly correlated with the silent and active chromatin states in the wild-type parasite, respectively. Collectively, the PfGCN5 complex represents a novel HAT complex with a unique subunit composition including an AP2 transcription factor, which signifies a new paradigm for targeting the co-activator complex to regulate general and parasite-specific cellular processes in this low-branching parasitic protist.

Epigenetic regulation of gene expression plays essential roles in orchestrating the general and parasite-specific cellular pathways in the malaria parasite Plasmodium falciparum. To better understand the epigenetic mechanisms in this parasite, we characterized the histone acetyltransferase GCN5-mediated transcription regulation during intraerythrocytic development of the parasite. Using tandem affinity purification and proteomic characterization, we identified that the PfGCN5-associated complex contains nine core components, including two PHD domain proteins (PfPHD1 and PfPHD2) and an AP2-domain transcription factor, which is divergent from the canonical GCN5 complexes evolutionarily conserved from yeast to human. To understand the functions of the PfGCN5 complex, we performed domain deletions in two subunits of this complex, PfGCN5 and PfPHD1. We found that the two deletion mutants displayed very similar growth phenotypes, including significantly reduced merozoite invasion rates and elevated sexual conversion. These two mutants were associated with dramatic decreases in histone H3K9 acetylation and H3K4 trimethylation, which led to global changes in chromatin states and gene expression. Consistent with the phenotypes, genes significantly affected by the PfGCN5 and PfPHD1 gene disruption include those participating in parasite-specific pathways such as invasion, virulence, and sexual development. In conclusion, this study presents a new model of the PfGCN5 complex for targeting the co-activator complex to regulate general and parasite-specific cellular processes in this low-branching parasitic protist.

The SAGA complex regulates early steps in transcription via its deubiquitylase module subunit USP22

The SAGA coactivator complex is essential for eukaryotic transcription and comprises four distinct modules, one of which contains the ubiquitin hydrolase USP22. In yeast, the USP22 ortholog deubiquitylates H2B, resulting in Pol II Ser2 phosphorylation and subsequent transcriptional elongation. In contrast to this H2B-associated role in transcription, we report here that human USP22 contributes to the early stages of stimulus-responsive transcription, where USP22 is required for pre-initiation complex (PIC) stability. Specifically, USP22 maintains long-range enhancer-promoter contacts and controls loading of Mediator tail and general transcription factors (GTFs) onto promoters, with Mediator core recruitment being USP22-independent. In addition, we identify Mediator tail subunits MED16 and MED24 and the Pol II subunit RBP1 as potential non-histone substrates of USP22. Overall, these findings define a role for human SAGA within the earliest steps of transcription.

Host Chromatin Regulators Required for Aggregatibacter actinomycetemcomitans Cytolethal Distending Toxin Activity in Saccharomyces cerevisiae Model

Cytolethal distending toxin (CDT) is a bacterial genotoxin that causes host cell cycle arrest and death. We previously employed a Saccharomyces cerevisiae model with inducible expression of the CDT catalytic subunit from Aggregatibacter actinomycetemcomitans, AaCdtB, and showed that a wide variety of host factors play a role in facilitating the activity of CdtB. Our observation that a yeast H2B mutant defective in chromatin condensation was partially resistant to CdtB implies that chromatin structure may affect CDT function. In this study, we identified host chromatin regulatory genes required for CdtB cytotoxicity. We found that the deletion of HTZ1 or certain subunits of SWR, INO80, and SIR complexes increased cellular resistance to CdtB. We hypothesized that CdtB may interact with Htz1 or the chromatin, but immunoprecipitation experiments failed to detect physical interaction between CdtB and Htz1 or the chromatin. However, we observed reduced nuclear localization of CdtB in several mutants, suggesting that impaired nuclear translocation may, at least partly, explain the mechanisms of CdtB resistance. In addition, mutations in chromatin regulatory genes induce changes in the global gene expression profile, and these may indirectly affect CdtB toxicity. Our results suggest that decreased expression of endoplasmic reticulum (ER)-Golgi transport-related genes that may be involved in CdtB transport and/or increased expression of DNA repair genes may contribute to CdtB resistance. These results suggest that the functions of chromatin regulators may contribute to the activity of CDT in host cells.

Three functionally redundant plant-specific paralogs are core subunits of the SAGA histone acetyltransferase complex in Arabidopsis

The SAGA (Spt-Ada-Gcn5 acetyltransferase) complex is an evolutionarily conserved histone acetyltransferase complex that has a critical role in histone acetylation, gene expression, and various developmental processes in eukaryotes. However, little is known about the composition and function of the SAGA complex in plants. In this study, we found that the SAGA complex in Arabidopsis thaliana contains not only conserved subunits but also four plant-specific subunits: three functionally redundant paralogs, SCS1, SCS2A, and SCS2B (SCS1/2A/2B), and a TAF-like subunit, TAFL. Mutations in SCS1/2A/2B lead to defective phenotypes similar to those caused by mutations in the genes encoding conserved SAGA subunits HAG1 and ADA2B, including delayed juvenile-to-adult phase transition, late flowering, and increased trichome density. Furthermore, we demonstrated that SCS1/2A/2B are required for the function of the SAGA complex in histone acetylation, thereby promoting the transcription of development-related genes. These results together suggest that SCS1/2A/2B are core subunits of the SAGA complex in Arabidopsis. Compared with SAGA complexes in other eukaryotes, the SAGA complexes in plants have evolved unique features that are necessary for normal growth and development.

DOT1L complex regulates transcriptional initiation in human erythroleukemic cells

DOT1L, the only H3K79 methyltransferase in human cells and a homolog of the yeast Dot1, normally forms a complex with AF10, AF17, and ENL or AF9, is dysregulated in most cases of mixed-lineage leukemia (MLLr), and has been believed to regulate transcriptional elongation on the basis of its colocalization with RNA polymerase II (Pol II), the sharing of subunits (AF9 and ENL) between the DOT1L and super elongation complexes, and the distribution of H3K79 methylation on both promoters and transcribed regions of active genes. Here we show that DOT1L depletion in erythroleukemic cells reduces its global occupancy without affecting the traveling ratio or the elongation rate (assessed by 4sUDRB-seq) of Pol II, suggesting that DOT1L does not play a major role in elongation in these cells. In contrast, analyses of transcription initiation factor binding reveal that DOT1L and ENL depletions each result in reduced TATA binding protein (TBP) occupancies on thousands of genes. More importantly, DOT1L and ENL depletions concomitantly reduce TBP and Pol II occupancies on a significant fraction of direct (DOT1L-bound) target genes, indicating a role for the DOT1L complex in transcription initiation. Mechanistically, proteomic and biochemical studies suggest that the DOT1L complex may regulate transcriptional initiation by facilitating the recruitment or stabilization of transcription factor IID, likely in a monoubiquitinated H2B (H2Bub1)-enhanced manner. Additional studies show that DOT1L enhances H2Bub1 levels by limiting recruitment of the Spt-Ada-Gcn5-acetyltransferase (SAGA) complex. These results advance our understanding of roles of the DOT1L complex in transcriptional regulation and have important implications for MLLr leukemias.