The yeast histone acetyltransferase A2 complex, but not free Gcn5p, binds stably to nucleosomal arrays

The role of histone acetylation in transcriptional regulation and seed development

Histone acetylation is highly conserved across eukaryotes and has been linked to gene activation since its discovery nearly 60 years ago. Over the past decades, histone acetylation has been evidenced to play crucial roles in plant development and response to various environmental cues. Emerging data indicate that histone acetylation is one of the defining features of "open chromatin," while the role of histone acetylation in transcription remains controversial. In this review, we briefly describe the discovery of histone acetylation, the mechanism of histone acetylation regulating transcription in yeast and mammals, and summarize the research progress of plant histone acetylation. Furthermore, we also emphasize the effect of histone acetylation on seed development and its potential use in plant breeding. A comprehensive knowledge of histone acetylation might provide new and more flexible research perspectives to enhance crop yield and stress resistance.

The histone code of the fungal genus Aspergillus uncovered by evolutionary and proteomic analyses

Chemical modifications of DNA and histone proteins impact the organization of chromatin within the nucleus. Changes in these modifications, catalysed by different chromatin-modifying enzymes, influence chromatin organization, which in turn is thought to impact the spatial and temporal regulation of gene expression. While combinations of different histone modifications, the histone code, have been studied in several model species, we know very little about histone modifications in the fungal genus Aspergillus, whose members are generally well studied due to their importance as models in cell and molecular biology as well as their medical and biotechnological relevance. Here, we used phylogenetic analyses in 94 Aspergilli as well as other fungi to uncover the occurrence and evolutionary trajectories of enzymes and protein complexes with roles in chromatin modifications or regulation. We found that these enzymes and complexes are highly conserved in Aspergilli, pointing towards a complex repertoire of chromatin modifications. Nevertheless, we also observed few recent gene duplications or losses, highlighting Aspergillus species to further study the roles of specific chromatin modifications. SET7 (KMT6) and other components of PRC2 (Polycomb Repressive Complex 2), which is responsible for methylation on histone H3 at lysine 27 in many eukaryotes including fungi, are absent in Aspergilli as well as in closely related Penicillium species, suggesting that these lost the capacity for this histone modification. We corroborated our computational predictions by performing untargeted MS analysis of histone post-translational modifications in Aspergillus nidulans. This systematic analysis will pave the way for future research into the complexity of the histone code and its functional implications on genome architecture and gene regulation in fungi.

Identification of Candida glabrata Transcriptional Regulators That Govern Stress Resistance and Virulence

The mechanisms by which Candida glabrata resists host defense peptides and caspofungin are incompletely understood. To identify transcriptional regulators that enable C. glabrata to withstand these classes of stressors, a library of 215 C. glabrata transcriptional regulatory deletion mutants was screened for susceptibility to both protamine and caspofungin. We identified eight mutants that had increased susceptibility to both host defense peptides and caspofungin. Of these mutants, six were deleted for genes that were predicted to specify proteins involved in histone modification. These genes were ADA2, GCN5, SPT8, HOS2, RPD3, and SPP1 Deletion of ADA2, GCN5, and RPD3 also increased susceptibility to mammalian host defense peptides. The Δada2 and Δgcn5 mutants had increased susceptibility to other stressors, such as H₂O₂ and SDS. In the Galleria mellonella model of disseminated infection, the Δada2 and Δgcn5 mutants had attenuated virulence, whereas in neutropenic mice, the virulence of the Δada2 and Δrpd3 mutants was decreased. Thus, histone modification plays a central role in enabling C. glabrata to survive host defense peptides and caspofungin, and Ada2 and Rpd3 are essential for the maximal virulence of this organism during disseminated infection.

Characterization of a metazoan ADA acetyltransferase complex

The Gcn5 acetyltransferase functions in multiple acetyltransferase complexes in yeast and metazoans. Yeast Gcn5 is part of the large SAGA (Spt-Ada-Gcn5 acetyltransferase) complex and a smaller ADA acetyltransferase complex. In flies and mammals, Gcn5 (and its homolog pCAF) is part of various versions of the SAGA complex and another large acetyltransferase complex, ATAC (Ada2A containing acetyltransferase complex). However, a complex analogous to the small ADA complex in yeast has never been described in metazoans. Previous studies in Drosophila hinted at the existence of a small complex which contains Ada2b, a partner of Gcn5 in the SAGA complex. Here we have purified and characterized the composition of this complex and show that it is composed of Gcn5, Ada2b, Ada3 and Sgf29. Hence, we have named it the metazoan 'ADA complex'. We demonstrate that the fly ADA complex has histone acetylation activity on histones and nucleosome substrates. Moreover, ChIP-Sequencing experiments identified Ada2b peaks that overlap with another SAGA subunit, Spt3, as well as Ada2b peaks that do not overlap with Spt3 suggesting that the ADA complex binds chromosomal sites independent of the larger SAGA complex.

The bromodomain of Gcn5 regulates site specificity of lysine acetylation on histone H3

In yeast, the conserved histone acetyltransferase (HAT) Gcn5 associates with Ada2 and Ada3 to form the catalytic module of the ADA and SAGA transcriptional coactivator complexes. Gcn5 also contains an acetyl-lysine binding bromodomain that has been implicated in regulating nucleosomal acetylation in vitro, as well as at gene promoters in cells. However, the contribution of the Gcn5 bromodomain in regulating site specificity of HAT activity remains unclear. Here, we used a combined acid-urea gel and quantitative mass spectrometry approach to compare the HAT activity of wild-type and Gcn5 bromodomain-mutant ADA subcomplexes (Gcn5-Ada2-Ada3). Wild-type ADA subcomplex acetylated H3 lysines with the following specificity; H3K14 > H3K23 > H3K9 ≈ H3K18 > H3K27 > H3K36. However, when the Gcn5 bromodomain was defective in acetyl-lysine binding, the ADA subcomplex demonstrated altered site-specific acetylation on free and nucleosomal H3, with H3K18ac being the most severely diminished. H3K18ac was also severely diminished on H3K14R, but not H3K23R, substrates in wild-type HAT reactions, further suggesting that Gcn5-catalyzed acetylation of H3K14 and bromodomain binding to H3K14ac are important steps preceding H3K18ac. In sum, this work details a previously uncharacterized cross-talk between the Gcn5 bromodomain "reader" function and enzymatic HAT activity that might ultimately affect gene expression. Future studies of how mutations in bromodomains or other histone post-translational modification readers can affect chromatin-templated enzymatic activities will yield unprecedented insight into a potential "histone/epigenetic code." MS data are available via ProteomeXchange with identifier PXD001167.

Histone target selection within chromatin: an exemplary case of teamwork

Histone modifiers like acetyltransferases, methyltransferases, and demethylases are critical regulators of most DNA-based nuclear processes, de facto controlling cell cycle progression and cell fate. These enzymes perform very precise post-translational modifications on specific histone residues, which in turn are recognized by different effector modules/proteins. We now have a better understanding of how these enzymes exhibit such specificity. As they often reside in multisubunit complexes, they use associated factors to target their substrates within chromatin structure and select specific histone mark-bearing nucleosomes. In this review, we cover the current understanding of how histone modifiers select their histone targets. We also explain how different experimental approaches can lead to conflicting results about the histone specificity and function of these enzymes.

CCDC134 interacts with hADA2a and functions as a regulator of hADA2a in acetyltransferase activity, DNA damage-induced apoptosis and cell cycle arrest

Human transcriptional adaptor hADA2a is an important component of the general control nonderepressible 5 (GCN5) histone acetyltransferase complex. Here, we report that coiled-coil domain containing 134 (CCDC134), a novel nuclear protein, binds to hADA2a and enhances the stability of the hADA2a protein in unstressed conditions. Furthermore, CCDC134 was found to participate in the p300/CBP-associated factor (PCAF) complex via hADA2a and affect the histone acetyltransferase activity of the complex. We also found that CCDC134 increased the PCAF-dependent K320 acetylation of p53 and p53 protein stability in the presence of hADA2a overexpression. Moreover, we demonstrated the biological significance of the interaction between CCDC134 and hADA2a. CCDC134 showed obvious nuclear accumulation after ultraviolet (UV) irradiation, and the knockdown of endogenous CCDC134 suppressed hADA2a-induced cell apoptosis activity and G1/S cell cycle arrest. Together, our findings indicate that CCDC134 might act as a novel regulator of hADA2a, and plays roles in the PCAF complex via hADA2a to affect its acetyltransferase activity and UV-induced DNA damage repair.

Antagonistic Gcn5-Hda1 interactions revealed by mutations to the Anaphase Promoting Complex in yeast

BACKGROUND

Histone post-translational modifications are critical for gene expression and cell viability. A broad spectrum of histone lysine residues have been identified in yeast that are targeted by a variety of modifying enzymes. However, the regulation and interaction of these enzymes remains relatively uncharacterized. Previously we demonstrated that deletion of either the histone acetyltransferase (HAT) GCN5 or the histone deacetylase (HDAC) HDA1 exacerbated the temperature sensitive (ts) mutant phenotype of the Anaphase Promoting Complex (APC) apc5CA allele. Here, the apc5CA mutant background is used to study a previously uncharacterized functional antagonistic genetic interaction between Gcn5 and Hda1 that is not detected in APC5 cells.

RESULTS

Using Northerns, Westerns, reverse transcriptase PCR (rtPCR), chromatin immunoprecipitation (ChIP), and mutant phenotype suppression analysis, we observed that Hda1 and Gcn5 appear to compete for recruitment to promoters. We observed that the presence of Hda1 can partially occlude the binding of Gcn5 to the same promoter. Occlusion of Gcn5 recruitment to these promoters involved Hda1 and Tup1. Using sequential ChIP we show that Hda1 and Tup1 likely form complexes at these promoters, and that complex formation can be increased by deleting GCN5.

CONCLUSIONS

Our data suggests large Gcn5 and Hda1 containing complexes may compete for space on promoters that utilize the Ssn6/Tup1 repressor complex. We predict that in apc5CA cells the accumulation of an APC target may compensate for the loss of both GCN5 and HDA1.

Histone dynamics and roles of histone acetyltransferases during cold-induced gene regulation in Arabidopsis

In Arabidopsis, CBF transcription factors bind to and activate certain cold-regulated (COR) gene promoters during cold acclimation. Consistent with the prevailing model that histone acetylation and nucleosomal depletion correspond with transcriptionally active genes, we now report that H3 acetylation increases and nucleosome occupancy decreases at COR gene promoters upon cold acclimation. Overexpression of CBF1 resulted in a constitutive increase in H3 acetylation and decrease in nucleosome occupancy, consistent with the constitutive activation of COR gene expression. Overexpression of a truncated form of CBF2 lacking its transcriptional activation domain resulted in a cold-stimulated increase in H3 acetylation, but no change in nucleosomal occupancy or COR gene expression, indicating that histone acetylation is congruent with but not sufficient for cold-activation of COR gene expression. Plants homozygous for T-DNA disruption alleles of GCN5 (encoding a histone acetyltransferase) or ADA2b (a GCN5-interacting protein) show diminished expression of COR genes during cold acclimation. Contrary to expectations, H3 acetylation at COR gene promoters was stimulated upon cold acclimation in ada2b and gcn5 plants as in wild type plants, but the decrease in nucleosome occupancy was diminished. Thus, GCN5 is not the HAT responsible for histone acetylation at COR gene promoters during cold acclimation. Several other HAT mutant plants were also tested; although some do affect COR gene expression, none affected histone acetylation. Therefore, H3 acetylation at the COR gene promoters is not solely dependent on any of the HATs tested.

Multiple histone modifications in euchromatin promote heterochromatin formation by redundant mechanisms in Saccharomyces cerevisiae

BACKGROUND

Methylation of lysine 79 on histone H3 by Dot1 is required for maintenance of heterochromatin structure in yeast and humans. However, this histone modification occurs predominantly in euchromatin. Thus, Dot1 affects silencing by indirect mechanisms and does not act by the recruitment model commonly proposed for histone modifications. To better understand the role of H3K79 methylation gene silencing, we investigated the silencing function of Dot1 by genetic suppressor and enhancer analysis and examined the relationship between Dot1 and other global euchromatic histone modifiers.

RESULT

We determined that loss of H3K79 methylation results in a partial silencing defect that could be bypassed by conditions that promote targeting of Sir proteins to heterochromatin. Furthermore, the silencing defect in strains lacking Dot1 was dependent on methylation of H3K4 by Set1 and histone acetylation by Gcn5, Elp3, and Sas2 in euchromatin. Our study shows that multiple histone modifications associated with euchromatin positively modulate the function of heterochromatin by distinct mechanisms. Genetic interactions between Set1 and Set2 suggested that the H3K36 methyltransferase Set2, unlike most other euchromatic modifiers, negatively affects gene silencing.

CONCLUSION

Our genetic dissection of Dot1's role in silencing in budding yeast showed that heterochromatin formation is modulated by multiple euchromatic histone modifiers that act by non-overlapping mechanisms. We discuss how euchromatic histone modifiers can make negative as well as positive contributions to gene silencing by competing with heterochromatin proteins within heterochromatin, within euchromatin, and at the boundary between euchromatin and heterochromatin.

Expression and purification of recombinant yeast Ada2/Ada3/Gcn5 and Piccolo NuA4 histone acetyltransferase complexes

Acetylation of histone tails by histone acetyltransferase (HAT) enzymes is a key post-translational modification of histones associated with transcriptionally active genes. Acetylation of the physiological nucleosome substrate is performed in cells by megadalton complexes such as SAGA and NuA4. To understand how HAT enzymes specifically recognize their nucleosome and not just histone tail substrates, we have identified the catalytic SAGA and NuA4 subcomplexes sufficient to act on nucleosomes. We describe here expression and purification procedures to prepare recombinant yeast Ada2/Ada3/Gcn5 subcomplex of SAGA which acetylates histones H3 and H2B on nucleosomes, and the Piccolo NuA4 complex which acetylates histones H4 and H2A on nucleosomes. We demonstrate an unexpected benefit of using the BL21-CodonPlus strain to enhance the purity of metal affinity purified Ada2/Ada3/Gcn5 complex. We also identify Escherichia coli EF-Tu as a contaminant that copurifies with both complexes over multiple chromatographic steps and use of hydrophobic interaction chromatography to remove the contaminant from the Piccolo NuA4 complex. The methods described here will be useful for studies into the molecular mechanism of these enzymes and for preparing the enzymes as reagents to study the interplay of nucleosome acetylation with other chromatin modification and remodeling enzymes.

Yng1 PHD finger binding to H3 trimethylated at K4 promotes NuA3 HAT activity at K14 of H3 and transcription at a subset of targeted ORFs

Posttranslational histone modifications participate in modulating the structure and function of chromatin. Promoters of transcribed genes are enriched with K4 trimethylation and hyperacetylation on the N-terminal tail of histone H3. Recently, PHD finger proteins, like Yng1 in the NuA3 HAT complex, were shown to interact with H3K4me3, indicating a biochemical link between K4 methylation and hyperacetylation. By using a combination of mass spectrometry, biochemistry, and NMR, we detail the Yng1 PHD-H3K4me3 interaction and the importance of NuA3-dependent acetylation at K14. Furthermore, genome-wide ChIP-Chip analysis demonstrates colocalization of Yng1 and H3K4me3 in vivo. Disrupting the K4me3 binding of Yng1 altered K14ac and transcription at certain genes, thereby demonstrating direct in vivo evidence of sequential trimethyl binding, acetyltransferase activity, and gene regulation by NuA3. Our data support a general mechanism of transcriptional control through which histone acetylation upstream of gene activation is promoted partially through availability of H3K4me3, "read" by binding modules in select subunits.

The essential gene wda encodes a WD40 repeat subunit of Drosophila SAGA required for histone H3 acetylation

Histone acetylation provides a switch between transcriptionally repressive and permissive chromatin. By regulating the chromatin structure at specific promoters, histone acetyltransferases (HATs) carry out important functions during differentiation and development of higher eukaryotes. HAT complexes are present in organisms as diverse as Saccharomyces cerevisiae, humans, and flies. For example, the well-studied yeast SAGA is related to three mammalian complexes. We previously identified Drosophila melanogaster orthologues of yeast SAGA components Ada2, Ada3, Spt3, and Tra1 and demonstrated that they associate with dGcn5 in a high-molecular-weight complex. To better understand the function of Drosophila SAGA (dSAGA), we sought to affinity purify and characterize this complex in more detail. A proteomic approach led to the identification of an orthologue of the yeast protein Ada1 and the novel protein encoded by CG4448, referred to as WDA (will decrease acetylation). Embryos lacking both alleles of the wda gene exhibited reduced levels of histone H3 acetylation and could not develop into adult flies. Our results point to a critical function of dSAGA and histone acetylation during Drosophila development.

The Drosophila histone acetyltransferase Gcn5 and transcriptional adaptor Ada2a are involved in nucleosomal histone H4 acetylation

The histone acetyltransferase (HAT) Gcn5 plays a role in chromatin structure and gene expression regulation as a catalytic component of multiprotein complexes, some of which also contain Ada2-type transcriptional coactivators. Data obtained mostly from studies on yeast (Saccharomyces cerevisiae) suggest that Ada2 potentiates Gcn5 activity and substrate recognition. dAda2b, one of two related Ada2 proteins of Drosophila melanogaster, was recently found to play a role in complexes acetylating histone 3 (H3). Evidence of an in vivo functional link between the related coactivator dAda2a and dGcn5, however, is lacking. Here we present data on the genetic interaction of dGcn5 and dAda2a. The loss of either dGcn5 or dAda2a function results in similar chromosome structural and developmental defects. In dAda2a mutants, the nucleosomal H4 acetylation at lysines 12 and 5 is significantly reduced, while the acetylation established by dAda2b-containing Gcn5 complexes at H3 lysines 9 and 14 is unaffected. The data presented here, together with our earlier data on the function of dAda2b, provide evidence that related Ada2 proteins of Drosophila, together with Gcn5 HAT, are involved in the acetylation of specific lysine residues in the N-terminal tails of nucleosomal H3 and H4. Our data suggest dAda2a involvement in both uniformly distributed H4 acetylation and gene-specific transcription regulation.

Sensors and signals: a coactivator/corepressor/epigenetic code for integrating signal-dependent programs of transcriptional response

A decade of intensive investigation of coactivators and corepressors required for regulated actions of DNA-binding transcription factors has revealed a network of sequentially exchanged cofactor complexes that execute a series of enzymatic modifications required for regulated gene expression. These coregulator complexes possess "sensing" activities required for interpretation of multiple signaling pathways. In this review, we examine recent progress in understanding the functional consequences of "molecular sensor" and "molecular adaptor" actions of corepressor/coactivator complexes in integrating signal-dependent programs of transcriptional responses at the molecular level. This strategy imposes a temporal order for modifying programs of transcriptional regulation in response to the cellular milieu, which is used to mediate developmental/homeostatic and pathological events.

Methylation of histone H3 mediates the association of the NuA3 histone acetyltransferase with chromatin

The SAS3-dependent NuA3 histone acetyltransferase complex was originally identified on the basis of its ability to acetylate histone H3 in vitro. Whether NuA3 is capable of acetylating histones in vivo, or how the complex is targeted to the nucleosomes that it modifies, was unknown. To address this question, we asked whether NuA3 is associated with chromatin in vivo and how this association is regulated. With a chromatin pulldown assay, we found that NuA3 interacts with the histone H3 amino-terminal tail, and loss of the H3 tail recapitulates phenotypes associated with loss of SAS3. Moreover, mutation of histone H3 lysine 14, the preferred site of acetylation by NuA3 in vitro, phenocopies a unique sas3Delta phenotype, suggesting that modification of this residue is important for NuA3 function. The interaction of NuA3 with chromatin is dependent on the Set1p and Set2p histone methyltransferases, as well as their substrates, histone H3 lysines 4 and 36, respectively. These results confirm that NuA3 is functioning as a histone acetyltransferase in vivo and that histone H3 methylation provides a mark for the recruitment of NuA3 to nucleosomes.

Mechanism of the long range anti-silencing function of targeted histone acetyltransferases in yeast

Transcriptionally silent chromatin in Saccharomyces cerevisiae is associated with histone hypoacetylation and is formed through the action of the Sir histone deacetylase complex. A histone acetyltransferase (HAT) targeted near silent chromatin can overcome silencing at a distance by increasing histone acetylation in a sizable region. However, how a tethered HAT acetylates distant nucleosomes has not been resolved. We demonstrate here that targeting the histone H3-specific HAT Gcn5p promotes acetylation of not only histone H3 but also histone H4 in a broad region. We also show that long range anti-silencing and histone acetylation by targeted HATs can be blocked by nucleosome-excluding sequences. These results are consistent with the contention that a tethered HAT promotes stepwise propagation of histone acetylation along the chromatin. Because histone hypoacetylation is key to the formation and maintenance of transcriptionally silent chromatin, it is believed that acetylation promoted by a targeted HAT disrupts silent chromatin thereby overcoming silencing. However, we show that the acetylated and transcriptionally active region created by a tethered HAT retains structural hallmarks of Sir-dependent silent chromatin and remains associated with Sir proteins indicating that tethered HATs overcome silencing without completely dismantling silent chromatin.

A Decade of Histone Acetylation: Marking Eukaryotic Chromosomes with Specific Codes

Post-translational modification of histones, a major protein component of eukaryotic chromosomes, contributes to the epigenetic regulation of gene expression. Distinct patterns of histone modification are observed at specific chromosomal regions and affect various reactions on chromosomes (transcription, replication, repair, and recombination). Histone modification has long been proposed to have a profound effect on eukaryotic gene expression since its discovery in 1964. Verification of this idea, however, was difficult until the identification of enzymes responsible for histone modifications. Ten years ago (1995), histone acetyltransferases (HATs), which acetylate lysine residues in histone amino-terminal tail regions, were isolated. HATs are involved in the regulation of both promoter-specific transcription and long-range/chromosome-wide transcription. Analyses of HATs and other modification enzymes have revealed mechanisms of epigenetic regulation that are mediated by post-translational modifications of histones. Here we review some major advances in the field, with emphasis on the lysine specificity of the acetylation reaction and on the regulation of gene expression over broad regions.

The homologous Drosophila transcriptional adaptors ADA2a and ADA2b are both required for normal development but have different functions

In Drosophila and several other metazoan organisms, there are two genes that encode related but distinct homologs of ADA2-type transcriptional adaptors. Here we describe mutations of the two Ada2 genes of Drosophila melanogaster. By using mutant Drosophila lines, which allow the functional study of individual ADA2s, we demonstrate that both Drosophila Ada2 genes are essential. Ada2a and Ada2b null homozygotes are late-larva and late-pupa lethal, respectively. Double mutants have a phenotype identical to that of the Ada2a mutant. The overproduction of ADA2a protein from transgenes cannot rescue the defects resulting from the loss of Ada2b, nor does complementation work vice versa, indicating that the two Ada2 genes of Drosophila have different functions. An analysis of germ line mosaics generated by pole-cell transplantation revealed that the Ada2a function (similar to that reported for Ada2b) is required in the female germ line. A loss of the function of either of the Ada2 genes interferes with cell proliferation. Interestingly, the Ada2b null mutation reduces histone H3 K14 and H3 K9 acetylation and changes TAF10 localization, while the Ada2a null mutation does not. Moreover, the two ADA2s are differently required for the expression of the rosy gene, involved in eye pigment production, and for Dmp53-mediated apoptosis. The data presented here demonstrate that the two genes encoding homologous transcriptional adaptor ADA2 proteins in Drosophila are both essential but are functionally distinct.

Characterization of yeast histone H3-specific type B histone acetyltransferases identifies an ADA2-independent Gcn5p activity

Background

The acetylation of the core histone NH₂-terminal tails is catalyzed by histone acetyltransferases. Histone acetyltransferases can be classified into two distinct groups (type A and B) on the basis of cellular localization and substrate specificity. Type B histone acetyltransferases, originally defined as cytoplasmic enzymes that acetylate free histones, have been proposed to play a role in the assembly of chromatin through the acetylation of newly synthesized histones H3 and H4. To date, the only type B histone acetyltransferase activities identified are specific for histone H4.

Results

To better understand the role of histone acetylation in the assembly of chromatin structure, we have identified additional type B histone acetyltransferase activities specific for histone H3. One such activity, termed HatB3.1, acetylated histone H3 with a strong preference for free histones relative to chromatin substrates. Deletion of the GCN5 and ADA3 genes resulted in the loss of HatB3.1 activity while deletion of ADA2 had no effect. In addition, Gcn5p and Ada3p co-fractionated with partially purified HatB3.1 activity while Ada2p did not.

Conclusions

Yeast extracts contain several histone acetyltransferase activities that show a strong preference for free histone H3. One such activity, termed HatB3.1, appears to be a novel Gcn5p-containing complex which does not depend on the presence of Ada2p.

The diverse superfamily of lysine acetyltransferases and their roles in leukemia and other diseases

Acetylation of the epsilon-amino group of lysine residues, or N(epsilon)-lysine acetylation, is an important post-translational modification known to occur in histones, transcription factors and other proteins. Since 1995, dozens of proteins have been discovered to possess intrinsic lysine acetyltransferase activity. Although most of these enzymes were first identified as histone acetyltransferases and then tested for activities towards other proteins, acetyltransferases only modifying non-histone proteins have also been identified. Lysine acetyltransferases form different groups, three of which are Gcn5/PCAF, p300/CBP and MYST proteins. While members of the former two groups mainly function as transcriptional co-activators, emerging evidence suggests that MYST proteins, such as Esa1, Sas2, MOF, TIP60, MOZ and MORF, have diverse roles in various nuclear processes. Aberrant lysine acetylation has been implicated in oncogenesis. The genes for p300, CBP, MOZ and MORF are rearranged in recurrent leukemia-associated chromosomal abnormalities. Consistent with their roles in leukemogenesis, these acetyltransferases interact with Runx1 (or AML1), one of the most frequent targets of chromosomal translocations in leukemia. Therefore, the diverse superfamily of lysine acetyltransferases executes an acetylation program that is important for different cellular processes and perturbation of such a program may cause the development of cancer and other diseases.

Yeast enhancer of polycomb defines global Esa1-dependent acetylation of chromatin

Drosophila Enhancer of Polycomb, E(Pc), is a suppressor of position-effect variegation and an enhancer of both Polycomb and trithorax mutations. A homologous yeast protein, Epl1, is a subunit of the NuA4 histone acetyltransferase complex. Epl1 depletion causes cells to accumulate in G2/M and global loss of acetylated histones H4 and H2A. In relation to the Drosophila protein, mutation of Epl1 suppresses gene silencing by telomere position effect. Epl1 protein is found in the NuA4 complex and a novel highly active smaller complex named Piccolo NuA4 (picNuA4). The picNuA4 complex contains Esa1, Epl1, and Yng2 as subunits and strongly prefers chromatin over free histones as substrate. Epl1 conserved N-terminal domain bridges Esa1 and Yng2 together, stimulating Esa1 catalytic activity and enabling acetylation of chromatin substrates. A recombinant picNuA4 complex shows characteristics similar to the native complex, including strong chromatin preference. Cells expressing only the N-terminal half of Epl1 lack NuA4 HAT activity, but possess picNuA4 complex and activity. These results indicate that the essential aspect of Esa1 and Epl1 resides in picNuA4 function. We propose that picNuA4 represents a nontargeted histone H4/H2A acetyltransferase activity responsible for global acetylation, whereas the NuA4 complex is recruited to specific genomic loci to perturb locally the dynamic acetylation/deacetylation equilibrium.

The diverse functions of histone acetyltransferase complexes

Although histone acetylation has historically been linked to transcription activation, recent studies indicate that this modification and the enzymes that catalyze it have much broader and diverse functions. Histone acetyltransferase complexes are involved in such diverse processes as transcription activation, gene silencing, DNA repair and cell-cycle progression. The high conservation of the acetyltransferase complexes and their functions illustrates their central role in cell growth and development.

A native peptide ligation strategy for deciphering nucleosomal histone modifications

Post-translational modifications of histones influence both chromatin structure and the binding and function of chromatin-associated proteins. A major limitation to understanding these effects has been the inability to construct nucleosomes in vitro that harbor homogeneous and site-specific histone modifications. Here, we describe a native peptide ligation strategy for generating nucleosomal arrays that can harbor a wide range of desired histone modifications. As a first test of this method, we engineered model nucleosomal arrays in which each histone H3 contains a phosphorylated serine at position 10 and performed kinetic analyses of Gcn5-dependent histone acetyltransferase activities. Recombinant Gcn5 shows increased histone acetyltransferase activity on nucleosomal arrays harboring phosphorylated H3 serine 10 and is consistent with peptide studies. However, in contrast to analyses using peptide substrates, we find that the histone acetyltransferase activity of the Gcn5-containing SAGA complex is not stimulated by H3 phosphorylation in the context of nucleosomal arrays. This difference between peptide and array substrates suggests that the ability to generate specifically modified nucleosomal arrays should provide a powerful tool for understanding the effects of post-translational histone modifications.

Two Drosophila Ada2 homologues function in different multiprotein complexes

The reversible acetylation of the N-terminal tails of histones is crucial for transcription, DNA repair, and replication. The enzymatic reaction is catalyzed by large multiprotein complexes, of which the best characterized are the Gcn5-containing N-acetyltransferase (GNAT) complexes. GNAT complexes from yeast to humans share several conserved subunits, such as Ada2, Ada3, Spt3, and Tra1/TRRAP. We have characterized these factors in Drosophila and found that the flies have two distinct Ada2 variants (dAda2a and dAda2b). Using a combination of biochemical and cell biological approaches we demonstrate that only one of the two Drosophila Ada2 homologues, dAda2b, is a component of Spt-Ada-Gcn5-acetyltransferase (SAGA) complexes. The other Ada2 variant, dAda2a, can associate with dGcn5 but is not incorporated into dSAGA-type complexes. This is the first example of a complex-specific association of the Ada-type transcriptional adapter proteins with GNATs. In addition, dAda2a is part of Gcn5-independent complexes, which are concentrated at transcriptionally active regions on polytene chromosomes. This implicates novel functions for dAda2a in transcription. Humans and mice also possess two Ada2 variants with high homology to dAda2a and dAda2b, respectively. This suggests that the mammalian and fly homologues of the transcriptional adapter Ada2 form two functionally distinct subgroups with unique characteristics.

The novel SLIK histone acetyltransferase complex functions in the yeast retrograde response pathway

The SAGA complex is a conserved histone acetyltransferase-coactivator that regulates gene expression in Saccharomyces cerevisiae. SAGA contains a number of subunits known to function in transcription including Spt and Ada proteins, the Gcn5 acetyltransferase, a subset of TATA-binding-protein-associated factors (TAF(II)s), and Tra1. Here we report the identification of SLIK (SAGA-like), a complex related in composition to SAGA. Notably SLIK uniquely contains the protein Rtg2, linking the function of SLIK to the retrograde response pathway. Yeast harboring mutations in both SAGA and SLIK complexes displays synthetic phenotypes more severe than those of yeast with mutation of either complex alone. We present data indicating that distinct forms of the SAGA complex may regulate specific subsets of genes and that SAGA and SLIK have multiple partly overlapping activities, which play a critical role in transcription by RNA polymerase II.

Yng1p modulates the activity of Sas3p as a component of the yeast NuA3 Hhistone acetyltransferase complex

The mammalian ING1 gene encodes a tumor suppressor required for the function of p53. In this study we report a novel function for YNG1, a yeast homolog of ING1. Yng1p is a stable component of the NuA3 histone acetyltransferase complex, which contains Sas3p, the yeast homolog of the mammalian MOZ proto-oncogene product, as its catalytic subunit. Yng1p is required for NuA3 function in vivo but surprisingly is not required for the integrity of the complex. Instead, we find that Yng1p mediates the interaction of Sas3p with nucleosomes and is thus required for the ability of NuA3 to modify histone tails. These data, and the observations that other ING1 homologs are found in additional yeast complexes that posttranslationally modify histones, suggest that members of the ING1 class of proteins may have broad roles in enhancing or modifying the activities of chromatin-modifying complexes, thereby regulating their activities in transcription control.

Histone acetyltransferases

Transcriptional regulation in eukaryotes occurs within a chromatin setting and is strongly influenced by nucleosomal barriers imposed by histone proteins. Among the well-known covalent modifications of histones, the reversible acetylation of internal lysine residues in histone amino-terminal domains has long been positively linked to transcriptional activation. Recent biochemical and genetic studies have identified several large, multisubunit enzyme complexes responsible for bringing about the targeted acetylation of histones and other factors. This review discusses our current understanding of histone acetyltransferases (HATs) or acetyltransferases (ATs): their discovery, substrate specificity, catalytic mechanism, regulation, and functional links to transcription, as well as to other chromatin-modifying activities. Recent studies underscore unexpected connections to both cellular regulatory processes underlying normal development and differentiation, as well as abnormal processes that lead to oncogenesis. Although the functions of HATs and the mechanisms by which they are regulated are only beginning to be understood, these fundamental processes are likely to have far-reaching implications for human biology and disease.

Role of the Ada2 and Ada3 transcriptional coactivators in histone acetylation

Previous studies have shown that the transcriptional coactivator protein Gcn5 functions as a catalytic histone acetyltransferase (HAT). In this work, we examine the roles of the Ada2 and Ada3 coactivator proteins that are functionally linked to Gcn5. We show that yeast Ada2, Ada3, and Gcn5 form a catalytic core of the ADA and Spt-Ada-Gcn5-acetyltransferase HAT complexes, which is necessary and sufficient in vitro for nucleosomal HAT activity and lysine specificity of the intact HAT complexes. We also demonstrate that Ada3 is necessary for Gcn5-dependent nucleosomal HAT activity in yeast extracts. Our results suggest that Ada2 potentiates the Gcn5 catalytic activity and that Ada3 facilitates nucleosomal acetylation and an expanded lysine specificity.

Population analysis of subsaturated 172-12 nucleosomal arrays by atomic force microscopy detects nonrandom behavior that is favored by histone acetylation and short repeat length

Concatameric 5 S rDNA templates reconstituted in vitro into nucleosomal arrays provide very popular chromatin models for many kinds of studies. Here, atomic force microscopy is used to determine the population distributions for one such nucleosomal array, the 172-12, reconstituted to various subsaturated levels with nonacetylated or hyperacetylated HeLa histones. This array is a model for short linker length genomes and transcriptionally active and newly replicated chromatins. The analysis shows that as input histone levels increase, template occupation increases progressively as discrete population distributions. The distributions are random at low (n(av) < 4) and high (n(av) > 8) loadings but display specific nonrandom features, such as a deficit of molecules with one nucleosome more or less than the peak species in the distribution and enhanced distribution breadths, in the mid-range (n(av) = 4-8). Thus, the mid-range of occupation on polynucleosomal arrays may be a special range for chromatin structure and/or assembly. The mid-range nonrandom features are enhanced in distributions from short repeat (172-12) arrays, particularly for unacetylated chromatin, and in distributions from hyperacetylated chromatin, particularly for long repeat (208-12) arrays. Thus, short repeat length and acetylation can affect basic chromatin properties, like population tendencies, in very similar ways and therefore may cause similar changes in chromatin structure. Some possible effects are suggested. The data also indicate that it is thermodynamically more difficult for hyperacetylated nucleosomes to assemble onto the 172-12 templates, a result having implications for in vivo chromatin assembly.

Histone H3 specific acetyltransferases are essential for cell cycle progression

Longstanding observations suggest that acetylation and/or amino-terminal tail structure of histones H3 and H4 are critical for eukaryotic cells. For Saccharomyces cerevisiae, loss of a single H4-specific histone acetyltransferase (HAT), Esa1p, results in cell cycle defects and death. In contrast, although several yeast HAT complexes preferentially acetylate histone H3, the catalytic subunits of these complexes are not essential for viability. To resolve the apparent paradox between the significance of H3 versus H4 acetylation, we tested the hypothesis that H3 modification is essential, but is accomplished through combined activities of two enzymes. We observed that Sas3p and Gcn5p HAT complexes have overlapping patterns of acetylation. Simultaneous disruption of SAS3, the homolog of the MOZ leukemia gene, and GCN5, the hGCN5/PCAF homolog, is synthetically lethal due to loss of acetyltransferase activity. This key combination of activities is specific for these two HATs because neither is synthetically lethal with mutations of other MYST family or H3-specific acetyltransferases. Further, the combined loss of GCN5 and SAS3 functions results in an extensive, global loss of H3 acetylation and arrest in the G(2)/M phase of the cell cycle. The strikingly similar effect of loss of combined essential H3 HAT activities and the loss of a single essential H4 HAT underscores the fundamental biological significance of each of these chromatin-modifying activities.

Linking global histone acetylation to the transcription enhancement of X-chromosomal genes in Drosophila males

It has become well established for several genes that targeting of histone acetylation to promoters is required for the activation of transcription. In contrast, global patterns of acetylation have not been ascribed to any particular regulatory function. In Drosophila, a specific modification of H4, acetylation at lysine 16, is enriched at hundreds of sites on the male X chromosome due to the activity of the male-specific lethal (MSL) dosage compensation complex. Utilizing chromatin immunoprecipitation, we have determined that H4Ac16 is present along the entire length of X-linked genes targeted by the MSL complex with relatively modest levels of acetylation at the promoter regions and high levels in the middle and/or 3' end of the transcription units. We propose that global acetylation by the MSL complex increases the expression of X-linked genes by facilitating transcription elongation rather than by enhancing promoter accessibility. We have also determined that H4Ac16 is absent from a region of the X chromosome that includes a gene known to be dosage-compensated by a MSL-independent mechanism. This study represents the first biochemical interpretation of the very large body of cytological observations on the chromosomal distribution of the MSL complex.

Nucleosomes and the chromatin fiber

During the past year and a half, significant progress has been made in understanding the structure and dynamics of nucleosomes and the chromatin fiber, the mechanism of action of the core histone amino termini, the structure and function of histone variants, and the function of linker histones in the chromatin fiber.

Sds3 (suppressor of defective silencing 3) is an integral component of the yeast Sin3[middle dot]Rpd3 histone deacetylase complex and is required for histone deacetylase activity

SDS3 (suppressor of defective silencing 3) was originally identified in a screen for mutations that cause increased silencing of a crippled HMR silencer in a rap1 mutant background. In addition, sds3 mutants have phenotypes very similar to those seen in sin3 and rpd3 mutants, suggesting that it functions in the same genetic pathway. In this manuscript we demonstrate that Sds3p is an integral subunit of a previously identified high molecular weight Rpd3p.Sin3p containing yeast histone deacetylase complex. By analyzing an sds3Delta strain we show that, in the absence of Sds3p, Sin3p can be chromatographically separated from Rpd3p, indicating that Sds3p promotes the integrity of the complex. Moreover, the remaining Rpd3p complex in the sds3Delta strain had little or no histone deacetylase activity. Thus, Sds3p plays important roles in the integrity and catalytic activity of the Rpd3p.Sin3p complex.