Yeast chromatin assembly complex 1 protein excludes nonacetylatable forms of histone H4 from chromatin and the nucleus

Identification of multiple roles for histone acetyltransferase 1 in replication-coupled chromatin assembly

Histone acetyltransferase 1 (Hat1) catalyzes the acetylation of newly synthesized histone H4 at lysines 5 and 12 that accompanies replication-coupled chromatin assembly. The acetylation of newly synthesized H4 occurs in the cytoplasm and the function of this acetylation is typically ascribed to roles in either histone nuclear import or deposition. Using cell lines from Hat1^+/+ and Hat1^−/− mouse embryos, we demonstrate that Hat1 is not required for either histone nuclear import or deposition. We employed quantitative proteomics to characterize Hat1-dependent changes in the composition of nascent chromatin structure. Among the proteins depleted from nascent chromatin isolated from Hat1^−/− cells are several bromodomain-containing proteins, including Brg1, Baz1A and Brd3. Analysis of the binding specificity of their bromodomains suggests that Hat1-dependent acetylation of H4 is directly involved in their recruitment. Hat1^−/− nascent chromatin is enriched for topoisomerase 2α and 2β. The enrichment of topoisomerase 2 is functionally relevant as Hat1^−/− cells are hyper-sensitive to topoisomerase 2 inhibition suggesting that Hat1 is required for proper chromatin topology. In addition, our results indicate that Hat1 is transiently recruited to sites of chromatin assembly, dissociating prior to the maturation of chromatin structure.

Structure-based nuclear import mechanism of histones H3 and H4 mediated by Kap123

Kap123, a major karyopherin protein of budding yeast, recognizes the nuclear localization signals (NLSs) of cytoplasmic histones H3 and H4 and translocates them into the nucleus during DNA replication. Mechanistic questions include H3- and H4-NLS redundancy toward Kap123 and the role of the conserved diacetylation of cytoplasmic H4 (K5ac and K12ac) in Kap123-mediated histone nuclear translocation. Here, we report crystal structures of full-length Kluyveromyces lactis Kap123 alone and in complex with H3- and H4-NLSs. Structures reveal the unique feature of Kap123 that possesses two discrete lysine-binding pockets for NLS recognition. Structural comparison illustrates that H3- and H4-NLSs share at least one of two lysine-binding pockets, suggesting that H3- and H4-NLSs are mutually exclusive. Additionally, acetylation of key lysine residues at NLS, particularly H4-NLS diacetylation, weakens the interaction with Kap123. These data support that cytoplasmic histone H4 diacetylation weakens the Kap123-H4-NLS interaction thereby facilitating histone Kap123-H3-dependent H3:H4/Asf1 complex nuclear translocation.

A Novel Histone Crosstalk Pathway Important for Regulation of UV-Induced DNA Damage Repair in Saccharomyces cerevisiae

Histone post-translational modifications play vital roles in a variety of nuclear processes, including DNA repair. It has been previously shown that histone H3K79 methylation is important for the cellular response to DNA damage caused by ultraviolet (UV) radiation, with evidence that specific methylation states play distinct roles in UV repair. Here, we report that H3K79 methylation is reduced in response to UV exposure in Saccharomyces cerevisiae This reduction is specific to the dimethylated state, as trimethylation levels are minimally altered by UV exposure. Inhibition of this reduction has a deleterious effect on UV-induced sister chromatid exchange, suggesting that H3K79 dimethylation levels play a regulatory role in UV repair. Further evidence implicates an additional role for H3K79 dimethylation levels in error-free translesion synthesis, but not in UV-induced G1/S checkpoint activation or double-stranded break repair. Additionally, we find that H3K79 dimethylation levels are influenced by acetylatable lysines on the histone H4 N-terminal tail, which are hyperacetylated in response to UV exposure. Preclusion of H4 acetylation prevents UV-induced reduction of H3K79 dimethylation, and similarly has a negative effect on UV-induced sister chromatid exchange. These results point to the existence of a novel histone crosstalk pathway that is important for the regulation of UV-induced DNA damage repair.

Chromatin Replication and Histone Dynamics

Inheritance of the DNA sequence and its proper organization into chromatin is fundamental for genome stability and function. Therefore, how specific chromatin structures are restored on newly synthesized DNA and transmitted through cell division remains a central question to understand cell fate choices and self-renewal. Propagation of genetic information and chromatin-based information in cycling cells entails genome-wide disruption and restoration of chromatin, coupled with faithful replication of DNA. In this chapter, we describe how cells duplicate the genome while maintaining its proper organization into chromatin. We reveal how specialized replication-coupled mechanisms rapidly assemble newly synthesized DNA into nucleosomes, while the complete restoration of chromatin organization including histone marks is a continuous process taking place throughout the cell cycle. Because failure to reassemble nucleosomes at replication forks blocks DNA replication progression in higher eukaryotes and leads to genomic instability, we further underline the importance of the mechanistic link between DNA replication and chromatin duplication.

The Cac1 subunit of histone chaperone CAF-1 organizes CAF-1-H3/H4 architecture and tetramerizes histones

The histone chaperone Chromatin Assembly Factor 1 (CAF-1) deposits tetrameric (H3/H4)₂ histones onto newly-synthesized DNA during DNA replication. To understand the mechanism of the tri-subunit CAF-1 complex in this process, we investigated the protein-protein interactions within the CAF-1-H3/H4 architecture using biophysical and biochemical approaches. Hydrogen/deuterium exchange and chemical cross-linking coupled to mass spectrometry reveal interactions that are essential for CAF-1 function in budding yeast, and importantly indicate that the Cac1 subunit functions as a scaffold within the CAF-1-H3/H4 complex. Cac1 alone not only binds H3/H4 with high affinity, but also promotes histone tetramerization independent of the other subunits. Moreover, we identify a minimal region in the C-terminus of Cac1, including the structured winged helix domain and glutamate/aspartate-rich domain, which is sufficient to induce (H3/H4)₂ tetramerization. These findings reveal a key role of Cac1 in histone tetramerization, providing a new model for CAF-1-H3/H4 architecture and function during eukaryotic replication.

DOI:http://dx.doi.org/10.7554/eLife.18023.001

The DNA of a human, yeast or other eukaryotic cell is bound to proteins called histones to form repeating units called nucleosomes. Every time a eukaryotic cell divides, it must duplicate its DNA. Old histones are first removed from the nucleosomes before being re-assembled onto the newly duplicated DNA along with new histone proteins, producing a full complement of nucleosomes.

A group of proteins called the chromatin assembly factor 1 (or CAF-1 for short) helps to assemble the histones onto the DNA. CAF-1 is made up of three proteins, and binds to two copies of each of the histones known as H3 and H4. These are the first histones to be assembled onto the nucleosomes. It was not clear how the components of CAF-1 are organized, or how CAF-1 recognizes histones.

Liu et al. have now investigated the structure of CAF-1 and its interactions with the H3 and H4 histones by studying yeast proteins and cells. Yeast is a good model system because yeast CAF-1 is smaller and easier to isolate than human CAF-1, yet still performs the same essential activities. Using a combination of biochemical and biophysical techniques, Liu et al. found that one of the three proteins that makes up yeast CAF-1 – called Cac1 – forms a scaffold that supports the other CAF-1 proteins and histones H3 and H4. Moreover, a specific part of Cac1 is able to bind to these histones and assemble two copies of each of them to prepare for efficient nucleosome assembly.

Further experiments revealed the specific areas where the CAF-1 proteins interact with each other and with the histones, determined how strong those interactions are, and confirmed that these interactions play important roles in yeast.

Overall, the results presented by Liu et al. provide new insights into the structure of CAF-1 bound to H3 and H4. In order to understand in detail how CAF-1 helps to assemble histones onto DNA, future work needs to capture three-dimensional snapshots of the different steps in this process. Further investigation is also needed to discover how CAF-1 cooperates with other factors that promote DNA duplication.

DOI:http://dx.doi.org/10.7554/eLife.18023.002

Neocentromeres Provide Chromosome Segregation Accuracy and Centromere Clustering to Multiple Loci along a Candida albicans Chromosome

Assembly of kinetochore complexes, involving greater than one hundred proteins, is essential for chromosome segregation and genome stability. Neocentromeres, or new centromeres, occur when kinetochores assemble de novo, at DNA loci not previously associated with kinetochore proteins, and they restore chromosome segregation to chromosomes lacking a functional centromere. Neocentromeres have been observed in a number of diseases and may play an evolutionary role in adaptation or speciation. However, the consequences of neocentromere formation on chromosome missegregation rates, gene expression, and three-dimensional (3D) nuclear structure are not well understood. Here, we used Candida albicans, an organism with small, epigenetically-inherited centromeres, as a model system to study the functions of twenty different neocentromere loci along a single chromosome, chromosome 5. Comparison of neocentromere properties relative to native centromere functions revealed that all twenty neocentromeres mediated chromosome segregation, albeit to different degrees. Some neocentromeres also caused reduced levels of transcription from genes found within the neocentromere region. Furthermore, like native centromeres, neocentromeres clustered in 3D with active/functional centromeres, indicating that formation of a new centromere mediates the reorganization of 3D nuclear architecture. This demonstrates that centromere clustering depends on epigenetically defined function and not on the primary DNA sequence, and that neocentromere function is independent of its distance from the native centromere position. Together, the results show that a neocentromere can form at many loci along a chromosome and can support the assembly of a functional kinetochore that exhibits native centromere functions including chromosome segregation accuracy and centromere clustering within the nucleus.

The accurate segregation of chromosomes during cell division is essential for maintaining genome integrity. The centromere is the DNA region on each chromosome where assembly of a large protein complex, the kinetochore, is required to maintain proper chromosome segregation. In addition, active centromeres exhibit a specific three-dimensional organization within the nucleus: the centromeres associate with one another in a clustered manner. Neocentromeres, or new centromeres, appear at new places along the chromosome when a native centromere becomes non-functional. We used a yeast model, Candida albicans, and isolated twenty instances in which neocentromeres had formed at different positions. All of these neocentromeres were able to direct chromosome segregation, but some had increased error rates. Like native centromeres, these neocentromeres cluster in the nucleus with the other active centromeres. This implies that formation of a neocentromere leads to reorganization of the three-dimensional structure of the nucleus so that different regions of the chromosome are in closer contact to regions of other chromosomes. Recent work suggests that approximately 3% of cancers may contain chromosomes with neocentromeres. Our observations that many neocentromeres have increased error rates provides insight into genome instability in cancer cells. Changes in chromosome copy number may benefit the cancer cells by increasing numbers of oncogenes and/or drug resistance genes, but may also sensitize the cells to chemotherapy approaches that target chromosome segregation mechanisms.

Reduced Histone Expression or a Defect in Chromatin Assembly Induces Respiration

Regulation of mitochondrial biogenesis and respiration is a complex process that involves several signaling pathways and transcription factors as well as communication between the nuclear and mitochondrial genomes. Here we show that decreased expression of histones or a defect in nucleosome assembly in the yeast Saccharomyces cerevisiae results in increased mitochondrial DNA (mtDNA) copy numbers, oxygen consumption, ATP synthesis, and expression of genes encoding enzymes of the tricarboxylic acid (TCA) cycle and oxidative phosphorylation (OXPHOS). The metabolic shift from fermentation to respiration induced by altered chromatin structure is associated with the induction of the retrograde (RTG) pathway and requires the activity of the Hap2/3/4/5p complex as well as the transport and metabolism of pyruvate in mitochondria. Together, our data indicate that altered chromatin structure relieves glucose repression of mitochondrial respiration by inducing transcription of the TCA cycle and OXPHOS genes carried by both nuclear and mitochondrial DNA.

Histone H3 K79 methylation states play distinct roles in UV-induced sister chromatid exchange and cell cycle checkpoint arrest in Saccharomyces cerevisiae

Histone post-translational modifications have been shown to contribute to DNA damage repair. Prior studies have suggested that specific H3K79 methylation states play distinct roles in the response to UV-induced DNA damage. To evaluate these observations, we examined the effect of altered H3K79 methylation patterns on UV-induced G1/S checkpoint response and sister chromatid exchange (SCE). We found that the di- and trimethylated states both contribute to activation of the G1/S checkpoint to varying degrees, depending on the synchronization method, although methylation is not required for checkpoint in response to high levels of UV damage. In contrast, UV-induced SCE is largely a product of the trimethylated state, which influences the usage of gene conversion versus popout mechanisms. Regulation of H3K79 methylation by H2BK123 ubiquitylation is important for both checkpoint function and SCE. H3K79 methylation is not required for the repair of double-stranded breaks caused by transient HO endonuclease expression, but does play a modest role in survival from continuous exposure. The overall results provide evidence for the participation of H3K79 methylation in UV-induced recombination repair and checkpoint activation, and further indicate that the di- and trimethylation states play distinct roles in these DNA damage response pathways.

Assembling chromatin: the long and winding road

It has been over 35 years since the acceptance of the "chromatin subunit" hypothesis, and the recognition that nucleosomes are the fundamental repeating units of chromatin fibers. Major subjects of inquiry in the intervening years have included the steps involved in chromatin assembly, and the chaperones that escort histones to DNA. The following commentary offers an historical perspective on inquiries into the processes by which nucleosomes are assembled on replicating and nonreplicating chromatin. This article is part of a Special Issue entitled: Histone chaperones and Chromatin assembly.

Towards a mechanism for histone chaperones

Histone chaperones can be broadly defined as histone-binding proteins that influence chromatin dynamics in an ATP-independent manner. Their existence reflects the importance of chromatin homeostasis and the unique and unusual biochemistry of the histone proteins. Histone supply and demand at chromatin is regulated by a network of structurally and functionally diverse histone chaperones. At the core of this network is a mechanistic variability that is only beginning to be appreciated. In this review, we highlight the challenges in determining histone chaperone mechanism and discuss possible mechanisms in the context of nucleosome thermodynamics. We discuss how histone chaperones prevent promiscuous histone interactions, and consider if this activity represents the full extent of histone chaperone function in governing chromatin dynamics. This article is part of a Special Issue entitled: Histone chaperones and Chromatin assembly.

Curcumin attenuation of lipopolysaccharide induced cardiac hypertrophy in rodents

To study the ameliorating effects of curcumin in lipopolysaccharide (LPS) induced cardiac hypertrophy, mice were assigned to 4 groups (3 males and 3 females in each group): (A) control, (B) curcumin: 100  μ g/kg of body weight by intraperitoneal route (IP), (C) LPS: 60 mg/kg (IP), and (D) LPS + curcumin: both at previously stated concentrations by IP route. All mice were sacrificed as 12 hr and 24 hrs groups accordingly after LPS injection. The hearts were collected, photographed for cardiomegaly, and weighed to compare heart weight/brain weight (HW/BW) in mg/mg. For immunohistochemistry, the tissue sections were exposed to histone H3, H4 and acetylated histone H3, H4 antibody. LPS induced a significant increase in histone acetylation as shown by intense staining. In curcumin + LPS treated mice nuclear staining was similar to the control group indicating that curcumin traversed the histone acetylation activity of the LPS. To further check the mechanism of action of curcumin, p300 protein acetylation levels were analyzed. This study suggests that the probable mechanism of action of curcumin is via the reduction of p300 HAT activity.

Shuttle vectors for facile gap repair cloning and integration into a neutral locus in Candida albicans

Candida albicans is the most prevalent fungal pathogen of humans. The current techniques used to construct C. albicans strains require integration of exogenous DNA at ectopic locations, which can exert position effects on gene expression that can confound the interpretation of data from critical experiments such as virulence assays. We have identified a large intergenic region, NEUT5L, which facilitates the integration and expression of ectopic genes. To construct and integrate inserts into this novel locus, we re-engineered yeast/bacterial shuttle vectors by incorporating 550 bp of homology to NEUT5L. These vectors allow rapid, facile cloning through in vivo recombination (gap repair) in Saccharomyces cerevisiae and efficient integration of the construct into the NEUT5L locus. Other useful features of these vectors include a choice of three selectable markers (URA3, the recyclable URA3-dpl200 or NAT1), and rare restriction enzyme recognition sites for releasing the insert from the vector prior to transformation into C. albicans, thereby reducing the insert size and preventing integration of non-C. albicans DNA. Importantly, unlike the commonly used RPS1/RP10 locus, integration at NEUT5L has no negative effect on growth rates and allows native-locus expression levels, making it an ideal genomic locus for the integration of exogenous DNA in C. albicans.

Schizosaccharomyces pombe Hat1 (Kat1) is associated with Mis16 and is required for telomeric silencing

The Hat1 histone acetyltransferase has been implicated in the acetylation of histone H4 during chromatin assembly. In this study, we have characterized the Hat1 complex from the fission yeast Schizosaccharomyces pombe and have examined its role in telomeric silencing. Hat1 is found associated with the RbAp46 homologue Mis16, an essential protein. The Hat1 complex acetylates lysines 5 and 12 of histone H4, the sites that are acetylated in newly synthesized H4 in a wide range of eukaryotes. Deletion of hat1 in S. pombe is itself sufficient to cause the loss of silencing at telomeres. This is in contrast to results obtained with an S. cerevisiae hat1Δ strain, which must also carry mutations of specific acetylatable lysines in the H3 tail domain for loss of telomeric silencing to occur. Notably, deletion of hat1 from S. pombe resulted in an increase of acetylation of histone H4 in subtelomeric chromatin, concomitant with derepression of this region. A similar loss of telomeric silencing was also observed after growing cells in the presence of the deacetylase inhibitor trichostatin A. However, deleting hat1 did not cause loss of silencing at centromeres or the silent mating type locus. These results point to a direct link between Hat1, H4 acetylation, and the establishment of repressed telomeric chromatin in fission yeast.

Histone chaperones link histone nuclear import and chromatin assembly

Histone chaperones are proteins that shield histones from nonspecific interactions until they are assembled into chromatin. After their synthesis in the cytoplasm, histones are bound by different histone chaperones, subjected to a series of posttranslational modifications and imported into the nucleus. These evolutionarily conserved modifications, including acetylation and methylation, can occur in the cytoplasm, but their role in regulating import is not well understood. As part of histone import complexes, histone chaperones may serve to protect the histones during transport, or they may be using histones to promote their own nuclear localization. In addition, there is evidence that histone chaperones can play an active role in the import of histones. Histone chaperones have also been shown to regulate the localization of important chromatin modifying enzymes. This review is focused on the role histone chaperones play in the early biogenesis of histones, the distinct cytoplasmic subcomplexes in which histone chaperones have been found in both yeast and mammalian cells and the importins/karyopherins and nuclear localization signals that mediate the nuclear import of histones. We also address the role that histone chaperone localization plays in human disease. This article is part of a Special Issue entitled: Histone chaperones and chromatin assembly.

Human histone acetyltransferase 1 protein preferentially acetylates H4 histone molecules in H3.1-H4 over H3.3-H4

In mammalian cells, canonical histone H3 (H3.1) and H3 variant (H3.3) differ by five amino acids and are assembled, along with histone H4, into nucleosomes via distinct nucleosome assembly pathways. H3.1-H4 molecules are assembled by histone chaperone CAF-1 in a replication-coupled process, whereas H3.3-H4 are assembled via HIRA in a replication-independent pathway. Newly synthesized histone H4 is acetylated at lysine 5 and 12 (H4K5,12) by histone acetyltransferase 1 (HAT1). However, it remains unclear whether HAT1 and H4K5,12ac differentially regulate these two nucleosome assembly processes. Here, we show that HAT1 binds and acetylates H4 in H3.1-H4 molecules preferentially over H4 in H3.3-H4. Depletion of Hat1, the catalytic subunit of HAT1 complex, results in reduced H3.1 occupancy at H3.1-enriched genes and reduced association of Importin 4 with H3.1, but not H3.3. Finally, depletion of Hat1 or CAF-1p150 leads to changes in expression of a H3.1-enriched gene. These results indicate that HAT1 differentially impacts nucleosome assembly of H3.1-H4 and H3.3-H4.

Replication-coupled chromatin assembly of newly synthesized histones: distinct functions for the histone tail domains

The maintenance of the genome during replication requires the assembly of nucleosomes with newly synthesized histones. Achieving the deposition of newly synthesized histones in chromatin implies their transport from the cytoplasm to the nucleus at the replication sites. Several lines of evidence have revealed critical functions of the histone tail domains in these conserved cellular processes. In this review, we discuss the role of the amino termini of the nucleosome building blocks, H2A/H2B and H3/H4, in different model systems. The experimental data showed that H2A/H2B tails and H3/H4 tails display distinct functions in nuclear import and chromatin assembly. Furthermore, we describe recent studies exploiting the unique properties of the slime mold, Physarum polycephalum , that have advanced understanding of the function of the highly conserved replication-dependent diacetylation of H4.

A barcode screen for epigenetic regulators reveals a role for the NuB4/HAT-B histone acetyltransferase complex in histone turnover

Dynamic modification of histone proteins plays a key role in regulating gene expression. However, histones themselves can also be dynamic, which potentially affects the stability of histone modifications. To determine the molecular mechanisms of histone turnover, we developed a parallel screening method for epigenetic regulators by analyzing chromatin states on DNA barcodes. Histone turnover was quantified by employing a genetic pulse-chase technique called RITE, which was combined with chromatin immunoprecipitation and high-throughput sequencing. In this screen, the NuB4/HAT-B complex, containing the conserved type B histone acetyltransferase Hat1, was found to promote histone turnover. Unexpectedly, the three members of this complex could be functionally separated from each other as well as from the known interacting factor and histone chaperone Asf1. Thus, systematic and direct interrogation of chromatin structure on DNA barcodes can lead to the discovery of genes and pathways involved in chromatin modification and dynamics.

The cyclophilin AtCYP71 interacts with CAF-1 and LHP1 and functions in multiple chromatin remodeling processes

Chromatin is the primary carrier of epigenetic information in higher eukaryotes. AtCYP71 contains both cyclophilin domain and WD40 repeats. Loss of AtCYP71 function causes drastic pleiotropic phenotypic defects. Here, we show that AtCYP71 physically interacts with FAS1 and LHP1, respectively, to modulate their distribution on chromatin. The lhp1 cyp71 double mutant showed more severe phenotypes than the single mutants, suggesting that AtCYP71 and LHP1 synergistically control plant development. Such synergism was in part illustrated by the observation that LHP1 association with its specific target loci requires AtCYP71 function. We also demonstrate that AtCYP71 physically interacts with FAS1 and is indispensable for FAS1 targeting to the KNAT1 locus. Together, our data suggest that AtCYP71 is involved in fundamental processes of chromatin assembly and histone modification in plants.

Histone chaperones: modulators of chromatin marks

The many factors that control chromatin biology play key roles in essential nuclear functions like transcription, DNA damage response and repair, recombination, and replication and are critical for proper cell-cycle progression, stem cell renewal, differentiation, and development. These players belong to four broad classes: histone modifiers, chromatin remodelers, histone variants, and histone chaperones. A large number of studies have established the existence of an intricate functional crosstalk between the different factors, not only within a single class but also between different classes. In light of this, while many recent reviews have focused on structure and functions of histone chaperones, the current text highlights novel and striking links that have been established between these proteins and posttranslational modifications of histones and discusses the functional consequences of this crosstalk. These findings feed a current hot question of how cell memory may be maintained through epigenetic mechanisms involving histone chaperones.

H4 replication-dependent diacetylation and Hat1 promote S-phase chromatin assembly in vivo

This study examined the function of H3 and H4 tail domains in replication-dependent chromatin assembly. Results show distinct functions of H3 and H4 tails in nuclear import and chromatin assembly. Further investigations show that H4 diacetylation is essential but not sufficient for nuclear import, as preventing Hat1 binding impedes histone transport in nuclei.

While specific posttranslational modification patterns within the H3 and H4 tail domains are associated with the S-phase, their actual functions in replication-dependent chromatin assembly have not yet been defined. Here we used incorporation of trace amounts of recombinant proteins into naturally synchronous macroplasmodia of Physarum polycephalum to examine the function of H3 and H4 tail domains in replication-coupled chromatin assembly. We found that the H3/H4 complex lacking the H4 tail domain was not efficiently recovered in nuclei, whereas depletion of the H3 tail domain did not impede nuclear import but chromatin assembly failed. Furthermore, our results revealed that the proper pattern of acetylation on the H4 tail domain is required for nuclear import and chromatin assembly. This is most likely due to binding of Hat1, as coimmunoprecipitation experiments showed Hat1 associated with predeposition histones in the cytoplasm and with replicating chromatin. These results suggest that the type B histone acetyltransferase assists in shuttling the H3/H4 complex from cytoplasm to the replication forks.

Histone modifications during DNA replication

Faithful and accurate replication of the DNA molecule is essential for eukaryote organisms. Nonetheless, in the last few years it has become evident that inheritance of the chromatin states associated with different regions of the genome is as important as the faithful inheritance of the DNA sequence itself. Such chromatin states are determined by a multitude of factors that act to modify not only the DNA molecule, but also the histone proteins associated with it. For instance, histones can be posttranslationally modified, and it is well established that these posttranslational marks are involved in several essential nuclear processes such as transcription and DNA repair. However, recent evidence indicates that posttranslational modifications of histones might be relevant during DNA replication. Hence, the aim of this review is to describe the most recent publications related to the role of histone posttranslational modifications during DNA replication.

Molecular functions of the histone acetyltransferase chaperone complex Rtt109-Vps75

Histone acetylation and nucleosome remodeling regulate DNA damage repair, replication and transcription. Rtt109, a recently discovered histone acetyltransferase (HAT) from Saccharomyces cerevisiae, functions with the histone chaperone Asf1 to acetylate lysine K56 on histone H3 (H3K56), a modification associated with newly synthesized histones. In vitro analysis of Rtt109 revealed that Vps75, a Nap1 family histone chaperone, could also stimulate Rtt109-dependent acetylation of H3K56. However, the molecular function of the Rtt109-Vps75 complex remains elusive. Here we have probed the molecular functions of Vps75 and the Rtt109-Vps75 complex through biochemical, structural and genetic means. We find that Vps75 stimulates the k_cat of histone acetylation by ∼100-fold relative to Rtt109 alone and enhances acetylation of K9 in the H3 histone tail. Consistent with the In vitro evidence, cells lacking Vps75 showed a substantial reduction (60%) in H3K9 acetylation during S phase. X-ray structural, biochemical and genetic analyses of Vps75 indicate a unique, structurally dynamic Nap1-like fold that suggests a potential mechanism of Vps75-dependent activation of Rtt109. Together, these data provide evidence for a multifunctional HAT-chaperone complex that acetylates histone H3 and deposits H3-H4 onto DNA, linking histone modification and nucleosome assembly.

Analysis of Saccharomyces cerevisiae histone H3 mutants reveals the role of the alphaN helix in nucleosome function

To understand the role of histone H3 sub-domains in chromatin function, 35 histone H3 tandem alanine mutants were generated and tested for their viability and sensitivity to DNA damaging agents. Among 13 non-viable H3 mutants, 6 were mapped around the alphaN helix and preceding tail region. Mutants with individual alanine substitutions in this region were viable but developed multiple sensitivities to DNA damaging agents. The only viable triple mutant, REI49-50A, in the alphaN helix region could not grow when combined with histone chaperone mutations, such as asf1Delta, cac1Delta, or hir1Delta, suggesting that this particular region is important when the histone assembly/disassembly pathway is compromised. In addition, further analysis showed that T45, E50, or F54 of the alphaN helix genetically interacted with a histone chaperone (Asf1) and transcription elongation factors (Paf1 and Hpr1). These results suggest a specific role of the H3 alphaN helix in histone dynamics mediated by histone chaperones, which might be important during transcription elongation.

UV sensitive mutations in histone H3 in Saccharomyces cerevisiae that alter specific K79 methylation states genetically act through distinct DNA repair pathways

Chromatin serves as a regulator of various nuclear processes, with post-translational modifications of histone proteins serving as modulators to influence chromatin function. We have previously shown that histone H3 K79 methylation is important for repair of UV-induced DNA damage in Saccharomyces cerevisiae, acting through multiple repair pathways. To evaluate the potential role of distinct K79 methylation states in DNA repair, we identified four mutations in histone H3 that confer sensitivity to UV, each of which also has a distinct effect on specific K79 methylation states. Epistasis analyses indicate that each mutation exerts its phenotypic effects through distinct subsets of the various DNA damage response pathways, suggesting the existence of discrete roles for histone H3 in DNA damage checkpoint and repair pathways. Furthermore, we find that the distribution of K79 methylation states is altered by mutation of the acetylatable N terminal lysines in histone H4. The combined results suggest that K79 methylation states may be modulated in response to UV damage via a trans-histone regulatory pathway, and that distinct methylation states may provide a means of coordinating specific DNA repair and damage checkpoint pathways.

Dynein-dependent nuclear dynamics affect morphogenesis in Candida albicans by means of the Bub2p spindle checkpoint

Candida albicans, the most prevalent fungal pathogen of humans, grows with multiple morphologies. The dynamics of nuclear movement are similar in wild-type yeast and pseudohyphae: nuclei divide across the bud neck. By contrast, in hyphae, nuclei migrate 10-20 microm into the growing germ tube before dividing. We analyzed the role of the dynein-dynactin complex in hyphal and yeast cells using time-lapse fluorescence microscopy. Cells lacking the heavy chain of cytoplasmic dynein or the p150(Glued) subunit of dynactin were defective in the position and orientation of the spindle. Hyphal cells often failed to deliver a nucleus to the daughter cell, resulting in defects in morphogenesis. Under yeast growth conditions, cultures included a mixture of yeast and pseudohyphal-like cells that exhibited distinctive defects in nuclear dynamics: in yeast, nuclei divided within the mother cell, and the spindle position checkpoint protein Bub2p ensured the delivery of the daughter nucleus to the daughter cell before cytokinesis; in pseudohyphal-like cells, pre-mitotic nuclei migrated into the daughter and no checkpoint ensured return of a nucleus to the mother cell before cytokinesis. Analysis of double mutants indicated that Bub2p also mediated the pre-anaphase arrest and polarization of pseudohyphal-like cells. Thus, Bub2p has two distinct roles in C. albicans cells lacking dynein: it mediates pre-anaphase arrest and it coordinates spindle disassembly with mitotic exit.

NAD+-dependent deacetylation of H4 lysine 16 by class III HDACs

Histone deacetylases (HDACs) catalyse the removal of acetyl groups from the N-terminal tails of histones. All known HDACs can be categorized into one of four classes (I-IV). The class III HDAC or silencing information regulator 2 (Sir2) family exhibits characteristics consistent with a distinctive role in regulation of chromatin structure. Accumulating data suggest that these deacetylases acquired new roles as genomic complexity increased, including deacetylation of non-histone proteins and functional diversification in mammals. However, the intrinsic regulation of chromatin structure in species as diverse as yeast and humans, underscores the pressure to conserve core functions of class III HDACs, which are also known as Sirtuins. One of the key factors that might have contributed to this preservation is the intimate relationship between some members of this group of proteins (SirT1, SirT2 and SirT3) and deacetylation of a specific residue in histone H4, lysine 16 (H4K16). Evidence accumulated over the years has uncovered a unique role for H4K16 in chromatin structure throughout eukaryotes. Here, we review the recent findings about the functional relationship between H4K16 and the Sir2 class of deacetylases and how that relationship might impact aging and diseases including cancer and diabetes.

Mutational analysis of H3 and H4 N termini reveals distinct roles in nuclear import

Core histones H3 and H4 are rapidly imported into the nucleus by members of the karyopherin (Kap)/importin family. We showed that H3 and H4 interact with Kap123p, histone acetyltransferase-B complex (HAT-B), and Asf1p in cytosol. In vivo analysis indicated that Kap123p is required for H3-mediated import, whereas H4 utilizes multiple Kaps including Kap123p. The evolutionary conservation of H3 and H4 cytoplasmic acetylation led us to analyze the role of acetylation in nuclear transport. We determined that lysine 14 is critical for H3 NLS function in vivo and demonstrated that mutation of H3 lysine 14 to the acetylation-mimic glutamine decreased association with Kap123p in vitro. Several lysines in the H4 NLS are important for its function. We showed that mutation of key lysines to glutamine resulted in a greater import defect than mutation to arginine, suggesting that positive charge promotes NLS function. Lastly we determined that six of ten N-terminal acetylation sites in H3 and H4 can be mutated to arginine, indicating that deposition acetylation is not absolutely necessary in vivo. However, the growth defect of these mutants suggests that acetylation does play an important role in import. These findings suggest a model where cytosolic histones bind import karyopherins prior to acetylation. Other factors are recruited to this complex such as HAT-B and Asf1p; these factors in turn promote acetylation. Acetylation may be important for modulating the interaction with transport factors and may play a role in the release of histones from karyopherins in the nucleus.

Chromatin challenges during DNA replication and repair

Inheritance and maintenance of the DNA sequence and its organization into chromatin are central for eukaryotic life. To orchestrate DNA-replication and -repair processes in the context of chromatin is a challenge, both in terms of accessibility and maintenance of chromatin organization. To meet the challenge of maintenance, cells have evolved efficient nucleosome-assembly pathways and chromatin-maturation mechanisms that reproduce chromatin organization in the wake of DNA replication and repair. The aim of this Review is to describe how these pathways operate and to highlight how the epigenetic landscape may be stably maintained even in the face of dramatic changes in chromatin structure.

Simple kinetic relationships and nonspecific competition govern nuclear import rates in vivo

Many cargoes destined for nuclear import carry nuclear localization signals that are recognized by karyopherins (Kaps). We present methods to quantitate import rates and measure Kap and cargo concentrations in single yeast cells in vivo, providing new insights into import kinetics. By systematically manipulating the amounts, types, and affinities of Kaps and cargos, we show that import rates in vivo are simply governed by the concentrations of Kaps and their cargo and the affinity between them. These rates fit to a straightforward pump–leak model for the import process. Unexpectedly, we deduced that the main limiting factor for import is the poor ability of Kaps and cargos to find each other in the cytoplasm in a background of overwhelming nonspecific competition, rather than other more obvious candidates such as the nuclear pore complex and Ran. It is likely that most of every import round is taken up by Kaps and nuclear localization signals sampling other cytoplasmic proteins as they locate each other in the cytoplasm.

Properties of the Type B Histone Acetyltransferase Hat1

The Hat1 histone acetyltransferase catalyzes the acetylation of H4 at lysines 5 and 12, the same sites that are acetylated in newly synthesized histone H4. By performing histone acetyltransferase (HAT) assays on various synthetic H4 N-terminal peptides, we have examined the interactions between Hat1 and the H4 tail domain. It was found that acetylation requires the presence of positively charged amino acids at positions 8 and 16 of H4, positions that are normally occupied by lysine; however, lysine per se is not essential and can be replaced by arginine. In contrast, replacing Lys-8 and -16 of H4 with glutamines reduces acetylation to background levels. Similarly, phosphorylation of Ser-1 of the H4 tail depresses acetylation by both yeast Hat1p and the human HAT-B complex. These results strongly support the model proposed by Ramakrishnan and colleagues for the interaction between Hat1 and the H4 tail (Dutnall, R. N., Tafrov, S. T., Sternglanz, R., and Ramakrishnan, V. (1998) Cell 94, 427-438) and may have implications for the genetic analysis of histone acetylation. It was also found that Lys-12 of H4 is preferentially acetylated by human HAT-B, in further agreement with the proposed model of H4 tail binding. Finally, we have demonstrated that deletion of the hat1 gene from the fission yeast Schizosaccharomyces pombe causes increased sensitivity to the DNA-damaging agent methyl methanesulfonate in the absence of any additional mutations. This is in contrast to results obtained with a Saccharomyces cerevisiae hat1Delta strain, which must also carry mutations of the acetylatable lysines of H3 for heightened methyl methanesulfonate sensitivity to be observed. Thus, although the role of Hat1 in DNA damage repair is evolutionarily conserved, the ability of H3 acetylation to compensate for Hat1 deletion appears to be more variable.

The histone chaperone anti-silencing function 1 stimulates the acetylation of newly synthesized histone H3 in S-phase

Anti-silencing function 1 (Asf1) is a highly conserved chaperone of histones H3/H4 that assembles or disassembles chromatin during transcription, replication, and repair. We have found that budding yeast lacking Asf1 has greatly reduced levels of histone H3 acetylated at lysine 9. Lysine 9 is acetylated on newly synthesized budding yeast histone H3 prior to its assembly onto newly replicated DNA. Accordingly, we found that the vast majority of H3 Lys-9 acetylation peaked in S-phase, and this S-phase peak of H3 lysine 9 acetylation was absent in yeast lacking Asf1. By contrast, deletion of ASF1 has no effect on the S-phase specific peak of H4 lysine 12 acetylation; another modification carried by newly synthesized histones prior to chromatin assembly. We show that Gcn5 is the histone acetyltransferase responsible for the S-phase-specific peak of H3 lysine 9 acetylation. Strikingly, overexpression of Asf1 leads to greatly increased levels of H3 on acetylation on lysine 56 and Gcn5-dependent acetylation on lysine 9. Analysis of a panel of Asf1 mutations that modulate the ability of Asf1 to bind to histones H3/H4 demonstrates that the histone binding activity of Asf1 is required for the acetylation of Lys-9 and Lys-56 on newly synthesized H3. These results demonstrate that Asf1 does not affect the stability of the newly synthesized histones per se, but instead histone binding by Asf1 promotes the efficient acetylation of specific residues of newly synthesized histone H3.

Chromatin assembly: a basic recipe with various flavours

Packaging of eukaryotic genomes into chromatin is a hierarchical mechanism, starting with histone deposition onto DNA to produce nucleosome arrays, which then further fold and ultimately form functional domains. Recent studies provide interesting insight into how nucleosome assembly is coordinated with histone and DNA metabolism and underline the combined contribution of histone chaperones and chromatin remodelers. How these factors operate at a molecular level is a matter of current investigation. New data highlight the importance of histone dimers as deposition entities for de novo nucleosome assembly and identify dedicated machineries involved in histone variant deposition.

Modifications of H3 and H4 during chromatin replication, nucleosome assembly, and histone exchange

Histone posttranslational modifications that accompany DNA replication, nucleosome assembly, and H2A/H2B exchange were examined in human tissue culture cells. Through microsequencing analysis and chromatin immunoprecipitation, it was found that a subset of newly synthesized H3.2/H3.3 is modified by acetylation and methylation at sites that correlate with transcriptional competence. Immunoprecipitation experiments suggest that cytosolic predeposition complexes purified from cells expressing FLAG-H4 contain H3/H4 dimers, not tetramers. Studies of the deposition of newly synthesized H2A/H2B onto replicating and nonreplicating chromatin demonstrated that H2A/H2B exchange takes place in chromatin regions that contain acetylated H4; however, there is no single pattern of H4 acetylation that accompanies exchange. H2A/H2B exchange is also largely independent of the deposition of replacement histone variant, H3.3. Finally, immunoprecipitation of nucleosomes replicated in the absence of de novo nucleosome assembly showed that histone modifications do not prevent the transfer of parental histones to newly replicated DNA and thus have the potential to serve as means of epigenetic inheritance. Our experiments provide an in-depth analysis of the "histone code" associated with chromatin replication and dynamic histone exchange in human cells.

Contribution of CAF-I to anaphase-promoting-complex-mediated mitotic chromatin assembly in Saccharomyces cerevisiae

The anaphase-promoting complex (APC) is required for mitotic progression and genomic stability. Recently, we demonstrated that the APC is also required for mitotic chromatin assembly and longevity. Here, we investigated the role the APC plays in chromatin assembly. We show that apc5(CA) mutations genetically interact with the CAF-I genes as well as ASF1, HIR1, and HIR2. When present in multiple copies, the individual CAF-I genes, CAC1, CAC2, and MSI1, suppress apc5(CA) phenotypes in a CAF-1- and Asf1p-independent manner. CAF-I and the APC functionally overlap, as cac1delta cac2delta msi1delta (caf1delta) cells expressing apc5(CA) exhibit a phenotype more severe than that of apc5(CA) or caf1delta. The Ts- phenotypes observed in apc5(CA) and apc5(CA) caf mutants may be rooted in compromised histone metabolism, as coexpression of histones H3 and H4 suppressed the Ts- defects. Synthetic genetic interactions were also observed in apc5(CA) asf1delta cells. Furthermore, increased expression of genes encoding Asf1p, Hir1p, and Hir2p suppressed the apc5(CA) Ts- defect in a CAF-I-dependent manner. Together, these results suggest the existence of a complex molecular mechanism controlling APC-dependent chromatin assembly. Our data suggest the APC functions with the individual CAF-I subunits, Asf1p, and the Hir1p and Hir2p proteins. However, Asf1p and an intact CAF-I complex are dispensable for CAF-I subunit suppression, whereas CAF-I is necessary for ASF1, HIR1, and HIR2 suppression of apc5(CA) phenotypes. We discuss the implications of our observations.