Chromatin assembly factor I mutants defective for PCNA binding require Asf1/Hir proteins for silencing

Acute multi-level response to defective de novo chromatin assembly in S-phase

Long-term perturbation of de novo chromatin assembly during DNA replication has profound effects on epigenome maintenance and cell fate. The early mechanistic origin of these defects is unknown. Here, we combine acute degradation of Chromatin Assembly Factor 1 (CAF-1), a key player in de novo chromatin assembly, with single-cell genomics, quantitative proteomics, and live-microscopy to uncover these initiating mechanisms in human cells. CAF-1 loss immediately slows down DNA replication speed and renders nascent DNA hyperaccessible. A rapid cellular response, distinct from canonical DNA damage signaling, is triggered and lowers histone mRNAs. As a result, histone variants usage and their modifications are altered, limiting transcriptional fidelity and delaying chromatin maturation within a single S-phase. This multi-level response induces a cell-cycle arrest after mitosis. Our work reveals the immediate consequences of defective de novo chromatin assembly during DNA replication, explaining how at later times the epigenome and cell fate can be altered.

Disordered regions and folded modules in CAF-1 promote histone deposition in Schizosaccharomyces pombe

Genome and epigenome integrity in eukaryotes depends on the proper coupling of histone deposition with DNA synthesis. This process relies on the evolutionary conserved histone chaperone CAF-1 for which the links between structure and functions are still a puzzle. While studies of the Saccharomyces cerevisiae CAF-1 complex enabled to propose a model for the histone deposition mechanism, we still lack a framework to demonstrate its generality and in particular, how its interaction with the polymerase accessory factor PCNA is operating. Here, we reconstituted a complete SpCAF-1 from fission yeast. We characterized its dynamic structure using NMR, SAXS and molecular modeling together with in vitro and in vivo functional studies on rationally designed interaction mutants. Importantly, we identify the unfolded nature of the acidic domain which folds up when binding to histones. We also show how the long KER helix mediates DNA binding and stimulates SpCAF-1 association with PCNA. Our study highlights how the organization of CAF-1 comprising both disordered regions and folded modules enables the dynamics of multiple interactions to promote synthesis-coupled histone deposition essential for its DNA replication, heterochromatin maintenance, and genome stability functions.

Chromatin assembly factor-1 preserves genome stability in ctf4Δ cells by promoting sister chromatid cohesion

Chromatin assembly and the establishment of sister chromatid cohesion are intimately connected to the progression of DNA replication forks. Here we examined the genetic interaction between the heterotrimeric chromatin assembly factor-1 (CAF-1), a central component of chromatin assembly during replication, and the core replisome component Ctf4. We find that CAF-1 deficient cells as well as cells affected in newly-synthesized H3-H4 histones deposition during DNA replication exhibit a severe negative growth with ctf4Δ mutant. We dissected the role of CAF-1 in the maintenance of genome stability in ctf4Δ yeast cells. In the absence of CTF4, CAF-1 is essential for viability in cells experiencing replication problems, in cells lacking functional S-phase checkpoint or functional spindle checkpoint, and in cells lacking DNA repair pathways involving homologous recombination. We present evidence that CAF-1 affects cohesin association to chromatin in a DNA-damage-dependent manner and is essential to maintain cohesion in the absence of CTF4. We also show that Eco1-catalyzed Smc3 acetylation is reduced in absence of CAF-1. Furthermore, we describe genetic interactions between CAF-1 and essential genes involved in cohesin loading, cohesin stabilization, and cohesin component indicating that CAF-1 is crucial for viability when sister chromatid cohesion is affected. Finally, our data indicate that the CAF-1-dependent pathway required for cohesion is functionally distinct from the Rtt101-Mms1-Mms22 pathway which functions in replicated chromatin assembly. Collectively, our results suggest that the deposition by CAF-1 of newly-synthesized H3-H4 histones during DNA replication creates a chromatin environment that favors sister chromatid cohesion and maintains genome integrity.

Nuclear pore complexes mediate subtelomeric gene silencing by regulating PCNA levels on chromatin

The nuclear pore complex (NPC) physically interacts with chromatin and regulates gene expression. The Saccharomyces cerevisiae inner ring nucleoporin Nup170 has been implicated in chromatin organization and the maintenance of gene silencing in subtelomeric regions. To gain insight into how Nup170 regulates this process, we used protein-protein interactions, genetic interactions, and transcriptome correlation analyses to identify the Ctf18-RFC complex, an alternative proliferating cell nuclear antigen (PCNA) loader, as a facilitator of the gene regulatory functions of Nup170. The Ctf18-RFC complex is recruited to a subpopulation of NPCs that lack the nuclear basket proteins Mlp1 and Mlp2. In the absence of Nup170, PCNA levels on DNA are reduced, resulting in the loss of silencing of subtelomeric genes. Increasing PCNA levels on DNA by removing Elg1, which is required for PCNA unloading, rescues subtelomeric silencing defects in nup170Δ. The NPC, therefore, mediates subtelomeric gene silencing by regulating PCNA levels on DNA.

Structural and functional studies of PCNA from African swine fever virus

Proliferating cell nuclear antigen (PCNA) belongs to the DNA sliding clamp family. Via interacting with various partner proteins, PCNA plays critical roles in DNA replication, DNA repair, chromatin assembly, epigenetic inheritance, chromatin remodeling, and many other fundamental biological processes. Although PCNA and PCNA-interacting partner networks are conserved across species, PCNA of a given species is rarely functional in heterologous systems, emphasizing the importance of more representative PCNA studies. Here, we report two crystal structures of PCNA from African swine fever virus (ASFV), which is the only member of the Asfarviridae family. Compared to the eukaryotic and archaeal PCNAs and the sliding clamp structural homologs from other viruses, AsfvPCNA possesses unique sequences and/or conformations at several regions, such as the J-loop, interdomain-connecting loop (IDCL), P-loop, and C-tail, which are involved in partner recognition or modification of sliding clamps. In addition to double-stranded DNA binding, we also demonstrate that AsfvPCNA can modestly enhance the ligation activity of the AsfvLIG protein. The unique structural features of AsfvPCNA can serve as a potential target for the development of ASFV-specific inhibitors and help combat the deadly virus. IMPORTANCE Two high-resolution crystal structures of African swine fever virus proliferating cell nuclear antigen (AsfvPCNA) are presented here. Structural comparison revealed that AsfvPCNA is unique at several regions, such as the J-loop, the interdomain-connecting loop linker, and the P-loop, which may play important roles in ASFV-specific partner selection of AsfvPCNA. Unlike eukaryotic and archaeal PCNAs, AsfvPCNA possesses high double-stranded DNA-binding affinity. Besides DNA binding, AsfvPCNA can also modestly enhance the ligation activity of the AsfvLIG protein, which is essential for the replication and repair of ASFV genome. The unique structural features make AsfvPCNA a potential target for drug development, which will help combat the deadly virus.

Biochemical and Structural Insights into the Winged Helix Domain of P150, the Largest Subunit of the Chromatin Assembly Factor 1

The Chromatin Assembly Factor 1 is a heterotrimeric complex responsible for the nucleosome assembly during DNA replication and DNA repair. In humans, the largest subunit P150 is the major actor of this process. It has been recently considered as a tumor-associated protein due to its overexpression in many malignancies. Structural and functional studies targeting P150 are still limited and only scarce information about this subunit is currently available. Literature data and bioinformatics analysis assisted the identification of a stable DNA binding domain, encompassing residues from 721 to 860 of P150 within the full-length protein. This domain was recombinantly produced and in vitro investigated. An acidic region modulating its DNA binding ability was also identified and characterized. Results showed similarities and differences between the P150 and its yeast homologue, namely Cac-1, suggesting that, although sharing a common biological function, the two proteins may also possess different features.

Chromatin dynamics and DNA replication roadblocks

A broad spectrum of spontaneous and genotoxin-induced DNA lesions impede replication fork progression. The DNA damage response that acts to promote completion of DNA replication is associated with dynamic changes in chromatin structure that include two distinct processes which operate genome-wide during S-phase. The first, often referred to as histone recycling or parental histone segregation, is characterized by the transfer of parental histones located ahead of replication forks onto nascent DNA. The second, known as de novo chromatin assembly, consists of the deposition of new histone molecules onto nascent DNA. Because these two processes occur at all replication forks, their potential to influence a multitude of DNA repair and DNA damage tolerance mechanisms is considerable. The purpose of this review is to provide a description of parental histone segregation and de novo chromatin assembly, and to illustrate how these processes influence cellular responses to DNA replication roadblocks.

Variation, Variegation and Heritable Gene Repression in S. cerevisiae

Phenotypic heterogeneity provides growth advantages for a population upon changes of the environment. In S. cerevisiae, such heterogeneity has been observed as "on/off" states in the expression of individual genes in individual cells. These variations can persist for a limited or extended number of mitotic divisions. Such traits are known to be mediated by heritable chromatin structures, by the mitotic transmission of transcription factors involved in gene regulatory circuits or by the cytoplasmic partition of prions or other unstructured proteins. The significance of such epigenetic diversity is obvious, however, we have limited insight into the mechanisms that generate it. In this review, we summarize the current knowledge of epigenetically maintained heterogeneity of gene expression and point out similarities and converging points between different mechanisms. We discuss how the sharing of limiting repression or activation factors can contribute to cell-to-cell variations in gene expression and to the coordination between short- and long- term epigenetic strategies. Finally, we discuss the implications of such variations and strategies in adaptation and aging.

CAF-1 and Rtt101p function within the replication-coupled chromatin assembly network to promote H4 K16ac, preventing ectopic silencing

Replication-coupled chromatin assembly is achieved by a network of alternate pathways containing different chromatin assembly factors and histone-modifying enzymes that coordinate deposition of nucleosomes at the replication fork. Here we describe the organization of a CAF-1-dependent pathway in Saccharomyces cerevisiae that regulates acetylation of histone H4 K16. We demonstrate factors that function in this CAF-1-dependent pathway are important for preventing establishment of silenced states at inappropriate genomic sites using a crippled HMR locus as a model, while factors specific to other assembly pathways do not. This CAF-1-dependent pathway required the cullin Rtt101p, but was functionally distinct from an alternate pathway involving Rtt101p-dependent ubiquitination of histone H3 and the chromatin assembly factor Rtt106p. A major implication from this work is that cells have the inherent ability to create different chromatin modification patterns during DNA replication via differential processing and deposition of histones by distinct chromatin assembly pathways within the network.

Cac1 WHD and PIP domains have distinct roles in replisome progression and genomic stability

Replication-coupled (RC) nucleosome assembly is an essential process in eukaryotic cells to maintain chromatin structure during DNA replication. The deposition of newly-synthesized H3/H4 histones during DNA replication is facilitated by specialized histone chaperones. CAF-1 is an important histone chaperone complex and its main subunit, Cac1p, contains a PIP and WHD domain for interaction with PCNA and the DNA, respectively. While Cac1p subunit was extensively studied in different systems much less is known regarding the importance of the PIP and WHD domains in replication fork progression and genome stability. By exploiting a time-lapse microscopy system for monitoring DNA replication in individual live cells, we examined how mutations in these Cac1p domains affect replication fork progression and post-replication characteristics. Our experiments revealed that mutations in the Cac1p WHD domain, which abolished the CAF-1-DNA interaction, slows down replication fork progression. In contrast, mutations in Cac1p PIP domain, abolishing Cac1p-PCNA interaction, lead to extended late-S/Anaphase duration, elevated number of RPA foci and increased spontaneous mutation rate. Our research shows that Cac1p WHD and PIP domains have distinct roles in high replisome progression and maintaining genome stability during cell cycle progression.

The replisome guides nucleosome assembly during DNA replication

Nucleosome assembly during DNA replication is tightly coupled to ongoing DNA synthesis. This process, termed DNA replication-coupled (RC) nucleosome assembly, is essential for chromatin replication and has a great impact on both genome stability maintenance and epigenetic inheritance. This review discusses a set of recent findings regarding the role of replisome components contributing to RC nucleosome assembly. Starting with a brief introduction to the factors involved in nucleosome assembly and some aspects of the architecture of the eukaryotic replisome, we discuss studies from yeast to mammalian cells and the interactions of replisome components with histones and histone chaperones. We describe the proposed functions of replisome components during RC nucleosome assembly and discuss their impacts on histone segregation and implications for epigenetic inheritance.

Dysfunctional CAF-I reveals its role in cell cycle progression and differential regulation of gene silencing

Chromatin Assembly Factor I (CAF-I) plays a central role in the reassembly of H3/H4 histones during DNA replication. In S. cerevisiae CAF-I is not essential and its loss is associated with reduced gene silencing at telomeres and increased sensitivity to DNA damage. Two kinases, Cyclin Dependent Kinase (CDK) and Dbf4-Dependent Kinase (DDK), are known to phosphorylate the Cac1p subunit of CAF-I, but their role in the regulation of CAF-I activity is not well understood. In this study we systematically mutated the phosphorylation target sites of these kinases. We show that concomitant mutations of the CDK and DDK target sites of Cac1p lead to growth retardation and significant cell cycle defects, altered cell morphology and increased sensitivity to DNA damage. Surprisingly, some mutations also produced flocculation, a phenotype that is lost in most laboratory strains, and displayed elevated expression of FLO genes. None of these effects is observed upon the destruction of CAF-I. In contrast, the mutations that caused flocculation did not affect gene silencing at the mating type and subtelomeric loci. We conclude that dysfunctional CAF-I produces severe phenotypes, which reveal a possible role of CAF-I in the coordination of DNA replication, chromatin reassembly and cell cycle progression. Our study highlights the role of phosphorylation of Cac1p by CDK and a putative role for DDK in the transmission and re-assembly of chromatin during DNA replication.

Histone chaperones and the Rrm3p helicase regulate flocculation in S. cerevisiae

Background

Biofilm formation or flocculation is a major phenotype in wild type budding yeasts but rarely seen in laboratory yeast strains. Here, we analysed flocculation phenotypes and the expression of FLO genes in laboratory strains with various genetic backgrounds.

Results

We show that mutations in histone chaperones, the helicase RRM3 and the Histone Deacetylase HDA1 de-repress the FLO genes and partially reconstitute flocculation. We demonstrate that the loss of repression correlates to elevated expression of several FLO genes, to increased acetylation of histones at the promoter of FLO1 and to variegated expression of FLO11. We show that these effects are related to the activity of CAF-1 at the replication forks. We also demonstrate that nitrogen starvation or inhibition of histone deacetylases do not produce flocculation in W303 and BY4742 strains but do so in strains compromised for chromatin maintenance. Finally, we correlate the de-repression of FLO genes to the loss of silencing at the subtelomeric and mating type gene loci.

Conclusions

We conclude that the deregulation of chromatin maintenance and transmission is sufficient to reconstitute flocculation in laboratory yeast strains. Consequently, we propose that a gain in epigenetic silencing is a major contributing factor for the loss of flocculation phenotypes in these strains. We suggest that flocculation in yeasts provides an excellent model for addressing the challenging issue of how epigenetic mechanisms contribute to evolution.

Chromatin Modifiers Alter Recombination Between Divergent DNA Sequences

Recombination between divergent DNA sequences is actively prevented by heteroduplex rejection mechanisms. In baker's yeast, such antirecombination mechanisms can be initiated by the recognition of DNA mismatches in heteroduplex DNA by MSH proteins, followed by recruitment of the Sgs1-Top3-Rmi1 helicase-topoisomerase complex to unwind the recombination intermediate. We previously showed that the repair/rejection decision during single-strand annealing recombination is temporally regulated by MSH (MutShomolog) protein levels and by factors that excise nonhomologous single-stranded tails. These observations, coupled with recent studies indicating that mismatch repair (MMR) factors interact with components of the histone chaperone machinery, encouraged us to explore roles for epigenetic factors and chromatin conformation in regulating the decision to reject vs. repair recombination between divergent DNA substrates. This work involved the use of an inverted repeat recombination assay thought to measure sister chromatid repair during DNA replication. Our observations are consistent with the histone chaperones CAF-1 and Rtt106, and the histone deacetylase Sir2, acting to suppress heteroduplex rejection and the Rpd3, Hst3, and Hst4 deacetylases acting to promote heteroduplex rejection. These observations, and double-mutant analysis, have led to a model in which nucleosomes located at DNA lesions stabilize recombination intermediates and compete with MMR factors that mediate heteroduplex rejection.

The histone chaperone CAF-1 cooperates with the DNA methyltransferases to maintain Cd4 silencing in cytotoxic T cells

In this study, Ng et al. investigated the maintenance of silent gene states and how the Cd4 gene is stably repressed in CD8⁺ T cells. Using CRISPR and shRNA screening, they identified the histone chaperone CAF-1 as a critical component for Cd4 repression and propose that the heritable silencing of the Cd4 gene in CD8⁺ T cells exploits cooperative functions among the DNA methyltransferases, CAF-1, and histone-modifying enzymes.

The transcriptional repression of alternative lineage genes is critical for cell fate commitment. Mechanisms by which locus-specific gene silencing is initiated and heritably maintained during cell division are not clearly understood. To study the maintenance of silent gene states, we investigated how the Cd4 gene is stably repressed in CD8⁺ T cells. Through CRISPR and shRNA screening, we identified the histone chaperone CAF-1 as a critical component for Cd4 repression. We found that the large subunit of CAF-1, Chaf1a, requires the N-terminal KER domain to associate with the histone deacetylases HDAC1/2 and the histone demethylase LSD1, enzymes that also participate in Cd4 silencing. When CAF-1 was lacking, Cd4 derepression was markedly enhanced in the absence of the de novo DNA methyltransferase Dnmt3a but not the maintenance DNA methyltransferase Dnmt1. In contrast to Dnmt1, Dnmt3a deficiency did not significantly alter levels of DNA methylation at the Cd4 locus. Instead, Dnmt3a deficiency sensitized CD8⁺ T cells to Cd4 derepression mediated by compromised functions of histone-modifying factors, including the enzymes associated with CAF-1. Thus, we propose that the heritable silencing of the Cd4 gene in CD8⁺ T cells exploits cooperative functions among the DNA methyltransferases, CAF-1, and histone-modifying enzymes.

Modulation of Gene Silencing by Cdc7p via H4 K16 Acetylation and Phosphorylation of Chromatin Assembly Factor CAF-1 in Saccharomyces cerevisiae

CAF-1 is an evolutionarily conserved H3/H4 histone chaperone that plays a key role in replication-coupled chromatin assembly and is targeted to the replication fork via interactions with PCNA, which, if disrupted, leads to epigenetic defects. In Saccharomyces cerevisiae, when the silent mating-type locus HMR contains point mutations within the E silencer, Sir protein association and silencing is lost. However, mutation of CDC7, encoding an S-phase-specific kinase, or subunits of the H4 K16-specific acetyltransferase complex SAS-I, restore silencing to this crippled HMR, HMR a e** Here, we observed that loss of Cac1p, the largest subunit of CAF-1, also restores silencing at HMR a e**, and silencing in both cac1Δ and cdc7 mutants is suppressed by overexpression of SAS2 We demonstrate Cdc7p and Cac1p interact in vivo in S phase, but not in G1, consistent with observed cell cycle-dependent phosphorylation of Cac1p, and hypoacetylation of chromatin at H4 K16 in both cdc7 and cac1Δ mutants. Moreover, silencing at HMR a e** is restored in cells expressing cac1p mutants lacking Cdc7p phosphorylation sites. We also discovered that cac1Δ and cdc7-90 synthetically interact negatively in the presence of DNA damage, but that Cdc7p phosphorylation sites on Cac1p are not required for responses to DNA damage. Combined, our results support a model in which Cdc7p regulates replication-coupled histone modification via a CAC1-dependent mechanism involving H4 K16ac deposition, and thereby silencing, while CAF-1-dependent replication- and repair-coupled chromatin assembly per se are functional in the absence of phosphorylation of Cdc7p consensus sites on CAF-1.

Mechanistic insights into histone deposition and nucleosome assembly by the chromatin assembly factor-1

Eukaryotic chromatin is a highly dynamic structure with essential roles in virtually all DNA-dependent cellular processes. Nucleosomes are a barrier to DNA access, and during DNA replication, they are disassembled ahead of the replication machinery (the replisome) and reassembled following its passage. The Histone chaperone Chromatin Assembly Factor-1 (CAF-1) interacts with the replisome and deposits H3-H4 directly onto newly synthesized DNA. Therefore, CAF-1 is important for the establishment and propagation of chromatin structure. The molecular mechanism by which CAF-1 mediates H3-H4 deposition has remained unclear. However, recent studies have revealed new insights into the architecture and stoichiometry of the trimeric CAF-1 complex and how it interacts with and deposits H3-H4 onto substrate DNA. The CAF-1 trimer binds to a single H3-H4 dimer, which induces a conformational rearrangement in CAF-1 promoting its interaction with substrate DNA. Two CAF-1•H3-H4 complexes co-associate on nucleosome-free DNA depositing (H3-H4)2 tetramers in the first step of nucleosome assembly. Here, we review the progress made in our understanding of CAF-1 structure, mechanism of action, and how CAF-1 contributes to chromatin dynamics during DNA replication.

Chromatin and Metabolism

Chromatin is a mighty consumer of cellular energy generated by metabolism. Metabolic status is efficiently coordinated with transcription and translation, which also feed back to regulate metabolism. Conversely, suppression of energy utilization by chromatin processes may serve to preserve energy resources for cell survival. Most of the reactions involved in chromatin modification require metabolites as their cofactors or coenzymes. Therefore, the metabolic status of the cell can influence the spectra of posttranslational histone modifications and the structure, density and location of nucleosomes, impacting epigenetic processes. Thus, transcription, translation, and DNA/RNA biogenesis adapt to cellular metabolism. In addition to dysfunctions of metabolic enzymes, imbalances between metabolism and chromatin activities trigger metabolic disease and life span alteration. Here, we review the synthesis of the metabolites and the relationships between metabolism and chromatin function. Furthermore, we discuss how the chromatin response feeds back to metabolic regulation in biological processes.

Overexpression of chromatin assembly factor-1/p60 predicts biological behaviour of laryngeal carcinomas

This study analysed the immunohistochemical expression of the CAF-1/p60 protein in laryngeal cancers. CAF-1/p60 assumes an independent discriminative and prognostic value in laryngeal neoplasms; the presence of this protein in carcinoma in situ compared with laryngeal precancerous and larynx infiltrating tumours. We assessed the immunohistochemical expression of CAF-1/p60 in 30 cases of moderate and/or severe dysplasia, 30 cases of carcinoma in situ and 30 cases of laryngeal squamous cell carcinoma (LSCCs). CAF-1/p60 expression increased significantly according to the high index of neoplastic cellular replication; therefore, CAF-1/p60 was overexpressed in neoplastic cells and its moderate-severe expression is correlated with poorer prognosis compared to less expression. In conclusion, overexpression of the CAF-1/p60 protein is related to a risk of higher morbidity and mortality and is a reliable independent prognostic index of laryngeal carcinoma. CAF1-p60 protein overexpression can be used in cancer management as an indicator of malignant evolution, especially in carcinoma in situ.

Crystal structures of PCNA mutant proteins defective in gene silencing suggest a novel interaction site on the front face of the PCNA ring

Proliferating cell nuclear antigen (PCNA), a homotrimeric protein, is the eukaryotic sliding clamp that functions as a processivity factor for polymerases during DNA replication. Chromatin association factor 1 (CAF-1) is a heterotrimeric histone chaperone protein that is required for coupling chromatin assembly with DNA replication in eukaryotes. CAF-1 association with replicating DNA, and the targeting of newly synthesized histones to sites of DNA replication and repair requires its interaction with PCNA. Genetic studies have identified three mutant forms of PCNA in yeast that cause defects in gene silencing and exhibit altered association of CAF-1 to chromatin in vivo, as well as inhibit binding to CAF-1 in vitro. Three of these mutant forms of PCNA, encoded by the pol30-6, pol30-8, and the pol30-79 alleles, direct the synthesis of PCNA proteins with the amino acid substitutions D41A/D42A, R61A/D63A, and L126A/I128A, respectively. Interestingly, these double alanine substitutions are located far away from each other within the PCNA protein. To understand the structural basis of the interaction between PCNA and CAF-1 and how disruption of this interaction leads to reduced gene silencing, we determined the X-ray crystal structures of each of these mutant PCNA proteins. All three of the substitutions caused disruptions of a surface cavity on the front face of the PCNA ring, which is formed in part by three loops comprised of residues 21–24, 41–44, and 118–134. We suggest that this cavity is a novel binding pocket required for the interaction between PCNA and CAF-1, and that this region in PCNA also represents a potential binding site for other PCNA-binding proteins.

Pivotal roles of PCNA loading and unloading in heterochromatin function

In Saccharomyces cerevisiae, heterochromatin structures required for transcriptional silencing of the HML and HMR loci are duplicated in coordination with passing DNA replication forks. Despite major reorganization of chromatin structure, the heterochromatic, transcriptionally silent states of HML and HMR are successfully maintained throughout S-phase. Mutations of specific components of the replisome diminish the capacity to maintain silencing of HML and HMR through replication. Similarly, mutations in histone chaperones involved in replication-coupled nucleosome assembly reduce gene silencing. Bridging these observations, we determined that the proliferating cell nuclear antigen (PCNA) unloading activity of Elg1 was important for coordinating DNA replication forks with the process of replication-coupled nucleosome assembly to maintain silencing of HML and HMR through S-phase. Collectively, these data identified a mechanism by which chromatin reassembly is coordinated with DNA replication to maintain silencing through S-phase.

H2B ubiquitylation and the histone chaperone Asf1 cooperatively mediate the formation and maintenance of heterochromatin silencing

Heterochromatin is a heritable form of gene repression, with critical roles in development and cell identity. Understanding how chromatin factors results in such repression is a fundamental question. Chromatin is assembled and disassembled during transcription, replication and repair by anti-silencing function 1 (Asf1), a highly conserved histone chaperone. Transcription and DNA replication are also affected by histone modifications that modify nucleosome dynamics, such as H2B ubiquitylation (H2Bub). We report here that H2Bub and Asf1 cooperatively promote transcriptional silencing at yeast telomeres and mating loci. Through real time monitoring of HML (Hidden MAT Left) locus silencing, we found that transcriptional repression was slowly initiated and never fully established in mutants lacking both Asf1 and H2Bub. These findings are consistent with impaired HML silencer-binding and spreading of repressor proteins, Sir2 and Sir3. In addition, mutants lacking H2Bub and Asf1 show defects in both nucleosome assembly and higher-order heterochromatin organization at the HML locus. Our findings reveal a novel role for H2Bub and Asf1 in epigenetic silencing at mating loci. Thus, the interplay between H2Hbub and Asf1 may fine-tune nucleosome dynamics and SIR protein recruitment, and represent an ongoing requirement for proper formation and maintenance of heterochromatin.

Rtt101-Mms1-Mms22 coordinates replication-coupled sister chromatid cohesion and nucleosome assembly

Two sister chromatids must be held together by a cohesion process from their synthesis during S phase to segregation in anaphase. Despite its pivotal role in accurate chromosome segregation, how cohesion is established remains elusive. Here, we demonstrate that yeast Rtt101-Mms1, Cul4 family E3 ubiquitin ligases are stronger dosage suppressors of loss-of-function eco1 mutants than PCNA The essential cohesion reaction, Eco1-catalyzed Smc3 acetylation is reduced in the absence of Rtt101-Mms1. One of the adaptor subunits, Mms22, associates directly with Eco1. Point mutations (L61D/G63D) in Eco1 that abolish the interaction with Mms22 impair Smc3 acetylation. Importantly, an eco1LGpol30A251V double mutant displays additive Smc3ac reduction. Moreover, Smc3 acetylation and cohesion defects also occur in the mutants of other replication-coupled nucleosome assembly (RCNA) factors upstream or downstream of Rtt101-Mms1, indicating unanticipated cross talk between histone modifications and cohesin acetylation. These data suggest that fork-associated Cul4-Ddb1 E3s, together with PCNA, coordinate chromatin reassembly and cohesion establishment on the newly replicated sister chromatids, which are crucial for maintaining genome and chromosome stability.

Differential effects of a post-anthesis heat stress on wheat (Triticum aestivum L.) grain proteome determined by iTRAQ

Heat stress, a major abiotic stressor of wheat (Triticum aestivum L.), often results in reduced yield and decreased quality. In this study, a proteomic method, Tags for Relative and Absolute Quantitation Isobaric (iTRAQ), was adopted to analyze the protein expression profile changes among wheat cultivar Jing411 under heat stress. Results indicated that there were 256 different proteins expressed in Jing411 under heat stress. According to the result of gene annotation and functional classification, 239 proteins were annotated by 856 GO function entries, including growth and metabolism proteins, energy metabolism proteins, processing and storage proteins, defense-related proteins, signal transduction, unknown function proteins and hypothetical proteins. GO enrichment analysis suggested that the differentially expressed proteins in Jing411 under heat stress were mainly involved in stimulus response (67), abiotic stress response (26) and stress response (58), kinase activity (12), and transferase activity (12). Among the differentially expressed proteins in Jing411, 115 were attributed to 119 KEGG signaling/metabolic pathways. KEGG pathway enrichment analysis in Jing411 showed that heat stress mainly affected the starch and sucrose metabolism as well as protein synthesis pathway in the endoplasmic reticulum. The protein interaction network indicated that there were 8 differentially expressed proteins that could form an interaction network in Jing411.

RRM3 regulates epigenetic conversions in Saccharomyces cerevisiae in conjunction with Chromatin Assembly Factor I

Chromatin structures are transmitted to daughter cells through a complex system of nucleosome disassembly and re-assembly at the advancing replication forks. However, the role of replication pausing in the transmission and perturbation of chromatin structures has not been addressed. RRM3 encodes a DNA helicase, which facilitates replication at sites covered with non-histone protein complexes (tRNA genes, active gene promoters, telomeres) in Saccharomyces cerevisiae. In this report we show that the deletion of RRM3 reduces the frequency of epigenetic conversions in the subtelomeric regions of the chromosomes. This phenotype is strongly dependent on 2 histone chaperones, CAF-I and ASF1, which are involved in the reassembly of nucleosomes behind replication forks, but not on the histone chaperone HIR1. We also show that the deletion of RRM3 increases the spontaneous mutation rates in conjunction with CAF-I and ASF1, but not HIR1. Finally, we demonstrate that Rrm3p and CAF-I compete for the binding to the DNA replication clamp PCNA (Proliferating Cell Nuclear Antigen). We propose that the stalling of DNA replication predisposes to epigenetic conversions and that RRM3 and CAF-I play key roles in this process.

The Cac2 subunit is essential for productive histone binding and nucleosome assembly in CAF-1

Nucleosome assembly following DNA replication controls epigenome maintenance and genome integrity. Chromatin assembly factor 1 (CAF-1) is the histone chaperone responsible for histone (H3-H4)₂ deposition following DNA synthesis. Structural and functional details for this chaperone complex and its interaction with histones are slowly emerging. Using hydrogen-deuterium exchange coupled to mass spectrometry, combined with in vitro and in vivo mutagenesis studies, we identified the regions involved in the direct interaction between the yeast CAF-1 subunits, and mapped the CAF-1 domains responsible for H3-H4 binding. The large subunit, Cac1 organizes the assembly of CAF-1. Strikingly, H3-H4 binding is mediated by a composite interface, shaped by Cac1-bound Cac2 and the Cac1 acidic region. Cac2 is indispensable for productive histone binding, while deletion of Cac3 has only moderate effects on H3-H4 binding and nucleosome assembly. These results define direct structural roles for yeast CAF-1 subunits and uncover a previously unknown critical function of the middle subunit in CAF-1.

DNA-mediated association of two histone-bound complexes of yeast Chromatin Assembly Factor-1 (CAF-1) drives tetrasome assembly in the wake of DNA replication

Nucleosome assembly in the wake of DNA replication is a key process that regulates cell identity and survival. Chromatin assembly factor 1 (CAF-1) is a H3-H4 histone chaperone that associates with the replisome and orchestrates chromatin assembly following DNA synthesis. Little is known about the mechanism and structure of this key complex. Here we investigate the CAF-1•H3-H4 binding mode and the mechanism of nucleosome assembly. We show that yeast CAF-1 binding to a H3-H4 dimer activates the Cac1 winged helix domain interaction with DNA. This drives the formation of a transient CAF-1•histone•DNA intermediate containing two CAF-1 complexes, each associated with one H3-H4 dimer. Here, the (H3-H4)2 tetramer is formed and deposited onto DNA. Our work elucidates the molecular mechanism for histone deposition by CAF-1, a reaction that has remained elusive for other histone chaperones, and it advances our understanding of how nucleosomes and their epigenetic information are maintained through DNA replication.

Insights into the molecular architecture and histone H3-H4 deposition mechanism of yeast Chromatin assembly factor 1

How the very first step in nucleosome assembly, deposition of histone H3-H4 as tetramers or dimers on DNA, is accomplished remains largely unclear. Here, we report that yeast chromatin assembly factor 1 (CAF1), a conserved histone chaperone complex that deposits H3-H4 during DNA replication, binds a single H3-H4 heterodimer in solution. We identify a new DNA-binding domain in the large Cac1 subunit of CAF1, which is required for high-affinity DNA binding by the CAF1 three-subunit complex, and which is distinct from the previously described C-terminal winged-helix domain. CAF1 binds preferentially to DNA molecules longer than 40 bp, and two CAF1-H3-H4 complexes concertedly associate with DNA molecules of this size, resulting in deposition of H3-H4 tetramers. While DNA binding is not essential for H3-H4 tetrasome deposition in vitro, it is required for efficient DNA synthesis-coupled nucleosome assembly. Mutant histones with impaired H3-H4 tetramerization interactions fail to release from CAF1, indicating that DNA deposition of H3-H4 tetramers by CAF1 requires a hierarchical cooperation between DNA binding, H3-H4 deposition and histone tetramerization.

PCNA dependent cellular activities tolerate dramatic perturbations in PCNA client interactions

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We assess the cellular effects of the mutation that causes PARD (PCNA^S228I).

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Cells from affected individuals are sensitive to T2AA and T3.

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PCNA^S228I impairs interactions between PCNA and Cdt1, DNMT1, PolD3 and PolD4.

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The PIP-box of p21 retains binding to PCNA^S228I.

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PCNA-dependent degradation and the cell cycle are only subtly altered by PCNA^S228I.

Proliferating cell nuclear antigen (PCNA) is an essential cofactor for DNA replication and repair, recruiting multiple proteins to their sites of action. We examined the effects of the PCNA^S228I mutation that causes PCNA-associated DNA repair disorder (PARD). Cells from individuals affected by PARD are sensitive to the PCNA inhibitors T3 and T2AA, showing that the S228I mutation has consequences for undamaged cells. Analysis of the binding between PCNA and PCNA-interacting proteins (PIPs) shows that the S228I change dramatically impairs the majority of these interactions, including that of Cdt1, DNMT1, PolD3^p66 and PolD4^p12. In contrast p21 largely retains the ability to bind PCNA^S228I. This property is conferred by the p21 PIP box sequence itself, which is both necessary and sufficient for PCNA^S228I binding. Ubiquitination of PCNA is unaffected by the S228I change, which indirectly alters the structure of the inter-domain connecting loop. Despite the dramatic in vitro effects of the PARD mutation on PIP-degron binding, there are only minor alterations to the stability of p21 and Cdt1 in cells from affected individuals. Overall our data suggests that reduced affinity of PCNA^S228I for specific clients causes subtle cellular defects in undamaged cells which likely contribute to the etiology of PARD.

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

Polycomb-group protein SlMSI1 represses the expression of fruit-ripening genes to prolong shelf life in tomato

Polycomb-group (PcG) protein MULTICOPY SUPPRESSOR OF IRA1 (MSI1) protein is an evolutionarily conserved developmental suppressor and plays a crucial role in regulating epigenetic modulations. However, the potential role and function of MSI1 in fleshy fruits remain unknown. In this study, SlMSI1 was cloned and transformed into tomato to explore its function. The quantitative real-time PCR results showed that SlMSI1 was highly expressed in flowers and fruits and that its transcript and protein levels were significantly decreased in fruits after the breaker stage. Additionally, SlMSI1-overexpressing transgenic tomatoes displayed abnormal non-ripening fruit formation, whereas its suppression promoted fruit ripening in transgenic tomatoes. Quantitative real-time PCR assays also showed that RIN and its regulons were decreased in SlMSI1 overexpression transgenic tomato fruits. Furthermore, RNA-seq analysis demonstrated that SlMSI1 inhibits fruit ripening by negatively regulating a large set of fruit-ripening genes in addition to RIN and its regulons. Finally, genetic manipulation of SlMSI1 and RIN successfully prolonged the fruit shelf life by regulating the fruit-ripening genes in tomato. Our findings reveal a novel regulatory function of SlMSI1 in fruit ripening and provide a new regulator that may be useful for genetic engineering and modification of fruit shelf life.

A DNA binding winged helix domain in CAF-1 functions with PCNA to stabilize CAF-1 at replication forks

Chromatin assembly factor 1 (CAF-1) is a histone H3–H4 chaperone that deposits newly synthesized histone (H3–H4)₂ tetramers during replication-coupled nucleosome assembly. However, how CAF-1 functions in this process is not yet well understood. Here, we report the crystal structure of C terminus of Cac1 (Cac1C), a subunit of yeast CAF-1, and the function of this domain in stabilizing CAF-1 at replication forks. We show that Cac1C forms a winged helix domain (WHD) and binds DNA in a sequence-independent manner. Mutations in Cac1C that abolish DNA binding result in defects in transcriptional silencing and increased sensitivity to DNA damaging agents, and these defects are exacerbated when combined with Cac1 mutations deficient in PCNA binding. Similar phenotypes are observed for corresponding mutations in mouse CAF-1. These results reveal a mechanism conserved in eukaryotic cells whereby the ability of CAF-1 to bind DNA is important for its association with the DNA replication forks and subsequent nucleosome assembly.

CDC28 phosphorylates Cac1p and regulates the association of chromatin assembly factor I with chromatin

Chromatin Assembly Factor I (CAF-I) plays a key role in the replication-coupled assembly of nucleosomes. It is expected that its function is linked to the regulation of the cell cycle, but little detail is available. Current models suggest that CAF-I is recruited to replication forks and to chromatin via an interaction between its Cac1p subunit and the replication sliding clamp, PCNA, and that this interaction is stimulated by the kinase CDC7. Here we show that another kinase, CDC28, phosphorylates Cac1p on serines 94 and 515 in early S phase and regulates its association with chromatin, but not its association with PCNA. Mutations in the Cac1p-phosphorylation sites of CDC28 but not of CDC7 substantially reduce the in vivo phosphorylation of Cac1p. However, mutations in the putative CDC7 target sites on Cac1p reduce its stability. The association of CAF-I with chromatin is impaired in a cdc28-1 mutant and to a lesser extent in a cdc7-1 mutant. In addition, mutations in the Cac1p-phosphorylation sites by both CDC28 and CDC7 reduce gene silencing at the telomeres. We propose that this phosphorylation represents a regulatory step in the recruitment of CAF-I to chromatin in early S phase that is distinct from the association of CAF-I with PCNA. Hence, we implicate CDC28 in the regulation of chromatin reassembly during DNA replication. These findings provide novel mechanistic insights on the links between cell-cycle regulation, DNA replication and chromatin reassembly.

The Fork in the Road: Histone Partitioning During DNA Replication

In the following discussion the distribution of histones at the replication fork is examined, with specific attention paid to the question of H3/H4 tetramer "splitting." After a presentation of early experiments surrounding this topic, more recent contributions are detailed. The implications of these findings with respect to the transmission of histone modifications and epigenetic models are also addressed.

The role of the chromatin assembly complex (CAF-1) and its p60 subunit (CHAF1b) in homeostasis and disease

Nucleosome assembly following DNA synthesis is critical for maintaining genomic stability. The proteins directly responsible for shuttling newly synthesized histones H3 and H4 from the cytoplasm to the assembly fork during DNA replication comprise the Chromatin Assembly Factor 1 complex (CAF-1). Whereas the diverse functions of the large (CAF-1-p150, CHAF1a) and small (RbAp48, p48) subunits of the CAF-1 complex have been well-characterized in many tissues and extend beyond histone chaperone activity, the contributions of the medium subunit (CAF-1-p60, CHAF1b) are much less well understood. Although it is known that CHAF1b has multiple functional domains (7× WD repeat domain, B-like domain, and a PEST domain), how these components come together to elicit the functions of this protein are still unclear. Here, we review the biology of the CAF-1 complex, with an emphasis on CHAF1b, including its structure, regulation, and function. In addition, we discuss the possible contributions of CHAF1b and the CAF-1 complex to human diseases. Of note, CHAF1b is located within the Down syndrome critical region (DSCR) of chromosome 21. Therefore, we also address the putative contributions of its trisomy to the various manifestations of DS.

The CAF-1 and Hir Histone Chaperones Associate with Sites of Meiotic Double-Strand Breaks in Budding Yeast

In the meiotic prophase, programmed DNA double-strand breaks (DSB) are introduced along chromosomes to promote homolog pairing and recombination. Although meiotic DSBs usually occur in nucleosome-depleted, accessible regions of chromatin, their repair by homologous recombination takes place in a nucleosomal environment. Nucleosomes may represent an obstacle for the recombination machinery and their timely eviction and reincorporation into chromatin may influence the outcome of recombination, for instance by stabilizing recombination intermediates. Here we show in budding yeast that nucleosomes flanking a meiotic DSB are transiently lost during recombination, and that specific histone H3 chaperones, CAF-1 and Hir, are mobilized at meiotic DSBs. However, the absence of these chaperones has no effect on meiotic recombination, suggesting that timely histone reincorporation following their eviction has no influence on the recombination outcome, or that redundant pathways are activated. This study is the first example of the involvement of histone H3 chaperones at naturally occurring, developmentally programmed DNA double-strand breaks.

The chromatin assembly factor 1 promotes Rad51-dependent template switches at replication forks by counteracting D-loop disassembly by the RecQ-type helicase Rqh1

A molecular switch for times of replication stress - Chromatin Assembly Factor 1 helps to protect DNA during recombination-mediated template-switching, favoring the rescue of stalled replication forks by both beneficial and detrimental homologous recombination events.

At blocked replication forks, homologous recombination mediates the nascent strands to switch template in order to ensure replication restart, but faulty template switches underlie genome rearrangements in cancer cells and genomic disorders. Recombination occurs within DNA packaged into chromatin that must first be relaxed and then restored when recombination is completed. The chromatin assembly factor 1, CAF-1, is a histone H3-H4 chaperone involved in DNA synthesis-coupled chromatin assembly during DNA replication and DNA repair. We reveal a novel chromatin factor-dependent step during replication-coupled DNA repair: Fission yeast CAF-1 promotes Rad51-dependent template switches at replication forks, independently of the postreplication repair pathway. We used a physical assay that allows the analysis of the individual steps of template switch, from the recruitment of recombination factors to the formation of joint molecules, combined with a quantitative measure of the resulting rearrangements. We reveal functional and physical interplays between CAF-1 and the RecQ-helicase Rqh1, the BLM homologue, mutations in which cause Bloom's syndrome, a human disease associating genome instability with cancer predisposition. We establish that CAF-1 promotes template switch by counteracting D-loop disassembly by Rqh1. Consequently, the likelihood of faulty template switches is controlled by antagonistic activities of CAF-1 and Rqh1 in the stability of the D-loop. D-loop stabilization requires the ability of CAF-1 to interact with PCNA and is thus linked to the DNA synthesis step. We propose that CAF-1 plays a regulatory role during template switch by assembling chromatin on the D-loop and thereby impacting the resolution of the D-loop.

Obstacles to the progression of DNA replication forks can result in genome rearrangements that are often observed in cancer cells and genomic disorders. Homologous recombination is a mechanism of restarting stalled replication fork that involves synthesis of the new DNA strands switching templates to a second (allelic) copy of the DNA sequence. However, the new strands can also occasionally recombine with nonallelic repeats (distinct regions of the genome that resemble the correct one) and thereby cause the inappropriate fusion of normally distant DNA segments; this is known as faulty template switching. The chromatin assembly factor 1 (CAF-1) is already known to be involved in depositing nucleosomes on DNA during DNA replication and repair. We have found that CAF-1 is also involved in the recombination-mediated template switch pathway in response to replication stress. Using both genetic and physical assays that allow the different steps of template switch to be analyzed, we reveal that CAF-1 protects recombination intermediates from disassembly by the RecQ-type helicase Rqh1, the homologue of BLM (people with mutations that affect BLM have Bloom's syndrome, an inherited predisposition to genome instability and cancer). Consequently, the likelihood of faulty template switch is controlled by the antagonistic activities of CAF-1 and Rqh1. We thus identified an evolutionarily conserved interplay between CAF-1 and RecQ-type helicases that helps to maintain genome stability in the face of replication stress.

Antibody against chromatin assembly factor-1 is a novel autoantibody specifically recognized in systemic lupus erythematosus

Autoantibodies to proliferating cell nuclear antigen (PCNA) are specifically, if rarely, present in systemic lupus erythematosus (SLE) patient sera. Even SLE patients lacking PCNA reactivity often show reaction to PCNA-binding protein. Here, immunoreactivity to chromatin assembly factor-1 (CAF-1), an essential molecule for DNA replication and a PCNA-binding protein, was compared for the sera of SLE patients, normal healthy controls (NHCs) and other disease controls, and in autoimmune sera reactive to standard autoantigens, by enzyme-linked immunosorbent assay (ELISA), indirect immunofluorescence, and immunoblotting. CAF1 and IRF1 expression in SLE and NHC peripheral mononuclear cells were compared by quantitative real-time polymerase chain reaction. Serum interferon-γ-inducing protein-10 and anti-double-stranded (ds)DNA antibody levels were measured by ELISA. Increased CAF-1 autoimmune reactivity was recognized in SLE or serum anti-dsDNA antibody-positive patients. Significantly greater central nervous system (CNS) involvement (aseptic meningitis) and serum anti-dsDNA antibody titers were present more often in anti-CAF-1 antibody-positive than antibody-negative SLE patients. IFN-γ positively regulated CAF-1 expression in vitro and was associated with anti-CAF-1 antibody production in SLE. Thus, a novel anti-CAF-1 autoantibody is frequently found in patients with SLE and is a useful biomarker for diagnosis, especially in cases with CNS involvement. Aberrant IFN-γ regulation appears to play an important role in anti-CAF-1 antibody production in SLE.

Cell cycle regulation of silent chromatin formation

Identical genes in two different cells can stably exist in alternate transcriptional states despite the dynamic changes that will occur to chromatin at that locus throughout the cell cycle. In mammals, this is achieved through epigenetic processes that regulate key developmental transitions and ensure stable patterns of gene expression during growth and differentiation. The budding yeast Saccharomyces cerevisiae utilizes silencing to control the expression state of genes encoding key regulatory factors for determining cell-type, ribosomal RNA levels and proper telomere function. Here, we review the composition of silent chromatin in S. cerevisiae, how silent chromatin is influenced by chromatin assembly and histone modifications and highlight several observations that have contributed to our understanding of the interplay between silent chromatin formation and stability and the cell cycle. This article is part of a Special Issue entitled: Histone chaperones and Chromatin assembly.

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.

A conserved sequence extending motif III of the motor domain in the Snf2-family DNA translocase Rad54 is critical for ATPase activity

Rad54 is a dsDNA-dependent ATPase that translocates on duplex DNA. Its ATPase function is essential for homologous recombination, a pathway critical for meiotic chromosome segregation, repair of complex DNA damage, and recovery of stalled or broken replication forks. In recombination, Rad54 cooperates with Rad51 protein and is required to dissociate Rad51 from heteroduplex DNA to allow access by DNA polymerases for recombination-associated DNA synthesis. Sequence analysis revealed that Rad54 contains a perfect match to the consensus PIP box sequence, a widely spread PCNA interaction motif. Indeed, Rad54 interacts directly with PCNA, but this interaction is not mediated by the Rad54 PIP box-like sequence. This sequence is located as an extension of motif III of the Rad54 motor domain and is essential for full Rad54 ATPase activity. Mutations in this motif render Rad54 non-functional in vivo and severely compromise its activities in vitro. Further analysis demonstrated that such mutations affect dsDNA binding, consistent with the location of this sequence motif on the surface of the cleft formed by two RecA-like domains, which likely forms the dsDNA binding site of Rad54. Our study identified a novel sequence motif critical for Rad54 function and showed that even perfect matches to the PIP box consensus may not necessarily identify PCNA interaction sites.

Human CAF-1-dependent nucleosome assembly in a defined system

Replication-coupled nucleosome assembly is a critical step in packaging newly synthesized DNA into chromatin. Previous studies have defined the importance of the histone chaperones CAF-1 and ASF1A, the replicative clamp PCNA, and the clamp loader RFC for the assembly of nucleosomes during DNA replication. Despite significant progress in the field, replication-coupled nucleosome assembly is not well understood. One of the complications in elucidating the mechanisms of replication-coupled nucleosome assembly is the lack of a defined system that faithfully recapitulates this important biological process in vitro. We describe here a defined system that assembles nucleosomal arrays in a manner dependent on the presence of CAF-1, ASF1A-H3-H4, H2A-H2B, PCNA, RFC, NAP1L1, ATP, and strand breaks. The loss of CAF-1 p48 subunit causes a strong defect in packaging DNA into nucleosomes by this system. We also show that the defined system forms nucleosomes on nascent DNA synthesized by the replicative polymerase δ. Thus, the developed system reproduces several key features of replication-coupled nucleosome assembly.

Systematic triple-mutant analysis uncovers functional connectivity between pathways involved in chromosome regulation

Genetic interactions reveal the functional relationships between pairs of genes. In this study, we describe a method for the systematic generation and quantitation of triple mutants, termed triple-mutant analysis (TMA). We have used this approach to interrogate partially redundant pairs of genes in S. cerevisiae, including ASF1 and CAC1, two histone chaperones. After subjecting asf1Δ cac1Δ to TMA, we found that the Swi/Snf Rdh54 protein compensates for the absence of Asf1 and Cac1. Rdh54 more strongly associates with the chromatin apparatus and the pericentromeric region in the double mutant. Moreover, Asf1 is responsible for the synthetic lethality observed in cac1Δ strains lacking the HIRA-like proteins. A similar TMA was carried out after deleting both CLB5 and CLB6, cyclins that regulate DNA replication, revealing a strong functional connection to chromosome segregation. This approach can reveal functional redundancies that cannot be uncovered through traditional double-mutant analyses.

Histone chaperones in nucleosome assembly and human disease

Nucleosome assembly following DNA replication, DNA repair and gene transcription is critical for the maintenance of genome stability and epigenetic information. Nucleosomes are assembled by replication-coupled or replication-independent pathways with the aid of histone chaperone proteins. How these different nucleosome assembly pathways are regulated remains relatively unclear. Recent studies have provided insight into the mechanisms and the roles of histone chaperones in regulating nucleosome assembly. Alterations or mutations in factors involved in nucleosome assembly have also been implicated in cancer and other human diseases. This review highlights the recent progress and outlines future challenges in the field.

CAF-1-induced oligomerization of histones H3/H4 and mutually exclusive interactions with Asf1 guide H3/H4 transitions among histone chaperones and DNA

Anti-silencing function 1 (Asf1) and Chromatin Assembly Factor 1 (CAF-1) chaperone histones H3/H4 during the assembly of nucleosomes on newly replicated DNA. To understand the mechanism of histone H3/H4 transfer among Asf1, CAF-1 and DNA from a thermodynamic perspective, we developed and employed biophysical approaches using full-length proteins in the budding yeast system. We find that the C-terminal tail of Asf1 enhances the interaction of Asf1 with CAF-1. Surprisingly, although H3/H4 also enhances the interaction of Asf1 with the CAF-1 subunit Cac2, H3/H4 forms a tight complex with CAF-1 exclusive of Asf1, with an affinity weaker than Asf1–H3/H4 or H3/H4–DNA interactions. Unlike Asf1, monomeric CAF-1 binds to multiple H3/H4 dimers, which ultimately promotes the formation of (H3/H4)₂ tetramers on DNA. Thus, transition of H3/H4 from the Asf1-associated dimer to the DNA-associated tetramer is promoted by CAF-1-induced H3/H4 oligomerization.

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.

The histone chaperone ASF1 is essential for sexual development in the filamentous fungus Sordaria macrospora

Ascomycetes develop four major types of fruiting bodies that share a common ancestor, and a set of common core genes most likely controls this process. One way to identify such genes is to search for conserved expression patterns. We analysed microarray data of Fusarium graminearum and Sordaria macrospora, identifying 78 genes with similar expression patterns during fruiting body development. One of these genes was asf1 (anti-silencing function 1), encoding a predicted histone chaperone. asf1 expression is also upregulated during development in the distantly related ascomycete Pyronema confluens. To test whether asf1 plays a role in fungal development, we generated an S. macrospora asf1 deletion mutant. The mutant is sterile and can be complemented to fertility by transformation with the wild-type asf1 and its P. confluens homologue. An ASF1-EGFP fusion protein localizes to the nucleus. By tandem-affinity purification/mass spectrometry as well as yeast two-hybrid analysis, we identified histones H3 and H4 as ASF1 interaction partners. Several developmental genes are dependent on asf1 for correct transcriptional expression. Deletion of the histone chaperone genes rtt106 and cac2 did not cause any developmental phenotypes. These data indicate that asf1 of S. macrospora encodes a conserved histone chaperone that is required for fruiting body development.