RESUMO
Disruption of chromatin organization during replication poses a major challenge to the maintenance and integrity of genome organization. It creates the need to accurately reconstruct the chromatin landscape following DNA duplication but there is little mechanistic understanding of how chromatin based modifications are restored on newly synthesized DNA. ATP-dependent chromatin remodeling activities serve multiple roles during replication and recent work underscores their requirement in the maintenance of proper chromatin organization. A new component of chromatin replication, the SWI/SNF-like chromatin remodeler SMARCAD1, acts at replication sites to facilitate deacetylation of newly assembled histones. Deacetylation is a pre-requisite for the restoration of epigenetic signatures in heterochromatin regions following replication. In this way, SMARCAD1, in concert with histone modifying activities and transcriptional repressors, reinforces epigenetic instructions to ensure that silenced loci are correctly perpetuated in each replication cycle. The emerging concept is that remodeling of nucleosomes is an early event imperative to promote the re-establishment of histone modifications following DNA replication.
Assuntos
Cromatina/química , Reparo do DNA , Replicação do DNA , Regulação Enzimológica da Expressão Gênica , Heterocromatina/química , Nucleossomos/química , Acetilação , Trifosfato de Adenosina/química , Cromatina/genética , Montagem e Desmontagem da Cromatina , DNA Helicases/química , DNA Helicases/genética , Ativação Enzimática , Epigênese Genética , Inativação Gênica , Heterocromatina/genética , Histonas/química , Humanos , Nucleossomos/genética , Mapeamento de Interação de ProteínasAssuntos
Núcleo Celular/metabolismo , Montagem e Desmontagem da Cromatina , Proteínas Nucleares/metabolismo , Adenosina Trifosfatases/química , Adenosina Trifosfatases/metabolismo , Animais , Proteínas Cromossômicas não Histona/química , Proteínas Cromossômicas não Histona/metabolismo , Humanos , Camundongos , Modelos Biológicos , Nucleossomos/metabolismoRESUMO
During DNA replication, chromatin states have to be accurately transmitted from the parental to the daughter strands for faithful epigenetic inheritance. Chromatin remodelling factors at the replication site are thought to be involved in this process. Recent work adds ATP-dependent nucleosome remodelling factors to this category of enzymes. The WICH complex, consisting of the ISWI-type ATPase SNF2H and the Williams Syndrome Transcription Factor (WSTF), binds to replication foci using PCNA, a key factor in DNA- and chromatin replication and DNA repair, as an interaction platform. Depletion of WSTF results in decreased chromatin accessibility, which is evident already in newly replicated DNA. This leads to heterochromatin formation on a global scale and a decrease in overall transcriptional activity. Here, we propose that WICH, by keeping nucleosomes mobile, provides access to the newly replicated DNA and may thereby create a window of opportunity after DNA replication for rebinding of factors that maintain the epigenetic state, and thus prevents aberrant heterochromatin formation. Our model may provide an explanation for the long-standing observation of a delay in chromatin "maturation" on newly replicated DNA, by connecting this delay with the action of PCNA-bound WSTF-ISWI, and highlights chromatin remodeling shortly after DNA replication as a critical point for regulation.
Assuntos
Adenosina Trifosfatases/fisiologia , Cromatina/química , Regulação da Expressão Gênica , Fatores de Transcrição/fisiologia , Animais , Sítios de Ligação , Ciclo Celular , DNA/química , Replicação do DNA , Epigênese Genética , Heterocromatina/química , Humanos , Modelos Biológicos , Nucleossomos/química , Ligação ProteicaRESUMO
Chromatin states have to be faithfully duplicated during DNA replication to maintain cell identity. It is unclear whether or how ATP-dependent chromatin-remodelling factors are involved in this process. Here we provide evidence that the Williams syndrome transcription factor (WSTF) is targeted to replication foci through direct interaction with the DNA clamp PCNA, an important coordinator of DNA and chromatin replication. WSTF, in turn, recruits imitation switch (ISWI)-type nucleosome-remodelling factor SNF2H to replication sites. These findings reveal a novel recruitment mechanism for ATP-dependent chromatin-remodelling factors that is fundamentally different from the previously documented targeting by sequence-specific transcriptional regulators. RNA-interference-mediated depletion of WSTF or SNF2H causes a compaction of newly replicated chromatin and increases the amount of heterochromatin markers, including HP1beta. This increase in the amount of HP1beta protein is mediated by progression through S phase and is not the result of an increase in HP1beta mRNA levels. We propose that the WSTF-ISWI complex has a role in the maintenance of chromatin structures during DNA replication.
Assuntos
Adenosina Trifosfatases/metabolismo , Montagem e Desmontagem da Cromatina/fisiologia , Replicação do DNA/genética , Antígeno Nuclear de Célula em Proliferação/metabolismo , Fatores de Transcrição/metabolismo , Síndrome de Williams/genética , Adenosina Trifosfatases/genética , Trifosfato de Adenosina/metabolismo , Cromatina/genética , Cromatina/metabolismo , Montagem e Desmontagem da Cromatina/genética , Homólogo 5 da Proteína Cromobox , Proteínas Cromossômicas não Histona/genética , Proteínas Cromossômicas não Histona/metabolismo , Cromossomos Humanos Par 7/genética , Marcadores Genéticos/genética , Células HeLa , Humanos , Proteínas Nucleares/genética , Proteínas Nucleares/metabolismo , Antígeno Nuclear de Célula em Proliferação/genética , Interferência de RNA , Fatores de Transcrição/genética , Síndrome de Williams/metabolismoRESUMO
The mechanism by which the eukaryotic DNA-replication machinery penetrates condensed chromatin structures to replicate the underlying DNA is poorly understood. Here we provide evidence that an ACF1-ISWI chromatin-remodeling complex is required for replication through heterochromatin in mammalian cells. ACF1 (ATP-utilizing chromatin assembly and remodeling factor 1) and an ISWI isoform, SNF2H (sucrose nonfermenting-2 homolog), become specifically enriched in replicating pericentromeric heterochromatin. RNAi-mediated depletion of ACF1 specifically impairs the replication of pericentromeric heterochromatin. Accordingly, depletion of ACF1 causes a delay in cell-cycle progression through the late stages of S phase. In vivo depletion of SNF2H slows the progression of DNA replication throughout S phase, indicating a functional overlap with ACF1. Decondensing the heterochromatin with 5-aza-2-deoxycytidine reverses the effects of ACF1 and SNF2H depletion. Expression of an ACF1 mutant that cannot interact with SNF2H also interferes with replication of condensed chromatin. Our data suggest that an ACF1-SNF2H complex is part of a dedicated mechanism that enables DNA replication through highly condensed regions of chromatin.