RESUMO
Chromatin is dynamically reorganized when DNA replication forks are challenged. However, the process of epigenetic reorganization and its implication for fork stability is poorly understood. Here we discover a checkpoint-regulated cascade of chromatin signalling that activates the histone methyltransferase EHMT2/G9a to catalyse heterochromatin assembly at stressed replication forks. Using biochemical and single molecule chromatin fibre approaches, we show that G9a together with SUV39h1 induces chromatin compaction by accumulating the repressive modifications, H3K9me1/me2/me3, in the vicinity of stressed replication forks. This closed conformation is also favoured by the G9a-dependent exclusion of the H3K9-demethylase JMJD1A/KDM3A, which facilitates heterochromatin disassembly upon fork restart. Untimely heterochromatin disassembly from stressed forks by KDM3A enables PRIMPOL access, triggering single-stranded DNA gap formation and sensitizing cells towards chemotherapeutic drugs. These findings may help in explaining chemotherapy resistance and poor prognosis observed in patients with cancer displaying elevated levels of G9a/H3K9me3.
Assuntos
Heterocromatina , Histonas , Humanos , Histonas/genética , Histonas/metabolismo , Heterocromatina/genética , Cromatina/genética , Montagem e Desmontagem da Cromatina , Replicação do DNA , Antígenos de Histocompatibilidade , Histona-Lisina N-Metiltransferase/genética , Histona-Lisina N-Metiltransferase/metabolismo , Histona Desmetilases com o Domínio Jumonji/genéticaRESUMO
From biosynthesis to assembly into nucleosomes, histones are handed through a cascade of histone chaperones, which shield histones from non-specific interactions. Whether mechanisms exist to safeguard the histone fold during histone chaperone handover events or to release trapped intermediates is unclear. Using structure-guided and functional proteomics, we identify and characterize a histone chaperone function of DNAJC9, a heat shock co-chaperone that promotes HSP70-mediated catalysis. We elucidate the structure of DNAJC9, in a histone H3-H4 co-chaperone complex with MCM2, revealing how this dual histone and heat shock co-chaperone binds histone substrates. We show that DNAJC9 recruits HSP70-type enzymes via its J domain to fold histone H3-H4 substrates: upstream in the histone supply chain, during replication- and transcription-coupled nucleosome assembly, and to clean up spurious interactions. With its dual functionality, DNAJC9 integrates ATP-resourced protein folding into the histone supply pathway to resolve aberrant intermediates throughout the dynamic lives of histones.
Assuntos
Proteínas de Choque Térmico HSP40/metabolismo , Chaperonas de Histonas/metabolismo , Linhagem Celular Tumoral , Cromatina , Montagem e Desmontagem da Cromatina , Replicação do DNA , Proteínas de Choque Térmico HSP40/fisiologia , Proteínas de Choque Térmico HSP70/metabolismo , Células HeLa , Chaperonas de Histonas/fisiologia , Histonas/metabolismo , Humanos , Componente 2 do Complexo de Manutenção de Minicromossomo/metabolismo , Modelos Moleculares , Chaperonas Moleculares/metabolismo , Nucleossomos , Ligação Proteica , Proteômica/métodosRESUMO
Chromatin organization is disrupted genome-wide during DNA replication. On newly synthesized DNA, nucleosomes are assembled from new naive histones and old modified histones. It remains unknown whether the landscape of histone post-translational modifications (PTMs) is faithfully copied during DNA replication or the epigenome is perturbed. Here we develop chromatin occupancy after replication (ChOR-seq) to determine histone PTM occupancy immediately after DNA replication and across the cell cycle. We show that H3K4me3, H3K36me3, H3K79me3, and H3K27me3 positional information is reproduced with high accuracy on newly synthesized DNA through histone recycling. Quantitative ChOR-seq reveals that de novo methylation to restore H3K4me3 and H3K27me3 levels occurs across the cell cycle with mark- and locus-specific kinetics. Collectively, this demonstrates that accurate parental histone recycling preserves positional information and allows PTM transmission to daughter cells while modification of new histones gives rise to complex epigenome fluctuations across the cell cycle that could underlie cell-to-cell heterogeneity.
Assuntos
Replicação do DNA/genética , Histonas/genética , Ciclo Celular/genética , Linhagem Celular Tumoral , Cromatina/genética , Epigênese Genética/genética , Feminino , Células HeLa , Humanos , Metilação , Nucleossomos/genética , Processamento de Proteína Pós-Traducional/genéticaRESUMO
After DNA replication, chromosomal processes including DNA repair and transcription take place in the context of sister chromatids. While cell cycle regulation can guide these processes globally, mechanisms to distinguish pre- and post-replicative states locally remain unknown. Here we reveal that new histones incorporated during DNA replication provide a signature of post-replicative chromatin, read by the human TONSLMMS22L homologous recombination complex. We identify the TONSL ankyrin repeat domain (ARD) as a reader of histone H4 tails unmethylated at K20 (H4K20me0), which are specific to new histones incorporated during DNA replication and mark post-replicative chromatin until the G2/M phase of the cell cycle. Accordingly, TONSLMMS22L binds new histones H3H4 both before and after incorporation into nucleosomes, remaining on replicated chromatin until late G2/M. H4K20me0 recognition is required for TONSLMMS22L binding to chromatin and accumulation at challenged replication forks and DNA lesions. Consequently, TONSL ARD mutants are toxic, compromising genome stability, cell viability and resistance to replication stress. Together, these data reveal a histone-reader-based mechanism for recognizing the post-replicative state, offering a new angle to understand DNA repair with the potential for targeted cancer therapy.