Replication of heterochromatin: insights into mechanisms of epigenetic inheritance.
ABSTRACT Heterochromatin is composed of tightly condensed chromatin in which the histones are deacetylated and methylated, and specific nonhistone proteins are bound. Additionally, in vertebrates and plants, the DNA within heterochromatin is methylated. As the heterochromatic state is stably inherited, replication of heterochromatin requires not only duplication of the DNA but also a reinstallment of the appropriate protein and DNA modifications. Thus replication of heterochromatin provides a framework for understanding mechanisms of epigenetic inheritance. In recent studies, roles have been identified for replication factors in reinstating heterochromatin, particularly functions for origin recognition complex, proliferating cell nuclear antigen, and chromatin-assembly factor 1 in recruiting the heterochromatin binding protein HP1, a histone methyltransferase, a DNA methyltransferase, and a chromatin remodeling complex. Potential mechanistic links between these factors are discussed. In some cells, replication of the heterochromatin is blocked, and in Drosophila this inhibition is mediated by a chromatin binding protein SuUR.
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ABSTRACT: Maintenance of gene expression through epigenetic mechanisms such as DNA- and histone-methylation is essential for preserving cellular identity and function. Multiplication of eukaryotic cells requires that the DNA content of the cell is duplicated through replication, which is coupled to incorporation of de novo synthesized core histones into nucleosomal structures. One of the challenging questions in biology is to explain how the organism ensures that regulatory epigenetic marks, once established, are transferred from one cell generation to the next. Based on studies in our laboratory, we have recently proposed a model for how the methylated lysine 27 of histone H3 (H3K27) can be stably transmitted through the cell division cycle. We found that the Polycomb Repressive Complex 2 (PRC2), which is responsible for di- and trimethylation of H3K27 (H3K27me2/me3), binds to its own site of methylation. Moreover, our results suggested that maintenance of transcriptional repression by PRC2 requires the binding of the PRC2 complex to H3K27me3/me2. Based on these two key observations we propose that PRC2 is able to copy the mark from an old parental H3 molecule to a newly synthesized H3 molecule as DNA replication proceeds. In addition, our results support a model for how the H3K27me3 mark could be preserved in the interphase of the cell cycle, where other events such as histone exchange and demethylation could counteract PRC2 function. Here we discuss the implications of our results in further detail.Epigenetics: official journal of the DNA Methylation Society 05/2009; 4(3):133-8. · 4.58 Impact Factor
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ABSTRACT: Despite extensive study of heterochromatin, relatively little is known about the mechanisms by which such a structure forms. We show that the Drosophila homologue of the human alpha-thalassemia and mental retardation X-linked protein (dATRX), is important in the formation or maintenance of heterochromatin through modification of position effect variegation. We further show that there are two isoforms of the dATRX protein, the longer of which interacts directly with heterochromatin protein 1 (dHP-1) through a CxVxL motif both in vitro and in vivo. These two proteins co-localise at heterochromatin in a manner dependent on this motif. Consistent with this observation, the long isoform of the dATRX protein localises primarily to the heterochromatin at the chromocentre on salivary gland polytene chromosomes, whereas the short isoform binds to many sites along the chromosome arms. We suggest that the establishment of a regular nucleosomal organisation may be common to heterochromatin and transcriptionally repressed chromatin in other locations, and may require the action of ATP dependent chromatin remodelling factors.PLoS ONE 02/2008; 3(5):e2099. · 4.09 Impact Factor
Article: Roles of the Clr4 methyltransferase complex in nucleation, spreading and maintenance of heterochromatin.[show abstract] [hide abstract]
ABSTRACT: Heterochromatin assembly, involving methylation of histone H3 lysine 9 (H3K9me), regulates various chromosomal processes. In fission yeast, heterochromatin targeted to specific repeat loci in an RNAi-dependent manner spreads across extended domains to exert regional epigenetic control. The Clr4 methyltransferase complex (ClrC) is responsible for nucleation and spreading of heterochromatin; however, its recruitment to heterochromatic repeats is poorly understood. Here we demonstrate that ClrC components are distributed throughout heterochromatic domains. To nucleate heterochromatin, Rik1, a WD domain-containing subunit of ClrC, is loaded onto the transcribed repeats via RNAi machinery including the RNA-induced transcriptional silencing (RITS) complex. Furthermore, we show that the chromodomain of Clr4 binds specifically to H3K9me that is essential for the spreading of heterochromatin. Our analyses delineate sequential steps for the assembly of heterochromatic domains and suggest that the ability of Clr4 to both 'write' and 'read' H3K9me facilitates heterochromatin maintenance through successive cell divisions.Nature Structural & Molecular Biology 05/2008; 15(4):381-8. · 12.71 Impact Factor