In Vivo HP1 Targeting Causes Large-Scale Chromatin Condensation and Enhanced Histone Lysine Methylation

Abstract

Changes in chromatin structure are a key aspect in the epigenetic regulation of gene expression. We have used a lac operator array system to visualize by light microscopy the effect of heterochromatin protein 1 (HP1) ␣ (HP1␣) and HP1␤ on large-scale chromatin structure in living mammalian cells. The structure of HP1, containing a chromodomain, a chromoshadow domain, and a hinge domain, allows it to bind to a variety of proteins. In vivo targeting of an enhanced green fluorescent protein-tagged HP1-lac repressor fusion to a lac operator-containing, gene-amplified chromosome region causes local condensation of the higher-order chromatin structure, recruitment of the histone methyltransferase SETDB1, and enhanced trimethylation of histone H3 lysine 9. Polycomb group proteins of both the HPC/HPH and the EED/EZH2 complexes, which are involved in the heritable repression of gene activity, are not recruited to the amplified chromosome region by HP1␣ and HP1␤ in vivo targeting. HP1␣ targeting causes the recruitment of endogenous HP1␤ to the chromatin region and vice versa, indicating a direct interaction between the two HP1 homologous proteins. Our findings indicate that HP1␣ and HP1␤ targeting is sufficient to induce heterochromatin formation.Packing of the eukaryotic genome into higher-order chromatin structures is tightly related to gene expression (reviewed in references 17 and 37). Changes in chromatin structure are a key element in epigenetic gene control. Transcriptional activation is associated with large-scale chromatin decondensation, whereas silencing is related to condensation (2, 31, 36, 51, 52). The molecular mechanisms that establish and maintain functionally distinct higher-order chromatin states in the interphase nucleus are poorly understood (8,42). Posttranslational modifications of histones have been directly linked with transcriptional regulation and with changes in chromatin structure (reviewed in references 22 and 24). Such modifications alter the interactions between histones and DNA and between chromatin-associated proteins. The "histone code" apparently defines functionally distinct chromatin domains of different transcriptional states (15,21,35,49). Many key questions concerning the role of histone modifications regulating chromatin structure and gene expression are unsolved.A number of heterochromatin-associated proteins, including heterochromatin protein 1 (HP1) and polycomb group (PcG) proteins, which all are involved in epigenetic silencing, share a common evolutionarily conserved domain, the chromodomain (CD), which is responsible for the association with chromatin (4,9,19,24,48). HP1 also contains a chromoshadow domain (CSD), which mediates protein-protein interactions, including homodimerization (1). Moreover, HP1 also binds to DNA and linker histones through its hinge domain (HD), positioned between the CD and the CSD. This HD may also be involved in targeting HP1 to heterochromatin through an RNA binding activity (30). A link between histone modifications and the formation of a repres...

Increased Citrullination of Histone H3 in Multiple Sclerosis Brain and Animal Models of Demyelination: A Role for Tumor Necrosis Factor-Induced Peptidylarginine Deiminase 4 Translocation

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Drosophila HP1c isoform interacts with the zinc-finger proteins WOC and Relative-of-WOC to regulate gene expression

“…In these lines, expression of fused proteins carrying the lacI-DNA-binding domain results in their targeting to the reporter gene. Previous studies showed that, in general, targeting HP1 proteins to a reporter gene induces silencing (Ayyanathan et al 2003;Li et al 2003;Verschure et al 2005;Smallwood et al 2007). Actually, in line S9.2, expression of HP1a-lacI results in silencing of the reporter gene, which is stronger in males (Fig.…”

Methylation of Lysine 9 in Histone H3 Directs Alternative Modes of Highly Dynamic Interaction of Heterochromatin Protein hHP1β with the Nucleosome

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