Evidence for the existence of an HP1-mediated subcode within the histone code

Abstract

Currently, the mammalian heterochromatic proteins HP1alpha, HP1beta and the pan-nuclear HP1gamma are considered 'gatekeepers' of methyl-K9-H3-mediated silencing. Understanding how the binding of these proteins to post-translationally modified histones is switched on and off will further our knowledge of how the histone code is modulated. Here, we report that all three HP1 isoforms can be extensively modified, similar to histones, suggesting that the silencing of gene expression may be further regulated beyond the histone code. To assess the potential impact of these modifications, we analysed the phosphorylation of HP1gamma at Ser 83 as a 'model modification'. We demonstrate that P-Ser 83-HP1gamma has an exclusively euchromatic localization, interacts with Ku70 (a regulatory protein involved in multiple nuclear procesess), has impaired silencing activity and serves as a marker for transcription elongation. These observations predict that regulation of silencing by methyl-K9-H3 through modification of mammalian HP1 proteins may be more complex than previously thought and suggests the existence of an HP1-mediated 'silencing subcode' that underlies the instructions of the histone code.

A phosphorylated subpopulation of the histone variant macroH2A1 is excluded from the inactive X chromosome and enriched during mitosis

“…S137ph is found in the hinge region of mH2A, which has been suggested to bind nucleic acids (6). Interestingly, heterochromatin protein 1 (HP1) also has a lysinerich hinge domain (between its chromo-and chromoshadow domains) that is phosphorylated in vivo (36). The HP1 hinge domain has been shown to bind RNA in vitro, and RNase treatment of mammalian cells releases HP1 from chromatin (37,38).…”

Differentially expressed genes are marked by histone 3 lysine 9 trimethylation in human cancer cells

“…HP1g is present in mammals but not in the widely studied fission yeast. Recent studies suggest phosphorylation of HP1g at Ser 83 is responsible for targeting the protein to euchromatin and it's binding to the Ku70 protein (Lomberk et al, 2006). There are no known functional roles for H3K9Me3 and HP1g in transcribing genes although it has been suggested that the presence of H3K9Me3 may prevent cryptic promoter elements from becoming aberrantly activated following the passage of RNA polymerase II (Vakoc et al, 2006).…”

Isoform-specific Intermolecular Disulfide Bond Formation of Heterochromatin Protein 1 (HP1)

“…We hypothesized that HP1␣-FLAG lacking the modified residue would fractionate into a single peak by reverse-phase HPLC. First, we thoroughly screened the CD and hinge region, both of which are reported to be posttranslationally modified (17,22). However, we could not determine any specific amino acid residue from the mutational analysis (supplemental Fig.…”

“…Tethering any HP1 isoform upstream of a promoter equally triggers gene silencing concomitant with local chromatin condensation and an increase in H3K9 methylation (10 -12), indicating their common silencing ability. However, nonredundant functions (13,14), different binding affinities to other proteins (15)(16)(17) and different localizations in tissues (18,19), of these three HP1 isoforms imply that ␣, ␤, and ␥ are functionally diverse. Furthermore, recent evidence clarified apparently opposite functions of HP1 isoforms, e.g.…”

“…In fact, reversible modifications of HP1 (e.g. phosphorylation) can modulate its function in response to various stimuli or cellular environments, suggesting an active role for HP1 beyond its known function as a marker of heterochromatin (17,22). However, the precise modulatory mechanism across three HP1 isoforms that leads to functional differences remains to be elucidated.…”