Project description:The family of Heterochromatin Protein 1 (HP1) consists of highly conserved proteins, which have important functions in the nucleus of eukaryotic cells. In mammals there are three HP1 paralogs: HP1(alpha), Hp1(beta), and Hp1(gamma)They are encoded by the Cbx5, Cbx1, and Cbx3 genes, respectively. Hp1 and Hp1 stably interact with Chd4 and Adnp to form the ChAHP complex. In this project, Chd4, Adnp, and the three Cbx genes were endogenously tagged with a FLAG-Avi tag in mouse embryonic stem cells. The tagged proteins were subjected to tandem-affinity purification and analysis by mass spectrometry.
Project description:H3K9 methylation (H3K9me) marks transcriptionally silent genomic regions called heterochromatin. A conserved class of HP1 proteins are critically required to establish and maintain heterochromatin. HP1 proteins bind to H3K9me, recruit factors that promote heterochromatin formation, and oligomerize to form phase-separated condensates. We do not understand how HP1 protein binding to heterochromatin establishes and maintains transcriptional silencing. Here, we demonstrate that the S.pombe HP1 homolog, Swi6, can be completely bypassed to establish silencing at ectopic and endogenous loci when an H3K4 methyltransferase, Set1 and an H3K14 acetyltransferase, Mst2 are deleted. Deleting Set1 and Mst2 enhances Clr4 enzymatic activity, leading to higher H3K9me levels and increased spreading. In contrast, Swi6 and its capacity to oligomerize were indispensable during epigenetic maintenance. Our results demonstrate the role of HP1 proteins in regulating histone modification crosstalk during establishment and identifies a genetically separable function in maintaining epigenetic memory.
Project description:Heterochromatin protein 1 (HP1) is commonly seen as a key factor of repressive heterochromatin, even though a few genes are known to require HP1-chromatin for their expression. In order to obtain insight into the targeting of HP1 and its interplay with other chromatin components, we have mapped HP1 binding sites on chromosome 2 and 4 in Drosophila Kc cells using high-density oligonucleotide arrays and the DamID technique. The resulting high-resolution maps show that HP1 forms large domains in pericentric regions, but is targeted to single genes on chromosome arms. Intriguingly, HP1 shows a striking preference for exon-dense genes on chromosome arms. Furthermore, HP1 binds along entire transcription units, except for 5’ regions. Comparison with expression data shows that most of these genes are actively transcribed. HP1 target genes are also marked by the histone variant H3.3 and dimethylated histone 3 lysine 4 (H3K4me2), which are both typical of active chromatin. Interestingly, H3.3 deposition, which is usually observed along entire transcription units, is limited to the 5’ ends of HP1-bound genes. Thus, H3.3 and HP1 are mutually exclusive marks on active chromatin. Additionally, we observed that HP1-chromatin and Polycomb-chromatin are non-overlapping, but often closely juxtaposed, suggesting an interplay between both types of chromatin. These results demonstrate that HP1-chromatin is transcriptionally active and has extensive links with several other chromatin components. Keywords: DamID
Project description:Heath et al (2021) performed proximity based labeling to probe for protein-protein interactions of the HP1 isoforms (alpha, beta, gamma) with and with-out genotoxic stress.
Project description:H3K9 methylation (H3K9me) marks transcriptionally silent genomic regions called heterochromatin. A conserved class of HP1 proteins are critically required to establish and maintain heterochromatin. HP1 proteins bind to H3K9me, recruit factors that promote heterochromatin formation, and oligomerize to form phase-separated condensates. We do not understand how HP1 protein binding to heterochromatin establishes and maintains transcriptional silencing. Here, we demonstrate that the S.pombe HP1 homolog, Swi6, can be completely bypassed to establish silencing at ectopic and endogenous loci when an H3K4 methyltransferase, Set1 and an H3K14 acetyltransferase, Mst2 are deleted. Deleting Set1 and Mst2 enhances Clr4 enzymatic activity, leading to higher H3K9me levels and increased spreading. In contrast, Swi6 and its capacity to oligomerize were indispensable during epigenetic maintenance. Our results demonstrate the role of HP1 proteins in regulating histone modification crosstalk during establishment and identifies a genetically separable function in maintaining epigenetic memory.
Project description:Embryonic stem cells (ESCs) have a hyperdynamic chromatin structure characterized by fewer discrete foci of heterochromatin protein 1 family of proteins (HP1) compared to somatic cells. During reprogramming of somatic cells, depletion of HP1γ early, reduced, while depletion later, enhanced the generation of induced pluripotent stem cells (iPSCs); concomitant with a change from a centromeric to nucleoplasmic localization. To identify the interactome of HP1γ in different biochemical environment in ESCs, we compared protein complexes of HP1γ at 0.42M salt (Dignam et al, 1983), with micrococcal nuclease (MCN) digestion with 0.3M NaCl, and MCN with 0.5M NaCl. To understand the effectors of the change in localization, we isolated protein complexes containing HP1γ in ESCs and compared the profile to that in partially reprogrammed intermediates called pre-iPSCs. Given the differential distribution of the HP1 family in ESCs, we also compared the protein interactome of HP1γ in ESCs with that of HP1α and HP1β. In addition, we probed histone associations of HP1γ in ESCs further by querying the histone post-translational modifications that were detectable. The HP1 proteins themselves are decorated with multiple PTMs, several of which we have found to be novel. Taken together our results reveal the complex contribution of the HP1 proteins to pluripotency.
Project description:Heterochromatin protein 1 (HP1) is a conserved chromatin-associated factor implicated in the establishment and maintenance of H3K9me-marked heterochromatin, potentially through phase separation–mediated condensation. Whether HP1 promotes heterochromatin condensation primarily through its dimerization or liquid–liquid phase separation (LLPS) remains unresolved. Using the C. elegans HP1 orthologue HPL-2 and a combined in vitro–in vivo approach, we systematically dissected the molecular determinants of HPL-2 function in heterochromatin condensation. Through specific mutants, we demonstrate that HPL-2 dimerization is essential for condensing H3K9me chromatin arrays in vitro and for maintaining H3K9me heterochromatin foci in C. elegans embryos. We further show that HPL-2 dimerization is sufficient to mediate segregation of H3K9me from unmodified chromatin arrays in vitro, forming spatially distinct H3K9me-enriched condensates. Surprisingly, HPL-2 mutants defective in heterochromatin foci formation cause only minor transcriptional changes among genes associated with H3K9me-marked heterochromatin, implying that HP1-dependent heterochromatin foci and gene silencing are not tightly coupled in vivo. Nonetheless, these mutant C. elegans exhibit profound physiological and developmental defects. Our findings establish dimerization as the principal molecular mechanism of HP1-driven H3K9me-chromatin condensation, elucidate an auxiliary role of LLPS, and reveal the uncoupling between HP1-dependent heterochromatin and transcriptional regulation.