Heterochromatin Protein HP1α Research Articles

The condensation of HP1α/Swi6 imparts nuclear stiffness

Biomolecular condensates have emerged as major drivers of cellular organization. It remains largely unexplored, however, whether these condensates can impart mechanical function(s) to the cell. The heterochromatin protein HP1α (Swi6 in Schizosaccharomyces pombe) crosslinks histone H3K9 methylated nucleosomes and has been proposed to undergo condensation to drive the liquid-like clustering of heterochromatin domains. Here, we leverage the genetically tractable S.pombe model and a separation-of-function allele to elucidate a mechanical function imparted by Swi6 condensation. Using single-molecule imaging, force spectroscopy, and high-resolution live-cell imaging, we show that Swi6 is critical for nuclear resistance to external force. Strikingly, it is the condensed yet dynamic pool of Swi6, rather than the chromatin-bound molecules, that is essential to imparting mechanical stiffness. Our findings suggest that Swi6 condensates embedded in the chromatin meshwork establish the emergent mechanical behavior of the nucleus as a whole, revealing that biomolecular condensation can influence organelle and cell mechanics.

CFDP1 regulates the stability of pericentric heterochromatin thereby affecting RAN GTPase activity and mitotic spindle formation.

The densely packed centromeric heterochromatin at minor and major satellites is comprised of H3K9me2/3 histones, the heterochromatin protein HP1α, and histone variants. In the present study, we sought to determine the mechanisms by which condensed heterochromatin at major and minor satellites stabilized by the chromatin factor CFDP1 affects the activity of the small GTPase Ran as a requirement for spindle formation. CFDP1 colocalized with heterochromatin at major and minor satellites and was essential for the structural stability of centromeric heterochromatin. Loss of CENPA, HP1α, and H2A.Z heterochromatin components resulted in decreased binding of the spindle nucleation facilitator RCC1 to minor and major satellite repeats. Decreased RanGTP levels as a result of diminished RCC1 binding interfered with chromatin-mediated microtubule nucleation at the onset of mitotic spindle formation. Rescuing chromatin H2A.Z levels in cells and mice lacking CFDP1 through knock-down of the histone chaperone ANP32E not only partially restored RCC1-dependent RanGTP levels but also alleviated CFDP1-knockout-related craniofacial defects and increased microtubule nucleation in CFDP1/ANP32E co-silenced cells. Together, these studies provide evidence for a direct link between condensed heterochromatin at major and minor satellites and microtubule nucleation through the chromatin protein CFDP1.

KCNQ1OT1 promotes genome-wide transposon repression by guiding RNA-DNA triplexes and HP1 binding.

Transposon (de)repression and heterochromatin reorganization are dynamically regulated during cell fate determination and are hallmarks of cellular senescence. However, whether they are sequence specifically regulated remains unknown. Here we uncover that the KCNQ1OT1 lncRNA, by sequence-specific Hoogsteen base pairing with double-stranded genomic DNA via its repeat-rich region and binding to the heterochromatin protein HP1α, guides, induces and maintains epigenetic silencing at specific repetitive DNA elements. Repressing KCNQ1OT1 or deleting its repeat-rich region reduces DNA methylation and H3K9me3 on KCNQ1OT1-targeted transposons. Engineering a fusion KCNQ1OT1 with an ectopically targeting guiding triplex sequence induces de novo DNA methylation at the target site. Phenotypically, repressing KCNQ1OT1 induces senescence-associated heterochromatin foci, transposon activation and retrotransposition as well as cellular senescence, demonstrating an essential role of KCNQ1OT1 to safeguard against genome instability and senescence.

AGO1 regulates pericentromeric regions in mouse embryonic stem cells.

Argonaute proteins (AGOs), which play an essential role in cytosolic post-transcriptional gene silencing, have been also reported to function in nuclear processes like transcriptional activation or repression, alternative splicing and, chromatin organization. As most of these studies have been conducted in human cancer cell lines, the relevance of AGOs nuclear functions in the context of mouse early embryonic development remains uninvestigated. Here, we examined a possible role of the AGO1 protein on the distribution of constitutive heterochromatin in mouse embryonic stem cells (mESCs). We observed a specific redistribution of the repressive histone mark H3K9me3 and the heterochromatin protein HP1α, away from pericentromeric regions upon Ago1 depletion. Furthermore, we demonstrated that major satellite transcripts are strongly up-regulated in Ago1_KO mESCs and that their levels are partially restored upon AGO1 rescue. We also observed a similar redistribution of H3K9me3 and HP1α in Drosha_KO mESCs, suggesting a role for microRNAs (miRNAs) in the regulation of heterochromatin distribution in mESCs. Finally, we showed that specific miRNAs with complementarity to major satellites can partially regulate the expression of these transcripts.

HP1α is a chromatin crosslinker that controls nuclear and mitotic chromosome mechanics.

Chromatin, which consists of DNA and associated proteins, contains genetic information and is a mechanical component of the nucleus. Heterochromatic histone methylation controls nucleus and chromosome stiffness, but the contribution of heterochromatin protein HP1α (CBX5) is unknown. We used a novel HP1α auxin-inducible degron human cell line to rapidly degrade HP1α. Degradation did not alter transcription, local chromatin compaction, or histone methylation, but did decrease chromatin stiffness. Single-nucleus micromanipulation reveals that HP1α is essential to chromatin-based mechanics and maintains nuclear morphology, separate from histone methylation. Further experiments with dimerization-deficient HP1αI165E indicate that chromatin crosslinking via HP1α dimerization is critical, while polymer simulations demonstrate the importance of chromatin-chromatin crosslinkers in mechanics. In mitotic chromosomes, HP1α similarly bolsters stiffness while aiding in mitotic alignment and faithful segregation. HP1α is therefore a critical chromatin-crosslinking protein that provides mechanical strength to chromosomes and the nucleus throughout the cell cycle and supports cellular functions.

Homotypic clustering of L1 and B1/Alu repeats compartmentalizes the 3D genome

Organization of the genome into euchromatin and heterochromatin appears to be evolutionarily conserved and relatively stable during lineage differentiation. In an effort to unravel the basic principle underlying genome folding, here we focus on the genome itself and report a fundamental role for L1 (LINE1 or LINE-1) and B1/Alu retrotransposons, the most abundant subclasses of repetitive sequences, in chromatin compartmentalization. We find that homotypic clustering of L1 and B1/Alu demarcates the genome into grossly exclusive domains, and characterizes and predicts Hi-C compartments. Spatial segregation of L1-rich sequences in the nuclear and nucleolar peripheries and B1/Alu-rich sequences in the nuclear interior is conserved in mouse and human cells and occurs dynamically during the cell cycle. In addition, de novo establishment of L1 and B1 nuclear segregation is coincident with the formation of higher-order chromatin structures during early embryogenesis and appears to be critically regulated by L1 and B1 transcripts. Importantly, depletion of L1 transcripts in embryonic stem cells drastically weakens homotypic repeat contacts and compartmental strength, and disrupts the nuclear segregation of L1- or B1-rich chromosomal sequences at genome-wide and individual sites. Mechanistically, nuclear co-localization and liquid droplet formation of L1 repeat DNA and RNA with heterochromatin protein HP1α suggest a phase-separation mechanism by which L1 promotes heterochromatin compartmentalization. Taken together, we propose a genetically encoded model in which L1 and B1/Alu repeats blueprint chromatin macrostructure. Our model explains the robustness of genome folding into a common conserved core, on which dynamic gene regulation is overlaid across cells.

Liquid-Liquid Phase Separation of Histone Proteins in Cells: Role in Chromatin Organization

Liquid-liquid phase separation (LLPS) of proteins and nucleic acids has emerged as an important phenomenon in membraneless intracellular organization. We demonstrate that the linker histone H1 condenses into liquid-like droplets in the nuclei of HeLa cells. The droplets, observed during the interphase of the cell cycle, are colocalized with DNA-dense regions indicative of heterochromatin. In vitro, H1 readily undergoes LLPS with both DNA and nucleosomes of varying lengths but does not phase separate in the absence of DNA. The nucleosome core particle maintains its structural integrity inside the droplets, as demonstrated by FRET. Unexpectedly, H2A also forms droplets in the presence of DNA and nucleosomes in vitro, whereas the other core histones precipitate. The phase diagram of H1 with nucleosomes is invariant to the nucleosome length at physiological salt concentration, indicating that H1 is capable of partitioning large segments of DNA into liquid-like droplets. Of the proteins tested (H1, core histones, and the heterochromatin protein HP1α), this property is unique to H1. In addition, free nucleotides promote droplet formation of H1 nucleosome in a nucleotide-dependent manner, with droplet formation being most favorable with ATP. Although LLPS of HP1α is known to contribute to the organization of heterochromatin, our results indicate that H1 also plays a role. Based on our study, we propose that H1 and DNA act as scaffolds for phase-separated heterochromatin domains.

Heterochromatin Protein HP1α Gelation Dynamics Revealed by Solid-State NMR Spectroscopy.

Heterochromatin protein 1α (HP1α) undergoes liquid-liquid phase separation (LLPS) and forms liquid droplets and gels in vitro, properties that also appear to be central to its biological function in heterochromatin compaction and regulation. Here we use solid-state NMR spectroscopy to track the conformational dynamics of phosphorylated HP1α during its transformation from the liquid to the gel state. Using experiments designed to probe distinct dynamic modes, we identify regions with varying mobilities within HP1α molecules and show that specific serine residues uniquely contribute to gel formation. The addition of chromatin disturbs the gelation process while preserving the conformational dynamics within individual bulk HP1α molecules. Our study provides a glimpse into the dynamic architecture of dense HP1α phases and showcases the potential of solid-state NMR to detect an elusive biophysical regime of phase separating biomolecules.

Heterochromatin Protein HP1α Gelation Dynamics Revealed by Solid‐State NMR Spectroscopy

AbstractHeterochromatin protein 1α (HP1α) undergoes liquid–liquid phase separation (LLPS) and forms liquid droplets and gels in vitro, properties that also appear to be central to its biological function in heterochromatin compaction and regulation. Here we use solid‐state NMR spectroscopy to track the conformational dynamics of phosphorylated HP1α during its transformation from the liquid to the gel state. Using experiments designed to probe distinct dynamic modes, we identify regions with varying mobilities within HP1α molecules and show that specific serine residues uniquely contribute to gel formation. The addition of chromatin disturbs the gelation process while preserving the conformational dynamics within individual bulk HP1α molecules. Our study provides a glimpse into the dynamic architecture of dense HP1α phases and showcases the potential of solid‐state NMR to detect an elusive biophysical regime of phase separating biomolecules.

RNA Binding Protein Rbmx Is Required in Acute Myeloid Leukemia By Regulating the Transcriptional Activity of the Heterochromatin Protein HP1α

Acute myeloid leukemia (AML) is characterized by a block in the development of myeloid cells, often due to dysregulation of genes involved in key processes including self-renewal, proliferation, and differentiation. Somatic mutations and aberrant expression of RNA binding proteins (RBPs) have recently been found to be important in hematological malignancies. For example, our group and others have recently determined that increased expression of MUSASHI-2 and SYNCRIP drives aggressive leukemia. To discover novel RBP regulators of leukemia, we performed an in vivo pooled shRNA screen of 127 MSI2 direct protein interactors and associated genes (Vu et al. Nat Gen. 2017). In this screen, shRNAs specific to the RBP RBMX (RNA binding motif protein, X-linked) were selectively depleted in murine MLL-AF9 driven leukemia. RBMX has been implicated in regulating alternative splicing, chromatin cohesion, and DNA-damage response, but its function in hematopoiesis and leukemia is not known. We confirmed that depletion of RBMX with shRNAs in murine MLL-AF9 leukemia cells resulted in reduced myeloid colony formation, increased apoptosis, and increased differentiation as determined by flow analysis of myeloid cell surface markers Gr-1 and Mac-1 (mean of 61-65% shRNA versus mean of 12.95% control). Furthermore, RBMX is highly expressed among human myeloid leukemia cell lines (n=10/11) and primary AML patient samples (n=2/4). Depletion of RBMX with shRNAs led to a dramatic decrease in cell proliferation and 3-fold induction of apoptosis in several human myeloid leukemia cell lines (MOLM-13, THP-1, K562, and KCL-22). Additionally, RBMX depletion in AML cells induced myeloid differentiation and significantly delayed leukemogenesis cells in vivo (median survival of 51.5 days in control versus median ‘not reached’ in shRNA1 and shRNA2). To determine if there is a differential requirement of RBMX in survival of leukemia cells compared to normal hematopoietic stem and progenitor cells (HSPCs), we depleted RBMX with shRNAs in normal murine bone marrow c-Kit+ cells and found no significant changes in colony formation. Depleting RBMX with shRNAs in human cord blood derived CD34+ HSPCs resulted in reduced colony formation but no increase in apoptosis. Thus, these data suggest that there is a differential requirement for RBMX in myeloid leukemia cells compared to normal cells. To uncover the mechanism of RBMX function, we performed RNA-sequencing of human AML cells (MOLM-13) depleted for RBMX. Gene set enrichment analysis demonstrated a loss of cell cycle and DNA repair associated programs in RBMX depleted cells. Complex chromosomal karyotyping analysis of these cells revealed increased metaphases with breaks and gaps (mean of 30.67% shRNA versus mean of 13.33% control) and irregular chromatin compaction (mean of 47.67%shRNA versus mean of 20% control), while cell cycle analysis showed significantly increased S-phase arrest as determined by flow analysis of Hoechst stained cells (mean of 37-40% shRNA versus of 24.18% control). Reanalysis of RBMX transcriptome-wide binding sites in 293T cells revealed that RBMX directly binds to heterochromatin protein HP1α transcripts (Liu et al. Nucleic Acids Res. 2017). HP1α, also called CBX5, is a key heterochromatin protein that binds to histone H3-K9 tri-methylation marks to promote heterochromatin formation, which is critical in chromatin condensation and chromosome segregation. HP1α has also been determined to be required for MLL leukemia stem cell maintenance. We demonstrated that RBMX depletion resulted in a significant decrease of HP1α mRNA expression without affecting its mRNA stability in AML cells. We confirmed that RBMX depletion reduced the protein abundance of HP1α. Moreover, overexpression of HP1α rescued the effect of RBMX depletion on cell growth and apoptosis. Our study finds that RBMX binds to HP1α mRNA and regulates the transcriptional activity of the HP1α locus, which then maintains proper chromatin compaction in leukemia cells. Overall, we determine that RBMX function is critical for myeloid leukemia survival and has potential as a novel therapeutic target in AML. DisclosuresNo relevant conflicts of interest to declare.

Local enrichment of HP1alpha at telomeres alters their structure and regulation of telomere protection

Enhanced telomere maintenance is evident in malignant cancers. While telomeres are thought to be inherently heterochromatic, detailed mechanisms of how epigenetic modifications impact telomere protection and structures are largely unknown in human cancers. Here we develop a molecular tethering approach to experimentally enrich heterochromatin protein HP1α specifically at telomeres. This results in increased deposition of H3K9me3 at cancer cell telomeres. Telomere extension by telomerase is attenuated, and damage-induced foci at telomeres are reduced, indicating augmentation of telomere stability. Super-resolution STORM imaging shows an unexpected increase in irregularity of telomeric structure. Telomere-tethered chromo shadow domain (CSD) mutant I165A of HP1α abrogates both the inhibition of telomere extension and the irregularity of telomeric structure, suggesting the involvement of at least one HP1α-ligand in mediating these effects. This work presents an approach to specifically manipulate the epigenetic status locally at telomeres to uncover insights into molecular mechanisms underlying telomere structural dynamics.

Phosphatidylinositol-4-phosphate 5-Kinase 1α Modulates Ribosomal RNA Gene Silencing through Its Interaction with Histone H3 Lysine 9 Trimethylation and Heterochromatin Protein HP1-α

Phosphoinositide signaling has been implicated in the regulation of numerous cellular processes including cytoskeletal dynamics, cellular motility, vesicle trafficking, and gene transcription. Studies have also shown that nuclear phosphoinositide(s) regulates processes such as mRNA export, cell cycle progression, gene transcription, and DNA repair. We have shown previously that the nuclear form of phosphatidylinositol-4-phosphate 5-kinase 1α (PIP5K), the enzyme responsible for phosphatidylinositol 4,5-bisphosphate synthesis, is modified by small ubiquitin-like modifier (SUMO)-1. In this study, we have shown that due to the site-specific Lys to Ala mutations of PIP5K at Lys-244 and Lys-490, it is unable to localize in the nucleus and nucleolus, respectively. Furthermore, by using chromatin immunoprecipitation assays, we have observed that PIP5K associates with the chromatin silencing complex constituted of H3K9me3 and heterochromatin protein 1α at multiple ribosomal DNA (rDNA) loci. These interactions followed a definite cyclical pattern of occupancy (mostly G1) and release from the rDNA loci (G1/S) throughout the cell cycle. Moreover, the immunoprecipitation results clearly demonstrate that PIP5K SUMOylated at Lys-490 interacts with components of the chromatin silencing machinery, H3K9me3 and heterochromatin protein 1α. However, PIP5K does not interact with the gene activation signature protein H3K4me3. This study, for the first time, demonstrates that PIP5K, an enzyme actively associated with lipid modification pathway, has additional roles in rDNA silencing.

Citrullination of Histone H3 Interferes with HP1-Mediated Transcriptional Repression

Multiple Sclerosis (MS) is an autoimmune disease associated with abnormal expression of a subset of cytokines, resulting in inappropriate T-lymphocyte activation and uncontrolled immune response. A key issue in the field is the need to understand why these cytokines are transcriptionally activated in the patients. Here, we have examined several transcription units subject to pathological reactivation in MS, including the TNFα and IL8 cytokine genes and also several Human Endogenous RetroViruses (HERVs). We find that both the immune genes and the HERVs require the heterochromatin protein HP1α for their transcriptional repression. We further show that the Peptidylarginine Deiminase 4 (PADI4), an enzyme with a suspected role in MS, weakens the binding of HP1α to tri-methylated histone H3 lysine 9 by citrullinating histone H3 arginine 8. The resulting de-repression of both cytokines and HERVs can be reversed with the PADI-inhibitor Cl-amidine. Finally, we show that in peripheral blood mononuclear cells (PBMCs) from MS patients, the promoters of TNFα, and several HERVs share a deficit in HP1α recruitment and an augmented accumulation of histone H3 with a double citrulline 8 tri-methyl lysine 9 modifications. Thus, our study provides compelling evidence that HP1α and PADI4 are regulators of both immune genes and HERVs, and that multiple events of transcriptional reactivation in MS patients can be explained by the deficiency of a single mechanism of gene silencing.

Telomere length regulates TERRA levels through increased trimethylation of telomeric H3K9 and HP1α

Gene silencing by the repressive telomeric chromatin environment, referred to as telomere position effect (TPE), has been well characterized in yeast and depends on telomere length. However, proof of its existence at native human chromosome ends has remained elusive, mainly owing to the paucity of genes near telomeres. The discovery of TERRAs, the telomeric noncoding RNAs transcribed from subtelomeric promoters, paved the way to probing for telomere-length impact on physiological TPE. Using cell lines of various origins, we show that telomere elongation consistently represses TERRA expression. Repression is mediated by increased trimethylated H3K9 density at telomeres and by heterochromatin protein HP1α, with no detectable spreading of the marks beyond the telomeric tract, restricting human TPE to telomere transcription. Our data further support the existence of a negative-feedback mechanism in which longer TERRA molecules repress their own transcription upon telomere elongation.

Quantitative fluorescence imaging of protein diffusion and interaction in living cells

Diffusion processes and local dynamic equilibria inside cells lead to nonuniform spatial distributions of molecules, which are essential for processes such as nuclear organization and signaling in cell division, differentiation and migration. To understand these mechanisms, spatially resolved quantitative measurements of protein abundance, mobilities and interactions are needed, but current methods have limited capabilities to study dynamic parameters. Here we describe a microscope based on light-sheet illumination that allows massively parallel fluorescence correlation spectroscopy (FCS) measurements and use it to visualize the diffusion and interactions of proteins in mammalian cells and in isolated fly tissue. Imaging the mobility of heterochromatin protein HP1α (ref. 4) in cell nuclei we could provide high-resolution diffusion maps that reveal euchromatin areas with heterochromatin-like HP1α-chromatin interactions. We expect that FCS imaging will become a useful method for the precise characterization of cellular reaction-diffusion processes.

Progress in the congenital dyserythropoietic anemias: Juicy but high-hanging fruits?

Progress in the congenital dyserythropoietic anemias: Juicy but high-hanging fruits?

Codanin-1 Mutations In Congenital Dyserythropoietic Anemia Type 1 Affect HP1α Localization In Erythroblasts

AbstractAbstract 1003 Introduction:The Congenital dyserythropoietic anemias (CDAs) are a heterogeneous group of rare inborn disorders mainly affecting erythropoiesis. Distinct from other inherited bone marrow failure syndromes, they are marked by morphological abnormalities of the erythroblasts leading to ineffective erythropoiesis/dyserythropoiesis, while the other hematopoietic lineages appear to be unaffected. Based largely on the dysplastic changes observed in erythroblasts by light and electron microscopy, the CDAs have been divided into 3 major types (CDA-1, 2 and 3). Causative genes have been identified for CDA-1 (CDAN1/15q15) and CDA-2 (CDAN2/20p11), while the chromosomal locus is known for CDA-3 (15q22). The majority of CDA-1 cases are associated with CDAN1 missense mutations, leading to single amino-acid substitutions in the 134 kDa Codanin-1 protein. Electron microscopy of CDA-1 erythroblasts reveals a characteristic pattern of spongy (“Swiss cheese”) heterochromatin. We investigated the chromatin in CDA-1 patients, the localization, distribution and interactions of Codanin-1 and generated a murine knock-out model for CDAN1. Results:Given the grossly abnormal chromatin ultrastructure in CDA-1 erythroblasts, we examined its overall content. No gross differences between normal and patient samples were seen, both in the amounts of histone proteins or various epigenetic marks of histone tails, suggesting that histone signatures, involved in maintenance of chromatin structure and epigenetic regulation, are globally maintained CDA-1. We confirmed the latter by ChIP-on-chip analysis where immunoprecipitated H4 acetylated, H3K4 dimethylated, H3K9 trimethylated and H3K27 trimethylated chromatin from CDA-1 erythroblasts and normal controls was hybridized to ENCODE genome-wide microrrays. Using a hybridoma technique, we produced 3 novel monoclonal antibodies to Codanin-1. These consistently demonstrate its distribution in both nucleus and cytoplasm of cells by immunofluorescence and Western Blotting. While Codanin-1 is found in the nucleus, it is more abundant in the cytoplasm in primary human erythroblasts. This localization pattern was unchanged in CDA-1 erythroblasts. Within the nucleus, Codanin-1 is localized in foci, while in the cytoplasm it shows an aggregated distribution, with the endoplasmic reticulum and Golgi apparatus being the most likely locations. No differences were found in the distribution patterns of RNA-polymerase-II, PML, nucleophosmin, HP1β and HP1γ between patients' and control erythroblasts. However, the localization of HP1α, a key component of heterochromatin, was found to be markedly perturbed. HP1α accumulates in the Golgi apparatus of CDA-1 but not normal erythroblasts. This was not seen in lymphoblasts derived from CDA-1 patients, nor in erythroblasts from a patient with CDA-2. We confirmed that the abnormal localization of HP1α in CDA-1 patients is confined to the intermediate erythroblast maturation stage, where the characteristic ultrastructural chromatin pattern of CDA-1 is observed. Immunoprecipitation of erythroblast protein extracts with anti- HP1α antibodies co-precipitated Codanin-1, suggesting that an abnormality in Codanin-1 could be responsible for the aberrant localization of HP1α. By confocal immunofluorescence, we also found Codanin-1 to co-localize partially with SEC23B, the protein mutated in CDA-2, suggesting a molecular link between the two major types of CDA. We generated the first murine Cdan1 gene-trap model demonstrating its widespread expression during embryonic development. Cdan1gt/gt homozygotes die in-utero before the onset of primitive erythropoiesis, suggesting that Cdan1 has other critical roles during embryogenesis. Adult heterozygous Cdan1wt/gt mice had normal hematopoietic function and morphology, normal life expectancy and fertility. No extra-hematopoietic macroscopic phenotype could be identified. Conclusions:Our results show that the absence of the highly conserved and ubiquitously expressed Codanin-1 is embryonic lethal, and suggest that missense CDAN1 mutations are responsible for the erythroid specific phenotype of CDA-1 via the abnormal cellular trafficking of the heterochromatin protein HP1α. Moreover, our findings suggest that common molecular pathways underlie the CDAs. Disclosures:No relevant conflicts of interest to declare.

Trimethylation of histone H3 lysine 4 impairs methylation of histone H3 lysine 9

Chromatin is broadly compartmentalized in two defined states: euchromatin and heterochromatin. Generally, euchromatin is trimethylated on histone H3 lysine 4 (H3K4me3) while heterochromatin contains the H3K9me3 marks. The H3K9me3 modification is added by lysine methyltransferases (KMTs) such as SETDB1. Herein, we show that SETDB1 interacts with its substrate H3, but only in the absence of the euchromatic mark H3K4me3. In addition, we show that SETDB1 fails to methylate substrates containing the H3K4me3 mark. Likewise, the functionally related H3K9 KMTs G9A, GLP, and SUV39H1 also fail to bind and to methylate H3K4me3 substrates. Accordingly, we provide in vivo evidence that H3K9me2-enriched histones are devoid of H3K4me2/3 and that histones depleted of H3K4me2/3 have elevated H3K9me2/3. The correlation between the loss of interaction of these KMTs with H3K4me3 and concomitant methylation impairment leads to the postulate that, at least these four KMTs, require stable interaction with their respective substrates for optimal activity. Thus, novel substrates could be discovered via the identification of KMT interacting proteins. Indeed, we find that SETDB1 binds to and methylates a novel substrate, the inhibitor of growth protein ING2, while SUV39H1 binds to and methylates the heterochromatin protein HP1α. Thus, our observations suggest a mechanism of post-translational regulation of lysine methylation and propose a potential mechanism for the segregation of the biologically opposing marks, H3K4me3 and H3K9me3. Furthermore, the correlation between H3-KMTs interaction and substrate methylation highlights that the identification of novel KMT substrates may be facilitated by the identification of interaction partners.

Janus Kinase 2: An Epigenetic 'Writer' that Activates Leukemogenic Genes

Activation of Janus kinase 2 (JAK2) plays a critical role in normal hematopoiesis and leukemogenesis. Dawson et al. (2009; JAK2 phosphorylates histone H3Y41 and excludes HP1alpha from chromatin. Nature 461, 819-822) report that JAK2 performs this function by displacing the heterochromatin protein HP1α from chromatin through phosphorylation of histone H3.

H1 histone subtype constitution and phosphorylation state of the ageing cell system of human peripheral blood lymphocytes

Until a few years ago, the H1 histones were exclusively considered to be the architectural proteins of chromatin involved in chromatin condensation. However there is now increasing data to support the hypothesis that the H1 subtypes are involved in genomic integrity and that they may have unexpected functional roles in various biological processes such as in differentiation and DNA repair, apoptosis and lifespan. Moreover, the H1 histones are phosphorylated to a great extent. Recent work has implicated phosphorylation of H1 in the regulation of chromatin remodeling. In light of the fact that chromatin reorganization and heterochromatin formation has been shown to take place during ageing and senescence, in the present investigation, we have analyzed the changes that take place in the somatic H1 linker histone subtype profile and their phosphorylation states in human peripheral blood lymphocytes as a function of donor age. Results from this work show that there is a significant age-related dephosphorylation of H1.4 and H1.5 and an increase in the heterochromatin protein HP1α as a function of donor age. These results indicate that dephosphorylation of H1 histones may be related to an increase in senescence-associated heterochromatin formation during the in vivo ageing of human peripheral blood lymphocytes.