Structural basis for specific binding of Polycomb chromodomain to histone H3 methylated at Lys 27.
ABSTRACT: The chromodomain of Drosophila Polycomb protein is essential for maintaining the silencing state of homeotic genes during development. Recent studies suggest that Polycomb mediates the assembly of repressive higher-order chromatin structures in conjunction with the methylation of Lys 27 of histone H3 by a Polycomb group repressor complex. A similar mechanism in heterochromatin assembly is mediated by HP1, a chromodomain protein that binds to histone H3 methylated at Lys 9. To understand the molecular mechanism of the methyl-Lys 27 histone code recognition, we have determined a 1.4-A-resolution structure of the chromodomain of Polycomb in complex with a histone H3 peptide trimethylated at Lys 27. The structure reveals a conserved mode of methyl-lysine binding and identifies Polycomb-specific interactions with histone H3. The structure also reveals a dPC dimer in the crystal lattice that is mediated by residues specifically conserved in the Polycomb family of chromodomains. The dimerization of dPC can effectively account for the histone-binding specificity and provides new mechanistic insights into the function of Polycomb. We propose that self-association is functionally important for Polycomb.
Project description:On the histone H3 tail, Lys 9 and Lys 27 are both methylation sites associated with epigenetic repression, and reside within a highly related sequence motif ARKS. Here we show that the chromodomain proteins Polycomb (Pc) and HP1 (heterochromatin protein 1) are highly discriminatory for binding to these sites in vivo and in vitro. In Drosophila S2 cells, and on polytene chromosomes, methyl-Lys 27 and Pc are both excluded from areas that are enriched in methyl-Lys 9 and HP1. Swapping of the chromodomain regions of Pc and HP1 is sufficient for switching the nuclear localization patterns of these factors, indicating a role for their chromodomains in both target site binding and discrimination. To better understand the molecular basis for the selection of methyl-lysine binding sites, we solved the 1.8 A structure of the Pc chromodomain in complex with a H3 peptide bearing trimethyl-Lys 27, and compared it with our previously determined structure of the HP1 chromodomain in complex with a H3 peptide bearing trimethyl-Lys 9. The Pc chromodomain distinguishes its methylation target on the H3 tail via an extended recognition groove that binds five additional residues preceding the ARKS motif.
Project description:Previous studies have shown two homologous chromodomain modules in the HP1 and Polycomb proteins exhibit discriminatory binding to related methyllysine residues (embedded in ARKS motifs) of the histone H3 tail. Methylated ARK(S/T) motifs have recently been identified in other chromatin factors (e.g. linker histone H1.4 and lysine methyltransferase G9a). These are thought to function as peripheral docking sites for the HP1 chromodomain. In vertebrates, HP1-like chromodomains are also present in the chromodomain Y chromosome (CDY) family of proteins adjacent to a putative catalytic motif. The human genome encodes three CDY family proteins, CDY, CDYL, and CDYL2. These have putative functions ranging from establishment of histone H4 acetylation during spermiogenesis to regulation of transcription co-repressor complexes. To delineate the biochemical functions of the CDY family chromodomains, we analyzed their specificity of methyllysine recognition. We detected substantial differences among these factors. The CDY chromodomain exhibits discriminatory binding to lysine-methylated ARK(S/T) motifs, whereas the CDYL2 chromodomain binds with comparable strength to multiple ARK(S/T) motifs. Interestingly, subtle amino acid changes in the CDYL chromodomain prohibit such binding interactions in vitro and in vivo. However, point mutations can rescue binding. In support of the in vitro binding properties of the chromodomains, the full-length CDY family proteins exhibit substantial variability in chromatin localization. Our studies underscore the significance of subtle sequence differences in a conserved signaling module for diverse epigenetic regulatory pathways.
Project description:To maintain genome stability, cells pack large portions of their genome into silent chromatin or heterochromatin. Histone H3 lysine 9 methylation, a hallmark of heterochromatin, is recognized by conserved readers called chromodomains. But how chromodomains interact with their actual binding partner, the H3K9 methylated nucleosome, remains elusive. We have determined the structure of a nucleosome trimethylated at lysine 9 of histone H3 (H3K9me3 Nucleosome) in a complex with the chromodomain of Chp1, a protein required for RNA interference-dependent heterochromatin formation in fission yeast. The cryo-electron microscopy structure reveals that the chromodomain of Chp1 binds the histone H3 lysine 9 methylated tail and the core of the nucleosome, primarily histones H3 and H2B. Mutations in chromodomain of Chp1 loops, which interact with the nucleosome core, abolished this interaction in vitro. Moreover, fission yeast cells with Chp1 loop mutations have a defect in Chp1 recruitment and heterochromatin formation. This study reveals the structural basis for heterochromatic silencing and suggests that chromodomains could read histone code in the H3 tail and the nucleosome core, which would provide an additional layer of regulation.
Project description:SUV39H1, the first identified histone lysine methyltransferase in human, is involved in chromatin modification and gene regulation. SUV39H1 contains a chromodomain in its N-terminus, which potentially plays a role in methyl-lysine recognition and SUV39H1 targeting. In this study, the structure of the chromodomain of human SUV39H1 was determined by X-ray crystallography. The SUV39H1 chromodomain displays a generally conserved structure fold compared with other solved chromodomains. However, different from other chromodomains, the SUV39H1 chromodomain possesses a much longer helix at its C-terminus. Furthermore, the SUV39H1 chromodomain was shown to recognize histone H3K9me2/3 specifically.
Project description:The eight mammalian Cbx proteins are chromodomain-containing proteins involved in regulation of heterochromatin, gene expression, and developmental programs. They are evolutionarily related to the Drosophila HP1 (dHP1) and Pc (dPc) proteins that are key components of chromatin-associated complexes capable of recognizing repressive marks such as trimethylated Lys-9 and Lys-27, respectively, on histone H3. However, the binding specificity and function of the human homologs, Cbx1-8, remain unclear. To this end we employed structural, biophysical, and mutagenic approaches to characterize the molecular determinants of sequence contextual methyllysine binding to human Cbx1-8 proteins. Although all three human HP1 homologs (Cbx1, -3, -5) replicate the structural and binding features of their dHP counterparts, the five Pc homologs (Cbx2, -4, -6, -7, -8) bind with lower affinity to H3K9me3 or H3K27me3 peptides and are unable to distinguish between these two marks. Additionally, peptide permutation arrays revealed a greater sequence tolerance within the Pc family and suggest alternative nonhistone sequences as potential binding targets for this class of chromodomains. Our structures explain the divergence of peptide binding selectivity in the Pc subfamily and highlight previously unrecognized features of the chromodomain that influence binding and specificity.
Project description:Histone tail peptides comprise the flexible portion of chromatin, the substance which serves as the packaging for the eukaryotic genome. According to the histone code hypothesis, reader protein domains (chromodomains) can recognize modifications of amino acid residues within these peptides, regulating the expression of genes. We have performed simulations on models of chromodomain helicase DNA-binding protein 1 complexed with a variety of histone H3 modifications. Binding free energies for both the overall complexes and the individual residues within the protein and peptides were computed with molecular mechanics-generalized Born surface area. The simulation results agree well with experimental data and identify several chromodomain helicase DNA-binding protein 1 residues that play key roles in the interaction with each of the H3 modifications. We identified one class of protein residues that bind to H3 in all of the complexes (generally interacting hydrophobically), and a second class of residues that bind only to particular H3 modifications (generally interacting electrostatically). Additionally, we found that modifications of H3R2 and H3T3 have a dominant effect on the binding affinity; methylation of H3K4 has little effect on the interaction strength when H3R2 or H3T3 is modified. Our findings with regard to the specificity shown by the latter class of protein residues in their binding affinity to certain modifications of H3 support the histone code hypothesis.
Project description:Polycomb group (PcG) proteins are epigenetic regulators that facilitate both embryonic development and cancer progression. PcG proteins form Polycomb repressive complexes 1 and 2 (PRC1 and PRC2). PRC2 trimethylates histone H3 lysine 27 (H3K27me3), a histone mark recognized by the N-terminal chromodomain (ChD) of the CBX subunit of canonical PRC1. There are five PcG CBX paralogs in humans. CBX2 in particular is upregulated in a variety of cancers, particularly in advanced prostate cancers. Using CBX2 inhibitors to understand and target CBX2 in prostate cancer is highly desirable; however, high structural similarity among the CBX ChDs has been challenging for developing selective CBX ChD inhibitors. Here, we utilize selections of focused DNA encoded libraries (DELs) for the discovery of a selective CBX2 chromodomain probe, SW2_152F. SW2_152F binds to CBX2 ChD with a K<sub>d</sub> of 80 nM and displays 24-1000-fold selectivity for CBX2 ChD over other CBX paralogs in vitro. SW2_152F is cell permeable, selectively inhibits CBX2 chromatin binding in cells, and blocks neuroendocrine differentiation of prostate cancer cell lines in response to androgen deprivation.
Project description:Posttranslational modifications (PTMs) play critical roles in regulating protein functions and mediating protein-protein interactions. An important PTM is lysine methylation that orchestrates chromatin modifications and regulates functions of non-histone proteins. Methyllysine peptides are bound by modular domains, of which chromodomains are representative. Here, we conducted the first large-scale study of chromodomains in the human proteome interacting with both histone and non-histone methyllysine peptides. We observed significant degenerate binding between chromodomains and histone peptides, i.e., different histone sites can be recognized by the same set of chromodomains, and different chromodomains can share similar binding profiles to individual histone sites. Such degenerate binding is not dictated by amino acid sequence or PTM motif but rather rooted in the physiochemical properties defined by the PTMs on the histone peptides. This molecular mechanism is confirmed by the accurate prediction of the binding specificity using a computational model that captures the structural and energetic patterns of the domain-peptide interaction. To further illustrate the power and accuracy of our model, we used it to effectively engineer an exceptionally strong H3K9me3-binding chromodomain and to label H3K9me3 in live cells. This study presents a systematic approach to deciphering domain-peptide recognition and reveals a general principle by which histone modifications are interpreted by reader proteins, leading to dynamic regulation of gene expression and other biological processes.
Project description:The male-specific lethal (MSL) complex upregulates the single male X chromosome to achieve dosage compensation in Drosophila melanogaster. We have proposed that MSL recognition of specific entry sites on the X is followed by local targeting of active genes marked by histone H3 trimethylation (H3K36me3). Here we analyze the role of the MSL3 chromodomain in the second targeting step. Using ChIP-chip analysis, we find that MSL3 chromodomain mutants retain binding to chromatin entry sites but show a clear disruption in the full pattern of MSL targeting in vivo, consistent with a loss of spreading. Furthermore, when compared to wild type, chromodomain mutants lack preferential affinity for nucleosomes containing H3K36me3 in vitro. Our results support a model in which activating complexes, similarly to their silencing counterparts, use the nucleosomal binding specificity of their respective chromodomains to spread from initiation sites to flanking chromatin.
Project description:The long terminal repeat (LTR) retrotransposon Tf1 of Schizosaccharomyces pombe integrates specifically into the promoters of pol II-transcribed genes. Its integrase (IN) contains a C-terminal chromodomain related to the chromodomains that bind to the N-terminal tail of histone H3. Although we have been unable to detect an interaction between histone tails and the chromodomain of Tf1 IN, it is possible that the chromodomain plays a role in directing IN to its target sites. To test this idea, we generated transposons with single amino acid substitutions in highly conserved residues of the chromodomain and created a chromodomain-deleted mutant. The mutations, V1290A, Y1292A, W1305A, and CHDDelta, substantially reduced transposition activity in vivo. Blotting assays showed that there was little or no reduction in the levels of IN or cDNA. By measuring the homologous recombination between cDNA and the plasmid copy of Tf1, we found that two of the mutations did not reduce the import of cDNA into the nucleus, while another caused a 33% reduction. Chromatin immunoprecipitation assays revealed that CHDDelta caused an approximately threefold reduction in the binding of IN to the downstream LTR of the cDNA. These data indicate that the chromodomain contributed directly to integration. We therefore tested whether the chromodomain contributed to selecting insertion sites. Results of a target plasmid assay showed that the deletion of the chromodomain resulted in a drastic reduction in the preference for pol II promoters. Collectively, these data indicate that the chromodomain promotes binding of cDNA and plays a key role in efficient targeting.