Matthieu Schapira

Somatic mutations at EZH2 Y641 act dominantly through a mechanism of selectively altered PRC2 catalytic activity, to increase H3K27 trimethylation

Next-generation sequencing of follicular lymphoma and diffuse-large B-cell lymphoma has revealed frequent somatic, heterozygous Y641 mutations in the histone methyltransferase EZH2. Heterozygosity and the presence of equal quantities of both mutant and wild-type mRNA and expressed protein suggest a dominant mode of action. Surprisingly, B-cell lymphoma cell lines and lymphoma samples harboring heterozygous EZH2(Y641) mutations have increased levels of histone H3 Lys-27-specific trimethylation (H3K27me3). Expression of EZH2(Y641F/N) mutants in cells with EZH2(WT) resulted in an increase of H3K27me3 levels in vivo. Structural modeling of EZH2(Y641) mutants suggests a Tyr/Phe switch model whereby structurally neutral, nontyrosine residues at position 641 would decrease affinity for unmethylated and monomethylated H3K27 substrates and potentially favor trimethylation. We demonstrate, using in vitro enzyme assays of reconstituted PRC2 complexes, that Y641 mutations result in a decrease in monomethylation and an increase in trimethylation activity of the enzyme relative to the wild-type enzyme. This represents the first example of a disease-associated gain-of-function mutation in a histone methyltransferase, whereby somatic EZH2 Y641 mutations in lymphoma act dominantly to increase, rather than decrease, histone methylation. The dominant mode of action suggests that allele-specific EZH2 inhibitors should be a future therapeutic strategy for this disease.

Methylation-state-specific recognition of histones by the MBT repeat protein L3MBTL2

The MBT repeat has been recently identified as a key domain capable of methyl–lysine histone recognition. Functional work has pointed to a role for MBT domain-containing proteins in transcriptional repression of developmental control genes such as Hox genes. In this study, L3MBTL2, a human homolog of Drosophila Sfmbt critical for Hox gene silencing, is demonstrated to preferentially recognize lower methylation states of several histone-derived peptides through its fourth MBT repeat. High-resolution crystallographic analysis of the four MBT repeats of this protein reveals its unique asymmetric rhomboid architecture, as well as binding mechanism, which preclude the interaction of the first three MBT repeats with methylated peptides. Structural elucidation of an L3MBTL2–H4K20me1 complex and comparison with other MBT-histone peptide complexes also suggests that an absence of distinct surface contours surrounding the methyl–lysine-binding pocket may underlie the lack of sequence specificity observed for members of this protein family.

ChromoHub: a data hub for navigators of chromatin-mediated signalling

Summary: The rapidly increasing research activity focused on chromatin-mediated regulation of epigenetic mechanisms is generating waves of data on writers, readers and erasers of the histone code, such as protein methyltransferases, bromodomains or histone deacetylases. To make these data easily accessible to communities of research scientists coming from diverse horizons, we have created ChromoHub, an online resource where users can map on phylogenetic trees disease associations, protein structures, chemical inhibitors, histone substrates, chromosomal aberrations and other types of data extracted from public repositories and the published literature. The interface can be used to define the structural or chemical coverage of a protein family, highlight domain architectures, interrogate disease relevance or zoom in on specific genes for more detailed information. This open-access resource should serve as a hub for cell biologists, medicinal chemists, structural biologists and other navigators that explore the biology of chromatin signalling. Availability: http://www.thesgc.org/chromohub/. Contact: matthieu.schapira@utoronto.ca Supplementary Information: Supplementary data are available at Bioinformatics online.

Novel structural and regulatory features of rhoptry secretory kinases in Toxoplasma gondii.

Serine/threonine kinases secreted from rhoptry organelles constitute important virulence factors of Toxoplasma gondii. Rhoptry kinases are highly divergent and their structures and regulatory mechanism are hitherto unknown. Here, we report the X‐ray crystal structures of two related pseudokinases named ROP2 and ROP8, which differ primarily in their substrate‐binding site. ROP kinases contain a typical bilobate kinase fold and a novel N‐terminal extension that both stabilizes the N‐lobe and provides a unique means of regulation. Although ROP2 and ROP8 were catalytically inactive, they provided a template for homology modelling of the active kinase ROP18, a major virulence determinant of T. gondii. Autophosphorylation of key residues in the N‐terminal extension resulted in ROP18 activation, which in turn phosphorylated ROP2 and ROP8. Mutagenesis and mass spectrometry experiments revealed that ROP18 was maximally activated when this phosphorylated N‐terminus relieved autoinhibition resulting from extension of aliphatic side chains into the ATP‐binding pocket. This novel means of regulation governs ROP kinases implicated in parasite virulence.

Structural insights into the inhibited states of the Mer receptor tyrosine kinase.

The mammalian ortholog of the retroviral oncogene v-Eyk, and a receptor tyrosine kinase upstream of antiapoptotic and transforming signals, Mer (MerTK) is a mediator of the phagocytic process, being involved in retinal and immune cell clearance and platelet aggregation. Mer knockout mice are viable and are protected from epinephrine-induced pulmonary thromboembolism and ferric chloride-induced thrombosis. Mer overexpression, on the other hand, is associated with numerous carcinomas. Although Mer adaptor proteins and signaling pathways have been identified, it remains unclear how Mer initiates phagocytosis. When bound to its nucleotide cofactor, the high-resolution structure of Mer shows an autoinhibited αC-Glu-out conformation with insertion of an activation loop residue into the active site. Mer complexed with compound-52 (C52: 2-(2-hydroxyethylamino)-6-(3-chloroanilino)-9-isopropylpurine), a ligand identified from a focused library, retains its DFG-Asp-in and αC-Glu-out conformation, but acquires other conformational changes. The αC helix and DFGL region is closer to the hinge region and the ethanolamine moiety of C52 binds in the groove formed between Leu593 and Val601 of the P-loop, causing a compression of the active site pocket. These conformational states reveal the mechanisms of autoinhibition, the pathophysiological basis of disease-causing mutations, and a platform for the development of chemical probes.

Molecular Characterization of a Novel Geranylgeranyl Pyrophosphate Synthase from Plasmodium Parasites

We present here a study of a eukaryotic trans-prenylsynthase from the malaria pathogen Plasmodium vivax. Based on the results of biochemical assays and contrary to previous indications, this enzyme catalyzes the production of geranylgeranyl pyrophosphate (GGPP) rather than farnesyl pyrophosphate (FPP). Structural analysis shows that the product length is constrained by a hydrophobic cavity formed primarily by a set of residues from the same subunit as the product as well as at least one other from the dimeric partner. Furthermore, Plasmodium GGPP synthase (GGPPS) can bind nitrogen-containing bisphosphonates (N-BPs) strongly with the energetically favorable cooperation of three Mg2+, resulting in inhibition by this class of compounds at IC50 concentrations below 100 nm. In contrast, human and yeast GGPPSs do not accommodate a third magnesium atom in the same manner, resulting in their insusceptibility to N-BPs. This differentiation is in part attributable to a deviation in a conserved motif known as the second aspartate-rich motif: whereas the aspartates at the start and end of the five-residue motif in FFPP synthases and P. vivax GGPPSs both participate in the coordination of the third Mg2+, an asparagine is featured as the last residue in human and yeast GGPPSs, resulting in a different manner of interaction with nitrogen-containing ligands.

Structural Basis for the Discriminative Recognition of N6-Methyladenosine RNA by the Human YT521-B Homology Domain Family of Proteins.

Background: Human YT521-B homology (YTH) domain selectively recognizes N6-methyladenosine (m6A) RNA. Results: YTHDF1 and Pho92 recognize m6A without sequence preference. Conclusion: The structure of YTHDF1 explained the key residue difference that results in the weaker binding of YTHDF1-3 compared with YTHDC1. Significance: This study systematically investigated the binding characteristics of the human YTH domain proteins, as well as yeast Pho92, and indicated the discriminative recognition of m6A among different YTH domains. N6-Methyladenosine (m6A) is the most abundant internal modification in RNA and is specifically recognized by YT521-B homology (YTH) domain-containing proteins. Recently we reported that YTHDC1 prefers guanosine and disfavors adenosine at the position preceding the m6A nucleotide in RNA and preferentially binds to the GG(m6A)C sequence. Now we systematically characterized the binding affinities of the YTH domains of three other human proteins and yeast YTH domain protein Pho92 and determined the crystal structures of the YTH domains of human YTHDF1 and yeast Pho92 in complex with a 5-mer m6A RNA, respectively. Our binding and structural data revealed that the YTH domain used a conserved aromatic cage to recognize m6A. Nevertheless, none of these YTH domains, except YTHDC1, display sequence selectivity at the position preceding the m6A modification. Structural comparison of these different YTH domains revealed that among those, only YTHDC1 harbors a distinctly selective binding pocket for the nucleotide preceding the m6A nucleotide.

Newly discovered and characterized antivirulence compounds inhibit bacterial mono-ADP-ribosyltransferase toxins.

ABSTRACT The mono-ADP-ribosyltransferase toxins are bacterial virulence factors that contribute to many disease states in plants, animals, and humans. These toxins function as enzymes that target various host proteins and covalently attach an ADP-ribose moiety that alters target protein function. We tested compounds from a virtual screen of commercially available compounds combined with a directed poly(ADP-ribose) polymerase (PARP) inhibitor library and found several compounds that bind tightly and inhibit toxins from Pseudomonas aeruginosa and Vibrio cholerae. The most efficacious compounds completely protected human lung epithelial cells against the cytotoxicity of these bacterial virulence factors. Moreover, we determined high-resolution crystal structures of the best inhibitors in complex with cholix toxin to reveal important criteria for inhibitor binding and mechanism of action. These results provide new insight into development of antivirulence compounds for treating many bacterial diseases.

Tamoxifen Enhances the Hsp90 Molecular Chaperone ATPase Activity

Background Hsp90 is an essential molecular chaperone that is also a novel anti-cancer drug target. There is growing interest in developing new drugs that modulate Hsp90 activity. Methodology/Principal Findings Using a virtual screening approach, 4-hydroxytamoxifen, the active metabolite of the anti-estrogen drug tamoxifen, was identified as a putative Hsp90 ligand. Surprisingly, while all drugs targeting Hsp90 inhibit the chaperone ATPase activity, it was found experimentally that 4-hydroxytamoxifen and tamoxifen enhance rather than inhibit Hsp90 ATPase. Conclusions/Significance Hence, tamoxifen and its metabolite are the first members of a new pharmacological class of Hsp90 activators.

Structural genomics of histone tail recognition

Summary: The structural genomics of histone tail recognition web server is an open access resource that presents within mini articles all publicly available experimental structures of histone tails in complex with human proteins. Each article is composed of interactive 3D slides that dissect the structural mechanism underlying the recognition of specific sequences and histone marks. A concise text html-linked to interactive graphics guides the reader through the main features of the interaction. This resource can be used to analyze and compare binding modes across multiple histone recognition modules, to evaluate the chemical tractability of binding sites involved in epigenetic signaling and design small molecule inhibitors. Availability: http://www.thesgc.org/resources/histone_tails/ Contact: matthieu.schapira@utoronto.ca Supplementary information: Supplementary data are available at Bioinformatics online.