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Copyright © 2008, European Molecular Biology Organization Scientific Report The autoimmune regulator PHD finger binds to non-methylated histone H3K4 to activate gene expression 1Molecular Pathology, University of Tartu, Tartu 50411, Estonia 2Biomolecular NMR Laboratory, Dulbecco Telethon Institute c/o S. Raffaele Scientific Institute, Milan 20132, Italy 3Institute of Chemical Physics, University of Tartu, Tartu 51010, Estonia 4Istituto di Ricerche di Biologia Molecolare, via Pontina km 30.600, Pomezia (Rome), 00040, Italy aTel: +39 0226434824; Fax: +39 0226434153; E-mail: musco.giovanna/at/hsr.it bTel: +372 7374 202; Fax: +372 7374 207; E-mail: part.peterson/at/ut.ee Received August 16, 2007; Revised December 7, 2007; Accepted January 9, 2008. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits distribution, and reproduction in any medium, provided the original author and source are credited. This license does not permit commercial exploitation or the creation of derivative works without specific permission. This article has been cited by other articles in PMC.Abstract Mutations in the gene autoimmune regulator (AIRE) cause autoimmune polyendocrinopathy candidiasis ectodermal dystrophy. AIRE is expressed in thymic medullary epithelial cells, where it promotes the expression of tissue-restricted antigens. By the combined use of biochemical and biophysical methods, we show that AIRE selectively interacts with histone H3 through its first plant homeodomain (PHD) finger (AIRE–PHD1) and preferentially binds to non-methylated H3K4 (H3K4me0). Accordingly, in vivo AIRE binds to and activates promoters containing low levels of H3K4me3 in human embryonic kidney 293 cells. We conclude that AIRE–PHD1 is an important member of a newly identified class of PHD fingers that specifically recognize H3K4me0, thus providing a new link between the status of histone modifications and the regulation of tissue-restricted antigen expression in thymus. Keywords: AIRE, negative selection, NMR, protein structure Introduction Autoimmune polyendocrinopathy candidiasis ectodermal dystrophy (APECED) is a monogenic autosomal recessive syndrome characterized by a breakdown of self-tolerance, leading to autoimmune reactions in several organs and providing a useful model for molecular studies of autoimmunity (Mathis & Benoist, 2007). The disease is caused by mutations in autoimmune regulator (AIRE; Fig 1A
Here, we show that AIRE binds to histone H3 through its first PHD finger (AIRE–PHD1). In contrast with BPTF, ING2 and RAG2, AIRE–PHD1 preferentially binds to histone H3 non-methylated at lysine 4 (H3K4me0). Our results, in agreement with recent studies of the DNMT3L and BHC80 PHD fingers (Lan et al, 2007; Ooi et al, 2007), show a new role for the PHD finger as an H3K4me0 reader. Results And Discussion AIRE–PHD1 binds to histone H3 To investigate the role of AIRE in chromatin-regulating complexes, we examined whether AIRE interacts with histones. Indeed, when incubated with whole histones, glutathione-S-transferase (GST)-AIRE (full-length) interacted with a histone that was identified as H3 by western blotting (Fig 1B AIRE–PHD1 preferentially binds to H3K4me0 Western blot analysis of H3/AIRE–PHD1 complex formation by using antibodies for H3K4me1, H3K4me3 and H3K9me3 indicated that H3K4 trimethylation hinders interaction (Fig 1F To confirm the specificity of AIRE–PHD1 for H3K4me0, we compared the binding of histone H3 N-terminal peptides—H3K4me0, H3K4me1, H3K4me2 and H3K4me3—to AIRE–PHD1 by using two dimensional 1H-15N nuclear magnetic resonance (NMR). A discrete set of chemical shift changes was observed on addition of all four histone H3 peptides to AIRE–PHD1 (supplementary Fig S2A,B online). However, the intensity of the changes was inversely related to the methylation level of the H3 peptide: the H3K4me0 peptide induced the largest changes (maximum average chemical shift change Δδmaxav=0.9 p.p.m.; Fig 2
The greater binding affinity of AIRE–PHD1 for H3K4me0 peptides was confirmed by both tryptophan fluorescence spectroscopy and isothermal titration calorimetry (ITC), yielding dissociation constants of ~4 μM, ~20 μM and >0.5 mM for H3K4me0, H3K4me1 and H3K4me2, respectively (supplementary Fig S2C online; Table 1). Notably, H3K4me3 did not show any significant interaction with AIRE–PHD1 in either binding assay.
In agreement with the GST fusion pull-down experiments, fluorescence spectroscopy showed no binding of H3K4me0 to AIRE–PHD1 containing the APECED-causing C311Y mutation (Bjorses et al, 2000). Nevertheless, a second pathological mutant, V301M (Soderbergh et al, 2000), was still able to bind to H3K4me0, indicating that this mutation is not located in the H3 interaction site (Table 1). The mapping of the H3/AIRE interaction site uniquely to AIRE–PHD1 was further confirmed by NMR titrations of histone H3 peptides into AIRE–PHD2, which bound neither methylated nor H3K4me0 peptides (data not shown). Model of AIRE–PHD1 and histone H3 interactions We generated a model of AIRE–PHD1 complexed with the H3K4me0 peptide on the basis of the crystal structure of the BPTF–PHD finger bound to H3K4me3 and performed molecular dynamics calculations for 10 ns. During the simulations, the peptide interacted stably with the first β-strand of AIRE–PHD1, creating a third antiparallel β-strand (Fig 3
Nature of the binding interface The model of AIRE–PHD1 complexed with H3K4me0 was in perfect agreement with the experimental chemical shift perturbation data, as the peptide-binding region coincided with the binding surface identified by NMR spectroscopy (Fig 3A H3K4me2 and H3K4me3 also induced similar patterns of chemical shift changes, indicating a similar interaction site with AIRE–PHD1. However, the changes were markedly reduced in size, in keeping with a weak interaction (Fig 2C,D Structural comparison with other PHD fingers Our data suggest a regulatory mechanism mediated by AIRE–PHD1 that differs from that of ING2 and BTPF, the PHD fingers of which bind to H3K4me3 and discriminate against H3K4me0. A structural based sequence alignment (supplementary Fig S6 online) suggests that AIRE–PHD1 is representative of a newly identified subclass of PHD fingers (Lan et al, 2007). AIRE–PHD1 differs structurally from the ING2 and BPTF PHD fingers owing to the lack of conserved aromatic residues used to coordinate the trimethyl ammonium ion of H3K4me3 by π-cation interactions. Instead, the crucial elements of the methylated lysine-binding aromatic cage seen in ING2 and BPTF (supplementary Fig S6 online) are substituted by negatively charged (D297) and small hydrophobic (A317) residues in AIRE–PHD1. Our data show that D297 is involved in the interaction of AIRE with H3K4me0, providing an alternative to the recognition of histone H3 by aromatic caging. Notably, D297 is conserved in other PHD finger proteins, for example, Sp110 and Sp140, which might constitute a subset of H3K4me0 readers (supplementary Fig S6 online). Recently, the PHD finger of BHC80 and the cysteine-rich domain of DNMT3L were shown to recognize H3K4me0 by an analogous mechanism, in which the H3 peptide binds to the surface of the domain, forming an additional β-strand that is anchored by the side chain and N-amine group of H3A1. Importantly, these proteins also have an acidic residue comparable to D297, which forms a salt bridge with K4. Although there are many similarities between these two structures and the AIRE–PHD1/H3K4me0 complex presented here, the AIRE–PHD1 finger differs in the additional recognition of the H3R2 side chain, which makes an important contribution to the high affinity of this interaction, as shown by our peptide mutagenesis experiments. AIRE interacts with chromatin We have shown previously that transiently transfected AIRE enhances target gene expression in human embryonic kidney (HEK)293 cells (Pitkanen et al, 2005). So far, no cell line has been described with endogenous AIRE expression; therefore, we transfected HEK293 cells with an AIRE-encoding or control plasmid and generated stable cell lines called HEK-AIRE and HEK-control. We first tested HEK-AIRE compared with HEK-control cell lines for expression levels of tissue-restricted antigens that are downregulated in AIRE-deficient mouse thymic medullary epithelial cells (Derbinski et al, 2005). Indeed, the HEK-AIRE cell line showed enhanced expression of such antigens, including insulin, the principal autoantigen in type I diabetes (Babaya et al, 2005), involucrin and S100A8 (Fig 5A
Our results provide new information on the role of AIRE in sensing epigenetic chromatin modifications through direct binding of AIRE–PHD1 to histone H3 N-terminal residues. Collectively, our data show that AIRE belongs to a new subset of PHD finger-containing proteins that preferentially recognize H3K4me0. Future studies should therefore explore the epigenetic role of AIRE in thymic expression of tissue-restricted antigens to advance further our understanding of this important regulator of autoimmunity. Methods Plasmid construction and in vitro binding assays. The construction of plasmids, information on antibodies and peptides used, as well as protein expression and binding assays are described in the supplementary information online. NMR binding, fluorescence titration assays and isothermal titration calorimetry thermodynamic analysis. Details on NMR titrations, fluorescence spectroscopy and thermodynamic measurements are described in the supplementary information online. Assembly of the complex structures and molecular dynamics calculations. The PHD finger structures from the human NURF BPTF PHD finger-H3K4me3 complex (2fuu) and AIRE1–PHD1 (1xwh) were superimposed by using the Lsqman program (Cα atom RMSD: 2.1 Å). Molecular dynamics simulations and analysis were performed using the GROMACS 3.1.3 package with GROMOS force field. The details of the protocol are available in the supplementary information online. Cell lines, expression analysis and chromatin immunoprecipitation. The establishment of HEK-AIRE and HEK-control cell lines is described in the supplementary information online. DNA ChIP was performed essentially according to Upstate Chromatin Immunoprecipitation Assay protocol. Quantitative PCR analysis and primer sequences are provided in the supplementary information online. Supplementary information is available at EMBO reports online (http://www.emboreports.org). supplementary Figs S1–S8 Click here to view.(488K, pdf) Acknowledgments T.O., A.R., I.L. and P.P. were supported by grants from the Wellcome Trust and European Union Framework programme 6 (Thymaide and Euraps), and from the Estonian Science Foundation (6663 and 6490). We acknowledge A. Häling for technical help. G.M. acknowledges support from Fondazione Telethon, Fondazione Cariplo and Compagnia S. Paolo, and thanks M. Bianchi and D. Gabellini for useful discussions. C.H. and U.M. thank the Foundation Innove (www.innove.ee) project no. 1.0101-0310 for financial support. Footnotes The authors declare that they have no conflict of interest. References
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