Abstract
Minutes after DNA damage, the variant histone H2AX is phosphorylated by protein kinases of the phosphoinositide kinase family, including ATM, ATR or DNA-PK1. Phosphorylated (γ)-H2AX—which recruits molecules that sense or signal the presence of DNA breaks, activating the response that leads to repair2,3—is the earliest known marker of chromosomal DNA breakage. Here we identify a dynamic change in chromatin that promotes H2AX phosphorylation in mammalian cells. DNA breaks swiftly mobilize heterochromatin protein 1 (HP1)-β (also called CBX1), a chromatin factor bound to histone H3 methylated on lysine 9 (H3K9me). Local changes in histone-tail modifications are not apparent. Instead, phosphorylation of HP1-β on amino acid Thr 51 accompanies mobilization, releasing HP1-β from chromatin by disrupting hydrogen bonds that fold its chromodomain around H3K9me. Inhibition of casein kinase 2 (CK2), an enzyme implicated in DNA damage sensing and repair4,5,6, suppresses Thr 51 phosphorylation and HP1-β mobilization in living cells. CK2 inhibition, or a constitutively chromatin-bound HP1-β mutant, diminishes H2AX phosphorylation. Our findings reveal an unrecognized signalling cascade that helps to initiate the DNA damage response, altering chromatin by modifying a histone-code mediator protein, HP1, but not the code itself.
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Acknowledgements
We thank L. Pellegrini for modelling the HP1–H3K9me2 structure; M. Daniels, S. Y. Peak-Chew, M. Lee, R. Kulkarni and P. Rowling for technical assistance; and T. Misteli, T. Jenuwein and D. Litchfield for gifts of material. N.A. acknowledges a fellowship from the International Agency for Research on Cancer, Lyon, France, J.A.B. a long-term fellowship from EMBO, Heidelberg, Germany, and A.D.J. a scholarship from the Gates Cambridge Trust. The UK Medical Research Council supports work in A.R.V.’s laboratory.
Author Contributions N.A. performed the experiments reported here, except that A.D.J. determined EGFP–HP1-β mobilization by FRAP and FLIP, demonstrated the release of Thr 51-phosphorylated HP1 from H3K9me, and performed the quantification of immunofluorescence, whereas J.A.B. helped to identify HP1-β phosphorylation sites, and measured the rCK2 kinetics. A.R.V. planned the project, helped to interpret the data and wrote the paper.
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The file contains Supplementary Figures S1-S10 with Legends, the legends for the Supplementary Movies 1 and 2 and detailed description of methods and reagents used in this work. (PDF 5980 kb)
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The file contains Supplementary Movie 1 showing HP1 dispersal in real-time after laser induced DNA damage. (MOV 2267 kb)
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The file contains Supplementary Movie 2 showing HP1 dispersal in real-time after laser induced DNA damage. (MOV 2331 kb)
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Ayoub, N., Jeyasekharan, A., Bernal, J. et al. HP1-β mobilization promotes chromatin changes that initiate the DNA damage response. Nature 453, 682–686 (2008). https://doi.org/10.1038/nature06875
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DOI: https://doi.org/10.1038/nature06875
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