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HP1-β mobilization promotes chromatin changes that initiate the DNA damage response

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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Figure 1: Alterations in HP1-β dynamics and localization accompany its mobilization after DNA damage.
Figure 2: Thr 51 substitution or phosphorylation alter the localization and dynamics of HP1-β by disrupting a hydrogen-bond network essential for the HP1–H3K9me interaction.
Figure 3: Thr 51 phosphorylation is induced after DNA damage and accompanies HP1-β dispersal from damage sites.
Figure 4: Inhibition of CK2 suppresses the phosphorylation of HP1-β on Thr 51 and mobilization after DNA damage.
Figure 5: HP1-β mobilization by CK2 promotes H2AX phosphorylation.

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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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Correspondence to Ashok R. Venkitaraman.

Supplementary information

Supplementary information

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)

Supplementary information

The file contains Supplementary Movie 1 showing HP1 dispersal in real-time after laser induced DNA damage. (MOV 2267 kb)

Supplementary information

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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