Epstein-Barr Virus Tegument Protein BKRF4 is a Histone Chaperone

Histone chaperones, which constitute an interaction and functional network involved in all aspects of histone metabolism, have to date been identified only in eukaryotes. The Epstein-Barr virus tegument protein BKRF4 is a histone-binding protein that engages histones H2A-H2B and H3-H4, and cellular chromatin, inhibiting the host DNA damage response. Here, we identified BKRF4 as a bona fide viral histone chaperone whose histone-binding domain (HBD) forms a co-chaperone complex with the human histone chaperone ASF1 in vitro. We determined the crystal structures of the quaternary complex of the BKRF4 HBD with human H3-H4 dimer and the histone chaperone ASF1b and the ternary complex of the BKRF4 HBD with human H2A-H2B dimer. Through structural and biochemical studies, we elucidated the molecular basis for H3-H4 and H2A-H2B recognition by BKRF4. We also revealed two conserved motifs, D/EL and DEF/Y/W, within the BKRF4 HBD, which may represent common motifs through which histone chaperones target H3-H4 and H2A-H2B, respectively. In conclusion, our results identify BKRF4 as a histone chaperone encoded by the Epstein-Barr virus, representing a typical histone chaperone found in a non-eukaryote. We envision that more histone chaperones await identification and characterization in DNA viruses and even archaea.