Live-Cell Imaging Reveals Multiple Interactions between Epstein-Barr Virus Nuclear Antigen 1 and Cellular Chromatin during Interphase and Mitosis

Epstein-Barr virus (EBV) establishes a life-long latent infection in humans. In proliferating latently infected cells, EBV genomes persist as multiple episomes that undergo one DNA replication event per cell cycle and remain attached to the mitotic chromosomes. EBV nuclear antigen 1 (EBNA-1) binding to the episome and cellular genome is essential to ensure proper episome replication and segregation. However, the nature and regulation of EBNA-1 interaction with chromatin has not been clearly elucidated. This activity has been suggested to involve EBNA-1 binding to DNA, duplex RNA, and/or proteins. EBNA-1 binding protein 2 (EBP2), a nucleolar protein, has been proposed to act as a docking protein for EBNA-1 on mitotic chromosomes. However, there is no direct evidence thus far for EBP2 being associated with EBNA-1 during mitosis. By combining video microscopy and Förster resonance energy transfer (FRET) microscopy, we demonstrate here for the first time that EBNA-1 and EBP2 interact in the nucleoplasm, as well as in the nucleoli during interphase. However, in strong contrast to the current proposed model, we were unable to observe any interaction between EBNA-1 and EBP2 on mitotic chromosomes. We also performed a yeast double-hybrid screening, followed by a FRET analysis, that led us to identify HMGB2 (high-mobility group box 2), a well-known chromatin component, as a new partner for EBNA-1 on chromatin during interphase and mitosis. Although the depletion of HMGB2 partly altered EBNA-1 association with chromatin in HeLa cells during interphase and mitosis, it did not significantly impact the maintenance of EBV episomes in Raji cells.