G/S Cell Cycle Induction by Epstein-Barr Virus BORF2 Is Mediated by P53 and APOBEC3B.

Herpesvirus lytic infection causes cells to arrest at the G/S phase of the cell cycle by poorly defined mechanisms. In a prior study using fluorescent ubiquitination-based cell cycle indicator (FUCCI) cells that express fluorescently tagged proteins marking different stages of the cell cycle, we showed that the Epstein-Barr virus (EBV) protein BORF2 induces the accumulation of G/S cells, and that BORF2 affects p53 levels without affecting the p53 target protein p21. We also found that BORF2 specifically interacted with APOBEC3B (A3B) and forms perinuclear bodies with A3B that prevent A3B from mutating replicating EBV genomes. We now show that BORF2 also interacts with p53 and that A3B interferes with the BORF2-p53 interaction, although A3B and p53 engage distinct surfaces on BORF2. Cell cycle analysis showed that G/S induction by BORF2 is abrogated when either p53 or A3B is silenced or when an A3B-binding mutant of BORF2 is used. Furthermore, silencing A3B in EBV lytic infection increased cell proliferation, supporting a role for A3B in G/S arrest. These data suggest that the p53 induced by BORF2 is inactive when it binds BORF2, but is released and induces G/S arrest when A3B is present and sequesters BORF2 in perinuclear bodies. Interestingly, this mechanism is conserved in the BORF2 homologue in HSV-1, which also re-localizes A3B, induces and binds p53, and induces G/S dependent on A3B and p53. In summary, we have identified a new mechanism by which G/S arrest can be induced in herpesvirus lytic infection. In lytic infection, herpesviruses cause cells to arrest at the G/S phase of the cell cycle in order to provide an optimal environment for viral replication; however, the mechanisms involved are not well understood. We have shown that the Epstein-Barr virus BORF2 protein and its homologue in herpes simplex virus 1 both induce G/S, and do this by similar mechanisms which involve binding p53 and APOBEC3B and induction of p53. Our study identifies a new mechanism by which G/S arrest can be induced in herpesvirus lytic infection and a new role of APOBEC3B in herpesvirus lytic infection.