Human cytomegalovirus mediates APOBEC3B relocalization early during infection through a ribonucleotide reductase-independent mechanism

The APOBEC3 family of DNA cytosine deaminases comprises an important arm of the innate antiviral defense system. The gamma-herpesviruses EBV and KSHV and the alpha-herpesviruses HSV-1 and HSV-2 have evolved an efficient mechanism to avoid APOBEC3 restriction by directly binding to APOBEC3B and facilitating its exclusion from the nuclear compartment. The only viral protein required for APOBEC3B relocalization is the large subunit of the ribonucleotide reductase (RNR). Here, we ask whether this APOBEC3B relocalization mechanism is conserved with the beta-herpesvirus human cytomegalovirus (HCMV). Although HCMV infection causes APOBEC3B relocalization from the nucleus to the cytoplasm in multiple cell types, the viral RNR (UL45) is not required. APOBEC3B relocalization occurs rapidly following infection suggesting involvement of an immediate early or early (IE-E) viral protein. In support of this mechanism, cycloheximide treatment of HCMV-infected cells prevents the expression of viral proteins and simultaneously blocks APOBEC3B relocalization. In comparison, the treatment of infected cells with phosphonoacetic acid, which is a viral DNA synthesis inhibitor affecting late protein expression, still permits A3B relocalization. These results combine to show that the beta-herpesvirus HCMV uses a fundamentally different, RNR-independent molecular mechanism to antagonize APOBEC3B.