Intracellular Localization of the Epstein-Barr Virus BFRF1 Gene Product in Lymphoid Cell Lines and Oral Hairy Leukoplakia Lesions

A novel protein encoded by the BFRF1 gene of the Epstein-Barr virus was identified recently [Farina et al. (2000) J Virol 74:3235-3244], which is antigenic "in vivo" and expressed early in the viral replicative cycle. In the present study, its subcellular localization was examined in greater detail comparing Epstein-Barr virus (EBV) induced producing and nonproducing cell lines by immunofluorescence: in 12-0-tetradecanoyl phorbol-13-acetate (TPA)-induced Raji and B95-8 cells, as well as in anti-IgG-stimulated Akata cells, the protein appeared to be localized over the cell nuclear membrane. A similar nuclear membrane localization was observed in epithelial cells of oral hairy leukoplakia, a pathological manifestation of permissive EBV infection. In contrast, upon transfection of BFRF1 in the EBV-negative Burkitt's lymphoma cell line DG75, the protein was localized predominantly over the plasma membrane. The membrane localization was abolished when DG75 cells were transfected with a C-terminal deletion mutant of BFRF1 lacking the transmembrane domain. Because induced Raji cells do not produce virus, the above observations indicate that the nuclear membrane localization is not associated with viral production, but requires the expression of EBV genes, and suggest that additional proteins, expressed early during viral lytic infection, might be necessary to target the protein to the nuclear membrane. Immunogold electron microscopy on ultrathin cryosections of induced B95-8 cells showed that BFRF1 on the nuclear membranes was concentrated over multilayered domains representing areas of active viral replication or at the sites of viral budding, suggesting that BFRF1 is involved in the process of viral assembly.