The processing, transport and heterologous expression of Epstein-Barr virus gp110

Epstein-Barr virus (EBV) glycoprotein gp110 has substantial structural and sequence homology with herpes simplex virus (HSV) gB and gBs of other alpha- and betaherpesviruses but unlike HSV gB localizes differently in infected cells and is absent from virions. To facilitate the analysis of EBV gp110, antisera were raised to fragments of gp110 expressed in a bacterial system. They recognized a protein of the predicted size in recombinant bacterial lysates, in lymphoblastoid cells and in recombinant vaccinia virus-gp110 infected cells. gp110 from all sources possessed a high-mannose type of N-glycosylation implying that gp110 has not passed through the Golgi. Immunofluorescence and immuno-electron microscopy confirmed this conclusion and demonstrated that, in contrast to HSV gB, the majority of immunoreactive gp110 was present at the nuclear membrane or endoplasmic reticulum (ER) but not at the cell membrane. Unexpectedly, a truncated version of gp110 lacking the hydrophobic C-terminal region, despite forming dimers analogous to HSV dimers, was transported in a similar manner to full-length gp110. Two chimeric proteins constructed by replacing the N- and C-terminal domains of gp110 with corresponding regions of gp340/220 were also transported to the nuclear membrane/ER. These data suggest that unlike HSV gB both the N- and C-terminal portions of EBV gp110 contain independent signals sufficient to direct the molecule to the ER/nuclear membrane. Specific transport of gammaherpesvirus gB homologues to the nuclear membrane, from where herpesviruses bud, suggests that they may be involved in the egress of virus from the nucleus.