Varicella-zoster virus gB and gE coexpression, but not gB or gE alone, leads to abundant fusion and syncytium formation equivalent to those from gH and gL coexpression

Varicella-zoster virus (VZV) is distinguished from herpes simplex virus typ e 1 (HSV-1) by the fact that cell-to-cell fusion and syncytium formation re quire only gH and gL within a transient-expression system. In the HSV syste m, four glycoproteins, namely, gH, gL, gB, and gD, are required to induce a similar fusogenic event. VZV lacks a gD homologous protein. In this report , the role of VZV gB as a fusogen was investigated arid compared to the gH- gL complex. First of ail, the VZV gH-gL experiment was repeated under a dif ferent set of conditions; namely, gH and gL were cloned into the same vacci nia virus (VV) genome. Surprisingly, the new expression system demonstrated that a recombinant VV-gH+gL construct was even more fusogenic than seen in the prior experiment with two individual expression plasmids containing gH and gL (K. M. Duns and C. Grose, J. Virol. 70:8961-8971, 1996). Recombinan t VV expressing VZV gB by itself, however, effected the formation of only s mall syncytia. When VZV gE and gB genes were cloned into one recombinant VV genome and another fusion assay was performed, extensive syncytium formati on was observed. The degree of fusion with VZV gE-gB coexpression was compa rable to that observed with VZV gH-gL: in both cases, >80% of the cells in a monolayer were fused. Thus, these studies established that VZV gE-gB coex pression greatly enhanced the fusogenic properties of gB. Control experimen ts documented that the fusion assay required a balance between the fusogeni c potential of the VZV glycoproteins and the fusion-inhibitory effect of th e VV infection itself.