Functional characterization of glycoprotein H chimeras composed of conserved domains of the pseudorabies virus and herpes simplex virus type 1 homologs

Membrane fusion is indispensable for entry of enveloped viruses into host cells. The conserved core fusion machinery of the Herpesviridae consists of glycoprotein gB and the gH/gL complex. Recently, crystal structures of gH/gL of herpes simplex virus type 2, Epstein-Barr-Virus, and of a core fragment of pseudorabies virus (PrV) gH identified four structurally conserved gH domains. To investigate functional conservation, chimeric genes encoding combinations of individual domains of PrV and herpes simplex virus type 1 (HSV-1) gH were expressed in rabbit kidney cells, and their processing and transport to the cell surface, as well as activity in fusion assays including gB, gD and gL of PrV or HSV-1 were analysed. Chimeric gH containing domain I of HSV-1 and domains II-IV of PrV exhibited limited fusion activity in the presence of PrV gB and gD and HSV-1 gL, but not of PrV gL. More strikingly, chimeric gH consisting of PrV domains I-III and HSV-1 domain IV exhibited considerable fusion activity together with PrV gB, gD and gL. Substitution of PrV gB by the HSV-1 protein significantly enhanced this activity. A cell line stably expressing this chimeric gH supported replication of gH-deleted PrV. Our results confirm the specificity of domain I for gL binding, demonstrate functional conservation of domain IV in two alphaherpesviruses from different genera, and indicate species-specific interactions of this domain with gB. They also suggest that gH domains II and III might form a structural and functional unit which does not tolerate major substitutions.