Herpes simplex virus type 1 glycoprotein K is not essential for infectious virus production in actively replicating cells but is required for efficient envelopment and translocation of infectious virions from the cytoplasm to the extracellular space

We characterized the glycoprotein K (gK)-null herpes simplex virus type 1 [HSV-1] (KOS) ΔgK and compared it to the gK-null virus HSV-1 F-gKβ (L. Hutchinson et al., J. Virol. 69:5401-5413, 1995). Δgk and F-gKβ mutant viruses produced small plaques on Vero cel monolayers at 48 h postinfection. F-gKβ caused extensive fusion of 143TK cells that was sensitive to melittin, a specific inhibitor of gK-induced cell fusion, while ΔgK virus did not fuse 143TK cells. A recombinant plasmid containing the truncated gK gene specified by F-gKβ failed to rescue the ICP27-null virus KOS (d27-1), while a plasmid with the ΔgK deletion rescued the d27-1 virus efficiently. ΔgK virus yield was approximately 100,00-fold lower in stationary cells than in actively replicating Vero cells. The plaquing efficiencies of ΔgK and F-gKβ virus stocks on VK302 cells were similar, while the plaquing efficiency of F-gKβ virus stocks on Vero cells was reduced nearly 10,000-fold in comparison to that of ΔgK virus. Mutant ΔgK and F-gKβ infectious virions accumulated within Vero and HEp-2 cells but failed to translocate to extracellular spaces. ΔgK capsids accumulated in the nuclei of Vero but not HEp-2 cells. Enveloped ΔgK virions were visualized in the cytoplasms of both Vero and HEp-2 cells, and viral capsids were found in the cytoplasm of HEP-2 cells within vesicles. Glycoproteins B, C, D, and H were expressed on the surface of ΔgK infected Vero cells in amounts similar to those for KOS-infected Vero cells. These results indicate that gK is involved in nucleocapsid envelopment, and more importantly in the translocation of infectious virions from the cytoplasm to the extracellular spaces, and that actively replicating cells can partially compensate for the envelopment but not for the cellular egress deficiency of the ΔgK virus. Comparison of ΔgK and F-gKβ viruses suggests that the inefficient viral replication and plaquing efficiency of F- gkβ virus in Vero cells and its syncytial phenotype in 143TK- cells are most likely due to expression of a truncated gK