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) Delta gK and compared it to the gK-null virus HSV-1 F-gK beta (L. Hutchinson et al., J. Virol. 69:5401-5413, 1995). Delta gK and F-gK beta mutant viruses produced small plaques on Vero cell monolayers at 48 h postinfertion. F-gK beta caused extensive fusion of 143TK cells that was sensitive to melittin, a specific inhibitor of gK-induced cell fusion, while Delta gK virus did not fuse 143TK cells, A recombinant plasmid containing the truncated gK gene specified by F-gK beta failed to rescue the ICP27-null virus KOS (d27-1), while a plasmid with the Delta gK deletion rescued the d27-1 virus efficiently, Delta gK virus yield was approximately 100,000-fold lower in stationary cells than in actively replicating Vero cells, The plaquing efficiencies of Delta gK and F-gK beta virus stocks on VK302 cells were similar, while the plaquing efficiency of F-gK beta virus stocks on Vero cells was reduced nearly 10,000-fold in comparison to that of Delta gK virus. Mutant Delta gK and F-gK beta infectious virions accumulated within Vero and HEp-2 cells but failed to translocate to extracellular spaces, Delta gK( capsids accumulated in the nuclei of Vero but not HEp-2 cells. Enveloped Delta gK 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 Delta gK-infected Vero cells in amounts similar Co 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 Tor the cellular egress deficiency; of the Delta gK virus. Comparison of Delta gK and F-gICP viruses suggests that the inefficient viral replication and plaquing efficiency of F-gK beta virus in Vero cells and its syncytial phenotype in 143TK(-) sells are most likely due to expression of a truncated gK.