Glycoprotein C Mutations Regulate the Plaque Size of Fusion-Defective Herpes Simplex Virus

Herpes simplex virus 1 (HSV-1) entry and virus-induced cell fusion require the coordinated activity of the essential glycoproteins gD, gH/gL, and gB. Mutations in this core fusion machinery reduce viral entry, fusion, and spread. Previously, we showed that introducing three mutations in gB (gB3A) or replacing HSV-1 gB with saimiriine herpesvirus 1 (SaHV-1) gB resulted in fusion-defective, small-plaque viruses. Serial passage of these viruses selected mutations in gH and gC, and we demonstrated that the gH mutations partially restored fusion function. In this study, we examined whether mutations in gC contribute to plaque formation, fusion, and entry. We generated an additional fusion-defective, small-plaque gD chimeric virus by replacing the profusion domain of HSV-1 gD with the corresponding region from SaHV-1 gD (gDchi). Serial passage of this virus partially restored plaque size and also selected for a mutation in gC. This study demonstrates that the selected gC mutants contributed to the partial restoration of plaque size. Plaque size of all three fusion-defective viruses was enhanced by exogenous expression of the corresponding selected gC mutants, whereas exogenous expression of wild-type gC reduced plaque size of the passaged isolates carrying gC mutations. Although gC mutants did not enhance fusion or entry mediated by the wild-type core fusion machinery, both wild-type and mutant gC enhanced cell-cell fusion when coexpressed with fusion-defective gH mutants. These results indicate that gC functions as a conditional accessory regulator that can partially compensate for defects in the HSV-1 core fusion machinery. IMPORTANCEHerpes simplex virus 1 (HSV-1) entry is mediated by a core fusion machinery comprised of glycoproteins gD, gH/gL, and gB. The contributions of additional viral glycoproteins to this process are not well defined. Glycoprotein C (gC) is known to function in cell attachment to heparan sulfate and immune evasion but is dispensable for viral entry under standard conditions. Here, we show that gC can act as a conditional accessory regulator of HSV-1 entry and fusion. Using serial passage of fusion-defective viruses, we demonstrate that mutations in gC are repeatedly selected and can partially compensate for defects in entry and fusion. These findings indicate that gC contributes to viral fusion under suboptimal conditions and highlight a role for this accessory glycoprotein in the adaptability of herpesvirus entry.