Analysis of contributions of herpes simplex virus type 1 UL43 protein to induction of cell-cell fusion

Purpose: To investigate whether UL43 protein, which is highly conserved in alpha- and gamma herpes viruses, and a non-glycosylated transmembrane protein, is involved in virus entry and virus-induced cell fusion. Methods: Mutagenesis was accomplished by a markerless two-step Red recombination mutagenesis system implemented on the Herpes simplex virus 1 (HSV-1) bacterial artificial chromosome (BAC). Growth properties of HSV-1 UL43 mutants were analyzed using plaque morphology and one-step growth kinetics. SDS-PAGE and Western blot was employed to assay the synthesis of the viral glycoproteins. Virus-penetration was assayed to determine if UL43 protein is required for efficient virus entry. Results: Lack of UL43 expression resulted in significantly reduced plaque sizes of syncytial mutant viruses and inhibited cell fusion induced by gBΔ28 or gKsyn20 (p < 0.05). Deletion of UL43 did not affect overall expression levels of viral glycoproteins gB, gC, gD, and gH on HSV-1(F) BAC infected cell surfaces. Moreover, mutant viruses lacking UL43 gene exhibited slower kinetics of entry into Vero cells than the parental HSV-1(F) BAC. Conclusion: Thus, these results suggest an important role for UL43 protein in mediating virus-induced membrane fusion and efficient entry of virion into target cells.