Identification of the fusion-from-without determinants of herpes simplex virus type 1 glycoprotein B

Fusion-from-without (FFWO) is the rapid induction of cell fusion at high multiplicities of infection and in the absence of viral protein synthesis. The ANG path strain and several other strains of herpes simplex virus type 1 (HSV-1) effectively cause FFWO of Vero cells. FFWO-inducing strains of HSV-1 contain syncytial mutations in the gB cytoplasmic domain; however, not all strains with such syncytial mutations cause FFWO. By characterization of recombinant viruses containing chimeric gB genes, it was shown that determinants in both the gB ectodomain and gB cytoplasmic domain control the FFWO phenotype of HSV-1. The complete nucleotide sequence of the ANG path gB gene was determined. Comparison of the predicted amino acid sequence of ANG path gB with other HSV-1 gB sequences showed that the gB genes of FFWO-inducing viruses must contain both syncytial mutations in the gB cytoplasmic domain and the fast rate-of-entry determinant at residue 553 in the gB ectodomain.