Fusion from without induced by herpes simplex virus type 1

Strains ANG and ANG path of herpes simplex virus type 1 (HSV1) produced fusion from without (FFWO) of cells in culture. FFWO required 45 min to become complete. In contrast, fusion from within (FFWI) was not detected until 3-4 h after infection, depending on the cell type. FFWO was temperature dependent: at 0 degrees no fusion could be observed, but increase of temperature increased the degree of fusion. The pH optimum for FFWO was 7.8-8.5. The FFWO activity of the virus was found to be slightly more heat stable at 46 degrees than was infectivity. FFWO was produced in Vero, CV-1 and BSC1 cells, but not in BHK clone 13 or in primary or secondary rabbit kidney cells. FFWO was linked to the presence of virus particles and perhaps to other sedimentable, infected-cell material but not to soluble factors. Actinomycin D, cycloheximide, and UV irradiation did not block this activity, indicating no direct activity of the HSV1 genome for FFWO.