Timing of Some of the Molecular Events Required for Cell Fusion Induced by Herpes Simplex Virus Type 1

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Abstract
The timing of some of the molecular events that are required for [human embryo lung HEL] cell fusion was investigated. Cell fusion was produced by a mutant of herpes simplex virus type 1 that causes extensive cell fusion during infection. The timing of molecular events required for fusion was established by the use of blocking agents. Phosphonoacetic acid blocks viral DNA synthesis; actinomycin D blocks RNA synthesis; cycloheximide blocks protein synthesis; 2-deoxyglucose blocks glycosylation of glycoproteins; high temperature, NH4Cl and adamantanone block unknown steps required for cell fusion. For cells infected at a low multiplicity of infection, phosphonoacetic acid decreased the rate but not the final amount of fusion, but at a multiplicity of infection of 10 it had no effect on the rate of cell fusion. RNA synthesis was required for fusion until 4 h after infection, protein synthesis until 5.5 h after infection, and glycosylation until 7 h after infection. The temperature-dependent step occurred before 6 h after infection, but NH4Cl and adamantanone acted at steps that occurred until 8 h after infection. Cycloheximide, temperature, NH4Cl and adamantanone acted reversibly; actinomycin D and 2-deoxyglucose acted irreversibly. The same order of action of the inhibitors was also determined by using pairs of inhibitors sequentially. These experiments also indicated that the fusion factor was not an .alpha.-polypeptide. Virus growth and cell fusion were both highly dependent on temperature in the range of 30-40.degree. C. Wild-type infections are apparently characterized by the presence of a fusion factor and a fusion inhibitor. The fusion-blocking agents were added to wild-type-infected cells under a variety of conditions in an attempt to selectively block the production of the fusion inhibitor molecule and thereby cause extensive cell fusion. Fusion was not observed in any of these experiments.