The timing of some of the molecular events that are required for 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, NH(4)Cl, 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, whereas NH(4)Cl and
adamantanone acted at steps that occurred until 8 h after
infection.
Cycloheximide, temperature, NH(4)Cl, 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 found to be highly dependent on temperature in the range of 30 to 40 degrees 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. However, fusion was not observed in any of these experiments.