The initial step in herpes simplex virus (HSV) entry is binding of virion glycoprotein (g)C and/or gB to cell surface heparan sulfate. After this initial attachment, gD interacts with cell surface receptor or receptors, and the virion envelope fuses with the cell membrane. Fusion requires viral glycoproteins gB, gD, gL, and gH, but the cellular factors that participate in or the pathways activated by viral entry have not been defined. To determine whether signal transduction pathways are triggered by viral-cell fusion, we examined the association of viral entry with tyrosine phosphorylation of cellular proteins. Using immunoprecipitation and Western blotting, we found that at least three cytoplasmic host cell proteins, designated p80, p104, and p140, become tyrosine phosphorylated within 5-10 min after exposure to HSV-1 or HSV-2. However, no phosphorylation is detected when cells are exposed to a mutant virus deleted in gL that binds but fails to penetrate. Phosphorylation is restored when the gL-deletion virus is grown on a complementing cell line. Viral entry and the phosphorylation of p80, p104, and p140 are inhibited when cells are infected with virus in the presence of protein tyrosine kinase inhibitors. Taken together, these studies suggest that tyrosine phosphorylation of host cellular proteins is triggered by viral entry.
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