Use of Expresor Cell Lines to Functionally Characterize the Herpes Simplex Virus Transcription-Activating Protein ICP4
During Herpes Simplex Virus type 1 (HSV-1) infections, many viral proteins are synthesized and several have proven or suspected roles in regulating viral gene expression. To facilitate the study of the individual activity of one such protein, ICP4, the ICP4 gene was cloned in a plasmid vector, and expresser cell lines containing 5-30% of infected cell levels of ICP4 were established. The ICP4 is functional, correctly processed, and located in the cell nuclei. The endogenous ICP4 gene retained its capacity to respond to viral trans-acting factors, since its expression after superinfection with HSV-2 mimicked that of the viral gene. Although cells infected with ICP4 mutant viruses overproduce ICP4 and other immediate-early proteins, cell lines synthesizing a mutant form of ICP4 did not overproduce this protein, suggesting that autoregulation of the ICP4 gene requires more than 30% of the infected cell level of ICP4 or, alternatively, requires the presence of other viral proteins. After superinfection in the presence of an inhibitor of protein synthesis, the endogenous ICP4 is capable of transactivating viral early genes incoding thymidine kinese, ICP6, ICP8, gB, gD, and gE. In contrast, the early gene for the viral alkaline exonuclease, the early-late gene for VP5 and the late genes for p40 and gC, respond poorly or not at all. This demonstrates that the most early genes can be induced by ICP4 in the absence of other viral immediate-early proteins, but that early-late and late genes require supplementary factors.