Wm. Xiao et al., IDENTIFICATION OF A PROMOTER-SPECIFIC TRANSACTIVATION DOMAIN IN THE HERPES-SIMPLEX VIRUS REGULATORY PROTEIN ICP4, Journal of virology, 71(3), 1997, pp. 1757-1765
ICP4 is expressed during the immediate-early phase of infection by her
pes simplex virus (HSV) and activates transcription of viral genes dur
ing subsequent phases of productive infection. Several members of the
alpha-herpesvirus family encode regulatory proteins that have extensiv
e homology with ICP4 and exhibit a transactivation domain (TAD) at the
N terminus. The portions of ICP4 required for nuclear localization, D
NA binding, and dimerization have been defined, but a domain that is s
pecifically required for transactivation has not been identified. We h
ave defined a promoter-specific ICP4 TAD by analysis of the activity o
f GAL4-ICP4 fusion proteins cotransfected into HeLa cells with a lucif
erase reporter gene linked to a promoter with five GAL4 binding sites.
The transactivation activity of GAL4-ICP4 hybrids is located entirely
within the first 139 residues of ICP4 and is significantly less poten
t than the activity of GAL4-TAD hybrids derived from ICP4 homologs. IC
P4 residues 97 to 109 are a critical component of this N-terminal TAD.
Transient transfection assays performed with nonfusion forms of ICP4
and luciferase genes linked to the HSV glycoprotein D (go) or thymidin
e kinase (tk) promoter revealed that ICP4 residues 97 to 109 are requi
red for induction of the go promoter but are not required for inductio
n of the tk promoter. Comparative experiments with ICP4 homologs revea
led that the pseudorabies virus TAD is a potent activator of the go pr
omoter and a weak activator of the fk promoter. Complementation assays
revealed that loss of ICP4 residues 97 to 109 reduced the yield of vi
rus from infected cells nearly 500-fold compared to wild-type ICP4. We
conclude that ICP4 residues 97 to 109 are a core component of a promo
ter-specific transactivation domain that is required for efficient rep
lication of herpes simplex virus.