The immediate early 62 protein (IE62) of varicella-zoster virus (VZV), a major 
viral trans-activator, initiates the virus life cycle and is a key component of 
pathogenesis. The IE62 possesses several domains essential for trans-activation, 
including an acidic trans-activation domain (TAD), a serine-rich tract (SRT), 
and binding domains for USF, TFIIB, and TATA box binding protein (TBP). 
Transient-transfection assays showed that the VZV IE62 lacking the SRT 
trans-activated the early VZV ORF61 promoter at only 16% of the level of the 
full-length IE62. When the SRT of IE62 was replaced with the SRT of equine 
herpesvirus 1 (EHV-1) IEP, its trans-activation activity was completely 
restored. Herpes simplex virus 1 (HSV-1) ICP4 that lacks a TAD very weakly 
(1.5-fold) trans-activated the ORF61 promoter. An IE62 TAD-ICP4 chimeric protein 
exhibited trans-activation ability (10.2-fold), indicating that the IE62 TAD 
functions with the SRT of HSV-1 ICP4 to trans-activate viral promoters. When the 
serine and acidic residues of the SRT were replaced with Ala, Leu, and Gly, 
trans-activation activities of the modified IE62 proteins IE62-SRTΔSe and 
IE62-SRTΔAc were reduced to 46% and 29% of wild-type activity, respectively. 
Bimolecular complementation assays showed that the TAD of IE62, EHV-1 IEP, and 
HSV-1 VP16 interacted with Mediator 25 in human melanoma MeWo cells. The SRT of 
IE62 interacted with the nucleolar-ribosomal protein EAP, which resulted in the 
formation of globular structures within the nucleus. These results suggest that 
the SRT plays an important role in VZV viral gene expression and replication.
IMPORTANCE: The immediate early 62 protein (IE62) of varicella-zoster virus 
(VZV) is a major viral trans-activator and is essential for viral growth. Our 
data show that the serine-rich tract (SRT) of VZV IE62, which is well conserved 
within the alphaherpesviruses, is needed for trans-activation mediated by the 
acidic trans-activation domain (TAD). The TADs of IE62, EHV-1 IEP, and HSV-1 
VP16 interacted with cellular Mediator 25 in bimolecular complementation assays. 
The interaction of the IE62 SRT with nucleolar-ribosomal protein EAP resulted in 
the formation of globular structures within the nucleus. Understanding the 
mechanisms by which the TAD and SRT of IE62 contribute to the function of this 
essential regulatory protein is important in understanding the gene program of 
this human pathogen.