Transcriptional transactivator. May interact with and recruit specific components of the general transcription machinery to viral promoters and stabilize their formation for transcription initiation. Negatively regulates its own transcription. This immediate early (EI) protein may be necessary in virion for viral pathogenesis (By similarity). Interacts with IE4 and IE63. Interacts with human USF1 and SP1 (By similarity). Localizes to the cytoplasm when phosphorylated. Phosphorylated by ORF66 protein kinase on Ser-686 and Ser-722. Also phosphorylated by ORF47 protein kinase and by human CSNK2A1/CKII (By similarity). Encoded both by ORF62 and ORF71 within the inverted repeat sequences bounding the Us region of the VZV genome. Belongs to the herpesviridae ICP4 family.