A cellular transcriptional coactivator that was originally identified by its requirement for the stable assembly immediate-early proteins of the HERPES SIMPLEX VIRUS. It is a nuclear protein that is a transcriptional coactivator for a number of transcription factors including VP16 PROTEIN; GA-BINDING PROTEIN; early growth response protein 2; and e2f4 transcription factor. It also interacts with and stabilizes herpes simplex virus protein vmw65 and helps regulate GENETIC TRANSCRIPTION of IMMEDIATE-EARLY genes in HERPES SIMPLEX VIRUS.