Stimulates viral early and late gene expression and thus play a crucial role in the regulation of productive infection. In addition, activates quiescent cells to reenter the cell cycle and up-regulates several E2F-responsive genes, which are responsible for pushing the cell into S phase. In S-phase, inhibits cellular DNA synthesis and blocks further cell cycle progression. Interacts with host SUMO-modified form of TATA-binding protein (TBP)-associated factor 12/TAF12 in a SIM-dependent manner; this interaction increases the transactivation activity of IE2. Interacts with host CHAF1A. Interacts with several components of the host transcriptional machinery including TBP, TF2B and CREB1. Interacts with host DNA replication licensing factor MCM3. Colocalizes with host PML-associated nuclear bodies. The SUMO-interacting motif (SIM) is required for efficient transactivation function. Sumoylated. The sumoylation is necessary for efficient replication of the virus and thus for the function of this viral transcription factor (By similarity). Belongs to the HHV-5 IE2 protein family.