Plays an important role in transactivating viral early genes as well as activating its own promoter, probably by altering the viral chromatin structure (By similarity). Expression of IE1 and IE2 proteins is critical for the establishment of lytic infection and reactivation from viral latency (PubMed:27466417). Disrupts PML-associated ND10 nuclear bodies by interfering with host PML and SP100 sumoylation thereby altering the regulation of type I and type II interferon-induced gene expression (Probable) (PubMed:27903803). Promotes efficient viral growth by interacting with and directing host SP100 to degradation, leading to enhanced acetylation level of histones (By similarity). In addition, functions in counteracting the host innate antiviral response. Inhibits the type I interferon pathway by directly interacting with and sequestrating host STAT2 (By similarity). Also targets type II interferon pathway by repressing IL6- and STAT3 target genes (By similarity). Repression of STAT3 genes is due to STAT3 nuclear accumulation and disruption of IL6-induced STAT3 phosphorylation by IE1 (PubMed:23903834). This repression is followed by phosphorylation and activation of STAT1 (By similarity). Inhibits host ISG transcription by sequestering host ISGF3 in a PML- and STAT2- binding dependent manner (By similarity). Alters host cell cycle progression, probably through its interaction with host E2F1 or RB1 that overcomes the RB1-mediated repression of E2F-responsive promoters (By similarity). Interacts with human p53/TP53; this interaction inhibits p53/TP53-dependent transactivation activity. Interacts with host STAT1. Interacts with host STAT2; this interaction promotes viral growth and counteracts the antiviral interferon response. May also interact with the host STAT1-STAT2 heterodimer. Interacts with host STAT3; this interaction leads to STAT3 nuclear accumulation and disruption of IL6-induced STAT3 phosphorylation (By similarity). Interacts with host PML; this interaction inhibits host PML de novo sumoylation and probably inhibits PML regulation of type I and type II interferon-induced gene expression (PubMed:27903803). Interacts with host DAXX. Interacts with host SP100. Interacts with host E2F1. Interacts with host RB1. Interacts with host HDAC1; this interaction inhibits histone deacetylation and promotes viral transcription. Interacts with host HDAC2; this interaction inhibits histone deacetylation and promotes viral transcription. Interacts with host HDAC3; this interaction inhibits histone deacetylation and promotes viral transcription (By similarity). Colocalizes with host PML-associated nuclear bodies very early post infection. The N-terminal region is required for nuclear targeting. The C-terminal 16-amino acid is termed the chromosome-tethering domain (CTD) and is required for the association of IE1, host PML and host STAT2 with the mitotic chromosomes. Targets host nucleosomes by directly binding to the acidic pocket of core histones. Sumoylated by host PML (PubMed:28250117). Sumoylation abolishes the interaction with host STAT2 and thus the IE1-mediated repression of interferon-stimulated genes (By similarity). Belongs to the HHV-5 IE1 protein family.