Plays an important role in transactivating viral early genes as well as activating its own promoter, probably by altering the viral chromatin structure (PubMed:17331553, PubMed:8876134, PubMed:23878222, PubMed:15572445, PubMed:26812545). Expression of IE1 and IE2 proteins is critical for the establishment of lytic infection and reactivation from viral latency (By similarity). 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 (PubMed:9151854, PubMed:15163746, PubMed:17367754, PubMed:10233977). Promotes efficient viral growth by interacting with and directing host SP100 to degradation, leading to enhanced acetylation level of histones (PubMed:21880768). In addition, functions in counteracting the host innate antiviral response. Inhibits the type I interferon pathway by directly interacting with and sequestrating host STAT2 (PubMed:18701593, PubMed:19812155, PubMed:16497831). Also targets type II interferon pathway by repressing IL6- and STAT3 target genes (PubMed:27387064). Repression of STAT3 genes is due to STAT3 nuclear accumulation and disruption of IL6-induced STAT3 phosphorylation by IE1 (By similarity). This repression is followed by phosphorylation and activation of STAT1 (PubMed:27387064). Inhibits host ISG transcription by sequestering host ISGF3 in a PML- and STAT2- binding dependent manner (PubMed:25812002). Alters host cell cycle progression, probably through its interaction with host E2F1 or RB1 that overcomes the RB1-mediated repression of E2F-responsive promoters (PubMed:8892909). Interacts with human p53/TP53; this interaction inhibits p53/TP53-dependent transactivation activity (PubMed:19776115). Interacts with host STAT1 (PubMed:16497831, PubMed:25812002). Interacts with host STAT2; this interaction promotes viral growth and counteracts the antiviral interferon response (PubMed:16497831, PubMed:18701593, PubMed:19812155, PubMed:25812002). May also interact with the host STAT1-STAT2 heterodimer (PubMed:16497831). Interacts with host STAT3; this interaction leads to STAT3 nuclear accumulation and disruption of IL6-induced STAT3 phosphorylation (PubMed:27387064). 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:25812002) (Probable). Interacts with host DAXX (PubMed:20444888). Interacts with host SP100 (PubMed:21880768). Interacts with host E2F1 (PubMed:7494286). Interacts with host RB1 (PubMed:8892909). Interacts with host HDAC1; this interaction inhibits histone deacetylation and promotes viral transcription (PubMed:25812002). Interacts with host HDAC2; this interaction inhibits histone deacetylation and promotes viral transcription (PubMed:25812002). Interacts with host HDAC3; this interaction inhibits histone deacetylation and promotes viral transcription (PubMed:15572445). Colocalizes with host PML-associated nuclear bodies very early post infection. The N-terminal region is required for nuclear targeting (PubMed:17367754). 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 (PubMed:22158879). Targets host nucleosomes by directly binding to the acidic pocket of core histones (PubMed:24227840). Sumoylated by host PML/nuclear domain 10 (By similarity). Sumoylation abolishes the interaction with host STAT2 and thus the IE1-mediated repression of interferon-stimulated genes (PubMed:18701593). Belongs to the HHV-5 IE1 protein family.