Multifunctional regulator of the expression of viral genes that contributes to the shutoff of host protein synthesis and mediates nuclear export of viral intronless mRNAs. Early in infection, this immediate early (EI) protein mediates the inhibition of cellular splicing. This results in the accumulation of unprocessed 3'end pre-mRNAs which can't be exported from the nucleus. Cellular protein synthesis is thereby shut off early after virus infection. Later in the infection, it helps recruit cellular RNA polymerase II to viral replication sites and promotes the nuclear export of viral intronless mRNAs by interacting with mRNAs and host NXF1/TAP. ICP27 binds to NUP62 which may provide facilitated viral mRNA export and may indirectly compete with some host cell transport receptors for binding and inhibit cellular nucleocytoplasmic transport pathways (PubMed:22334672). Also stimulates translation of viral transcripts. Repression of host gene expression blocks the cell cycle at the G1 phase and prevents apoptosis. Seems to silence the 3' splice site of the promyelocytic leukemia (PML) intron 7a, thereby switching PML isoforms from PML-II to PML-V. This could be linked to the accelerated mRNA export induced by ICP27 which might not provide sufficient time for PML pre-mRNA to be spliced in the nucleus. Homodimer (PubMed:26085142, PubMed:26062451). Interacts with host RBP1; this interaction facilitates the RNA polymerase recruitment to viral transcription sites. Interacts (via the RGG box) with host ALYREF/THOC4; this interaction recruits ALYREF to viral replication compartments and probably directs viral mRNA to the TAP/NFX1 pathway (PubMed:12438613). Interacts with host ALYREF2 (PubMed:21253573). Interacts (via the RGG box) with host SRPK1; this interaction relocalizes SRPK1 to the nucleus and seems to alter its activity (PubMed:12660167, PubMed:19553338). Interacts with ICP4; this interaction modulates ICP4 DNA-binding activity (PubMed:8995681). Interacts with host NXF1; this interaction allows efficient export of HHV-1 early and late transcripts (PubMed:19369354). Shuttles between the nucleus and the cytoplasm. Binds viral intronless RNAs through the RGG region. The C-terminus is essential for homodimerization. Methylated within the RGG box possibly by host PRMT1. When hypomethylated, ICP27 is exported to the cytoplasm earlier and more rapidly. Phosphorylated. Belongs to the HHV-1 ICP27 protein family. A role in degradation of host polymerase has been suggested in PubMed 16537625, but this could be a consequence of binding to immature mRNAs simultaneously to transcription by the polymerase.