Acts as a transactivator along with IE2, and is required for oriLyt-dependent DNA replication in the transient transfection replication assay using native promoters. Interacts (via N-terminus) with the viral DNA polymerase accessory subunit UL44. Interacts (via C-terminus) with host EIF2AK2. Isoform IRS1-263 is predominantly nuclear. Belongs to the herpesviridae US22 family.