The EICP27 protein of equine herpesvirus 1 is recruited to viral promoters by its interaction with the immediate-early protein

The equine herpesvirus 1 (EHV-1) EICP27 protein cooperates with either the immediate-early (IE) or the EICP0 protein to synergistically trans-activate viral promoters. GST-pulldown and co-immunoprecipitation assays revealed that the EICP27 protein's cooperation with the IE or the EICP0 protein involves its physical interaction with these viral proteins. In the case of the IE-EICP27 protein interaction, IE residues 424 to 826 and EICP27 residues 41 to 206 harbor the interactive domains. Electrophoretic mobility shift assays (EMSA) suggested that the EICP27 protein is not a sequence-specific DNA-binding protein as it fails to directly bind to the IE promoter, the early EICP27, EICP0, and TK promoters, or the late gD and IR5 promoters. However, EMSA studies also showed that the interaction of the IE and EICP27 proteins results in the recruitment of the EICP27 protein to representative early promoters. These results support our hypothesis that the EICP27 protein participates in the trans-activation of EHV-1 promoters, and suggest its presence within RNA polymerase II preinitiation complexes that assemble at viral promoters.