Herpes simplex virus 1 [Alpha] regulatory protein ICPO functionally interacts with cellular transcription factor BMAL1

The infected cell protein no. 0 (ICPO) of herpes simplex virus 1 (HSV-1) is a promiscuous transactivator shown to enhance the expression of gene introduced into cells by infection or transfection. At the molecular level, ICPO is a 775-aa ring finger protein localized initially in the nucleus and late in infection in the cytoplasm and mediates the degradation of several proteins and stabilization of others. None of the known functions at the molecular level account for the apparent activity of ICPO as a transactivator. Here we report that ICPO functionally interacts with cellular transcription factor BMAL1, a member of the basic helix-loop-helix PER-ARNT-SIM (PAS) super family of transcriptional regulators. Specifically, sequences mapped to the exon II of ICPO interacted with BMAL1 in the yeast two-hybrid system and in reciprocal pull-down experiments in vitro. Moreover, the enhancement of transcription of a luciferase reporter construct whose promoter contained multiple BMAL1-binding sites by ICPO and BMAL1 was significantly greater than that observed by ICPO or BMAL1 alone. Although the level of BMAL1 present in nuclei of infected cells remained unchanged between 3 and 8 h after infection, the level of cytoplasmic BMAL1 was reduced at 8 h after infection. The reduction of cytoplasmic BMAL1 was significantly greater in cells infected with the ICPO-null mutant than in the wild-type virus-infected cells, suggesting that ICPO mediates partial stabilization of the protein. These results indicate that ICPO interacts physically and functionally with at least one cellular transcription-regulatory factor.