Regulatory interaction between the cellular restriction factor IFI16 and viral pp65 (pUL83) modulates viral gene expression and IFI16 protein stability

Introduction. A key player in the intrinsic resistance against human cytomegalovirus (HCMV) is the interferon-γ-inducible protein 16 (IFI16), which behaves as a viral DNA sensor in the first hours post infection and as a repressor of viral gene transcription in the later stages. Previous studies on HCMV replication demonstrated that IFI16 binds to the viral protein kinase pUL97, undergoes phosphorylation and relocalizes to the cytoplasm of infected cells. Materials and methods. To assess the interplay between IFI16 and pUL83 we employed human foreskin fibroblasts (HFF) infected with the wild type HCMV strain (v65Rev), the HCMV v65Stop lacking pUL83 expression, or the HCMV mutant virus (RV-VM1) expressing a pUL83 lacking the nuclear egression signal (NES). Results. In this study, we demonstrate that the tegument protein pp65 (pUL83) recruits IFI16 to the promoter of the UL54 gene and downregulates viral replication as shown by use of the HCMV mutant v65Stop, which lacks pp65 expression. Interestingly, at late time-points of HCMV infection, IFI16 is stabilized by its interaction with pp65, which stood in contrast to IFI16 degradation, observed in herpes simplex virus (HSV-1)-infected cells. Moreover, we found that its translocation to the cytoplasm, in addition to pUL97, strictly depends on pp65, as demonstrated with the HCMV mutant RV-VM1, which expresses a form of pp65 unable to translocate into the cytoplasm. Discussion and conclusions. These data reveal a dual role for pp65: during early infection, it modulates IFI16 activity at the promoter of immediate-early and early genes; subsequently, it delocalizes IFI16 from the nucleus into the cytoplasm, thereby stabilizing and protecting it from degradation. Overall, these data identify a novel activity of the pp65/IFI16 interactome involved in the regulation of UL54 gene expression and IFI16 stability during early and late phases of HCMV replication.