Regulation of Innate Immune Responses by Bovine Herpesvirus 1 and Infected Cell Protein 0 (bICP0)

Abstract

Bovine herpesvirus 1 (BoHV-1) infected cell protein 0 (bICP0) is an important transcriptional regulatory protein that stimulates productive infection. In transient transfection assays, bICP0 also inhibits interferon dependent transcription. bICP0 can induce degradation of interferon stimulatory factor 3 (IRF3), a cellular transcription factor that is crucial for activating beta interferon (IFN-β) promoter activity. Recent studies also concluded that interactions between bICP0 and IRF7 inhibit trans-activation of IFN-β promoter activity. The C3HC4 zinc RING (really important new gene) finger located near the amino terminus of bICP0 is important for all known functions of bICP0. A recombinant virus that contains a single amino acid change in a well conserved cysteine residue of the C3HC4 zinc RING finger of bICP0 grows poorly in cultured cells, and does not reactivate from latency in cattle confirming that the C3HC4 zinc RING finger is crucial for viral growth and pathogenesis. A bICP0 deletion mutant does not induce plaques in permissive cells, but induces autophagy in a cell type dependent manner. In summary, the ability of bICP0 to stimulate productive infection, and repress IFN dependent transcription plays a crucial role in the BoHV-1 infection cycle.

Keywords: IRF3; IRF7; bICP0; bovine herpesvirus 1 (BoHV-1); interferon; transcriptional regulation.

Figure 1.

Schematic of bICP0 gene within the BoHV-1 genome. Panel A: Schematic of BoHV-1 genome and location of bICP0 gene. The Unique Long (L) and Unique Short (S) regions of BoHV-1 are denoted. Panel B: Position of bICP4 and bICP0 transcripts is shown. The immediate early transcription unit 1 (IEtu1) encodes bICP4 (IE/4.2) and bICP0 (IE/2.9) [148,149]. The IEtu1 promoter activates IE expression of IE/4.2 and IE/2.9 (IEtu1 pro). E/2.6 is the early transcript that encodes bICP0 and an early promoter (E pro) activates expression of this transcript [147]. Exon 2 (e2) of bICP0 contains all of the protein coding sequences of bICP0. The dashed lines are intron sequences. Panel C: Schematic of bICP0 protein and known functional domains. The functional domains include a NLS (nuclear localization sequence, TAD (transcriptional activation domain), Acidic Domain, and the C₃HC₄ zinc RING finger. Amino acid sequence (residue 13–51) of the C₃HC₄ zinc RING finger in bICP0 is shown. The consensus residues in a C₃HC₄ zinc RING finger are also shown. The mutated cysteine at position 51 is highlighted in yellow and is underlined.

Figure 2.

Activation of IFN-β promoter activity and how bICP0 inhibits IFN-β promoter activity. Panel A: Schematic of the human IFN-β promoter necessary for inducing an IFN response to virus infection [2,80,86]. The NF-κB binding site is bound by the two proteins, p60 and p50, that comprise the NF-κB transcription factor. A summary of signaling pathways that induces IFN-β promoter activity is presented [7,32,42,54,72,109,119,133,142,143,153]. Panel B: A schematic of the known steps by which bICP0 inhibits IFN-β promoter activity. For details, see text.

Figure 3.

Schematic of the BoHV-1 LR gene and primers used in this study. Partial restriction map, location of LR-RNA, organization of LR ORFs, and the bICP0 ORF. Start sites for LR transcription during latency and productive infection were previously described [44]. Reading Frames B and C contain an open reading frame, but lack an initiating Met. The (*) denotes the positions of stop codons that are in frame with the respective ORF.