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Bovine herpesvirus 1 (BHV-1) is a significant pathogen of cattle. Following acute infection, BHV-1 establishes a latent infection that persists for the life of the infected host. Stress induced factors cause the virus to reactivate from latency, resulting in virus transmission and transient immune suppression. BHV-1 encoded bICP0 is expressed early and constitutively throughout productive infection. bICP0 is critical for efficient viral replication, virulence, and reactivation in cattle because it stimulates viral transcription and interferes with innate immune responses. bICP0 potentially interacts with a variety of proteins to activate viral gene expression and inhibit innate antiviral defenses. bICP0 localizes to promyelocytic leukemia (PML) protein-containing nuclear domains, which are associated with antiviral activity and commonly targeted for disruption by a wide variety of viruses. The zinc RING finger motif within bICP0 plays a critical role in the biological functions of bICP0, and possesses intrinsic E3 ubiquitin ligase activity that is important for polyubiquitination and subsequent degradation of proteins.
Results from these studies demonstrated mutations within the zinc RING finger increased bICP0 protein levels, presumably due to disruption of the ability of bICP0 to induce its own ubiquitination. Sequences at its C-terminus are also important for regulating the half-life of bICP0. BHV-1 infection and bICP0 expression alone reduced PML protein levels. Unexpectedly, bICP0 mutants that localized primarily in the cytoplasm also induced PML degradation. bICP0 was readily detected in the cytoplasm of low passage bovine cells at later times post infection, suggesting bICP0 induces degradation of cytoplasmic isoforms of PML to promote viral replication. Identification of bICP0-interacting proteins was also investigated to further elucidate the mechanisms underlying bICP0 functions. The ability of BHV-1 and a bICP0 zinc RING finger mutant to grow in oncogenic cells was also examined to determine if defects in viral growth of the mutant could be relieved in cells with potential defects in their interferon response. Collectively, studies presented in this dissertation determine that nuclear and cytoplasmic bICP0 have functions that promote productive infection.
Advisor: Clinton Jones