UC San Diego

Viruses rely on host cell resources to execute the diverse functions required for propagation, and they must simultaneously counteract intrinsic antiviral defenses deployed by the cell. To commandeer and overtake cellular processes, viruses encode regulatory proteins able to bind and redirect host proteins that function at key nodes in cellular pathways. Many viruses harness the cellular ubiquitin-proteasome system to execute their counterattacks. Herpes simplex virus-1 (HSV-1) encodes a RING domain E3 ubiquitin ligase, ICP0, which promotes lytic infection and reactivation from latency by targeting cellular proteins for ubiquitination and degradation. In the work presented in this Dissertation, we identify two novel substrates of ICP0, the cellular E3 ligases and DNA damage response (DDR) proteins RNF8 and RNF168. In uncovering why these cellular proteins are targeted, we demonstrate that the cell is able to deploy the DDR not only to recognize damage to the host genome, but also to recognize incoming HSV-1 genomes and repress transcription from the viral genome. As such, we define the DDR to function as an intrinsic antiviral defense, coordinated through RNF8 and ubiquitin-dependent mechanisms. The degradation of RNF8 and RNF168 serves as the viral counterattack, as this prevents recruitment of DDR proteins to sites of incoming viral genomes, induces loss of ubiquitinated H2A and H2AX, relieves transcriptional repression of the viral genome, and promotes viral infection. We further elucidate the mechanism by which ICP0 targets RNF8 and demonstrate that ICP0 mimics a cellular phosphosite normally induced on cellular proteins in response to DNA damage to bind the RNF8 FHA domain. We demonstrate a role for the cellular CK1 kinase in catalyzing phosphorylation on ICP0 to facilitate the RNF8- ICP0 interaction. These observations highlight the power of viral mimicry in targeting cellular proteins and pathways during infection and suggest that other viruses may mimic cellular post-translational marks to insert themselves into key cellular pathways. Finally, our findings demonstrate that the ubiquitin system plays prominent roles in both cellular defense and viral counterattack during infection