Comparative proteomics identifies Schlafen 5 (SLFN5) as a herpes simplex virus restriction factor that suppresses viral transcription

Intrinsic antiviral host factors confer cellular defense by limiting virus replication and are often counteracted by viral countermeasures. We reasoned that host factors that inhibit viral gene expression could be identified by determining proteins bound to viral DNA (vDNA) in the absence of key viral antagonists. Herpes simplex virus 1 (HSV-1) expresses ICP0, which functions as an E3 ubiquitin ligase required to promote infection. Cellular substrates of ICP0 have been discovered as host barriers to infection, but mechanisms for inhibition of viral gene expression are not fully understood. To identify restriction factors antagonized by ICP0, we compared proteomes associated with vDNA during HSV-1 infection with wild-type (WT) virus and mutant lacking functional ICP0 (ΔICP0). We identified the cellular protein Schlafen 5 (SLFN5) as an ICP0 target that binds vDNA during HSV-1 ΔICP0 infection. We demonstrated that ICP0 mediates ubiquitination of SLFN5 which leads to its proteasomal degradation. In the absence of ICP0, SLFN5 binds vDNA to repress HSV-1 transcription by limiting accessibility of RNA polymerase II to viral promoters. These results highlight how comparative proteomics of proteins associated with viral genomes can identify host restriction factors, and reveal that viral countermeasure can overcome SLFN antiviral activity.