SLFN5 adenovirus

Intrinsic antiviral host factors confer cellular defenses by limiting viral replication, but these factors are often counteracted by viral countermeasures. Researchers reasoned that host factors that inhibit viral gene expression could be identified by determining proteins that bind to viral DNA (vDNA) in the absence of key viral antagonists. Herpes simplex virus 1 (HSV-1) expresses the E3 ubiquitin protein ligase ICP0 (ICP0), which functions as an essential E3 ubiquitin ligase to promote infection. To identify restriction factors antagonized by ICP0, researchers compared the proteome associated with vDNA during HSV-1 infection with wild-type virus and a mutant lacking functional ICP0 (ΔICP0). Researchers identified the cellular protein Schlafen family member 5 (SLFN5) as an ICP0 target that binds to vDNA during HSV-1 ΔICP0 infection. Researchers demonstrate that ICP0 mediates ubiquitination of SLFN5, leading to its degradation in the proteasome. In the absence of ICP0, SLFN5 binds vDNA and represses HSV-1 transcription by limiting the accessibility of RNA polymerase II to the viral promoter. These results highlight how comparative proteomics of proteins associated with the viral genome can identify host restriction factors and reveal viral countermeasures that can overcome the antiviral activity of SLFN.

To determine whether the Walker A helicase motif of SLFN5 affects HSV-1 replication, the researchers transduced SLFN5 knockout cells with adenoviral vectors expressing SLFN5 or a Walker A mutant (K584A) SLFN5. Expression of both wild-type and mutant SLFN5 reduced HSV-1 protein expression and progeny production compared to controls, indicating helicase-independent antiviral activity (Figure 1e, f). They also tested the effect of SLFN5 overexpression on HSV-1 replication and compared it with a Δ730–891 truncated SLFN5 lacking the ICP0 binding domain. When SLFN5 protein was induced in cells subsequently infected with HSV-1, only full-length SLFN5 inhibited ΔRING HSV-1 replication. Depletion of SLFN11 using siRNA did not affect HSV-1 protein expression or vDNA replication (Figure 1g, h). HSV-2 also reduced SLFN5 levels and increased viral protein expression when SFLN5 was depleted. In contrast, other DNA viruses (HCMV or Ad5) neither degraded SLFN5 nor were restricted by SLFN5. These results suggest that the SLFN family-mediated antiviral restriction is virus-specific.

Figure 1. ICP0 counteracts SLFN5-mediated suppression of HSV-1 replication. (Kim E T, et al., 2021)