E1B-55K-Mediated Regulation of RNF4 SUMO-Targeted Ubiquitin Ligase Promotes Human Adenovirus Gene Expression

Human adenovirus (HAdV) E1B-55K is a multifunctional regulator of productive viral replication and oncogenic transformation in nonpermissive mammalian cells. These functions depend on E1B-55K's posttranslational modification with the SUMO protein and its binding to HAdV E4orf6. Both early viral proteins recruit specific host factors to form an E3 ubiquitin ligase complex that targets antiviral host substrates for proteasomal degradation. Recently, we reported that the PML-NB-associated factor Daxx represses efficient HAdV productive infection and is proteasomally degraded via a SUMO-E1B-55K-dependent, E4orf6-independent pathway, the details of which remained to be established. RNF4, a cellular SUMO-targeted ubiquitin ligase (STUbL), induces ubiquitinylation of specific SUMOylated proteins and plays an essential role during DNA repair. Here, we show that E1B-55K recruits RNF4 to the insoluble nuclear matrix fraction of the infected cell to support RNF4/Daxx association, promoting Daxx PTM and thus inhibiting this antiviral factor. Removing RNF4 from infected cells using RNA interference resulted in blocking the proper establishment of viral replication centers and significantly diminished viral gene expression. These results provide a model for how HAdV antagonize the antiviral host responses by exploiting the functional capacity of cellular STUbLs. Thus, RNF4 and its STUbL function represent a positive factor during lytic infection and a novel candidate for future therapeutic antiviral intervention strategies.IMPORTANCE Daxx is a PML-NB-associated transcription factor that was recently shown to repress efficient HAdV productive infection. To counteract this antiviral measurement during infection, Daxx is degraded via a novel pathway including viral E1B-55K and host proteasomes. This virus-mediated degradation is independent of the classical HAdV E3 ubiquitin ligase complex, which is essential during viral infection to target other host antiviral substrates. To maintain a productive viral life cycle, HAdV E1B-55K early viral protein inhibits the chromatin-remodeling factor Daxx in a SUMO-dependent manner. In addition, viral E1B-55K protein recruits the STUbL RNF4 and sequesters it into the insoluble fraction of the infected cell. E1B-55K promotes complex formation between RNF4- and E1B-55K-targeted Daxx protein, supporting Daxx posttranslational modification prior to functional inhibition. Hence, RNF4 represents a novel host factor that is beneficial for HAdV gene expression by supporting Daxx counteraction. In this regard, RNF4 and other STUbL proteins might represent novel targets for therapeutic intervention

KAP1 Is a Host Restriction Factor That Promotes Human Adenovirus E1B-55K SUMO Modification

Once transported to the replication sites, human adenoviruses (HAdVs) need to ensure decondensation and transcriptional activation of their viral genomes to synthesize viral proteins and initiate steps to reprogram the host cell for viral replication. These early stages during adenoviral infection are poorly characterized but represent a decisive moment in the establishment of a productive infection. Here, we identify a novel host viral restriction factor, KAP1. This heterochromatin-associated transcription factor regulates the dynamic organization of the host chromatin structure via its ability to influence epigenetic marks and chromatin compaction. In response to DNA damage, KAP1 is phosphorylated and functionally inactive, resulting in chromatin relaxation. We discovered that KAP1 posttranslational modification is dramatically altered during HAdV infection to limit the antiviral capacity of this host restriction factor, which represents an essential step required for efficient viral replication. Conversely, we also observed during infection an HAdV-mediated decrease of KAP1 SUMO moieties, known to promote chromatin decondensation events. Based on our findings, we provide evidence that HAdV induces KAP1 deSUMOylation to minimize epigenetic gene silencing and to promote SUMO modification of E1B-55K by a so far unknown mechanism. IMPORTANCE Here we describe a novel cellular restriction factor for human adenovirus (HAdV) that sheds light on very early modulation processes in viral infection. We reported that chromatin formation and cellular SWI/SNF chromatin remodeling play key roles in HAdV transcriptional regulation. We observed that the cellular chromatin-associated factor and epigenetic reader SPOC1 represses HAdV infection and gene expression. Here, we illustrate the role of the SPOC1-interacting factor KAP1 during productive HAdV growth. KAP1 binds to the viral E1B-55K protein, promoting its SUMO modification, therefore illustrating a crucial step for efficient viral replication. Simultaneously, KAP1 posttranslational modification is dramatically altered during infection. We observed an HAdV-mediated decrease in KAP1 SUMOylation, known to promote chromatin decondensation events. These findings indicate that HAdV induces the loss of KAP1 SUMOylation to minimize epigenetic gene silencing and to promote the SUMO modification of E1B-55K by a so far unknown mechanism