Fowl adenovirus serotype 4 52/55k protein triggers PKR degradation by ubiquitin-proteasome system to evade effective innate immunity.

The pro- and inflammatory cytokines fail to effectively inhibit FAdV-4, which has always puzzled us. In the current study, the data determined that the mRNA levels of interferons were significantly enhanced in the livers and LMH cells from 24 h to 72 h post FAdV-4 infection. But the viral load of FAdV-4 was still significantly increased, which meant that FAdV-4 evaded innate immune response. We additionally revealed that the protein levels not mRNA levels of PKR were degraded in host cell at 48 h post FAdV-4 infection. Moreover, the results of over expression and silent expression of PKR revealed that PKR could inhibit FAdV-4 proliferation. These results indicated that FAdV-4 degraded the protein levels of PKR to evade innate immune response. We also found that the protein degradation levels of PKR induced by FAdV-4 were recovery in LHM cells after treatment with proteasome inhibitor MG132, and ubiquitin-specific proteases inhibitor DUB-IN-1. Furthermore, our current data presented that FAdV-4 52/55 K protein directly interacted with PKR and degraded it determined by Co-immunoprecipitation and immunofluorescence. We also determined that 52/55 K protein triggered PKR degradation, and the degradation of PKR could be recovery in LHM cells after treatment with MG132, or DUB-IN-1, respectively. Finally, our data demonstrated that 52/55 K protein was a ubiquitylase that could directly degrade PKR protein in host cells via the ubiquitin-proteasome pathway. Therefore, the current study firstly revealed that FAdV-4 52/55 K protein played the key role in triggering PKR degradation by ubiquitin-proteasome system pathway to escape from innate immunity response.