Enhanced replication of mouse adenovirus type 1 following virus-induced degradation of protein kinase R (PKR)

Protein kinase R (PKR) plays a major role in activating host immunity during infection by sensing dsRNA produced by viruses. Once activated by dsRNA, PKR phosphorylates the translation factor eIF2α, halting cellular translation. Many viruses have methods of inhibiting PKR activation or its downstream effects, circumventing protein synthesis shutdown. These include sequestering dsRNA or producing proteins that bind to and inhibit PKR activation. Here we describe our finding that in multiple cell types, PKR was depleted during mouse adenovirus type 1 (MAV-1) infection. MAV-1 did not appear to be targeting PKR at a transcriptional or translational level because total PKR mRNA levels and levels of PKR mRNA bound to polysomes were unchanged or increased during MAV-1 infection. However, inhibiting the proteasome reduced the PKR depletion seen in MAV-1-infected cells, whereas inhibiting the lysosome had no effect. This suggests that proteasomal degradation alone is responsible for PKR degradation during MAV-1 infection. Time course experiments indicate that the degradation occurs early after infection. Infecting cells with UV-inactivated virus prevented PKR degradation, whereas inhibiting viral DNA replication did not. Together these results suggest that an early viral gene is responsible. Degradation of PKR is a rare mechanism to oppose PKR activity, and it has only been described in six RNA viruses. To our knowledge, this is the first example of a DNA virus counteracting PKR by degrading it.