Figure 1.

Viral inhibitors of the PKR pathway. PKR is depicted as a monomer with a kinase domain (KD) and two dsRNA binding domains (RBD) bound to the 60S ribosomal subunit. The binding of dsRNA induces a conformational change to promote PKR dimerization, autophosphorylation, and activation of the eIF-2α kinase activity. Activated PKR also leads to activation of NF-κB, p53, and IFN regulatory factor 1 (IRF1). Viral inhibitors that act through different mechanisms are depicted. Adenovirus VA RNAs, Epstein–Barr virus EBER RNAs, and HIV transactivator responsive region TAR RNA bind and inhibit PKR and presumably displace PKR from the ribosome. Numerous viral proteins, such as vaccinia virus E3L, influenza virus NS1, and reovirus σ3 bind and sequester dsRNA, thereby preventing activation by dsRNA. Vaccinia virus K3L and HIV trans-activating transcriptional activator Tat act to inhibit PKR binding to eIF-2α. Protein phosphatase PP1 dephosphorylates phosphorylated eIF-2α as well as phosphorylated PKR. Herpes simplex virus 1 encodes a protein γ32.5 that facilitates activation of PP1. Hepatitis C virus nonstructural protein NS5A prevents PKR dimerization. In addition, influenza virus activates a cellular inhibitor P58^IPK that also inhibits PKR dimerization. Poliovirus induces PKR degradation. (See ref. 5 and references therein). met represents initiator methionyl tRNA, and AUG represents the initiator codon.