The dsRNA protein kinase PKR: virus and cell control.

The IFN-induced double-stranded RNA-dependent protein kinase (PKR) is one of the four mammalian serine-threonine kinases (the three others being HRI, GCN2 and PERK) that phosphorylate the eIF2 alpha translation initiation factor, in response to stress signals, mainly as a result of viral infections. eIF2 alpha phosphorylation results in arrest of translation of both cellular and viral mRNAs, an efficient way to inhibit virus replication. The particularity of PKR is to activate by binding to dsRNA through two N terminal dsRNA binding motifs (dsRBM). PKR activation during a viral infection represents a threat for several viruses, which have therefore evolved to express PKR inhibitors, such as the Vaccinia E3L and K3L proteins. The function of PKR can also be regulated by cellular proteins, either positively (RAX/PACT; Mda7) or negatively (p58IPK, TRBP, nucleophosmin, Hsp90/70). PKR can provoke apoptosis, in part through its ability to control protein translation, but the situation appears to be more complex, as NF-kappaB, ATF-3 and p53 have also been implicated. PKR-induced apoptosis involves mainly the FADD/caspase 8 pathway, while the mitochondrial APAF/caspase 9 pathway is also engaged. As a consequence of the effects of PKR on translation, transcription and apoptosis, PKR can function to control cell growth and cell differentiation, and its activity can be controlled by the action of several oncogenes.