The C-terminal, third conserved motif of the protein activator PACT plays an essential role in the activation of double-stranded-RNA-dependent protein kinase (PKR)

One of the key mediators of the antiviral and antiproliferative actions of interferon is double-stranded-RNA-dependent protein kinase (PKR), PKR activity is also involved in the regulation of cell proliferation, apoptosis and signal transduction. We have recently identified PACT, a novel protein activator of PKR, as an important modulator of PKR activity in cells in the absence of viral infection. PACT heterodimerizes with PKR and activates it by direct protein-protein interactions. Endogenous PACT acts as an activator of PKR in response to diverse stress signals, such as serum starvation and peroxide or arsenite treatment, and is therefore a novel, stress-modulated physiological activator of PKR. In this study, we have characterized the functional domains of PACT that are required for PKR activation. Our results have shown that, unlike the N-terminal conserved domains 1 and 2, the third conserved domain of PACT is dispensable for its binding of double-stranded RNA and interaction with PKR. However, a deletion of domain 3 results in a loss of PKR activation ability, in spite of a normal interaction with PKR, thereby indicating that domain 3 plays an essential role in PKR activation. Purified recombinant domain 3 could also activate PKR efficiently in vitro. Our results indicate that, although dispensable for PACT's high-affinity interaction with PKR, the third motif is essential for PKR activation. In addition, domain 3 and eukaryotic initiation factor 2,X both interact with PKR through the same region within PKR, which we have mapped to lie between amino acid residues 318 and 551.