PACT, a stress-modulated cellular activator of interferon-induced double-stranded RNA-activated protein kinase, PKR

The interferon (IFN)-induced, double-stranded (ds)RNA-activated serine-thre onine protein kinase, PKR, is a key mediator of the antiviral activities of IFNs. In addition, PKR activity is also involved in regulation of cell pro liferation, apoptosis, and signal transduction. In virally infected cells, dsRNA has been shown to bind and activate PKR kinase function. Implication of PKR activity in normal cellular processes has invoked activators other t han dsRNA because RNAs with perfectly duplexed regions of sufficient length that are able to activate PKR are absent in cellular RNAs. We have recentl y reported cloning of PACT, a novel protein activator of PKR. PACT heterodi merizes with PKR and activates it by direct protein-protein interaction. Ov erexpression of PACT in mammalian cells leads to phosphorylation of the alp ha subunit of the eukaryotic initiation factor 2 (eIF2 alpha), the cellular substrate for PKR, and leads to inhibition of protein synthesis. Here, we present evidence that endogenous PACT acts as a protein activator of PKR in response to diverse stress signals such as serum starvation, and peroxide or arsenite treatment. Following exposure of cells to these stress agents, PACT is phosphorylated and associates with PKR with increased affinity. PAC T-mediated activation of PKR leads to enhanced eIF2 alpha phosphorylation f ollowed by apoptosis. Based on the results presented here, we propose that PACT is a novel stress-modulated physiological activator of PKR.