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J. Biol. Chem. 275 (48): 37993-37998

© 2000 by The American Society for Biochemistry and Molecular Biology, Inc.

PACT, a Stress-modulated Cellular Activator of Interferon-induced Double-stranded RNA-activated Protein Kinase, PKR*

Chandrashekhar V. Patel, Indhira Handy, Tiffany Goldsmith, and Rekha C. PatelDagger

From the Department of Biological Sciences, University of South Carolina, Columbia, South Carolina 29208

The interferon (IFN)-induced, double-stranded (ds)RNA-activated serine-threonine protein kinase, PKR, is a key mediator of the antiviral activities of IFNs. In addition, PKR activity is also involved in regulation of cell proliferation, 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 than dsRNA because RNAs with perfectly duplexed regions of sufficient length that are able to activate PKR are absent in cellular RNAs. We have recently reported cloning of PACT, a novel protein activator of PKR. PACT heterodimerizes with PKR and activates it by direct protein-protein interaction. Overexpression of PACT in mammalian cells leads to phosphorylation of the alpha  subunit of the eukaryotic initiation factor 2 (eIF2alpha ), 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. PACT-mediated activation of PKR leads to enhanced eIF2alpha phosphorylation followed by apoptosis. Based on the results presented here, we propose that PACT is a novel stress-modulated physiological activator of PKR.

* This work was supported in part by Grants K176 and E183 from the Venture Fund/SC Educational Foundation and the SC Cancer Center, respectively (to R. C. P.).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Dagger To whom correspondence should be addressed: Dept. of Biological Sciences, University of South Carolina, 700 Sumter St., Columbia, SC 29208. Tel.: 803- 777-1853; Fax: 803-777-4002; E-mail:

Copyright © 2000 by The American Society for Biochemistry and Molecular Biology, Inc.

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