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Mol Cell Biol, February 1998, p. 859-871, Vol. 18, No. 2
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

Regulation of Interferon-Induced Protein Kinase PKR: Modulation of P58IPK Inhibitory Function by a Novel Protein, P52rIPK

Michael Gale Jr.,1 Collin M. Blakely,1 Deborah A. Hopkins,2 Mark W. Melville,1 Marlene Wambach,2 Patrick R. Romano,3 and Michael G. Katze1,2,*

Department of Microbiology, School of Medicine,1 and Regional Primate Research Center,2 University of Washington, Seattle, Washington 98195, and Laboratory of Eukaryotic Gene Regulation, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 208923

Received 15 August 1997/Returned for modification 14 October 1997/Accepted 4 November 1997

The cellular response to environmental signals is largely dependent upon the induction of responsive protein kinase signaling pathways. Within these pathways, distinct protein-protein interactions play a role in determining the specificity of the response through regulation of kinase function. The interferon-induced serine/threonine protein kinase, PKR, is activated in response to various environmental stimuli. Like many protein kinases, PKR is regulated through direct interactions with activator and inhibitory molecules, including P58IPK, a cellular PKR inhibitor. P58IPK functions to represses PKR-mediated phosphorylation of the eukaryotic initiation factor 2alpha subunit (eIF-2alpha ) through a direct interaction, thereby relieving the PKR-imposed block on mRNA translation and cell growth. To further define the molecular mechanism underlying regulation of PKR, we have utilized an interaction cloning strategy to identify a novel cDNA encoding a P58IPK-interacting protein. This protein, designated P52rIPK, possesses limited homology to the charged domain of Hsp90 and is expressed in a wide range of cell lines. P52rIPK and P58IPK interacted in a yeast two-hybrid assay and were recovered as a complex from mammalian cell extracts. When coexpressed with PKR in yeast, P58IPK repressed PKR-mediated eIF-2alpha phosphorylation, inhibiting the normally toxic and growth-suppressive effects associated with PKR function. Conversely, introduction of P52rIPK into these strains resulted in restoration of both PKR activity and eIF-2alpha phosphorylation, concomitant with growth suppression due to inhibition of P58IPK function. Furthermore, P52rIPK inhibited P58IPK function in a reconstituted in vitro PKR-regulatory assay. Our results demonstrate that P58IPK is inhibited through a direct interaction with P52rIPK which, in turn, results in upregulation of PKR activity. Taken together, our data describe a novel protein kinase-regulatory system which encompasses an intersection of interferon-, stress-, and growth-regulatory pathways.

* Corresponding author. Mailing address: Department of Microbiology, School of Medicine, University of Washington, Box 357242, Seattle, WA 98195-7242. Phone: (206) 543-8837. Fax: (206) 685-0305. E-mail: honey{at}u.washington.edu.




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Copyright © 1998 by the American Society for Microbiology. All rights reserved.