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J. Biol. Chem., Vol. 279, Issue 37, 38649-38657, September 10, 2004

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Phosphorylation of Mnk1 by Caspase-activated Pak2/-PAK Inhibits Phosphorylation and Interaction of eIF4G with Mnk^*

Kevin C. Orton, Jun Ling, Andrew J. Waskiewicz, Jonathan A. Cooper, William C. Merrick¶, Nadejda L. Korneeva||, Robert E. Rhoads||, Nahum Sonenberg**, and Jolinda A. Traugh

From the Department of Biochemistry, University of California, Riverside, Riverside, California 92521, the Fred Hutchinson Cancer Research Center, Seattle, Washington 98109, the ^¶Department of Biochemistry, School of Medicine, Case Western Reserve University, Cleveland, Ohio 44106, the ^||Department of Biochemistry and Molecular Biology, Louisiana State University Health Sciences Center, Shreveport, Louisiana 71130, and the ^**Department of Biochemistry, McGill Cancer Center, McGill University, Montreal, Quebec H3G 1Y6, Canada

The mitogen-activated protein kinase-interacting kinase 1 (Mnk1) is phosphorylated by caspase-cleaved protein kinase Pak2/-PAK but not by Cdc42-activated Pak2. Phosphorylation of Mnk1 is rapid, reaching 1 mol/mol within 15 min of incubation with Pak2. A kinetic analysis of the phosphorylation of Mnk1 by Pak2 yields a K_m of 0.6 µM and a V_max of 14.9 pmol of ³²P/min/µg of Pak2. Two-dimensional tryptic phosphopeptide mapping of Mnk1 phosphorylated by Pak2 yields two distinct phosphopeptides. Analysis of the phosphopeptides by automated microsequencing and manual Edman degradation identified the sites in Mnk1 as Thr²² and Ser²⁷. Mnk1, activated by phosphorylation with Erk2, phosphorylates the eukaryotic initiation factor (eIF) 4E and the eIF4G components of eIF4F. Phosphorylation of Mnk1 by Pak2 does not activate Mnk1, as measured with either eIF4E or eIF4F as substrate. Phosphorylation of Erk2-activated Mnk1 by Pak2 has no effect on phosphorylation of eIF4E but reduces phosphorylation of eIF4G by Mnk1 by up to 50%. Phosphorylation of Mnk1 by Pak2 inhibits binding of eIF4G peptides containing the Mnk1 binding site by up to 80%. When 293T cells are subjected to apoptotic induction by hydrogen peroxide, Mnk1 is phosphorylated at both Thr²² and Ser²⁷. These results indicate a role for Pak2 in the down-regulation of translation initiation in apoptosis by phosphorylation of Mnk1.

Received for publication, June 30, 2004

^* This work was supported by United States Public Health Service Grants GM26738 (to J. A. T.), CA73879 (to J. A. C.), GM20818 (to R. E. R.), and GM26796 (to W. C. M.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

To whom correspondence should be addressed: Dept. of Biochemistry, University of California, Riverside, CA 92521. Tel.: 951-827-4239; Fax: 951-827-4774; E-mail: jolinda.traugh{at}ucr.edu.

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