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J. Biol. Chem., Vol. 283, Issue 1, 222-230, January 4, 2008

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Access Denied: Snf1 Activation Loop Phosphorylation Is Controlled by Availability of the Phosphorylated Threonine 210 to the PP1 Phosphatase^*

Eric M. Rubenstein, Rhonda R. McCartney, Chao Zhang, Kevan M. Shokat, Margaret K. Shirra^¶, Karen M. Arndt^¶, and Martin C. Schmidt¹

From the Department of Molecular Genetics and Biochemistry, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania 15261, the Howard Hughes Medical Institute and Department of Molecular and Cellular Pharmacology, University of California, San Francisco, San Francisco, California 94143, and the ^¶Department of Biological Sciences, University of Pittsburgh, Pittsburgh, Pennsylvania 15260

Phosphorylation of the Saccharomyces cerevisiae Snf1 kinase activation loop is determined by the integration of two reaction rates: the rate of phosphorylation by upstream kinases and the rate of dephosphorylation by Glc7. The activities of the Snf1-activating kinases do not appear to be glucose-regulated, since immune complex kinase assays with each of the three Snf1-activating kinases show similar levels of activity when prepared from cells grown in either high or low glucose. In contrast, the dephosphorylation of the Snf1 activation loop was strongly regulated by glucose. When de novo phosphorylation of Snf1 was inhibited, phosphorylation of the Snf1 activation loop was found to be stable in low glucose but rapidly lost upon the addition of glucose. A greater than 10-fold difference in the rates of Snf1 activation loop dephosphorylation was detected. However, the activity of the Glc7-Reg1 phosphatase may not itself be directly regulated by glucose, since the Glc7-Reg1 enzyme was active in low glucose toward another substrate, the transcription factor Mig1. Glucose-mediated regulation of Snf1 activation loop dephosphorylation is controlled by changes in the ability of the Snf1 activation loop to act as a substrate for Glc7.

Received for publication, September 24, 2007 , and in revised form, November 6, 2007.

^* This work was supported by National Institutes of Health Grants GM46443 (to M. C. S.), GM52593 (to K. M. A.), DK74654 (to K. M. A.), and AI44009 (to K. M. S.) and American Heart Association Predoctoral Fellowship 0615379U (to E. M. R.). 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.

¹ To whom correspondence should be addressed: Dept. of Molecular Genetics and Biochemistry, University of Pittsburgh School of Medicine, Pittsburgh, PA 15261. Tel.: 412-648-9243; E-mail: mcs2{at}pitt.edu.

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