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J. Biol. Chem., Vol. 282, Issue 8, 5249-5255, February 23, 2007

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A Proteasome-regulated Glycogen Synthase Kinase-3 Modulates Disease Response in Plants^*

From the Max F. Perutz Laboratories, University of Vienna, Vienna Biocenter, Dr. Bohrgasse 9, A-1030 Vienna, Austria

Glycogen synthase kinase-3 (GSK-3) is a key player in various important signaling pathways in animals. The activity of GSK-3 is known to be modulated by protein phosphorylation and differential complex formation. However, little information is available regarding the function and regulation of plant GSK-3/shaggy-like kinases (GSKs). Analysis of the in vivo kinase activity of MsK1, a GSK from Medicago sativa, revealed that MsK1 is active in healthy plants and that MsK1 activity is down-regulated by the elicitor cellulase in a time- and dose-dependent manner. Surprisingly, cellulase treatment triggered the degradation of the MsK1 protein in a proteasome-dependent manner suggesting a novel mechanism of GSK-3 regulation. Inhibition of MsK1 kinase activity and degradation of the protein were two successive processes that could be uncoupled. In a transgenic approach, stimulus-induced inhibition of MsK1 was impeded by constant replenishment of MsK1 by a strong constitutive promoter. MsK1 overexpressing plants exhibited enhanced disease susceptibility to the virulent bacterial pathogen Pseudomonas syringae. MAP kinase activation in response to pathogen infection was compromised in plants with elevated MsK1 levels. These data strongly suggest that tight regulation of the plant GSK-3, MsK1, may be important for innate immunity to limit the severity of virulent bacterial infection.

Received for publication, October 30, 2006 , and in revised form, December 18, 2006.

^* This work was supported by the Austrian Science Fund. 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.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Figs. S1–S3.

¹ Current address: Dept. of Biological and Environmental Sciences, University of Helsinki, PL 65 Viikinkaari 1, Fi-00014 Finland.

² Current address: Gregor Mendel Institute of Molecular Plant Biology, Austrian Academy of Sciences, Vienna Biocenter, Dr. Bohrgasse 3, A-1030 Vienna, Austria.

³ To whom correspondence should be addressed. Current address: Gregor Mendel Institute of Molecular Plant Biology, Austrian Academy of Sciences, Vienna Biocenter, Dr. Bohrgasse 3, A-1030 Vienna, Austria. Tel.: 43-1-79044-9850; Fax: 43-1-79044-9001; E-mail: claudia.jonak{at}gmi.oeaw.ac.at.

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