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J. Biol. Chem., Vol. 279, Issue 41, 42552-42559, October 8, 2004

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Calcium/Calmodulin Up-regulates a Cytoplasmic Receptor-like Kinase in Plants^*

Tianbao Yang, Shubho Chaudhuri, Lihua Yang, Yanping Chen, and B. W. Poovaiah

From the Center for Integrated Biotechnology and Department of Horticulture, Washington State University, Pullman, Washington 99164-6414

Calcium/calmodulin-dependent kinases play an important role in protein phosphorylation in eukaryotes. However, not much is known about calcium/calmodulin-dependent protein phosphorylation and its role in signal transduction in plants. By using a protein-protein interaction-based approach, we have isolated a novel plant-specific calmodulin-binding receptor-like cytoplasmic kinase (CRCK1) from Arabidopsis thaliana, as well as its ortholog from Medicago sativa (alfalfa). CRCK1 does not show high homology to calcium/calmodulin-dependent protein kinases in animals. In contrast, it shows high homology in the kinase domain to serine/threonine receptor-like kinases in plants. However, it contains neither a transmembrane domain nor an extracellular domain. Calmodulin binds to CRCK1 in a calcium-dependent manner with an affinity of 20.5 nM. The calmodulin-binding site in CRCK1 is located in amino acids 160–183, which overlap subdomain II of the kinase domain. CRCK1 undergoes autophosphorylation in the presence of Mg²⁺ at the threonine residue(s). The K_m and V_max values of CRCK1 for ATP are 1 µM and 33.6 pmol/mg/min, respectively. Calcium/calmodulin stimulates the kinase activity of CRCK1, which increases the V_max of CRCK1 9-fold. The expression of CRCK1 is increased in response to stresses such as cold and salt and stress molecules such as abscisic acid and hydrogen peroxide. These results indicate the presence of a calcium/calmodulin-regulated receptor-like cytoplasmic kinase in plants. Furthermore, these results also suggest that calcium/calmodulin-regulated protein phosphorylation involving CRCK1 plays a role in stress signal transduction in plants.

Received for publication, March 12, 2004 , and in revised form, July 7, 2004.

^* This work was supported by National Science Foundation Grant MCB-0082256 and the United States Department of Agriculture Grant 2002-00741. 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 nucleotide sequence(s) reported in this paper has been submitted to the GenBank^TM/EBI Data Bank with accession number(s) AY568379 and AY563141.

To whom correspondence should be addressed: Center for Integrated Biotechnology and Dept. of Horticulture, Washington State University, Pullman, WA 99164-6414. Tel.: 509-335-2487; Fax: 509-335-8690; E-mail: poovaiah{at}wsu.edu.

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