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CELL BIOLOGY AND SIGNAL TRANSDUCTION

The Arabidopsis Calcium Sensor Calcineurin B-Like 3 Inhibits the 5'-Methylthioadenosine Nucleosidase in a Calcium-Dependent Manner^{1,[C],[W],[OA]}

Department of Molecular Biology, Sejong University, Seoul 143–747, Korea (S.-I.O., J.P., S.Y., Y.K., S.P., M.R., M.J.N., K.-N.K.); and School of Life Sciences and Biotechnology, Korea University, Seoul 136–701, Korea (S.-I.O., S.H.O., J.-K.K., J.-S.S.)

Calcineurin B-like (CBL) proteins represent a unique family of calcium sensors in plant cells. Sensing the calcium signals elicited by a variety of abiotic stresses, CBLs transmit the information to a group of serine/threonine protein kinases (CBL-interacting protein kinases [CIPKs]), which are currently known as the sole targets of the CBL family. Here, we report that the CBL3 member of this family has a novel interaction partner in addition to the CIPK proteins. Extensive yeast two-hybrid screenings with CBL3 as bait identified an interesting Arabidopsis (Arabidopsis thaliana) cDNA clone (named AtMTAN, for 5'-methylthioadenosine nucleosidase), which encodes a polypeptide similar to EcMTAN from Escherichia coli. Deletion analyses showed that CBL3 utilizes the different structural modules to interact with its distinct target proteins, CIPKs and AtMTAN. In vitro and in vivo analyses verified that CBL3 and AtMTAN physically associate only in the presence of Ca²⁺. In addition, we empirically demonstrated that the AtMTAN protein indeed possesses the MTAN activity, which can be inhibited specifically by Ca²⁺-bound CBL3. Overall, these findings suggest that the CBL family members can relay the calcium signals in more diverse ways than previously thought. We also discuss a possible mechanism by which the CBL3-mediated calcium signaling regulates the biosynthesis of ethylene and polyamines, which are involved in plant growth and development as well as various stress responses.

² These authors contributed equally to the article.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Kyung-Nam Kim (knkim{at}sejong.ac.kr).

^[C] Some figures in this article are displayed in color online but in black and white in the print edition.

^[W] The online version of this article contains Web-only data.

^[OA] Open Access articles can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.108.130419

^* Corresponding author; e-mail knkim{at}sejong.ac.kr.

Received September 26, 2008; accepted October 16, 2008; published October 22, 2008.