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J. Biol. Chem., Vol. 277, Issue 22, 19304-19314, May 31, 2002

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Mlo, a Modulator of Plant Defense and Cell Death, Is a Novel Calmodulin-binding Protein
ISOLATION AND CHARACTERIZATION OF A RICE Mlo HOMOLOGUE^*

Min Chul Kim, Sang Hyoung Lee, Jong Kyong Kim, Hyun Jin Chun, Man Soo Choi, Woo Sik Chung, Byeong Cheol Moon, Chang Ho Kang, Chan Young Park, Jae Hyuk Yoo, Yun Hwan Kang, Seong Cheol Koo, Yoon Duck Koo, Jae Cheol Jung, Sun Tae Kim, Paul Schulze-Lefert^§, Sang Yeol Lee, and Moo Je Cho^¶

From the  Division of Applied Life Science, Plant Molecular Biology and Biotechnology Research Center, Gyeongsang National University, Chinju 660-701, Korea and ^§ Max-Planck-Institut für Züchtungsforschung, Carl-von-Linné-Weg 10, D-50829 Köln, Germany

Transient influx of Ca²⁺ constitutes an early event in the signaling cascades that trigger plant defense responses. However, the downstream components of defense-associated Ca²⁺ signaling are largely unknown. Because Ca²⁺ signals are mediated by Ca²⁺-binding proteins, including calmodulin (CaM), identification and characterization of CaM-binding proteins elicited by pathogens should provide insights into the mechanism by which Ca²⁺ regulates defense responses. In this study, we isolated a gene encoding rice Mlo (Oryza sativa Mlo; OsMlo) using a protein-protein interaction-based screening of a cDNA expression library constructed from pathogen-elicited rice suspension cells. OsMlo has a molecular mass of 62 kDa and shares 65% sequence identity and scaffold topology with barley Mlo, a heptahelical transmembrane protein known to function as a negative regulator of broad spectrum disease resistance and leaf cell death. By using gel overlay assays, we showed that OsMlo produced in Escherichia coli binds to soybean CaM isoform-1 (SCaM-1) in a Ca²⁺-dependent manner. We located a 20-amino acid CaM-binding domain (CaMBD) in the OsMlo C-terminal cytoplasmic tail that is necessary and sufficient for Ca²⁺-dependent CaM complex formation. Specific binding of the conserved CaMBD to CaM was corroborated by site-directed mutagenesis, a gel mobility shift assay, and a competition assay with a Ca²⁺/CaM-dependent enzyme. Expression of OsMlo was strongly induced by a fungal pathogen and by plant defense signaling molecules. We propose that binding of Ca²⁺-loaded CaM to the C-terminal tail may be a common feature of Mlo proteins.

The nucleotide sequence(s) reported in this paper has been submitted to the GenBank^TM/EBI Data Bank with accession number(s) AF388195.

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