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J. Biol. Chem., Vol. 280, Issue 36, 31420-31427, September 9, 2005

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Apoplastic Calmodulin Receptor-like Binding Proteins in Suspension-cultured Cells of Arabidopsis thaliana^*

Sujuan Cui, Xiaoqiang Guo, Fang Chang, Yanwei Cui, Ligeng Ma, Ying Sun, and Daye Sun

From the Institute of Molecular Cell Biology, Hebei Normal University, Shijiazhuang, Hebei Province, 050016, China

Calmodulin, a highly conserved protein family that has long been well known as an intracellular calcium sensor, was identified in the culture medium and cell walls of Arabidopsis thaliana suspension-cultured cells by immunoblotting assay. A promotion effect by applying exogenous purified calmodulin and an inhibition effect by the addition of anti-calmodulin anti-serum or calmodulin antagonist to the medium on proliferation of suspension cells were found by monitoring incorporation of [methyl-³H]thymidine into nuclear DNA. Radioligand binding analysis with ³⁵S-labeled calmodulin indicated the presence of specific, reversible, and saturable calmodulin binding sites on the surface of both A. thaliana suspension-cultured cells and its protoplasts; among them at least one is on the surface of Arabidopsis protoplasts, with the K_d 9.2 nM, and two are on the out-surface of Arabidopsis suspension-cultured cells, with K_d values of 47.5 and 830 nM. Chemical crosslinking of ³⁵S-labeled calmodulin to protoplasts revealed 117- and 41-kDa plasma membrane proteins specifically bound to calmodulin, whereas cross-linking with intact suspension-cultured cells verified more calmodulin binding proteins which might be cell wall-associated in addition to membrane-localized. Taking together, our data provide first evidence for the presence of apoplastic calmodulin receptor-like binding proteins on the cell surface of Arabidopsis suspension-cultured cells, which strongly supports our previous idea that apoplastic calmodulin functions as a peptide signal involved in regulation of cell growth and development.

Received for publication, February 4, 2005 , and in revised form, June 24, 2005.

^* This work was supported by National Natural Science Foundation of China Grants 90208004 and 30470889, National Key Basic Research Special Funds of China Grants G19990117 and 2006CB100101, Outstanding Younger Scientist Foundation of China Grant 30025024, Natural Science Foundation of Hebei Province in China Grant C2004000152, and Doctor Foundation of Hebei Normal University Grant L2004B11. 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.

To whom correspondence may be addressed: Institute of Molecular Cell Biology, Hebei Normal University, No. 265 Yuhua Rd., Shijiazhuang 050016, China. Tel. or Fax: 86-311-5820649; E-mail: Yingsun{at}mail.hebtu.edu.ca. To whom correspondence may be addressed: Institute of Molecular Cell Biology, Hebei Normal University, No. 265 Yuhua Rd., Shijiazhuang 050016, China. Tel.: 86-311-85820649; Fax: 86-311-86269144; E-mail: cell{at}mail.hebtu.edu.cn.

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