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Calmodulin Binds to Extracellular Sites on the Plasma Membrane of Plant Cells and Elicits a Rise in Intracellular Calcium Concentration？

作　　者：Wang QL, Chen B, Liu P, Zheng MZ, Wang YQ, Cui SJ, Sun DY, Fang XH, Liu CM, Lucas WJ, Lin JX
影响因子：5.581？
刊物名称：Journal of Biological Chemistry？
出版年份：
卷：284  期：18  页码：12000-12007？

论文摘要：

Calmodulin (CaM) is a highly conserved intracellular calcium sensor. In plants, CaM also appears to be present in the apoplasm, and application of exogenous CaM has been shown to influence a number of physiological functions as a polypeptide signal; however, the existence and localization of its corresponding apoplasmic binding sites remain controversial. To identify the site(s) of action, a CaM-conjugated quantum dot (QD) system was employed for single molecule level detection at the surface of plant cells. Using this approach, we show that QD-CaM binds selectively to sites on the outer surface of the plasma membrane, which was further confirmed by high resolution transmission electron microscopy. Measurements of Ca2+ fluxes across the plasma membrane, using ion-selective microelectrodes, demonstrated that exogenous CaM induces a net influx into protoplasts. Consistent with these flux studies, calcium-green-dextran and FRET experiments confirmed that applied CaM/QD-CaM elicited an increase in cytoplasmic Ca2+ levels. These results support the hypothesis that apoplasmic CaM can act as a signaling agent. These findings are discussed in terms of CaM acting as an apoplasmic peptide ligand to mediate transmembrane signaling in the plant kingdom. 
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