title: Calmodulin binds to extracellular sites on the plasma membrane of plant cells and elicits a rise in intracellular calcium concentration.
abstract: 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 Ca(2+) 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 Ca(2+) 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.
authors.affiliation: Key Laboratory of Photosynthesis and Molecular Environment Physiology, Institute of Botany, Chinese Academy of Sciences, Beijing 100093, China.
authors.forename: Qinli
authors.initials: Q
authors.lastname: Wang
authors.forename: Bo
authors.initials: B
authors.lastname: Chen
authors.forename: Peng
authors.initials: P
authors.lastname: Liu
authors.forename: Maozhong
authors.initials: M
authors.lastname: Zheng
authors.forename: Yuqing
authors.initials: Y
authors.lastname: Wang
authors.forename: Sujuan
authors.initials: S
authors.lastname: Cui
authors.forename: Daye
authors.initials: D
authors.lastname: Sun
authors.forename: Xiaohong
authors.initials: X
authors.lastname: Fang
authors.forename: Chun-Ming
authors.initials: CM
authors.lastname: Liu
authors.forename: William J
authors.initials: WJ
authors.lastname: Lucas
authors.forename: Jinxing
authors.initials: J
authors.lastname: Lin
mesh_terms: D002118:Calcium / Q000378:metabolism*; D002147:Calmodulin / Q000378:metabolism* / Q000494:pharmacology; D002462:Cell Membrane / Q000378:metabolism* / Q000648:ultrastructure; D027762:Lilium / Q000378:metabolism* / Q000648:ultrastructure; D010940:Plant Proteins / Q000378:metabolism* / Q000494:pharmacology; D011523:Protoplasts / Q000378:metabolism / Q000648:ultrastructure; D045663:Quantum Dots; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology*; D014026:Nicotiana / Q000378:metabolism* / Q000648:ultrastructure
chemical_list: D002147:Calmodulin; D010940:Plant Proteins; D002118:Calcium