Calmodulin binds to extracellular sites on the plasma membrane of plant cells and elicits a rise in intracellular calcium concentration.
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.
Wang
Qinli
Q
Key Laboratory of Photosynthesis and Molecular Environment Physiology, Institute of Botany, Chinese Academy of Sciences, Beijing 100093, China.
Chen
Bo
B
Liu
Peng
P
Zheng
Maozhong
M
Wang
Yuqing
Y
Cui
Sujuan
S
Sun
Daye
D
Fang
Xiaohong
X
Liu
Chun-Ming
CM
Lucas
William J
WJ
Lin
Jinxing
J
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
D002147:Calmodulin; D010940:Plant Proteins; D002118:Calcium