Calcium plays a crucial role as a secondary messenger in all aspects of plant growth, development and survival. Calcium dependent protein kinases (CDPKs) are the major calcium decoders, which couple the changes in calcium level to an appropriate physiological response. The mechanism by which calcium regulates CDPK protein is not well understood. In this study, we investigated the interactions of Ca2+ ions with the CDPK1 isoform of Cicer arietinum (CaCDPK1) using a combination of biophysical tools. CaCDPK1 has four different EF hands as predicted by protein sequence analysis. The fluorescence emission spectrum of CaCDPK1 showed quenching with a 5nm red shift upon addition of calcium, indicating conformational changes in the tertiary structure. The plot of changes in intensity against calcium concentrations showed a biphasic curve with binding constants of 1.29μM and 120μM indicating two kinds of binding sites. Isothermal calorimetric (ITC) titration with CaCl2 also showed a biphasic curve with two binding constants of 0.027μM and 1.7μM. Circular dichroism (CD) spectra showed two prominent peaks at 208 and 222nm indicating that CaCDPK1 is a α-helical rich protein. Calcium binding further increased the α-helical content of CaCDPK1 from 75 to 81%. Addition of calcium to CaCDPK1 also increased fluorescence of 8-anilinonaphthalene-1-sulfonic acid (ANS) indicating exposure of hydrophobic surfaces. Thus, on the whole this study provides evidence for calcium induced conformational changes, exposure of hydrophobic surfaces and heterogeneity of EF hands in CaCDPK1.
Bibliographical noteFunding Information:
We thank Prof. S.K. Podder for his valuable suggestions throughout the work, Prof. Utpal Tatu for helping in MALDI data analysis and Prof. R. Vardharajan for ITC experiment and data analysis. This work was supported by a grant from the Department of Science and Technology, Government of India .
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- Abiotic stress
- Calcium dependent protein kinases
- Conformational change
- Isothermal titration calorimetry