Metal binding affinity and structural properties of calmodulin‐like protein 14 from Arabidopsis thaliana

In addition to the well‐known Ca²⁺ sensor calmodulin, plants possess many calmodulin‐like proteins (CMLs) that are predicted to have specific roles in the cell. Herein, we described the biochemical and biophysical characterization of recombinant Arabidopsis thaliana CML14. We applied isothermal titration calorimetry to analyze the energetics of Ca²⁺ and Mg²⁺ binding to CML14, and nuclear magnetic resonance spectroscopy, together with intrinsic and ANS‐based fluorescence, to evaluate the structural effects of metal binding and metal‐induced conformational changes. Furthermore, differential scanning calorimetry and limited proteolysis were used to characterize protein thermal and local stability. Our data demonstrate that CML14 binds one Ca²⁺ ion with micromolar affinity (K_d ～ 12 µM) and the presence of 10 mM Mg²⁺ decreases the Ca²⁺ affinity by ～5‐fold. Although binding of Ca²⁺ to CML14 increases protein stability, it does not result in a more hydrophobic protein surface and does not induce the large conformational rearrangement typical of Ca²⁺ sensors, but causes only localized structural changes in the unique functional EF‐hand. Our data, together with a molecular modelling prediction, provide interesting insights into the biochemical properties of Arabidopsis CML14 and may be useful to direct additional studies aimed at understanding its physiological role.