Biophysical characterization of calmodulin and calmodulin-like proteins from rice, Oryza sativa L.

Calmodulin (CaM) transduces the increase in cytosolic Ca2+ concentrations by binding to and altering the activities of target proteins, thereby affecting the physiological responses to the vast array of stimuli. Here, we examined the purified recombinant proteins encoded by three Cam and eight Cam-like (CML) genes from rice. With the exception of one OsCML, all recombinant proteins could be purified by Ca2+-dependent hydrophobic chromatography and exhibited an electrophoretic mobility shift when incubated with Ca2+. The three CaMs all bound CaM kinase II peptide, but none of the eight CMLs did, suggesting a possible differential target binding between the CaM and CML proteins. In addition, their conformational changes upon Ca2+-binding were evaluated by circular dichroism spectroscopy and fluorescence spectroscopy using 8-Anilino-1-naphthalene-sulfonic acid. Taken together, OsCMLs were found exhibiting a spectrum of both structural and functional characteristics that ranged from typical to atypical of CaMs. From structural comparison, the OsCMLs have overall main-chain conformation nearly identical to OsCaMs, but with distinct distribution of some charged and hydrophobic amino acids on their target-binding site. These results suggest that genetic polymorphism has promoted the functional diversity of the OsCML family, whose members possess modes of actions probably different from, though maybe overlapping with, those of OsCaMs.