Calmodulin 2 Functions as an RNA Chaperone in Prokaryotic Cells

Plants express many calmodulin (CaM) isoforms. These proteins regulate the growth, development and environmental stress responses of plants by modulating targets. Herein, the Arabidopsis CaM2 isoform was found to be crucial for cold adaptation in prokaryotic cells, similar to the Escherichia coli cold shock protein CspA. Expressing CaM2 or CspA in the cold-sensitive E. coli BX04 mutant complemented the cold-sensitive phenotype under cold stress, but expression of CaM1, CaM7 or CML8 (CaM8) did not. Similar to RNA chaperones such as CspA, CaM2 strongly interacted with nucleic acids and its nucleic acid-binding capacity was much higher than that of CaM7, despite there being only a single amino acid difference between these two isoforms. Microscopic observation of CaM2-GFP revealed that CaM2 plays roles in both the nucleus and cytosol where RNA molecules are abundant. These results suggest that CaM2 can positively modulate cold stress responses by interacting with nucleic acid targets. Furthermore, CaM2 has both nucleic acid targets, similar to CaM7, and protein targets such as CAMTA3.