Novel CIPK1-associated proteins in Arabidopsis contain an evolutionarily conserved C-terminal region that mediates nuclear localization

Environmental stimuli, including light, pathogens, hormones, and abiotic stresses, elicit changes in the cytosolic Ca²⁺ signatures of plant cells. However, little is known about the molecular mechanisms by which plants sense and transmit the specific cytoplasmic Ca²⁺ signal into the nucleus, where gene regulation occurs to respond appropriately to the stress. In this study, we have identified two novel Arabidopsis (Arabidopsis thaliana) proteins specifically associated with Calcineurin B-Like-Interacting Protein Kinase1 (CIPK1), a member of Ser/Thr protein kinases that interact with the calcineurin B-like Ca²⁺-binding proteins. These two proteins contain a very similar C-terminal region (180 amino acids in length, 81% similarity), which is required and sufficient for both interaction with CIPK1 and translocation to the nucleus. Interestingly, the conserved C-terminal region was also found in many proteins from various eukaryotic organisms, including humans. However, none of them have been characterized so far. Taken together, these findings suggest that the two proteins containing the evolutionarily conserved C-terminal region (ECT1 and ECT2) may play a critical role in relaying the cytosolic Ca²⁺ signals to the nucleus, thereby regulating gene expression.