Isolation and Characterization of a Novel Rice Ca2+-regulated Protein Kinase Gene Involved in Responses to Diverse Signals Including Cold, Light, Cytokinins, Sugars and Salts

We have isolated a cold-inducible gene (designated OsCK1) from Oryza sativa by a differential cDNA screening technique. Sequence analysis indicated that the open reading frame of the OsCK1 gene consists of 1350 bp encoding 449 amino acid residues, which is very similar to a family of calcineurin B-like protein (CBL)-interacting protein kinases (CIPKs) or salt overly sensitive 2 (SOS2)-like protein kinases (PKS) in Arabidopsis. CIPKs/PKS are a group of Ser/Thr protein kinases associated with the AtCBL/SOS3-like calcium-binding proteins (SCaBP). OsCK1 actually interacts with AtCBL3 through the C-terminal region in a yeast two-hybrid system, suggesting that OsCK1 is probably a rice orthologue of one of the CIPK/PKS members. Expression of the OsCK1 gene was detected mainly in the shoots and highly inducible by diverse signals such as cold, light, salt, sugar and cytokinins. In addition, calcium increased the OsCK1 transcript level, whereas a calcium ionophore, A23187, partially abolished stimulus-induced expressions. OsCK1 phosphorylated itself and a generic substrate, myelin basic protein, in the preference of Mn2+. Deletion of the C-terminal region from OsCK1 significantly decreased autophosphorylation activity without affecting the ability for substrate phosphorylation. These findings suggest that the CBL/CIPK or SCaBP/PKS signaling pathways recently found in Arabidopsis may also exist in rice and function in cold response in which calcium signal serves as a second messenger.