The Crystal Structure of Plant-Specific Calcium-Binding Protein AtCBL2 in Complex with the Regulatory Domain of AtCIPK14

Calcium signals mediate a multitude of plant responses to external stimuli. Calcineurin B-like (CBL) proteins and their target kinases, CBL-interacting protein kinases (CIPKs), represent important relays in plant calcium signaling. CBL interacts with CIPK through a conserved motif (NAF/FISL motif) within the C-terminal regulatory domain. To better understand the functional role of the CBL–CIPK system, we determined the crystal structure of AtCBL2 in complex with the regulatory domain of AtCIPK14 at 1.2 Å resolution. The NAF/FISL motif is inserted into a hydrophobic crevice within AtCBL2, accompanied by a large displacement of the helices and loop on the opposite side of the NAF/FISL motif from the C-terminal region, which shields the hydrophobic crevice in free form. Ca2+ are coordinated within four EF hands in AtCBL2 in bound form. This calcium coordination pattern differs from that in the structure of the SOS3–SOS2 complex previously reported. Structural comparison of the two structures shows that the recognition of CBL by CIPK is performed in a similar manner, but inherent interactions confer binding affinity and specificity.