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Sperm chemotaxis toward a chemoattractant is very important for the success of fertilization. Calaxin, a member of the neuronal calcium sensor protein family, directly acts on outer-arm dynein and regulates specific flagellar movement during sperm chemotaxis of ascidian,  Ciona intestinalis . Here, we present the crystal structures of calaxin both in the open and closed states upon Ca 2+  and Mg 2+  binding. The crystal structures revealed that three of the four EF-hands of a calaxin molecule bound Ca 2+  ions and that EF2 and EF3 played a critical role in the conformational transition between the open and closed states. The rotation of α7 and α8 helices induces a significant conformational change of a part of the α10 helix into the loop. The structural differences between the Ca 2+ - and Mg 2+ -bound forms indicates that EF3 in the closed state has a lower affinity for Mg 2+ , suggesting that calaxin tends to adopt the open state in Mg 2+ -bound form. SAXS data supports that Ca 2+ -binding causes the structural transition toward the closed state. The changes in the structural transition of the C-terminal domain may be required to bind outer-arm dynein. These results provide a novel mechanism for recognizing a target protein using a calcium sensor protein.

Crystal structure of a Ca2+-dependent regulator of flagellar motility reveals the open-closed structural transition