Structure, Function, and Phylogenetic Consideration of Calaxin

Sperm chemotaxis is widely seen both in animals and plants and is considered to be necessary for efficient success of fertilization. Although intracellular Ca2+ is known to play important roles in sperm chemotaxis, the molecular mechanism causing the change in flagellar waveform that drives sperm directed toward the egg is still unclear. Several Ca2+-binding proteins, especially calmodulin, have been discussed as an important regulator of the molecular motor dynein in flagellar motility during chemotactic movement of sperm. However, there has been no experimental evidence to show the binding of calmodulin to dyneins. Recently, we found a novel Ca2+-binding protein, termed calaxin, in the axonemes of sperm flagella in the ascidian Ciona intestinalis. Calaxin binds to the outer arm dynein in a Ca2+-dependent manner and suppresses its activity to slide microtubules at high Ca2+ concentration. Inhibition of calaxin results in significant loss of chemotactic behavior of sperm, indicating that calaxin is essential for sperm chemotaxis. In this chapter, we describe the finding history, molecular nature, and the roles in sperm chemotaxis of calaxin, as well as its phylogenetic consideration.