Background information. Spermatozoa show several changes in flagellar waveform, such as upon fertilization. Ca2+ has been shown to play critical roles in modulating the waveforms of sperm flagella. However, a Ca2+-binding protein in sperm flagella that regulates axonemal dyneins has not been fully characterized.
Results. We identified a novel neuronal calcium sensor family protein, named calaxin (Ca2+-binding axonemal protein), in sperm flagella of the ascidian Ciona intestinalis. Calaxin has three EF-hand Ca2+-binding motifs, and its orthologues are present in metazoan species, but not in yeast, green algae or plant. Immunolocalization revealed that calaxin is localized near the outer arm of the sperm flagellar axonemes. Moreover, it is distributed in adult tissues bearing epithelial cilia. An in vitro binding experiment indicated that calaxin binds to outer arm dynein. A cross-linking experiment showed that calaxin binds to β-tubulin in situ. Overlay experiments further indicated that calaxin binds the β-dynein heavy chain of outer arm dynein in the presence of Ca2+.
Conclusions. These results suggest that calaxin is a potential Ca2+-dependent modulator of outer arm dynein in metazoan cilia and flagella.