Calcium sensors of ciliary outer arm dynein: functions and phylogenetic considerations for eukaryotic evolution.

The motility of eukaryotic cilia and flagella is modulated in response to several extracellular stimuli. Ca(2+) is the most critical intracellular factor for these changes in motility, directly acting on the axonemes and altering flagellar asymmetry. Calaxin is an opisthokont-specific neuronal calcium sensor protein first described in the sperm of the ascidian Ciona intestinalis. It binds to a heavy chain of two-headed outer arm dynein in a Ca(2+)-dependent manner and regulates 'asymmetric' wave propagation at high concentrations of Ca(2+). A Ca(2+)-binding subunit of outer arm dynein in Chlamydomonas reinhardtii, the light chain 4 (LC4), which is a Ca(2+)-sensor phylogenetically different from calaxin, shows Ca(2+)-dependent binding to a heavy chain of three-headed outer arm dynein. However, LC4 appears to participate in 'symmetric' wave propagation at high concentrations of Ca(2+). LC4-type dynein light chain is present in bikonts, except for some subclasses of the Excavata. Thus, flagellar asymmetry-symmetry conversion in response to Ca(2+) concentration represents a 'mirror image' relationship between Ciona and Chlamydomonas. Phylogenetic analyses indicate the duplication, divergence, and loss of heavy chain and Ca(2+)-sensors of outer arm dynein among excavate species. These features imply a divergence point with respect to Ca(2+)-dependent regulation of outer arm dynein in cilia and flagella during the evolution of eukaryotic supergroups.