The internal structure of cilia and flagella, the axoneme, has a peculiar arrangement of nine outer-doublet microtubules surrounding a pair of single microtubules. The nine-fold symmetry originates from the structure of the basal body, a close relative of the centriole present in all animal cells. The basal body and the centriole are replicated once per cell cycle by an unknown mechanism; this mechanism is considered one of the greatest mysteries in current cell biology.
We started a project a few years ago aiming at the elucidation of the mechanism that regulates flagellar assembly. Through isolation and analysis of mutants lacking flagella, we have identified a gene involved in the maturation of the basal body. Also, we have isolated a cDNA clone of a protein apparently important in holding the nine outer-doublets together. A homolog of this protein is present in mammalian genomes.
The high-speed beating of cilia and flagella is produced by the sliding movements between microtubules and multiple motor proteins (dyneins). However, it is not understood how dynein-microtubule interaction is controlled spatially and temporally so as to produce regular bending waves. The axoneme is equipped with more than ten different species of dyneins. We have isolated and analyzed various mutants lacking specific dyneins and thereby shown that different dyneins differ in their functional properties. Importantly, our results indicate that axonemal beating requires the presence of different types of dyneins. How these dyneins cooperate in flagellar functioning is an intriguing future problem. In more biophysical studies, we have been studying in vitro motility produced by brain microtubules and isolated axonemal dyneins, functional reconstitution of isolated dyneins onto axonemes lacking them, and nanometer-scale oscillatory movements within the axoneme. Currently our research effort is focused on the mechanism by which dynein activity is regulated.