tArp Is an Actin-Related Protein Required for Ciliary Function That Is Conserved in Ciliates and Related Eukaryotic Groups.

Cells exhibit a wide range of behaviors due to their diverse molecular systems, which have been established through the molecular evolution of individual species. Protein families generated through gene duplications may be involved in this process. The actin cytoskeleton is no exception. The majority of eukaryotes express not only actin, but also several types of actin-related proteins (Arps). The ciliate Tetrahymena thermophila possesses a species-specific Arp, named tArp, which localizes to cilia. However, the evolutionary origin of the gene and its detailed cellular functions remain unclear. In the present study, we performed a comprehensive molecular phylogenetic analysis of actin and Arps across eukaryotes, with a focus on ciliates and their close relatives, and demonstrated that tArp is widely conserved in SAR, one of the largest eukaryotic supergroups consisting of Stramenopiles, Alveolata, and Rhizaria. Furthermore, the gene disruption of tARP reduced swimming velocity due to defects in ciliary beat frequency and waveforms. Therefore, we propose that tArp is a member of the novel Arp family that specifically contributes to efficient ciliary motility.