Cloning, localization, and axonemal function of tetrahymena centrin

Centrin, an EF hand Ca²⁺ binding protein, has been cloned in Tetrahymena thermophila. It is a 167 amino acid protein of 19.4 kDa with a unique N-terminal region, coded by a single gene containing an 85-base pair intron. It has > 80% homology to other centrins and high homology to Tetrahymena EF hand proteins calmodulin, TCBP23, and TCBP25. Specific cellular localizations of the closely related Tetrahymena EF hand proteins are different from centrin. Centrin is localized to basal bodies, cortical fibers in oral apparatus and ciliary rootlets, the apical filament ring and to inner arm (14S) dynein (IAD) along the ciliary axoneme. The function of centrin in Ca²⁺ control of IAD activity was explored using in vitro microtubule (MT) motility assays. Ca²⁺ or the Ca²⁺-mimicking peptide CALP1, which binds EF hand proteins in the absence of Ca²⁺, increased MT sliding velocity. Antibodies to centrin abrogated this increase. This is the first demonstration of a specific centrin function associated with axonemal dynein. It suggests that centrin is a key regulatory protein for Tetrahymena axonemal Ca²⁺ responses, including ciliary reversal or chemotaxis.