Twenty-five dyneins in Tetrahymena: A re-examination of the multidynein hypothesis.

Dyneins are responsible for essential movements in eukaryotic cells. The motor activity of each dynein complex resides in its complement of heavy chains. In the present study, we examined 136 heavy chain sequences from the completed genomes of 11 diverse model organisms, including examples from Viridiplantae, Excavata, Chromalveolata, and Metazoa. In many cases, we discovered dynein heavy chains previously not identified. For example, Tetrahymena expresses a total of 25 DYH genes rather than the previously identified 14. The Tetrahymena DYH genes are nonaxonemal DYH1 and DYH2; axonemal outer arm alpha, beta, and gamma; axonemal two-headed inner arm 1alpha and 1beta; and 18 single-headed inner arm heavy chains. The heavy chains divide into nine classes; six of these are highly conserved in sequence and number of isoforms in a given organism. The other three are single-headed inner arm dyneins, whose numbers vary significantly in different organisms. These findings lead to two conclusions. One, the last common ancestor of all eukaryotes expressed nine different dynein heavy chains. Two, subsequent to the divergences leading to different organisms, additional dynein heavy chains emerged. These newer dyneins are not well conserved across species and the variation may reflect different motility requirements in different organisms. Together, these results suggest that each of the nine classes of dyneins is functionally distinct, but members within some of the classes are not specialized. An understanding of the relationships among the various dynein heavy chains is important when deducing functions across species.