Gene knockouts reveal separate functions for two cytoplasmic dyneins in Tetrahymena thermophila

In many organisms, there are multiple isoforms of cytoplasmic dynein heavy chains, and division of labor among the isoforms would provide a mechanism to regulate dynein function. The targeted disruption of somatic genes in Te trahymena thermophila presents the opportunity to determine the contributio ns of individual dynein isoforms in a single cell that expresses multiple d ynein heavy chain genes. Substantial portions of two Tetrahymena cytoplasmi c dynein heavy chain genes were cloned, and their motor domains were sequen ced. Tetrahymena DYH1 encodes the ubiquitous cytoplasmic dynein Dyh1, and D YH2 encodes a second cytoplasmic dynein isoform, Dyh2. The disruption of DY H1, but not DYH2, resulted in cells with two detectable defects: 1) phagocy tic activity was inhibited, and 2) the cells failed to distribute their chr omosomes correctly during micronuclear mitosis. In contrast, the disruption of DYH2 resulted in a loss of regulation of cell size and cell shape and i n the apparent inability of the cells to repair their cortical cytoskeleton s. We conclude that the two dyneins perform separate tasks in Tetrahymena.