The Tctex1/Tctex2 and LC7/Roadblock families of dynein light chains are required for wild-type assembly, stability, and motor function within the Chlamydomonas flagellum

Chlamydomonas is a unicellular green algae that uses its two flagella to swim through its environment. The flagella are composed of a microtubule-based axoneme that provides a scaffold for greater than 450 proteins that collectively regulate motility. The ultimate force-producing components associated with the axoneme are dynein molecular motors. Dyneins are minus-end directed microtubule-based motors that perform an array of functions within the cytoplasm of eukaryotic cells, as well as within cilia and flagella. Cytoplasmic dyneins are involved in such processes as nuclear envelope breakdown, mitosis, maintenance of the golgi apparatus, intraflagellar transport, and the movement of an array of cargoes towards the nucleus, including membrane-bound vesicles, proteins, RNAs, and even viral components. Flagellar dyneins assemble along the length of the axoneme and generate the force necessary for flagellar bending and movement. Cytoplasmic, axonemal outer arm and inner arm I1 dyneins are composed of two or three heavy chains, two or three intermediate chains, and a series of light chains (LCs) that belong to three protein families: LC8, Tctex1/Tctex2, and LC7/Roadblock. Here I demonstrate that members of the Tctex1/Tctex2 family are differentially expressed in a tissue-specific and developmentally-regulated manner, suggesting that within cells, cargo binding specificity may be modulated by LC content. In addition, a novel member of this family associated with axonemal inner arm I1, termed Tctex2b, is required for the stability of this dynein and in its absence results in reduced motor function as measured by three different functional assays, both in vivo and in vitro. Two members of the LC7/Roadblock family associate with two dynein systems in the flagellum: the outer dynein arm and inner arm I1. In addition, one of these proteins is required for wild-type assembly and stability of both dyneins. The second interacts with regulatory components of each motor and identifies the first link between the outer arm motor unit and the trimeric docking complex which is required for the correct placement of the outer arm in the axoneme. Together, these data suggest that dynein LCs are integral components required for assembly, stability, and proper motor function.