Numerous studies have indicated that each of the seven projections associated with the central pair of microtubules plays a distinct role in regulating eukaryotic ciliary / flagellar motility. Mutants which lack specific projections have distinct motility phenotypes. For example, Chlamydomonas pf6 mutants lack the C1a projection and have twitchy, non-beating flagella. The C1a projection is a complex of proteins including PF6, C1a-86, C1a-34, C1a-32, C1a-18 and calmodulin. To define functional domains within PF6 and to potentially assign functions to specific C1a components, we generated deletion constructs of the PF6 gene and tested for their ability to assemble and rescue motility upon transformation of mutant pf6 cells. Our results demonstrate that domains near the carboxyl-terminus of PF6 are essential for motility and/or assembly of the projection. The amino terminal half of PF6 is not required for C1a assembly; however, this region is important for stability of the C1a-34, C1a-32, and C1a-18 sub-complex and wild-type beat frequency. Analysis of double mutants lacking the amino terminus of PF6 and outer dynein arms reveal that C1a may play a role in modulating both inner and outer dynein arm activity.

Kinesin-like calmodulin-binding protein, KCBP, is a novel member of the C-kinesin superfamily first discovered in flowering plants. This minus-end-directed kinesin exhibits Ca2+-calmodulin-sensitive motor activity in vitro and has been implicated in trichome morphogenesis and cell division. A homologue of KCBP is also found in the unicellular, biflagellate green alga Chlamydomonas reinhardtii (CrKCBP). Unlike plant cells, Chlamydomonas cells do not form trichomes and do not assemble a phragmoplast before cell division. To test whether CrKCBP is involved in additional microtubule-based processes not observed in plants, we generated antibodies against the putative calmodulin-binding domain and used these antibodies in biochemical and localization studies. In interphase cells CrKCBP primarily localizes near the base of the flagella, although surprisingly, a small fraction also localizes along the length of the flagella. CrKCBP is bound to isolated axonemes in an ATP-dependent fashion and is not a component of the dynein arms, radial spokes or central apparatus. During mitosis, CrKCBP appears concentrated at the centrosomes during prophase and metaphase. However, during telophase and cytokinesis CrKCBP co-localizes with the microtubules associated with the phycoplast. These studies implicate CrKCBP in flagellar functions as well as cell division.

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