Dissection of the intraflagellar transport protein machinery in Caenorhabditis elegans neurons

Cilia have diverse roles in motility and sensory reception and their dysfunction contributes to cilia-related diseases. Cilium biogenesis depends upon intraflagellar transport (IFT) motors which deliver ciliary precursors associated with IFT particles to the tip of the axoneme. In Caenorhabditis elegans, this is accomplished by two IFT-motors, kinesin-II and OSM-3 kinesin, both members of the kinesin-2 subfamily, and they cooperate to form two sequential anterograde IFT pathways that build distinct parts of cilia. Kinesin-II and OSM-3 kinesin function redundantly to transport the same IFT particles along doublet microtubules to form the axoneme middle segment, and subsequently, OSM-3 kinesin alone delivers IFT particles along the distal singlet microtubules to stabilize the distal segment. I identified three ciliary proteins that mediate the functional coordination of these motors: the Bardet-Biedl syndrome (BBS) ciliary disease proteins, BBS-7 and BBS-8 stabilize complexes of IFT-particles containing both IFT-motors; and a conserved ciliary protein, DYF-1, is specifically required for OSM-3 kinesin to dock onto and move IFT-particles. I assigned a direct ciliary function to a polycystic kidney disease protein, Qilin, by showing that its C. elegans homolog, DYF-3 participates in IFT and is essential for cilium formation. I and my colleagues addressed how BBS proteins maintain the integrity of IFT-particles, and BBS proteins antagonize a mechanical competition between kinesin-II and OSM-3 kinesin that tends to dissociate IFT-particle A and B subcomplexes. Lastly, I and my colleagues used expression profiling of the C. elegans ciliogenic transcription factor, DAF-19, to provide a comprehensive inventory of the proteins forming the IFT-machinery. The overall predictive value of this screen were assessed by uncovering two previously uncharacterized IFT-associated proteins, namely IFTA-1, which is required for cilia formation, and IFTA-2, a novel member of the RAB GTPase family that is implicated in vesicle trafficking. By assessing the IFT motility properties of multiple IFT components in bbs gene mutants, the C. elegans IFT-machinery is organized into modules, with each contributing to distinct functional aspects of IFT or cilium function.