DYF-5 regulates intraflagellar transport by affecting train turnaround

Intraflagellar transport (IFT) coordinates the transport of cargo in cilia and is essential for ciliary function. CILK1 has been identified as a key regulator of IFT. The mechanism by which it acts has, however, remained unclear. In this study, we use fluorescence imaging and single-molecule tracking in the phasmid cilia of live C. elegans to study the effect of the CILK1 homolog DYF-5 on the dynamics of the IFT. We show that in the absence of DYF-5, IFT components accumulate at the ciliary tip. kinesin-II is no longer restricted to the proximal segment of the cilium but is present all throughout the cilium, while its velocity is different from that of OSM-3. The frequency of IFT trains is reduced and in particular retrograde trains were rarely observed. In the absence of DYF-5, retrograde transport is vastly reduced, resulting in the accumulation of IFT components at the tip and depletion at the base. The latter results in impeded anterograde train assembly, resulting in fewer trains with irregular composition. Our results show that DYF-5 plays a key role in regulating the turnarounds of IFT trains at the ciliary tip.