Dynamics of the intraflagellar transport machinery at the ciliary tip

Cilia (and flagella) are hair-like organelles that extend from the plasma membrane of nearly all mammalian cells for sensory and motile functions. The assembly and maintenance of all cilia require intraflagellar transport (IFT) of its building blocks from the cell body to its tip. These cargoes bind to IFT complexes that are organized into larger IFT trains as they enter the cilium, transported to ciliary tip along axonemal microtubules by the kinesin-II motor. Once the trains reach the tip, they are reorganized and transported back to the ciliary base by dynein-1b. Due to the traffic jam of multiple trains at the ciliary tip, how IFT trains are remodeled in these turnaround zones cannot be determined by conventional imaging. Using Photogate imaging, we visualized the full range of movement of single IFT trains and motors in Chlamydomonas flagella. This revealed that at the tip of the flagellum, IFT trains split apart and mix with each other to assemble into new trains, which move back to the base. Kinesin-II carries dynein to the tip as inactive cargo, detaches from IFT trains at the tip and diffuses in the flagellum. As the flagellum grows longer, diffusion delays return of kinesin-II to the basal body, depleting kinesin-II available for transport of new material to the tip. Our results suggest that dissociation of kinesin-II from IFT trains serves as a negative feedback mechanism that facilitates flagellar length control in Chlamydomonas.