Intraflagellar transport trains can turn around without the ciliary tip complex

Cilia and flagella are microtubule doublet based organelles found across the eukaryotic tree of life. Their very high aspect ratio and crowded interior are unfavourable to diffusive transport for their assembly and maintenance. Instead, a highly dynamic system of intraflagellar transport (IFT) trains moves rapidly up and down the cilium. However, the mechanism of how these trains turn around upon reaching the ciliary tip has remained elusive. It has been hypothesized that there exists a dedicated calcium-dependent protein-based machinery at the ciliary tip to mediate this conversion. In this work, we use physical and chemical methods to manipulate IFT in the cilia of the unicellular green alga Chlamydomonas reinhardtii to show that no such stationary tip-machinery is required for IFT turnaround. Instead, we demonstrate that the conversion from anterograde to retrograde IFT trains is a calcium independent intrinsic ability of the IFT system.