Intraflagellar transport (IFT) is important for the maintenance of cilia and flagella, and is mediated by IFT trains that are composed of IFT-A and IFT-B subcomplexes, as well as the motors kinesin-2 and dynein. The general view in the field has been that anterograde trains disintegrate at the tip into smaller retrograde trains, which involves the release of kinesin-2 and train fragmentation. However, the underlying mechanisms, in particular the extent of train disassembly, are not clear. In this study, Karl Lechtreck and colleagues (Wingfield et al., 2021) make use of sophisticated live imaging of IFT particles and motor proteins in Chlamydomonas to characterise train behaviour at the tip in detail. They observe that all IFT proteins studied dwell at the tip, and proteins from one anterograde train are distributed into approximately three retrograde trains. Furthermore, during the turnaround at the tip, IFT-A, IFT-B and IFT dynein typically remain associated with each other. On the basis of their observations, the authors propose a model whereby anterograde trains directly convert into retrograde trains at the tip without any significant disassembly, instead fragmenting into a string of smaller IFT ‘carts’. Such a persistent association of the three subcomplexes might help to prevent the build-up of IFT material, and to balance its entry to and exit from the flagellum.
