In vivo imaging shows continued association of several IFT A, B and dynein complexes while IFT trains U-turn at the tip

Flagellar assembly depends on intraflagellar transport (IFT), a bidirectional motility of protein carriers, the IFT trains. The trains are periodic assemblies of IFT-A and IFT-B subcomplexes and the motors kinesin-2 and IFT dynein. At the tip, anterograde trains are remodeled for retrograde IFT, a process that in Chlamydomonas involves kinesin-2 release and train fragmentation. However, the degree of train disassembly at the tip remains unknown. Two-color imaging of fluorescent protein-tagged IFT components indicates that IFT-A and IFT-B proteins from a given anterograde train usually return in the same set of retrograde trains. Similarly, concurrent turnaround was typical for IFT-B proteins and the IFT dynein subunit D1bLIC-GFP but severance was observed as well. Our data support a simple model of IFT turnaround, in which IFT-A, IFT-B, and IFT dynein typically remain associated at the tip and anterograde trains convert directly into retrograde trains without disassembly but for possible splitting into strings of IFT complexes. Continuous association of IFT-A, IFT-B and IFT dynein during tip remodeling could balance protein entry and exit preventing the build-up of IFT material in flagella.