IFT-A complexes from the assembly of freshly expressed IFT-A subunits and the disassembly of retrograde IFT trains are concentrated in the basal bodies. Preassembled bimodular IFT-A are incorporated into anterograde IFT train by bending at the hinge regions and twists of the flexible TPRs. After reaching the tip, IFT-A and IFT-B dissociate from anterograde train, likely through adjustments at the hinge regions and the flexible TPRs to transform the anterograde train into the loosely zigzagged retrograde train. At last, retrograde trains move back to the basal bodies and IFT-A detaches from the retrograde train to finish a cycle. Opening questions about the mechanism of ciliary tip turnaround and retrograde train assembly, as well as the roles of folded IFT-A complex in IFT-A cycle await further studies.
