The molecular structure of IFT-A and IFT-B in anterograde intraflagellar transport trains

Anterograde intraflagellar transport (IFT) trains are essential for cilia assembly and maintenance. These trains are formed of 22 IFT-A and IFT-B proteins that link structural and signaling cargos to microtubule motors for import into cilia. It remains unknown how the IFT-A/-B proteins are arranged into complexes and how these complexes polymerize into functional trains. Here we use in situ cryo-electron tomography of Chlamydomonas reinhardtii cilia and AlphaFold2 protein structure predictions to generate a molecular model of the entire anterograde train. We show how the conformations of both IFT-A and IFT-B are dependent on lateral interactions with neighboring repeats, suggesting that polymerization is required to cooperatively stabilize the complexes. Following three-dimensional classification, we reveal how IFT-B extends two flexible tethers to maintain a connection with IFT-A that can withstand the mechanical stresses present in actively beating cilia. Overall, our findings provide a framework for understanding the fundamental processes that govern cilia assembly.

Automated summary

In this study, a molecular model of the anterograde train in cilia assembly was generated using cryo-electron tomography and AlphaFold2 protein structure predictions. The conformations of IFT-A and IFT-B were found to depend on lateral interactions with neighboring repeats, indicating the importance of polymerization for complex stability. Furthermore, IFT-B was shown to extend flexible tethers to maintain a connection with IFT-A under mechanical stresses. These findings provide insights into the fundamental processes of cilia assembly.