Robust interaction of IFT70 with IFT52-IFT88 in the IFT-B complex is required for ciliogenesis

In the intraflagellar transport (IFT) machinery, the IFT-B and IFT-A complexes mediate anterograde and retrograde ciliary protein trafficking, respectively. Among the 16 subunits of the IFT-B complex, several subunits are essential for ciliogenesis, whereas others, which are associated peripherally with the complex, are dispensable for ciliogenesis but play a role in protein trafficking. IFT22-knockout (KO) cells established in this study demonstrated no defects in ciliogenesis or ciliary protein trafficking. In stark contrast, IFT70A and IFT70B double-knockout cells did not form cilia, even though IFT70 is associated peripherally with the IFT-B complex via the IFT52-IFT88 dimer, and other IFT-B subunits assembled at the ciliary base in the absence of IFT70. Exogenous expression of either IFT70A or IFT70B restored the ciliogenesis defect of IFT70-KO cells, indicating their redundant roles. IFT70 has 15 consecutive tetratricopeptide repeats (TPRs) followed by a short helix (α36). Deletion of the first TPR or α36 of IFT70A greatly reduced its ability to interact with the IFT52-IFT88 dimer. Exogenous expression of any of the IFT70A deletion mutants in IFT70-KO cells could not restore ciliogenesis. These results show that IFT70 plays an essential role in ciliogenesis, although it is dispensable for assembly of the residual IFT-B subunits.