Tetrahymena IFT122A is not essential for cilia assembly but plays a role in returning IFT proteins from the ciliary tip to the cell body

Intraflagellar transport (IFT) moves multiple protein particles composed of two biochemically distinct complexes, IFT-A and IFT-B, bi-directionally within cilia and is essential for cilia assembly and maintenance. We identified an ORF from the Tetrahymena macronuclear genome sequence, encoding IFT122A, an ortholog of an IFT-A complex protein. Tetrahymena IFT122A is induced during cilia regeneration, and epitope-tagged Ift122Ap could be detected in isolated cilia. IFT122A knockout cells still assembled cilia, albeit with lower efficiency, and could regenerate amputated cilia. Ift172p and Ift88p, two IFT-B complex proteins that localized mainly to basal bodies and along the cilia in wild-type cells, became preferentially enriched at the ciliary tips in IFT122A knockout cells. Our results indicate that Tetrahymena IFT122A is not required for anterograde transport-dependent ciliary assembly but plays a role in returning IFT proteins from the ciliary tip to the cell body.