IFT139 regulates Hedgehog signaling and cilia structure through ciliary protein localization

The primary cilium, a microtubule-based membrane protrusion, is essential for eukaryotic development and health. Import and export of proteins in and out of the primary cilium relies on intraflagellar transport protein complexes (IFT) IFT-B and IFT-A, in conjunction with their respective motor proteins. Here, using mouse fibroblast cells, we investigated the function of IFT139 (Thm1, TTC21B) in Hedgehog signaling, cilia structure, and ciliary protein localization, as well as the effect of the P209L ciliopathy mutation on cell proliferation and Hedgehog signaling. In cells without IFT139, Ptch1 retains normal localization, Smo and Gli accumulate in the distal tips of cilia with or without pathway activation, while SuFu fails to accumulate in cilia upon pathway activation. We also found that Arl13b abnormally accumulates at the distal tips of cilia, but acetylated tubulin does not. Lastly, the ciliopathy mutation P209L impairs cell proliferation and Hedgehog transcriptional response, mimicking a loss of function in IFT139. Our work highlights the multifaceted roles IFT139 have on distinct ciliary proteins, and its importance in ciliopathies.