Graduate School of Pharmaceutical Sciences, Kyoto University, Kyoto 606-8501, Japan.
Mol Biol Cell. 2021 Jan 1;32(1):45-56. doi: 10.1091/mbc.E20-08-0556. Epub 2020 Nov 11.
Cilia sense and transduce extracellular signals via specific receptors. The intraflagellar transport (IFT) machinery mediates not only bidirectional protein trafficking within cilia but also the import/export of ciliary proteins across the ciliary gate. The IFT machinery is known to comprise two multisubunit complexes, namely, IFT-A and IFT-B; however, little is known about how the two complexes cooperate to mediate ciliary protein trafficking. We here show that IFT144-IFT122 from IFT-A and IFT88-IFT52 from IFT-B make major contributions to the interface between the two complexes. Exogenous expression of the IFT88(Δα) mutant, which has decreased binding to IFT-A, partially restores the ciliogenesis defect of -knockout (KO) cells. However, IFT88(Δα)-expressing -KO cells demonstrate a defect in IFT-A entry into cilia, aberrant accumulation of IFT-B proteins at the bulged ciliary tips, and impaired import of ciliary G protein-coupled receptors (GPCRs). Furthermore, overaccumulated IFT proteins at the bulged tips appeared to be released as extracellular vesicles. These phenotypes of IFT88(Δα)-expressing -KO cells resembled those of -KO cells. These observations together indicate that the IFT-A complex cooperates with the IFT-B complex to mediate the ciliary entry of GPCRs as well as retrograde trafficking of the IFT machinery from the ciliary tip.
纤毛通过特定受体感知和转导细胞外信号。内纤毛运输（IFT）机制不仅介导纤毛内双向蛋白质运输，还介导纤毛蛋白在纤毛门的进出口。已知 IFT 机制由两个多亚基复合物组成，即 IFT-A 和 IFT-B；然而，对于这两个复合物如何合作来介导纤毛蛋白运输知之甚少。我们在这里表明，IFT-A 中的 IFT144-IFT122 和 IFT-B 中的 IFT88-IFT52 对两个复合物之间的界面做出了主要贡献。外源性表达与 IFT-A 结合减少的 IFT88(Δα)突变体部分恢复了 -敲除（KO）细胞的纤毛发生缺陷。然而，IFT88(Δα)表达的 -KO 细胞表现出 IFT-A 进入纤毛的缺陷、IFT-B 蛋白在膨出的纤毛尖端异常积累以及纤毛 G 蛋白偶联受体（GPCR）的导入受损。此外，在膨出尖端过度积累的 IFT 蛋白似乎作为细胞外囊泡释放。IFT88(Δα)表达的 -KO 细胞的这些表型类似于 -KO 细胞。这些观察结果表明，IFT-A 复合物与 IFT-B 复合物合作，介导 GPCR 的纤毛进入以及从纤毛尖端逆行运输 IFT 机制。
