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 IFT88-knockout (KO) cells. However, IFT88(Δα)-expressing IFT88-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 GPCRs. Furthermore, overaccumulated IFT proteins at the bulged tips appeared to be released as extracellular vesicles. These phenotypes of IFT88(Δα)-expressing IFT88-KO cells resembled those of IFT144-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。鞭毛内运输（IFT）机制不仅介导纤毛内的双向蛋白质运输，还介导纤毛蛋白跨纤毛门的输入/输出。已知 IFT 机制包含两个多亚基复合体，即 IFT-A 和 IFT-B；然而，人们对这两种复合物如何合作介导纤毛蛋白运输知之甚少。我们在这里表明，来自 IFT-A 的 IFT144-IFT122 和来自 IFT-B 的 IFT88-IFT52 对两个复合物之间的界面做出了主要贡献。 IFT88(Δα)突变体的外源表达减少了与IFT-A的结合，部分恢复了IFT88敲除(KO)细胞的纤毛发生缺陷。然而，表达IFT88(Δα)的IFT88 -KO细胞表现出IFT-A进入纤毛的缺陷、IFT-B蛋白在凸出的纤毛尖端的异常积累以及纤毛GPCR的输入受损。此外，在凸出的尖端过度积累的 IFT 蛋白似乎作为细胞外囊泡释放。表达IFT88(Δα)的IFT88 -KO细胞的这些表型与IFT144 -KO细胞的表型相似。这些观察结果共同表明，IFT-A 复合体与 IFT-B 复合体合作介导 GPCR 的纤毛进入以及 IFT 机器从纤毛尖端的逆行运输。