Intraflagellar transport (IFT) is the highly conserved process by which proteins are transported along ciliary microtubules by a train-like polymeric assembly of IFT-A and IFT-B complexes. IFT-A is sandwiched between IFT-B and the ciliary membrane, consistent with its putative role in transporting transmembrane and membrane-associated cargoes. Here, we have used single-particle analysis electron cryomicroscopy (cryo-EM) to determine structures of native IFT-A complexes. We show that subcomplex rearrangements enable IFT-A to polymerize laterally on anterograde IFT trains, revealing a cooperative assembly mechanism. Surprisingly, we discover that binding of IFT-A to IFT-B shields the preferred lipid-binding interface from the ciliary membrane but orients an interconnected network of β-propeller domains with the capacity to accommodate diverse cargoes toward the ciliary membrane. This work provides a mechanistic basis for understanding IFT-train assembly and cargo interactions.

鞭毛内运输 (IFT) 是一种高度保守的过程，通过该过程，蛋白质通过 IFT-A 和 IFT-B 复合物的火车状聚合物组装体沿着纤毛微管运输。 IFT-A 夹在 IFT-B 和睫状膜之间，这与其在运输跨膜和膜相关货物中的假定作用一致。在这里，我们使用单颗粒分析电子冷冻显微镜 (cryo-EM) 来确定天然 IFT-A 复合物的结构。我们表明，子复合体重排使 IFT-A 能够在顺行 IFT 列车上横向聚合，揭示了一种协作组装机制。令人惊讶的是，我们发现 IFT-A 与 IFT-B 的结合屏蔽了睫状膜的首选脂质结合界面，但定向了 β-螺旋桨结构域的互连网络，具有将不同货物容纳到睫状膜的能力。这项工作为理解 IFT 列车组装和货物相互作用提供了机械基础。