Flagellar assembly depends on intraflagellar transport (IFT), a bidirectional motility of protein carriers, the IFT trains. The trains are periodic assemblies of IFT-A and IFT-B subcomplexes and the motors kinesin-2 and IFT dynein. At the tip, anterograde trains are remodeled for retrograde IFT, a process that in Chlamydomonas involves kinesin-2 release and train fragmentation. However, the degree of train disassembly at the tip remains unknown. Here, we performed two-color imaging of fluorescent protein-tagged IFT components, which indicates that IFT-A and IFT-B proteins from a given anterograde train usually return in the same set of retrograde trains. Similarly, concurrent turnaround was typical for IFT-B proteins and the IFT dynein subunit D1bLIC-GFP but severance was observed as well. Our data support a simple model of IFT turnaround, in which IFT-A, IFT-B and IFT dynein typically remain associated at the tip and segments of the anterograde trains convert directly into retrograde trains. Continuous association of IFT-A, IFT-B and IFT dynein during tip remodeling could balance protein entry and exit, preventing the build-up of IFT material in flagella.

中文翻译：

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体内成像显示几种 IFT-A、IFT-B 和动力蛋白复合物的持续关联，而 IFT 在尖端训练 U 形转弯。

鞭毛装配依赖于鞭毛内运输 (IFT)，一种蛋白质载体的双向运动，即 IFT 列车。这些列车是 IFT-A 和 IFT-B 子复合体以及电机 kinesin-2 和 IFT dynein 的定期组装。在尖端，顺行列车被改造为逆行 IFT，在衣藻中，这一过程涉及 kinesin-2 释放和列车碎片。然而，火车末端的拆卸程度仍然未知。在这里，我们对荧光蛋白标记的 IFT 组件进行了双色成像，这表明来自给定顺行列车的 IFT-A 和 IFT-B 蛋白通常在同一组逆行列车中返回。同样，IFT-B 蛋白和 IFT 动力蛋白亚基 D1bLIC-GFP 的典型情况是同时周转，但也观察到了中断。我们的数据支持 IFT 周转的简单模型，其中 IFT-A、IFT-B 和 IFT 动力蛋白通常在尖端保持关联，顺行列车的部分直接转换为逆行列车。在尖端重塑过程中，IFT-A、IFT-B 和 IFT 动力蛋白的持续结合可以平衡蛋白质的进入和退出，防止 IFT 物质在鞭毛中的积聚。