Most cells tightly control the length of their cilia. The regulation likely involves intraflagellar transport (IFT), a bidirectional motility of multi‐subunit particles organized into trains that deliver building blocks into the organelle. In Chlamydomonas, the anterograde IFT motor kinesin‐2 consists of the motor subunits FLA8 and FLA10 and the nonmotor subunit KAP. KAP dissociates from IFT at the ciliary tip and diffuses back to the cell body. This observation led to the diffusion‐as‐a‐ruler model of ciliary length control, which postulates that KAP is progressively sequestered into elongating cilia because its return to the cell body will require increasingly more time, limiting motor availability at the ciliary base, train assembly, building block supply, and ciliary growth. Here, we show that Chlamydomonas FLA8 also returns to the cell body by diffusion. However, more than 95% of KAP and FLA8 are present in the cell body and, at a given time, just ~1% of the motor participates in IFT. After repeated photobleaching of both cilia, IFT of fluorescent kinesin subunits continued indicating that kinesin‐2 cycles from the large cell‐body pool through the cilia and back. Furthermore, growing and full‐length cilia contained similar amounts of kinesin‐2 subunits and the size of the motor pool at the base changed only slightly with ciliary length. These observations are incompatible with the diffusion‐as‐a‐ruler model, but rather support an “on‐demand model,” in which the cargo load of the trains is regulated to assemble cilia of the desired length.

中文翻译：

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鞭毛内运输 (IFT) 运动驱动蛋白-2 的分布和整体流动分析支持衣藻纤毛长度控制的“按需”模型

大多数细胞严格控制纤毛的长度。这种调节可能涉及鞭毛内运输（IFT），即组织成列车的多亚基颗粒的双向运动，将构建块输送到细胞器中。在衣藻属，顺行 IFT 运动驱动蛋白-2 由运动亚基 FLA8 和 FLA10 以及非运动亚基 KAP 组成。KAP 在纤毛尖端与 IFT 分离并扩散回细胞体。这一观察结果导致了纤毛长度控制的扩散作为尺子模型，该模型假设 KAP 逐渐被隔离在伸长的纤毛中，因为它返回细胞体将需要越来越多的时间，限制了纤毛基部运动的可用性，训练组装、构建块供应和纤毛生长。在这里，我们表明衣藻属FLA8也通过扩散返回细胞体。然而，超过 95% 的 KAP 和 FLA8 存在于细胞体内，并且在给定时间，只有约 1% 的运动参与 IFT。对两个纤毛进行重复光漂白后，荧光驱动蛋白亚基的 IFT 继续表明驱动蛋白-2 从大细胞体库循环通过纤毛并返回。此外，生长中的纤毛和全长纤毛含有相似数量的驱动蛋白-2 亚基，并且基部运动池的大小仅随纤毛长度略有变化。这些观察结果与扩散作为统治者模型不相容，而是支持“按需模型”，在该模型中，调节火车的货物负载以组装所需长度的纤毛。