Yuqing Hou

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.