Regulation of tubulin transport in cilia. (A) Scatter plot of the frequencies of tubulin transport by anterograde IFT in shf2 (red symbols) and lf2-1 mutant cilia (blue symbols). Diamonds: steady-state cilia; triangles: regenerating cilia. Trend lines are given for each dataset. n = 27, 43, 29, and 44 for lf2-1 regenerating and steady-state and shf2 regenerating and steady-state, respectively, from a single experiment. The mean ciliary length of control cilia, and the frequencies of tubulin transport in steady-state and regenerating control cilia (arrowheads) are indicated. (B) Model of the regulation of tubulin transport. Incompletely assembled cilia (left) induce a signal resulting in increased loading of IFT particles with tubulin (curved arrow) or facilitated access of highly loaded IFT particles into cilia. Augmented tubulin transport will increase the concentration of soluble tubulin in the cilia matrix and promote the polymerization of axonemal microtubules. Mutants in SHF2 prematurely down-regulate tubulin transport (A), suggesting that SHF2 functions in maintaining a high tubulin flux into cilia until steady-state length is reached. Full-length cilia (right) inhibit tubulin loading onto IFT or admission of highly loaded IFT trains into cilia.