Intraflagellar transport of tubulin maintains steady-state axoneme integrity in C. elegans cilia

The integrity of the axoneme – the microtubule (MT)-based core of the cilium – and intraflagellar transport (IFT) are interdependent. The mechanisms determining axoneme structure and dynamics have remained largely unknown, especially in primary cilia with a more variable architecture and longer MT singlet parts. Using fluorescence imaging in the phasmid neurons of C. elegans , we here demonstrate that β-tubulin isotype TBB-4 diffuses through the dendrite and employs a combination of anterograde IFT and diffusion to reach the sites of incorporation in the steady-state axoneme. Disrupting tubulin’s ability to bind to the IFT significantly reduces its share in the axoneme. We suggest that, in phasmid cilia, a constant supply of tubulin by IFT is required for steady-state length maintenance, in order to elevate soluble tubulin concentration near the axonemal tips and to promote MT stability.