Constructing a eukaryotic cilium/flagellum is a demanding task requiring the transport of
proteins from their cytoplasmic synthesis site into a spatially and environmentally distinct cellular compartment. The clear potential hazard is that import of aberrant
proteins could seriously disable cilia/flagella assembly or turnover processes. Here, we reveal that
tubulin protein destined for incorporation into axonemal microtubules interacts with a
tubulin cofactor C (TBCC) domain-containing
protein that is specifically located at the mature basal body transitional fibres. RNA interference-mediated ablation of this
protein results in axonemal microtubule defects but no effect on other microtubule populations within the cell. Bioinformatics analysis indicates that this
protein belongs to a clade of flagellum-specific TBCC-like
proteins that includes the human
protein, XRP2, mutations which lead to certain forms of the
hereditary eye disease retinitis pigmentosa. Taken with other observations regarding the role of transitional fibres in cilium/flagellum assembly, we suggest that a localized
protein processing capacity embedded at transitional fibres ensures the 'quality' of
tubulin imported into the cilium/flagellum, and further, that loss of a ciliary/flagellar quality control capability may underpin a number of human
genetic disorders.