The C-terminal tails of heterotrimeric kinesin-2 motor subunits directly bind to α-tubulin1: Possible implications for cilia-specific tubulin entry

The assembly of microtubule-based cytoskeleton propels the cilia and flagella growth. Previous studies have indicated that the kinesin-2 family motors transport tubulin into the cilia through intraflagellar transport. Here, we report a direct interaction between the C-terminal tail fragments of heterotrimeric kinesin-2 and -tubulin1 isoforms in vitro. Blot overlay screen, affinity purification from tissue extracts, cosedimentation with subtilisin-treated microtubule and LC-ESI-MS/MS characterization of the tail-fragment-associated tubulin identified an association between the tail domains and -tubulin1A/D isotype. The interaction was confirmed by Forster's resonance energy transfer assay in tissue-cultured cells. The overexpression of the recombinant tails in NIH3T3 cells affected the primary cilia growth, which was rescued by coexpression of a -tubulin1 transgene. Furthermore, fluorescent recovery after photobleach analysis in the olfactory cilia of Drosophila indicated that tubulin is transported in a non-particulate form requiring kinesin-2. These results provide additional new insight into the mechanisms underlying selective tubulin isoform enrichment in the cilia.