UC Davis

Cilia have diverse roles in motility and sensory reception and their dysfunction contributes to cilia-related diseases. Assembly and maintenance of cilia depends on the intraflagellar transport (IFT) of axoneme, membrane, matrix and signalling proteins to appropriate destinations within the organelle(1-4). In the current model, these diverse cargo proteins bind to multiple sites on macromolecular IFT particles, which are moved by a single anterograde IFT motor, kinesin-II, from the ciliary base to its distal tip(5,6), where cargo-unloading occurs(1-4,7). Here, we describe the observation of fluorescent IFT motors and IFT particles moving along distinct domains within sensory cilia of wild-type and IFT-motor-mutant Caenorhabditis elegans. We show that two anterograde IFT motor holoenzymes, kinesin-II and Osm-3-kinesin(8), cooperate in a surprising way to control two pathways of IFT that build distinct parts of cilia. Instead of each motor independently moving its own specific cargo to a distinct destination, the two motors function redundantly to transport IFT particles along doublet microtubules adjacent to the transition zone to form the axoneme middle segment(9). Next, Osm-3-kinesin alone transports IFT particles along the distal singlet microtubules to stabilize the distal segment. Thus, the subtle coordinate activity of these IFT motors creates two sequential transport pathways.