Illuminating the Cooperative Action of Kinesin-II and OSM-3-Kinesin in the Chemosensory Cilia of Caenorhabditis Elegans

Abstract

Intraflagellar transport (IFT) is indispensable for the assembly and maintenance of cilia. In the chemosensory cilia of C. elegans, two motors of the kinesin-2-family, heterotrimeric kinesin-II and homodimeric OSM-3, collaborate to drive anterograde IFT. It is known that both kinesins associate with IFT-trains that move along the middle segment, and that OSM-3 alone drives transport in the distal segment. However, many questions remain concerning how these kinesins cooperate, how this is regulated and why both motors are required. To address this problem, we have improved the fidelity of IFT-assays by using single transgenes encoding for fluorescently-labeled kinesins. In combination with ultrasensitive, quantitative fluorescence microscopy and dynamic photoactivated localization microscopy allowing observation of single motors in living nematodes, we find that the motor composition of IFT-trains is highly dynamic. Kinesin-II undocks gradually from IFT-trains close to the ciliary base, and not suddenly upon reaching the middle segment tips as previously thought, whereas OSM-3 docks gradually resulting in accelerating IFT-trains. Undocked kinesin-II is transported back to the ciliary base by dynein-driven retrograde trains. Consequently, the IFT-system ensures that kinesin-II stays close to the ciliary base, where, it is responsible for the loading of IFT-trains onto the axoneme, and that OSM-3 stays around the distal segment, where it drives fast long-distance transport. Our work on motor cooperation and dynamics provides new insight into how IFT drives the proper development and functioning of cilia and, more broadly, how kinesin motors work together to generate intracellular transport pathways within cells.