Functional Exploration of Heterotrimeric Kinesin-II in IFT and Ciliary Length Control in Chlamydomonas

Heterotrimeric organization of kinesin-II is essential for its function in anterograde IFT in ciliogenesis. However, the molecular basis of forming this complex for its function is not well understood. In addition, the anterograde IFT velocity varies significantly in different organisms, but how motor speed affects ciliary length is not clear. We show that Chlamydomonas kinesin-II (CrKinesin-II) involves distinct mechanisms from mammals and C. elegans in its assembly to necessitate its function in IFT. Furthermore, chimeric CrKinesin-II with human kinesin-II motor domains functioned in vitro and in vivo, leading to a ~2.8-fold reduced anterograde IFT velocity and a similar fold reduction in IFT injection rate that supposedly correlates with ciliary assembly activity. However, the ciliary length was only mildly reduced (~15%). Modelling analyses suggest that such a non-linear scaling relationship between IFT velocity and ciliary length can be accounted for by limitation of the motors and/or its ciliary cargoes, e.g. tubulin.