Two structurally distinct filamentous tracks, namely singlet microtubules in the cytoplasm and axonemes in the cilium, serve as railroads for long-range transport processes in vivo In all organisms studied so far, the kinesin-2 family is essential for long-range transport on axonemes. Intriguingly, in higher eukaryotes, kinesin-2 has been adapted to work on microtubules in the cytoplasm as well. Here, we show that heterodimeric kinesin-2 motors distinguish between axonemes and microtubules. Unlike canonical kinesin-1, kinesin-2 takes directional, off-axis steps on microtubules, but it resumes a straight path when walking on the axonemes. The inherent ability of kinesin-2 to side-track on the microtubule lattice restricts the motor to one side of the doublet microtubule in axonemes. The mechanistic features revealed here provide a molecular explanation for the previously observed partitioning of oppositely moving intraflagellar transport trains to the A- and B-tubules of the same doublet microtubule. Our results offer first mechanistic insights into why nature may have co-evolved the heterodimeric kinesin-2 with the ciliary machinery to work on the specialized axonemal surface for two-way traffic.
Keywords: axonemes; heterodimeric kinesin‐2; intraflagellar transport; microtubules.
© 2017 The Authors.