Taxol is a critically important cancer drug that stabilises microtubules. We report that taxol acts differently on different metazoan tubulin isotypes. 50 nM taxol blocks catastrophe of human or zebrafish α1β4 but has no effect on human α1β3 microtubules. 500 nM taxol blocks catastrophe in both α1β3 and α1β4 microtubules but introduces kinks only into α1β4 microtubules. Taxol washout relaxes the kinks, suggesting taxol expands α1β4 but not α1β3 lattices. Kinesin-driven microtubule gliding detects this conformational shift - α1β4 microtubules glide at ~450 nm/sec in 400 nM taxol, but at ~750 nm/sec in 10 μM taxol, whereas α1β3 microtubules glide at ~450 nm/sec, even in 10 μM taxol. Thus, taxol readily stabilises α1β4 GDP-tubulin lattices and shifts them to a fast-gliding conformation, but stabilises α1β3 lattices much less readily and without shifting their conformation. These isotype-specific actions of taxol may drive the switch to β3 tubulin commonly seen in taxol-resistant tumours.

No abstract

Taxol is a small molecule effector that allosterically locks tubulin into the microtubule lattice. We report that taxol has different effects on different single isotype microtubule lattices. Using in vitro reconstitution, we show that α1β4 human and zebrafish GDP-tubulin lattices are stabilised and expanded by taxol, whereas α1β3 human GDP-tubulin lattices require tenfold more taxol for stability and are not expanded. In kinesin motility assays, this isotype-specific mechanical action of taxol causes segmented isotype microtubules to deviate into loops, because the expanded β4 and compacted β3 segments try to glide at different rates. To explain, we propose that taxol switches GDP-microtubules into one of two different lattice-mechanical states, a compacted/slow-gliding state or an expanded/fast-gliding state, with switching to the expanded/fast-gliding state dependant on taxol occupancy and available to only some tubulin isotypes. In mixed isotype lattices, we find evidence that this lattice-mechanical switching occurs cooperatively.