Microtubule lattice defects facilitate spastin-mediated severing

The length regulation of microtubules and their organization into complex arrays inside cells occurs through the activity of polymerases, depolymerases as well as severing enzymes such as katanin and spastin. The latter hexamerize on the microtubule lattice, pull out single tubulin dimers in an ATP-dependent manner and eventually generate internal breaks in the microtubule. While both, katanin and spastin, were shown to be regulated by posttranslational tubulin modifications, only katanin was reported to have microtubule lattice-defect- or crossover-sensing activity. Here, we employ in vitro assays to investigate the impact of microtubule lattice defects on the severing characteristics of spastin. Toward this end, we prepared GMPCPP-stabilized microtubules with varying defect densities. Thereby, microtubule defects were introduced either through specific polymerization conditions or by end-to-end annealing of microtubules. We found that (i) the presence of defects accelerated the onset of the severing process and (ii) severing was twice as frequent in microtubule segments with defect sites as compared to random lattice segments. However, there was no evidence of preferential binding of spastin to defect sites. We therefore propose a severing mechanism in which defects do not actively promote microtubule severing but rather passively contribute to microtubule lattice instability, facilitating the process as fewer tubulin subunits are required to be removed for microtubule severing.