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 and depolymerases, as well as severing enzymes, such as spastin and katanin. Spastin and katanin hexamerize on the microtubule lattice, pull out single tubulin dimers in an ATP-dependent manner and eventually generate internal breaks in the microtubule. Although spastin has been shown to be regulated by post-translational tubulin modifications, the impact of microtubule lattice defects on the severing characteristics of spastin has not been explored. To address this, we prepared GMPCPP-stabilized microtubules with varying defect densities - introduced either through specific polymerization conditions or by end-to-end annealing - for subsequent in vitro severing assays. We found that: (1) the presence of defects accelerated the onset of the severing process; and (2) severing was twice as frequent in microtubule segments with defect sites when 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, instead, passively contribute to microtubule lattice instability. The defects thus facilitate the severing process by reducing the number of tubulin subunits that must be removed for severing to occur.