# Microtubule lattice defects facilitate spastin-mediated severing

## TL;DR

This study shows that defects in microtubule structures speed up spastin's ability to cut microtubules, but not by increasing spastin's binding.

## Contribution

The study reveals that microtubule lattice defects passively enhance spastin-mediated severing by reducing required subunit removal.

## Key findings

- Lattice defects accelerate the onset of spastin-mediated severing.
- Severing occurs twice as frequently in microtubule segments with defects.
- Spastin does not preferentially bind to defect sites.

## Abstract

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.

Summary: Microtubule severing by spastin is strongly influenced by lattice integrity. Using GMPCPP-stabilized microtubules with defined defect densities, we show that lattice defects accelerate severing onset without enhancing spastin binding.