Spastin is a dual-function enzyme that severs microtubules and promotes their regrowth to increase the number and mass of microtubules

Significance

Microtubule-severing enzymes are ATPases that utilize ATP hydrolysis to fragment microtubules. These enzymes play central roles in various cellular functions, including cell growth and division and neural development. In vivo studies have shown that severing enzymes can lead to an increase in microtubule mass, but how severing activity and microtubule dynamics collectively expand the microtubule network remains unclear. Here, we demonstrate that the severing enzyme spastin is a dual-microtubule regulator: in addition to its severing activity, it promotes microtubule regrowth through an ATP-independent mechanism. Using a mathematical model, we show that the modulation of dynamics is essential for increasing microtubule mass and identify quantitative criteria that must be satisfied if severing is to expand the microtubule network.

Abstract

The remodeling of the microtubule cytoskeleton underlies dynamic cellular processes, such as mitosis, ciliogenesis, and neuronal morphogenesis. An important class of microtubule remodelers comprises the severases—spastin, katanin, and fidgetin—which cut microtubules into shorter fragments. While severing activity might be expected to break down the microtubule cytoskeleton, inhibiting these enzymes in vivo actually decreases, rather increases, the number of microtubules, suggesting that severases have a nucleation-like activity. To resolve this paradox, we reconstituted Drosophila spastin in a dynamic microtubule assay and discovered that it is a dual-function enzyme. In addition to its ATP-dependent severing activity, spastin is an ATP-independent regulator of microtubule dynamics that slows shrinkage and increases rescue. We observed that spastin accumulates at shrinking ends; this increase in spastin concentration may underlie the increase in rescue frequency and the slowdown in shortening. The changes in microtubule dynamics promote microtubule regrowth so that severed microtubule fragments grow, leading to an increase in the number and mass of microtubules. A mathematical model shows that spastin’s effect on microtubule dynamics is essential for this nucleation-like activity: spastin switches microtubules into a state where the net flux of tubulin onto each polymer is positive, leading to the observed exponential increase in microtubule mass. This increase in the microtubule mass accounts for spastin’s in vivo phenotypes.