The microtubule-binding domain of spastin participates in microtubule severing through electrostatic interactions

Spastin is a microtubule-severing enzyme and takes part in various microtubule-based events, but its microtubule-severing mechanism remains largely elusive. Spastin has an intrinsically unstructured microtubule-binding domain (MTBD) N-terminal to the AAA domain that is indispensable for the microtubule-severing activity. By performing a series of mutagenesis studies, we find that spastin can tolerate the mutation of a small number of basic residues in the MTBD, but mutating half of the basic residues abolishes the basal and microtubule-stimulated ATPase activities of spastin. The isolated MTBD pellets an equal molar amount of tubulin into curl and ring assemblies. Moreover, spastin with a sequence-reversed MTBD is active in ATP hydrolysis and microtubule severing. These results suggest that the MTBD of spastin participates in microtubule severing by making electrostatic interactions with microtubule protofilaments.