The nucleotide cycle of spastin correlates with its microtubule-binding properties

Spastin is an AAA (ATPase associated with diverse cellular activities) protein with microtubule (MT)-severing activity. The spastin-encoding gene was identified as the most often mutated gene in the human neurodegenerative disease hereditary spastic paraplegia. Although the structure of the AAA domain of spastin has been determined, the mechanism by which spastin severs MTs remains elusive. Here, we studied the MT-binding and nucleotide-binding properties of spastin, as well as their interplay. The results suggest that ATP-bound spastin interacts strongly and cooperatively with MTs; this interaction stimulates ATP hydrolysis by spastin. After ATP hydrolysis, spastin dissociates from MTs, and then exchanges ADP for ATP in solution for the next round of work. In particular, spastin in the ternary complex of MT-spastin-ATP is the most cooperative state during the working cycle, and is probably the force-generating state that is responsible for MT severing. The results presented in this study establish the nucleotide cycle of spastin in correlation with its MT-binding properties, and provide a biochemical framework for further studies of the working mechanism of spastin.