Structural basis of microtubule severing by the hereditary spastic paraplegia protein spastin

Publisher Website
Google Scholar
Abstract
Spastin, a microtubule severing protein involved in the assembly or function of nuclear protein complexes, has attracted interest because of its role in neuronal synapse formation, and the discovery that mutations in the gene cause hereditary neurodegenerative disease. The structure of spastin has now been determined to atomic resolution by X-ray crystallography, revealing an AAA ATPase cassette augmented by structural elements unique to the microtubule severing enzyme subfamily. Mapping of hereditary spastic paraplegia mutations onto the structure of spastin reveals how disease mutations damage the enzyme. Spastin and kastin are AAA-ATPases that function as microtubule severing enzymes. Mutations in spastin are the predominant cause of hereditary spastic paraplegias (HSP). The atomic structure of spastin monomer and coupled with atomic docking generate a model of spastin hexamer is solved. Spastin forms a ring with a prominent central pore and six radiating arms that dock onto the microtubule. Spastin, the most common locus for mutations in hereditary spastic paraplegias1, and katanin are related microtubule-severing AAA ATPases2,3,4,5,6 involved in constructing neuronal7,8,9,10 and non-centrosomal7,11 microtubule arrays and in segregating chromosomes12,13. The mechanism by which spastin and katanin break and destabilize microtubules is unknown, in part owing to the lack of structural information on these enzymes. Here we report the X-ray crystal structure of the Drosophila spastin AAA domain and provide a model for the active spastin hexamer generated using small-angle X-ray scattering combined with atomic docking. The spastin hexamer forms a ring with a prominent central pore and six radiating arms that may dock onto the microtubule. Helices unique to the microtubule-severing AAA ATPases surround the entrances to the pore on either side of the ring, and three highly conserved loops line the pore lumen. Mutagenesis reveals essential roles for these structural elements in the severing reaction. Peptide and antibody inhibition experiments further show that spastin may dismantle microtubules by recognizing specific features in the carboxy-terminal tail of tubulin. Collectively, our data support a model in which spastin pulls the C terminus of tubulin through its central pore, generating a mechanical force that destabilizes tubulin–tubulin interactions within the microtubule lattice. Our work also provides insights into the structural defects in spastin that arise from mutations identified in hereditary spastic paraplegia patients.