SPG4, the gene encoding for spastin, a member of the ATPases associated with various cellular activities (AAA) family, is mutated in around 40% of cases of autosomal dominant hereditary spastic paraplegia (AD‐HSP). This group of neurodegenerative diseases is characterized by a progressive spasticity and lower limb weakness with degeneration of terminal axons in cortico‐spinal tracts and dorsal columns. Spastin has two main domains, a microtubule interacting and endosomal trafficking (MIT) domain at the N‐terminus and the C‐terminus AAA domain. Early studies suggested that spastin interacts with microtubules similarly to katanin, a member of the same subgroup of AAA. Recent evidence confirmed that spastin possesses microtubule‐severing activity but can also bundle microtubules in vitro. Understanding the physiologic and pathologic involvement of these activities and their regulation is critical in the study of HSP. © 2007 Wiley‐Liss, Inc.