Catalytic subunit of a complex which severs microtubules in an ATP-dependent manner. Microtubule severing may promote rapid reorganization of cellular microtubule arrays and the release of microtubules from the centrosome following nucleation. n ATP + n H2O + a microtubule = n ADP + n phosphate + (n+1) alpha/beta tubulin heterodimers. ATPase activity is stimulated by microtubules, which promote homooligomerization. ATP-dependent microtubule severing is stimulated by interaction with katnb1. Can homooligomerize into hexameric rings, which may be promoted by interaction with microtubules. Interacts with katnb1, which may serve as a targeting subunit. Predominantly cytoplasmic. Also localized to the interphase centrosome and the mitotic spindle poles. Enhanced recruitment to the mitotic spindle poles requires microtubules and interaction with katnb1. Belongs to the AAA ATPase family. Katanin p60 subunit A1 subfamily.