KL1 is a novel microtubule severing enzyme that regulates mitotic spindle architecture

Katanin is a microtubule severing enzyme with demonstrated roles in a variety of cellular activities including mitosis. Here we identify the closely related, but relatively uncharacterized human protein, Katanin-like 1 (KL1), as a novel mitotic regulator. Over expression of KL1 in tissue culture cells results in the complete disassembly of cellular microtubules strongly suggesting that it is an active microtubule severing protein. During mitosis, the localization of KL1 is restricted to spindle poles and is notably absent from centrosomes. This is in clear contrast to conventional Katanin whose localization extends from centrosomes onto poles. Consistent with its localization, siRNA depletion of KL1 from U2OS cells results in a specific and significant reduction in the density of microtubules at spindle poles and significantly increases spindle length. Depletion of KL1 also alters the distribution of gamma-tubulin at centrosomes/spindle poles. Despite its impact on spindle morphology, we could find no evidence that KL1 influences anaphase chromosome motility. Based on our findings, we propose that KL1-mediated microtubule severing is utilized to generate microtubule seeds within the poles and that loss of this activity alters the normal balance of motor-generated forces that determine spindle length.