To Cut or not to Cut?: Physically Regulating Microtubule Severing Enzymes

Abstract

Microtubule-severing enzymes are AAA (ATPases associated with cellular activities) proteins that remove tubulin dimers from the microtubule lattice. Severing proteins are known to remodel the cytoskeleton during interphase and mitosis, and are required in proper axon morphology and mammalian bone and cartilage development. We have performed the first single molecule imaging to determine where and how severing enzymes act to cut microtubules. We have focused on the original member of the group, katanin, and the newest member, fidgetin to compare their biophysical activities in vitro. We find that, at lower concentrations, both katanin and fidgetin can depolymerize microtubules by removing terminal dimers. Katanin preferentially removes dimers from the plus-end, while fidgetin removes from the minus-end. This activity reflects their cellular localization and activity, where katanin is localized to the cell cortex and fidgetin at the microtubule-organizing center. At higher concentrations, both katanin and fidgetin can sever microtubules, but katanin cuts cleanly through, while fidgetin appear to remove long strips of protofilaments, without a clean break. Further, we find that katanin and fidgetin prefer different types of tubulin to bind and sever. These studies reveal the physical regulation schemes to control severing activity in cells, and ultimately regulate cytoskeletal architecture.