Two-Step Interphase Microtubule Disassembly Aids Spindle Morphogenesis

Entry into mitosis triggers profound changes in cell shape and cytoskeletal organisation. Here, by studying microtubule remodelling in human flat mitotic cells, we identify a two-step process of interphase microtubule disassembly. First, a microtubule stabilizing protein, Ensconsin, is inactivated in prophase as a consequence of its phosphorylation downstream of Cdk1/CyclinB. This leads to a reduction in interphase microtubule stability that may help to fuel the growth of centrosomally-nucleated microtubules. The peripheral interphase microtubules that remain are then rapidly lost as the concentration of tubulin heterodimers falls following dissolution of the nuclear compartment boundary. Finally, we show that a failure to destabilize microtubules in prophase leads to the formation of microtubule clumps, which interfere with spindle assembly. Overall, this analysis highlights the importance of the stepwise remodelling of the microtubule cytoskeleton, and the significance of permeabilization of the nuclear envelope in coordinating the changes in cellular organisation and biochemistry that accompany mitotic entry.