Phase separation of TPX2 enhances and spatially coordinates microtubule nucleation

Abstract

Phase separation of substrates and effectors is proposed to enhance biological reaction rates and efficiency. TPX2 is an effector of microtubule nucleation in spindles, and functions with the substrate tubulin by an unknown mechanism. Here, we show that TPX2 phase separates into a co-condensate with tubulin, which mediates microtubule nucleation in vitro and in isolated cytosol. TPX2-tubulin co-condensation preferentially occurs on pre-existing microtubules at the endogenous and physiologically relevant concentration of TPX2. Truncation and chimera versions of TPX2 directly demonstrate that TPX2-tubulin co-condensation enhances the efficiency of TPX2-mediated microtubule nucleation. Finally, the known inhibitor of TPX2, the importin-/{beta} heterodimer, regulates both co-condensation and activity. Our study demonstrates how regulated phase separation can simultaneously enhance reaction efficiency and spatially coordinate microtubule nucleation, which may facilitate rapid and accurate spindle formation.