Phase separation of substrates and effectors is proposed to enhance biological reaction rates and efficiency. Targeting protein for Xklp2 (TPX2) is an effector of branching 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, the site of branching microtubule nucleation, at the endogenous and physiologically relevant concentration of TPX2. Truncation and chimera versions of TPX2 suggest that TPX2-tubulin co-condensation enhances the efficiency of TPX2-mediated branching microtubule nucleation. Finally, the known inhibitor of TPX2, the importin-α/β heterodimer, regulates TPX2 condensation in vitro and, consequently, branching microtubule nucleation activity in isolated cytosol. 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.

中文翻译：

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TPX2的相分离增强并在空间上协调微管成核。

提出了底物和效应物的相分离以提高生物反应速率和效率。Xklp2（TPX2）的靶蛋白是纺锤体中分支微管成核的效应子，并通过未知机制与底物微管蛋白一起起作用。在这里，我们显示TPX2相与微管蛋白分离成共缩合物，后者在体外和分离的细胞质中介导微管成核。TPX2-微管蛋白共冷凝优先发生在预先存在的微管，分支微管成核位点，内源和生理相关的TPX2浓度下。截断和嵌合体版本的TPX2表明TPX2-微管蛋白共缩合可提高TPX2介导的分支微管成核的效率。最后，已知的TPX2抑制剂importin-α/β异二聚体 在体外调节TPX2缩合，并因此在分离的细胞质中分支微管成核活性。我们的研究表明，可控的相分离可如何同时提高反应效率并在空间上协调微管成核，从而有助于快速，准确地形成纺锤。