Microtubules are nucleated from specific locations at precise times in the cell cycle. However, the factors that constitute these microtubule nucleation pathways and their mode of action still need to be identified. Using purified Xenopus laevis proteins we biochemically reconstitute branching microtubule nucleation, which is critical for chromosome segregation. We found that besides the microtubule nucleator gamma-tubulin ring complex (γ-TuRC), the branching effectors augmin and TPX2 are required to efficiently nucleate microtubules from pre-existing microtubules. TPX2 has the unexpected capacity to directly recruit γ-TuRC as well as augmin, which in turn targets more γ-TuRC along the microtubule lattice. TPX2 and augmin enable γ-TuRC-dependent microtubule nucleation at preferred branching angles of less than 90 degrees from regularly-spaced patches along microtubules. This work provides a blueprint for other microtubule nucleation pathways and helps explain how microtubules are generated in the spindle.

中文翻译：

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分支微管成核的生化重建

微管在细胞周期的精确时间从特定位置成核。但是，仍然需要确定构成这些微管成核途径的因素及其作用方式。使用纯化的非洲爪蟾我们用生化方法重组了蛋白质的分支微管成核，这对染色体分离至关重要。我们发现，除了微管成核剂γ-微管蛋白环复合物（γ-TuRC）之外，还需要分支效应子augmin和TPX2才能有效地使现有微管中的微管成核。TPX2具有直接募集γ-TuRC和augmin的出乎意料的能力，而后者又沿微管晶格靶向更多的γ-TuRC。TPX2和augmin可使γ-TuRC依赖的微管成核，并沿着微管从规则间隔的贴片以小于90度的优选分支角成核。这项工作为其他微管成核途径提供了蓝图，并有助于解释纺锤体中微管的产生方式。