Division of Biological Science, Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Japan.
PLoS One. 2011;6(11):e28120. doi: 10.1371/journal.pone.0028120. Epub 2011 Nov 30.
Chromosome segregation during mitosis and meiosis relies on the spindle and the functions of numerous microtubule-associated proteins (MAPs). One of the best-studied spindle MAPs is the highly conserved TPX2, which has been reported to have characteristic intracellular dynamics and molecular activities, such as nuclear localisation in interphase, poleward movement in the metaphase spindle, microtubule nucleation, microtubule stabilisation, microtubule bundling, Aurora A kinase activation, kinesin-5 binding, and kinesin-12 recruitment. This protein has been shown to be essential for spindle formation in every cell type analysed so far. However, as yet, TPX2 homologues have not been found in the Drosophila genome. In this study, I found that the Drosophila protein Ssp1/Mei-38 has significant homology to TPX2. Sequence conservation was limited to the putative spindle microtubule-associated region of TPX2, and intriguingly, D-TPX2 (Ssp1/Mei-38) lacks Aurora A- and kinesin-5-binding domains, which are highly conserved in other animal and plant species, including many insects such as ants and bees. D-TPX2 uniformly localised to kinetochore microtubule-enriched regions of the metaphase spindle in the S2 cell line, and it had microtubule binding and bundling activities in vitro. In comparison with other systems, the contribution of D-TPX2 to cell division seems to be minor; live cell imaging of microtubules and chromosomes after RNAi knockdown identified significant delay in chromosome congression in only 18% of the cells. Thus, while this conserved spindle protein is present in Drosophila, other mechanisms may largely compensate for its spindle assembly and chromosome segregation functions.
有丝分裂和减数分裂过程中的染色体分离依赖于纺锤体和众多微管相关蛋白（MAPs）的功能。研究最为深入的纺锤体 MAP 之一是高度保守的 TPX2，它具有特征性的细胞内动力学和分子活性，例如在间期的核定位、在中期纺锤体中的极向运动、微管核形成、微管稳定、微管捆绑、Aurora A 激酶激活、驱动蛋白-5 结合和驱动蛋白-12 募集。迄今为止，该蛋白已被证明是所有分析过的细胞类型中纺锤体形成所必需的。然而，到目前为止，在果蝇基因组中尚未发现 TPX2 的同源物。在这项研究中，我发现果蝇蛋白 Ssp1/Mei-38 与 TPX2 具有显著的同源性。序列保守性仅限于 TPX2 的假定纺锤体微管相关区域，有趣的是，D-TPX2（Ssp1/Mei-38）缺乏 Aurora A 和驱动蛋白-5 结合结构域，这些结构域在其他动物和植物物种中高度保守，包括许多昆虫，如蚂蚁和蜜蜂。D-TPX2 在 S2 细胞系的中期纺锤体的着丝粒微管富集区域均匀定位，并具有体外微管结合和捆绑活性。与其他系统相比，D-TPX2 对细胞分裂的贡献似乎较小；在 RNAi 敲低后对微管和染色体进行活细胞成像，仅在 18%的细胞中发现染色体向心运动明显延迟。因此，虽然这种保守的纺锤体蛋白存在于果蝇中，但其他机制可能在很大程度上补偿了其纺锤体组装和染色体分离功能。
