在1月11日出版的《细胞》（Cell）封面文章上，来自德国、美国和日本的一组科学家提出了一种单分子技术，这种技术能直接观察XMAP215-GFP与微管的动态相互作用，从而帮助研究人员寻找XMAP215如何实现微管加速生长。
研究中，科学家使用了一种荧光显微方法观察微管生长，为了消除背景中杂散荧光的影响，小组利用全内反射荧光显微技术（TIRF），从而使激发光被限制在盖玻片附近很小的区域内。
研究结果表明，XMAP215可以与微管蛋白结合成1:1的复合物，XMAP215存在于微管的正极（plus end），从而促进微管不断生长。此外，实验还表明，XMAP215可以与微管的正极生长端（growing plus end）和正极收缩端（shrinking plus end）同时结合，并随着生长端而发生移动。除了促进微管的生长外，在某些情况下XMAP215也能导致逆反应——微管收缩的发生。
以上结果表明，XMAP215是一种直接催化微管生长的聚合酶，它促进微管蛋白聚合物结合到微管正极生长端。同时，这种全新观点也提供了利用其它细胞因子实现XMAP215调控的可能，例如，XTACC3蛋白可以激发XMAP215。此外，磷酸化过程也能改变XMAP215的诸多活性，并可能对XMAP215的结合微管蛋白的能力带来影响。而XMAP215与形成素（formins）、肌动蛋白聚合酶之间的相似性也充分说明，前向尖端追踪是激发细胞骨架聚合体生长的一种普遍机制。（科学网 何宏辉/编译）
绿谷生物网推荐原始出处：
封面图片：生长中的微管，突出的末端显示活跃着的XMAP215。
（图片来源：Gary Brouhard）
Cell, Vol 132, 79-88, 11 January 2008
Article
XMAP215 Is a Processive Microtubule Polymerase
1 Max Planck Institute of Molecular Cell Biology and Genetics, Dresden, Germany
2 Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, MA, USA
3 Graduate School of Biostudies, Kyoto University, Kyoto, Japan
Corresponding author
Jonathon Howard
howard@mpi-cbg.de
Corresponding author
Anthony A. Hyman
hyman@mpi-cbg.de
Fast growth of microtubules is essential for rapid assembly of the microtubule cytoskeleton during cell proliferation and differentiation. XMAP215 belongs to a conserved family of proteins that promote microtubule growth. To determine how XMAP215 accelerates growth, we developed a single-molecule assay to visualize directly XMAP215-GFP interacting with dynamic microtubules. XMAP215 binds free tubulin in a 1:1 complex that interacts with the microtubule lattice and targets the ends by a diffusion-facilitated mechanism. XMAP215 persists at the plus end for many rounds of tubulin subunit addition in a form of “tip tracking.” These results show that XMAP215 is a processive polymerase that directly catalyzes the addition of up to 25 tubulin dimers to the growing plus end. Under some circumstances XMAP215 can also catalyze the reverse reaction, namely microtubule shrinkage. The similarities between XMAP215 and formins, actin polymerases, suggest that processive tip tracking is a common mechanism for stimulating the growth of cytoskeletal polymers.
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