Varga Vladimir, Helenius Jonne, Tanaka Kozo, Hyman Anthony A, Tanaka Tomoyuki U, Howard Jonathon
Max Planck Institute of Molecular Cell Biology & Genetics, Pfotenhauerstr. 108, 01307 Dresden, Germany.
Nat Cell Biol. 2006 Sep;8(9):957-62. doi: 10.1038/ncb1462. Epub 2006 Aug 13.
The microtubule cytoskeleton and the mitotic spindle are highly dynamic structures, yet their sizes are remarkably constant, thus indicating that the growth and shrinkage of their constituent microtubules are finely balanced. This balance is achieved, in part, through kinesin-8 proteins (such as Kip3p in budding yeast and KLP67A in Drosophila) that destabilize microtubules. Here, we directly demonstrate that Kip3p destabilizes microtubules by depolymerizing them--accounting for the effects of kinesin-8 perturbations on microtubule and spindle length observed in fungi and metazoan cells. Furthermore, using single-molecule microscopy assays, we show that Kip3p has several properties that distinguish it from other depolymerizing kinesins, such as the kinesin-13 MCAK. First, Kip3p disassembles microtubules exclusively at the plus end and second, remarkably, Kip3p depolymerizes longer microtubules faster than shorter ones. These properties are consequences of Kip3p being a highly processive, plus-end-directed motor, both in vitro and in vivo. Length-dependent depolymerization provides a new mechanism for controlling the lengths of subcellular structures.
微管细胞骨架和有丝分裂纺锤体是高度动态的结构，但其大小却显著恒定，这表明其组成微管的生长和收缩得到了精确平衡。这种平衡部分是通过使微管不稳定的驱动蛋白8家族蛋白（如芽殖酵母中的Kip3p和果蝇中的KLP67A）实现的。在这里，我们直接证明Kip3p通过解聚微管使其不稳定，这解释了在真菌和后生动物细胞中观察到的驱动蛋白8扰动对微管和纺锤体长度的影响。此外，通过单分子显微镜检测，我们发现Kip3p具有一些使其有别于其他解聚驱动蛋白（如驱动蛋白13 MCAK）的特性。首先，Kip3p仅在微管正端解聚微管，其次，值得注意的是，Kip3p解聚较长微管的速度比解聚较短微管的速度更快。这些特性是Kip3p在体外和体内都是高度持续的、正端定向的马达蛋白的结果。长度依赖性解聚为控制亚细胞结构的长度提供了一种新机制。
