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Curr Biol. 2015 Jun 1;25(11):1509-14. doi: 10.1016/j.cub.2015.04.015. Epub 2015 May 14.

The Ran-GTP gradient spatially regulates XCTK2 in the spindle.

Author information

1
Department of Biology, Indiana University, 915 E. 3(rd) Street, Bloomington, IN 47405, USA.
2
Medical Sciences, Indiana University, 915 E. 3(rd) Street, Bloomington, IN 47405, USA.
3
Department of Biomedical Engineering and Computational Biology, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA 15213, USA.
4
Medical Sciences, Indiana University, 915 E. 3(rd) Street, Bloomington, IN 47405, USA. Electronic address: cwalczak@indiana.edu.

Abstract

Ran is a small GTP binding protein that was originally identified as a regulator of nucleocytoplasmic transport [1] and subsequently found to be important for spindle formation [2-5]. In mitosis, a gradient of Ran-GTP emanates from chromatin and diminishes toward spindle poles [6]. Ran-GTP promotes spindle self-organization through the release of importin-bound spindle assembly factors (SAFs), which stimulate microtubule (MT) nucleation and organization and regulate MT dynamics [7-9]. Although many SAFs are non-motile MT-associated proteins, such as NuMA, TPX2, and HURP [7, 10-12], Ran also controls motor proteins, including Kid and HSET/XCTK2 [13, 14]. The Kinesin-14 XCKT2 is important for spindle assembly and pole organization [15-20], and Ran-GTP is proposed to promote XCKT2 MT crosslinking activity by releasing importin α/β from a bipartite nuclear localization signal (NLS) located in the tail domain [14]. Here, we show that the Ran-GTP gradient spatially regulates XCTK2 within the spindle. A flattened Ran-GTP gradient blocked the ability of excess XCTK2 to stimulate bipolar spindle assembly and resulted in XCTK2-mediated bundling of free MTs. These effects required the XCTK2 tail, which promoted the motility of XCTK2 within the spindle independent of the Ran-GTP gradient. In addition, the turnover kinetics of XCTK2 were spatially controlled: they were faster near the poles relative to the chromatin, but not with a mutant XCTK2 that cannot bind to importin α/β. Our results support a model in which the Ran-GTP gradient spatially coordinates motor localization with motility to ensure efficient spindle formation.

PMID:
25981788
PMCID:
PMC4452415
DOI:
10.1016/j.cub.2015.04.015
[Indexed for MEDLINE]
Free PMC Article

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