The Ran-GTP Gradient Spatially Regulates XCTK2 in the Spindle

• Published in: Current biology Vol. 25; no. 11; pp. 1509 - 1514
• Main Authors: Weaver, Lesley N., Ems-McClung, Stephanie C., Chen, Sez-Hon R., Yang, Ge, Shaw, Sidney L., Walczak, Claire E.
• Format: Journal Article
• Language: English
• Published: England Elsevier Ltd 01.06.2015
• Subjects: Animals; Cell Line; Guanosine Triphosphate - metabolism; Karyopherins - metabolism; Kinesin - metabolism; ran GTP-Binding Protein - metabolism; Spindle Apparatus - metabolism; Spodoptera; Xenopus; Xenopus Proteins - metabolism
• ISSN: 0960-9822; 1879-0445
• DOI: 10.1016/j.cub.2015.04.015

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. [Display omitted] •XCTK2 requires a gradient of Ran-GTP to stimulate spindle assembly•The Ran-GTP gradient regulates XCTK2 microtubule binding affinity•XCTK2 turnover kinetics are spatially regulated in the spindle The Kinesin-14 XCTK2 is a minus-end-directed microtubule crosslinking motor that stimulates bipolar spindle assembly and focuses spindle poles. Weaver et al. demonstrate that a spatial gradient of Ran-GTP is necessary to control motor binding to the spindle and thus to regulate XCTK2 function during spindle assembly.