title: The Ran-GTP gradient spatially regulates XCTK2 in the spindle.
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.
authors.affiliation: Department of Biology, Indiana University, 915 E. 3(rd) Street, Bloomington, IN 47405, USA.
authors.forename: Lesley N
authors.initials: LN
authors.lastname: Weaver
authors.affiliation: Medical Sciences, Indiana University, 915 E. 3(rd) Street, Bloomington, IN 47405, USA.
authors.forename: Stephanie C
authors.initials: SC
authors.lastname: Ems-McClung
authors.affiliation: Department of Biomedical Engineering and Computational Biology, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA 15213, USA.
authors.forename: Sez-Hon R
authors.initials: SH
authors.lastname: Chen
authors.affiliation: Department of Biomedical Engineering and Computational Biology, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA 15213, USA.
authors.forename: Ge
authors.initials: G
authors.lastname: Yang
authors.affiliation: Department of Biology, Indiana University, 915 E. 3(rd) Street, Bloomington, IN 47405, USA.
authors.forename: Sidney L
authors.initials: SL
authors.lastname: Shaw
authors.affiliation: Medical Sciences, Indiana University, 915 E. 3(rd) Street, Bloomington, IN 47405, USA. Electronic address: cwalczak@indiana.edu.
authors.forename: Claire E
authors.initials: CE
authors.lastname: Walczak
mesh_terms: D000818:Animals; D002460:Cell Line; D006160:Guanosine Triphosphate / Q000378:metabolism; D028884:Karyopherins / Q000378:metabolism; D016547:Kinesins / Q000378:metabolism*; D008941:Spindle Apparatus / Q000378:metabolism*; D018411:Spodoptera; D014981:Xenopus; D029867:Xenopus Proteins / Q000378:metabolism*; D020931:ran GTP-Binding Protein / Q000378:metabolism*
chemical_list: C104595:KIFC1 protein, Xenopus; D028884:Karyopherins; D029867:Xenopus Proteins; D006160:Guanosine Triphosphate; D016547:Kinesins; D020931:ran GTP-Binding Protein