Ems-McClung SC et al. (2020), RanGTP induces an effector gradient of XCTK2 an...

XB-ART-56577

J Cell Biol 2020 Feb 03;2192:. doi: 10.1083/jcb.201906045.

Show Gene links Show Anatomy links

RanGTP induces an effector gradient of XCTK2 and importin α/β for spindle microtubule cross-linking.

Ems-McClung SC , Emch M , Zhang S , Mahnoor S , Weaver LN , Walczak CE .

???displayArticle.abstract???
High RanGTP around chromatin is important for governing spindle assembly during meiosis and mitosis by releasing the inhibitory effects of importin α/β. Here we examine how the Ran gradient regulates Kinesin-14 function to control spindle organization. We show that Xenopus Kinesin-14, XCTK2, and importin α/β form an effector gradient that is highest at the poles and diminishes toward the chromatin, which is opposite the RanGTP gradient. Importin α and β preferentially inhibit XCTK2 antiparallel microtubule cross-linking and sliding by decreasing the microtubule affinity of the XCTK2 tail domain. This change in microtubule affinity enables RanGTP to target endogenous XCTK2 to the spindle. We propose that these combined actions of the Ran pathway are critical to promote Kinesin-14 parallel microtubule cross-linking to help focus spindle poles for efficient bipolar spindle assembly. Furthermore, our work illustrates that RanGTP regulation in the spindle is not simply a switch, but rather generates effector gradients where importins α and β gradually tune the activities of spindle assembly factors.

???displayArticle.pubmedLink??? 31865374
???displayArticle.pmcLink??? PMC7041689
???displayArticle.link??? J Cell Biol
???displayArticle.grants??? [+]

Species referenced: Xenopus
Genes referenced: kif14 kifc1 kpna1 numa1 ran
GO keywords: cytoskeleton [+]

???attribute.lit??? ???displayArticles.show???

	Figure 1. FRET biosensors recapitulate Ran-regulated association of XCTK2 and importin α/β. (A–C) Schematic (left) and solution-based FRET assay (right) of importin α-CyPet with YPet-XCTK2 ± importin β (A) ± RanQ69L (B) or YPet-XCTK2-ΔNLS + importin β (C). The normalized FRET ratios are graphed as the mean ± SEM from 440 to 600 nm (n = 3 independent experiments).
	Figure 2. XCTK2 forms a gradient of association with importin α/β from the poles to the chromatin. (A) Representative confocal fluorescence (CyPet, YPet, and MTs) and lifetime (FLIM) images of importin α-CyPet + nontagged XCTK2 (importin α-CyPet), importin α-CyPet + YPet-XCTK2-ΔNLS, and importin α-CyPet + YPet-XCTK2 spindle assembly reactions. Scale bar: 10 µm. (B) YPet fluorescence line scans of spindles assembled in A of importin α-CyPet with YPet-XCTK2-ΔNLS or YPet-XCTK2 normalized to percentage of spindle length (25 bins). YPet fluorescence percentage spindle length is graphed as the mean ± SEM (n = 30 YPet-XCTK2-ΔNLS + importin α-CyPet and 58 YPet-XCTK2 + importin α-CyPet spindles from three independent experiments). (C) Lifetime line scans of spindles imaged in A and B, where lifetimes are represented as the amplitude averaged lifetime (τAV/AMP), and lifetimes per normalized percentage spindle length are graphed as the mean ± SEM (n = 38 importin α-CyPet + XCTK2 spindles, 30 YPet-XCTK2-ΔNLS + importin α-CyPet, and 58 YPet-XCTK2 + importin α-CyPet spindles). (D) Line scans of the lifetimes from spindles with YPet-XCTK2 + importin α-CyPet relative to the fluorescence of the spindle MTs and the YPet fluorescence plotted as in B and C. (E) Chromatin and pole lifetime differences for each spindle analyzed in C with the mean ± SD indicated (D’Agostino and Pearson normality test and Brown–Forsythe’s and Welch’s with Dunnett’s multiple comparisons test: ****, P < 0.0001).
	Figure S1. XCTK2 tail MT binding is tunable by importin α/β association. (A) Schematic of FRET biosensors to detect XCTK2 and importin α/β association where XCTK2 is N-terminally tagged with CyPet (CyPet-XCTK2) and importin α is C-terminally tagged with YPet (importin α-YPet). (B) Solution-based FRET assay of CyPet-XCTK2 with importin α-YPet ± importin β. The normalized FRET ratios are graphed as mean ± SEM from 440 to 600 nm (n = 4–5 independent experiments). (C and D) Affinity assays of CyPet-XCTK2 with importin α-YPet ± importin β. The mean ± SD of CyPet-XCTK2 bound for each importin α-YPet concentration and the best-fit quadratic binding curves are graphed from C with the binding data summarized in D (n = 4–5 independent experiments; extra sum-of-squares F test compared with CyPet-XCTK2 + importin α-YPet: Kd, P = 0.0088; Bmax, P < 0.0001). CyPet-XCTK2 bound is graphed from 0 to 0.3 µM importin α-YPet, excluding 1 and 2.5 µM importin α-YPet data points. (E and F) MT affinity assays of YPet-XCTK2-Tail in the absence and presence of equimolar (1×) or fourfold excess (4×) of importin α/β. The amount of YPet-XCTK2-Tail bound is plotted as the mean ± SD for each MT concentration, and the best-fit quadratic binding curves are graphed (E) with the binding data summarized in F (n = 5–8 independent experiments; extra sum-of-squares F test compared with YPet-XCTK2-Tail alone: YPet-XCTK2-Tail + 1× importin α/β, Kd, NS; Bmax, P < 0.0001; YPet-XCTK2-Tail + 4× importin α/β, Kd, P < 0.0001; P = 0.0002). (D and F) , P < 0.01; , P < 0.001; ***, P < 0.0001.
	Figure 3. Importin α/β inhibit XCTK2 antiparallel MT cross-linking and sliding. (A) Schematic of XCTK2 antiparallel (left) and parallel (right) MT cross-linking and sliding assay with segmented MTs. (B) Representative images and kymographs of MT cross-linking and sliding with YPet-XCTK2 (left; Video 1) or YPet-XCTK2 with 4–8 molar excess of importin α/β (right; Video 2). The template MT minus (−) and plus (+) ends are indicated on the top kymograph image, and the sliding cargo MT plus end is indicated by an arrowhead. The YPet-XCTK2 kymograph images increment by 30-s intervals, and the YPet-XCTK2 + importin α/β increment by 75-s intervals to illustrate the multiple events over the course of the time lapse. Scale bars: 5 µm. (C–G) Quantification of the indicated conditions plotted as the number of cross-links per experiment, with the mean ± SD indicated as a bar graph, with the dots representing the values of individual experiments (n = 729 YPet-XCTK2 and 352 YPet-XCTK2 + importin α/β cross-linked MTs, n = 63 YPet-XCTK2 and 31 YPet-XCTK2 + importin α/β antiparallel cross-links, n = 81 YPet-XCTK2 and 56 YPet-XCTK2 + importin α/β parallel cross-links from six independent experiments; F test to compare variances and two-tailed Student’s or Welch’s t tests were performed: , P < 0.05). (H) The number of sliding events per MT cross-link is graphed with the mean ± SD (n = 212 YPet-XCTK2 and 140 YPet-XCTK2 + importin α/β cross-links from three independent experiments; D’Agostino and Pearson normality and log-normal tests and two-tailed Mann–Whitney t test: **, P < 0.0001). (I) Velocity of MT sliding events for antiparallel and parallel MT cross-links is graphed as a frequency histogram with the best-fit log Gaussian curve (n = 42 YPet-XCTK2 antiparallel and 71 parallel events, and n = 60 YPet-XCTK2 + importin α/β antiparallel and 116 parallel events from three independent experiments; D’Agostino and Pearson log-normal test and extra sum-of-squares F test to compare geometric means of the log Gaussian distribution of events: XCTK2 antiparallel versus parallel, , P = 0.0362; XCTK2 + importin α/β antiparallel versus parallel, **, P = 0.0050). The frequency histogram is graphed up to 60 nm/s, excluding a single point at 120 nm/s.
	Figure 4. The RanGTP gradient promotes global and local targeting of XCTK2 within the spindle. (A) Representative MT confocal and Rango-2 FLIM images of spindles assembled with XB control buffer, 10 µM Ran, or 20 µM Ran addition. Lifetimes are represented as the amplitude averaged lifetime (τAV/AMP). (B) Line scans from pole to pole of the spindle lifetime images for the indicated conditions normalized for percent spindle length (25 bins). Lifetimes per spindle length are graphed as the mean ± SEM (n = 57–78 spindles per condition from three to four independent experiments). (C) Lifetime difference between the pole and chromatin regions for each spindle in each condition from B (D’Agostino and Pearson normality test and one-way ANOVA with Tukey’s multiple comparisons test compared with XB control: , P < 0.05). (D) Representative wide-field fluorescence microscopy images of spindle assembly reactions from parallel experiments with XB control buffer or with added Ran that were stained with α-XCTK2. (E) Spindles from D were analyzed using a Cell Profiler pipeline that measured the mean total XCTK2 spindle fluorescence based on α-XCTK2 staining in the indicated conditions and plotted with the mean ± SD (n = 314–564 spindles per condition from five independent experiments). (F) Line scans of bipolar spindles from D were performed, normalized for percentage spindle length (101 bins), and graphed as the mean ± SEM (n = 168–248 spindles per condition from four independent experiments). (G) XCTK2 peak pole fluorescence plotted as the average fluorescence from the two poles per spindle analyzed in F. (H) XCTK2 chromatin fluorescence plotted as the center spindle position for each spindle analyzed in F. In G and H, the mean ± SD is indicated. (E–H) D’Agostino and Pearson normality tests and Kruskal–Wallis with Dunn’s multiple comparisons tests compared with XB control were performed: , P < 0.05; ****, P < 0.0001. Scale bars: 10 µm.
	Figure S2. Enhancing the RanGTP gradient does not change spindle morphology or NuMA localization. (A–E) Spindle morphology analysis with Cell Profiler of spindles measured in Fig. 4 D and graphed with the mean ± SD (n = 314–564 spindles per condition from four to five independent experiments; D’Agostino and Pearson normality and Kruskal–Wallis with Dunn’s multiple comparisons tests were performed, and statistical significance is compared with XB control buffer: **, P < 0.0001). (F) Line scan analysis of MT polymer distribution of spindles analyzed in Fig. 4 F graphed as the mean ± SEM. (G) Representative wide-field fluorescence microscopy images of spindle assembly reactions with XB control buffer, 10 µM Ran, or 20 µM Ran addition stained with α-NuMA that were performed in parallel to those in Fig. 4. Scale bar: 10 µm. (H) Total α-NuMA fluorescence of spindles assembled in G and plotted with the mean ± SD (n = 209–240 spindles per condition from four independent experiments; D’Agostino and Pearson normality and Kruskal–Wallis with Dunn’s multiple comparisons tests were performed compared with XB control buffer: **, P < 0.0001). (I) Line scan analysis of bipolar spindles analyzed in G, normalized for percentage spindle length (101 bins), and graphed as the mean ± SEM (n = 86–134 spindles per condition from four independent experiments). (J and K) NuMA peak pole and chromatin fluorescence as calculated in Fig. 4 (G and H) and plotted with the mean ± SD (D’Agostino and Pearson normality and Kruskal–Wallis with Dunn’s multiple comparisons tests were performed compared with XB buffer control: NS).
	Figure 5. The RanGTP gradient generates an effector gradient of Kinesin-14s for preferential parallel MT cross-linking and sliding near the spindle poles. (A) Representative confocal fluorescence (CyPet, YPet, and MT) and lifetime (FLIM) images of spindles assembled with importin α-CyPet + nontagged XCTK2 + Ran (importin α-CyPet + 20 µM Ran), importin α-CyPet + YPet-XCTK2, or importin α-CyPet + YPet-XCTK2 + 20 µM Ran). Scale bar: 10 µm. (B) YPet fluorescence line scans of spindles assembled in A of importin α-CyPet with YPet-XCTK2 ± Ran normalized to percentage spindle length (25 bins) and graphed as the mean ± SEM (n = 60 YPet-XCTK2 and 97 YPet-XCTK2 + Ran spindles from four to five independent experiments). (C) Lifetime line scans of spindles imaged in A where lifetimes are represented as the amplitude averaged lifetime (τAV/AMP), and lifetimes per percentage of spindle length are graphed as the mean ± SEM (n = 68 importin α-CyPet + XCTK2 + Ran spindles, 27 importin α-CyPet + YPet-XCTK2, and 52 importin α-CyPet + YPet-XCTK2 + Ran spindles from four to five independent experiments). (D) XCTK2 cross-links and slides both antiparallel and parallel MTs near the chromatin where RanGTP is high and the association with importin α/β is low. Near the spindle poles where RanGTP is low, importin α/β can bind to the XCTK2 tail and can selectively inhibit XCTK2 antiparallel MT cross-linking. Thus, the RanGTP gradient sets up an opposing effector gradient of importin α/β association with XCTK2 that promotes pole focusing through preferential parallel MT cross-linking and sliding near the poles. (E) In cancer cells with centrosome amplification and high RanGTP, we propose that heightened RanGTP increases Kinesin-14 association to the spindle and that the RanGTP gradient biases Kinesin-14 localization toward the spindle poles, where reduced importin α/β association sets up an effector gradient of increased Kinesin-14 cross-linking and sliding activity that mediates centrosome clustering for cancer cell survival.
	Figure S3. Enhancing the RanGTP gradient decreases the association of XCTK2 with importin α/β without affecting the steepness of the effector gradient. (A) YPet-XCTK2 mean fluorescence across the spindle ± Ran from spindles imaged in Fig. 5 A, calculated from the line scans performed in Fig. 5 B, and graphed with the mean ± SD (n = 60 YPet-XCTK2 spindles and 97 YPet-XCTK2 + Ran spindles from four to five independent experiments; D’Agostino and Pearson normality test and two-tailed Mann–Whitney t test were performed: NS). (B) YPet-XCTK2 fluorescence difference between the pole and chromatin regions ± Ran for each spindle line scan in Fig. 5 B and graphed with the mean ± SD (D’Agostino and Pearson normality test and two-tailed Mann–Whitney t test were performed: **, P < 0.0001). (C) Mean line scan lifetime across the spindles of importin α-CyPet ± YPet-XCTK2 ± Ran calculated from the line scans performed in Fig. 5 C and graphed with the mean ± SD (n = 68 importin α-CyPet + Ran spindles, 27 YPet-XCTK2 + importin α-CyPet spindles, and 52 YPet-XCTK2 + Ran + importin α-CyPet spindles from four to five independent experiments; D’Agostino and Pearson normality tests, F tests to compare variances, Mann–Whitney t tests compared with importin α-CyPet + Ran control and a Student’s t test between YPet-XCTK2 and YPet-XCTK2 + Ran were performed: , P < 0.0001; , P < 0.01). (D) Lifetime difference between the pole and chromatin regions for each spindle from Fig. 5 C and graphed ± SD (D’Agostino and Pearson normality tests, F tests to compare variances, Welch’s t test compared with importin α-CyPet + Ran control, and Student’s t test between YPet-XCTK2 and YPet-XCTK2 + Ran were performed: , P < 0.05; *, P < 0.01).

References [+] :

Athale, Regulation of microtubule dynamics by reaction cascades around chromosomes. 2008, Pubmed, Xenbase

	Figure 1. FRET biosensors recapitulate Ran-regulated association of XCTK2 and importin α/β. (A–C) Schematic (left) and solution-based FRET assay (right) of importin α-CyPet with YPet-XCTK2 ± importin β (A) ± RanQ69L (B) or YPet-XCTK2-ΔNLS + importin β (C). The normalized FRET ratios are graphed as the mean ± SEM from 440 to 600 nm (n = 3 independent experiments).
	Figure 2. XCTK2 forms a gradient of association with importin α/β from the poles to the chromatin. (A) Representative confocal fluorescence (CyPet, YPet, and MTs) and lifetime (FLIM) images of importin α-CyPet + nontagged XCTK2 (importin α-CyPet), importin α-CyPet + YPet-XCTK2-ΔNLS, and importin α-CyPet + YPet-XCTK2 spindle assembly reactions. Scale bar: 10 µm. (B) YPet fluorescence line scans of spindles assembled in A of importin α-CyPet with YPet-XCTK2-ΔNLS or YPet-XCTK2 normalized to percentage of spindle length (25 bins). YPet fluorescence percentage spindle length is graphed as the mean ± SEM (n = 30 YPet-XCTK2-ΔNLS + importin α-CyPet and 58 YPet-XCTK2 + importin α-CyPet spindles from three independent experiments). (C) Lifetime line scans of spindles imaged in A and B, where lifetimes are represented as the amplitude averaged lifetime (τAV/AMP), and lifetimes per normalized percentage spindle length are graphed as the mean ± SEM (n = 38 importin α-CyPet + XCTK2 spindles, 30 YPet-XCTK2-ΔNLS + importin α-CyPet, and 58 YPet-XCTK2 + importin α-CyPet spindles). (D) Line scans of the lifetimes from spindles with YPet-XCTK2 + importin α-CyPet relative to the fluorescence of the spindle MTs and the YPet fluorescence plotted as in B and C. (E) Chromatin and pole lifetime differences for each spindle analyzed in C with the mean ± SD indicated (D’Agostino and Pearson normality test and Brown–Forsythe’s and Welch’s with Dunnett’s multiple comparisons test: ****, P < 0.0001).
	Figure S1. XCTK2 tail MT binding is tunable by importin α/β association. (A) Schematic of FRET biosensors to detect XCTK2 and importin α/β association where XCTK2 is N-terminally tagged with CyPet (CyPet-XCTK2) and importin α is C-terminally tagged with YPet (importin α-YPet). (B) Solution-based FRET assay of CyPet-XCTK2 with importin α-YPet ± importin β. The normalized FRET ratios are graphed as mean ± SEM from 440 to 600 nm (n = 4–5 independent experiments). (C and D) Affinity assays of CyPet-XCTK2 with importin α-YPet ± importin β. The mean ± SD of CyPet-XCTK2 bound for each importin α-YPet concentration and the best-fit quadratic binding curves are graphed from C with the binding data summarized in D (n = 4–5 independent experiments; extra sum-of-squares F test compared with CyPet-XCTK2 + importin α-YPet: Kd, P = 0.0088; Bmax, P < 0.0001). CyPet-XCTK2 bound is graphed from 0 to 0.3 µM importin α-YPet, excluding 1 and 2.5 µM importin α-YPet data points. (E and F) MT affinity assays of YPet-XCTK2-Tail in the absence and presence of equimolar (1×) or fourfold excess (4×) of importin α/β. The amount of YPet-XCTK2-Tail bound is plotted as the mean ± SD for each MT concentration, and the best-fit quadratic binding curves are graphed (E) with the binding data summarized in F (n = 5–8 independent experiments; extra sum-of-squares F test compared with YPet-XCTK2-Tail alone: YPet-XCTK2-Tail + 1× importin α/β, Kd, NS; Bmax, P < 0.0001; YPet-XCTK2-Tail + 4× importin α/β, Kd, P < 0.0001; P = 0.0002). (D and F) , P < 0.01; , P < 0.001; ***, P < 0.0001.
	Figure 3. Importin α/β inhibit XCTK2 antiparallel MT cross-linking and sliding. (A) Schematic of XCTK2 antiparallel (left) and parallel (right) MT cross-linking and sliding assay with segmented MTs. (B) Representative images and kymographs of MT cross-linking and sliding with YPet-XCTK2 (left; Video 1) or YPet-XCTK2 with 4–8 molar excess of importin α/β (right; Video 2). The template MT minus (−) and plus (+) ends are indicated on the top kymograph image, and the sliding cargo MT plus end is indicated by an arrowhead. The YPet-XCTK2 kymograph images increment by 30-s intervals, and the YPet-XCTK2 + importin α/β increment by 75-s intervals to illustrate the multiple events over the course of the time lapse. Scale bars: 5 µm. (C–G) Quantification of the indicated conditions plotted as the number of cross-links per experiment, with the mean ± SD indicated as a bar graph, with the dots representing the values of individual experiments (n = 729 YPet-XCTK2 and 352 YPet-XCTK2 + importin α/β cross-linked MTs, n = 63 YPet-XCTK2 and 31 YPet-XCTK2 + importin α/β antiparallel cross-links, n = 81 YPet-XCTK2 and 56 YPet-XCTK2 + importin α/β parallel cross-links from six independent experiments; F test to compare variances and two-tailed Student’s or Welch’s t tests were performed: , P < 0.05). (H) The number of sliding events per MT cross-link is graphed with the mean ± SD (n = 212 YPet-XCTK2 and 140 YPet-XCTK2 + importin α/β cross-links from three independent experiments; D’Agostino and Pearson normality and log-normal tests and two-tailed Mann–Whitney t test: **, P < 0.0001). (I) Velocity of MT sliding events for antiparallel and parallel MT cross-links is graphed as a frequency histogram with the best-fit log Gaussian curve (n = 42 YPet-XCTK2 antiparallel and 71 parallel events, and n = 60 YPet-XCTK2 + importin α/β antiparallel and 116 parallel events from three independent experiments; D’Agostino and Pearson log-normal test and extra sum-of-squares F test to compare geometric means of the log Gaussian distribution of events: XCTK2 antiparallel versus parallel, , P = 0.0362; XCTK2 + importin α/β antiparallel versus parallel, **, P = 0.0050). The frequency histogram is graphed up to 60 nm/s, excluding a single point at 120 nm/s.
	Figure 4. The RanGTP gradient promotes global and local targeting of XCTK2 within the spindle. (A) Representative MT confocal and Rango-2 FLIM images of spindles assembled with XB control buffer, 10 µM Ran, or 20 µM Ran addition. Lifetimes are represented as the amplitude averaged lifetime (τAV/AMP). (B) Line scans from pole to pole of the spindle lifetime images for the indicated conditions normalized for percent spindle length (25 bins). Lifetimes per spindle length are graphed as the mean ± SEM (n = 57–78 spindles per condition from three to four independent experiments). (C) Lifetime difference between the pole and chromatin regions for each spindle in each condition from B (D’Agostino and Pearson normality test and one-way ANOVA with Tukey’s multiple comparisons test compared with XB control: , P < 0.05). (D) Representative wide-field fluorescence microscopy images of spindle assembly reactions from parallel experiments with XB control buffer or with added Ran that were stained with α-XCTK2. (E) Spindles from D were analyzed using a Cell Profiler pipeline that measured the mean total XCTK2 spindle fluorescence based on α-XCTK2 staining in the indicated conditions and plotted with the mean ± SD (n = 314–564 spindles per condition from five independent experiments). (F) Line scans of bipolar spindles from D were performed, normalized for percentage spindle length (101 bins), and graphed as the mean ± SEM (n = 168–248 spindles per condition from four independent experiments). (G) XCTK2 peak pole fluorescence plotted as the average fluorescence from the two poles per spindle analyzed in F. (H) XCTK2 chromatin fluorescence plotted as the center spindle position for each spindle analyzed in F. In G and H, the mean ± SD is indicated. (E–H) D’Agostino and Pearson normality tests and Kruskal–Wallis with Dunn’s multiple comparisons tests compared with XB control were performed: , P < 0.05; ****, P < 0.0001. Scale bars: 10 µm.
	Figure S2. Enhancing the RanGTP gradient does not change spindle morphology or NuMA localization. (A–E) Spindle morphology analysis with Cell Profiler of spindles measured in Fig. 4 D and graphed with the mean ± SD (n = 314–564 spindles per condition from four to five independent experiments; D’Agostino and Pearson normality and Kruskal–Wallis with Dunn’s multiple comparisons tests were performed, and statistical significance is compared with XB control buffer: **, P < 0.0001). (F) Line scan analysis of MT polymer distribution of spindles analyzed in Fig. 4 F graphed as the mean ± SEM. (G) Representative wide-field fluorescence microscopy images of spindle assembly reactions with XB control buffer, 10 µM Ran, or 20 µM Ran addition stained with α-NuMA that were performed in parallel to those in Fig. 4. Scale bar: 10 µm. (H) Total α-NuMA fluorescence of spindles assembled in G and plotted with the mean ± SD (n = 209–240 spindles per condition from four independent experiments; D’Agostino and Pearson normality and Kruskal–Wallis with Dunn’s multiple comparisons tests were performed compared with XB control buffer: **, P < 0.0001). (I) Line scan analysis of bipolar spindles analyzed in G, normalized for percentage spindle length (101 bins), and graphed as the mean ± SEM (n = 86–134 spindles per condition from four independent experiments). (J and K) NuMA peak pole and chromatin fluorescence as calculated in Fig. 4 (G and H) and plotted with the mean ± SD (D’Agostino and Pearson normality and Kruskal–Wallis with Dunn’s multiple comparisons tests were performed compared with XB buffer control: NS).
	Figure 5. The RanGTP gradient generates an effector gradient of Kinesin-14s for preferential parallel MT cross-linking and sliding near the spindle poles. (A) Representative confocal fluorescence (CyPet, YPet, and MT) and lifetime (FLIM) images of spindles assembled with importin α-CyPet + nontagged XCTK2 + Ran (importin α-CyPet + 20 µM Ran), importin α-CyPet + YPet-XCTK2, or importin α-CyPet + YPet-XCTK2 + 20 µM Ran). Scale bar: 10 µm. (B) YPet fluorescence line scans of spindles assembled in A of importin α-CyPet with YPet-XCTK2 ± Ran normalized to percentage spindle length (25 bins) and graphed as the mean ± SEM (n = 60 YPet-XCTK2 and 97 YPet-XCTK2 + Ran spindles from four to five independent experiments). (C) Lifetime line scans of spindles imaged in A where lifetimes are represented as the amplitude averaged lifetime (τAV/AMP), and lifetimes per percentage of spindle length are graphed as the mean ± SEM (n = 68 importin α-CyPet + XCTK2 + Ran spindles, 27 importin α-CyPet + YPet-XCTK2, and 52 importin α-CyPet + YPet-XCTK2 + Ran spindles from four to five independent experiments). (D) XCTK2 cross-links and slides both antiparallel and parallel MTs near the chromatin where RanGTP is high and the association with importin α/β is low. Near the spindle poles where RanGTP is low, importin α/β can bind to the XCTK2 tail and can selectively inhibit XCTK2 antiparallel MT cross-linking. Thus, the RanGTP gradient sets up an opposing effector gradient of importin α/β association with XCTK2 that promotes pole focusing through preferential parallel MT cross-linking and sliding near the poles. (E) In cancer cells with centrosome amplification and high RanGTP, we propose that heightened RanGTP increases Kinesin-14 association to the spindle and that the RanGTP gradient biases Kinesin-14 localization toward the spindle poles, where reduced importin α/β association sets up an effector gradient of increased Kinesin-14 cross-linking and sliding activity that mediates centrosome clustering for cancer cell survival.
	Figure S3. Enhancing the RanGTP gradient decreases the association of XCTK2 with importin α/β without affecting the steepness of the effector gradient. (A) YPet-XCTK2 mean fluorescence across the spindle ± Ran from spindles imaged in Fig. 5 A, calculated from the line scans performed in Fig. 5 B, and graphed with the mean ± SD (n = 60 YPet-XCTK2 spindles and 97 YPet-XCTK2 + Ran spindles from four to five independent experiments; D’Agostino and Pearson normality test and two-tailed Mann–Whitney t test were performed: NS). (B) YPet-XCTK2 fluorescence difference between the pole and chromatin regions ± Ran for each spindle line scan in Fig. 5 B and graphed with the mean ± SD (D’Agostino and Pearson normality test and two-tailed Mann–Whitney t test were performed: **, P < 0.0001). (C) Mean line scan lifetime across the spindles of importin α-CyPet ± YPet-XCTK2 ± Ran calculated from the line scans performed in Fig. 5 C and graphed with the mean ± SD (n = 68 importin α-CyPet + Ran spindles, 27 YPet-XCTK2 + importin α-CyPet spindles, and 52 YPet-XCTK2 + Ran + importin α-CyPet spindles from four to five independent experiments; D’Agostino and Pearson normality tests, F tests to compare variances, Mann–Whitney t tests compared with importin α-CyPet + Ran control and a Student’s t test between YPet-XCTK2 and YPet-XCTK2 + Ran were performed: , P < 0.0001; , P < 0.01). (D) Lifetime difference between the pole and chromatin regions for each spindle from Fig. 5 C and graphed ± SD (D’Agostino and Pearson normality tests, F tests to compare variances, Welch’s t test compared with importin α-CyPet + Ran control, and Student’s t test between YPet-XCTK2 and YPet-XCTK2 + Ran were performed: , P < 0.05; *, P < 0.01).