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. Author manuscript; available in PMC: 2017 Jun 2.
Published in final edited form as: Cell. 2016 May 5;165(6):1428–1439. doi: 10.1016/j.cell.2016.04.030

A TOG Protein Confers Tension Sensitivity to Kinetochore-Microtubule Attachments

Matthew P Miller 1, Charles L Asbury 2,*, Sue Biggins 1,*
PMCID: PMC4892958  NIHMSID: NIHMS778545  PMID: 27156448

Abstract

The development and survival of all organisms depends on equal partitioning of their genomes during cell division. Accurate chromosome segregation requires selective stabilization of kinetochore-microtubule attachments that come under tension due to opposing pulling forces exerted on sister kinetochores by dynamic microtubule tips. Here, we show that the XMAP215 family member, Stu2, makes a major contribution to kinetochore-microtubule coupling. Stu2 and its human ortholog, ch-TOG, exhibit a conserved interaction with the Ndc80 kinetochore complex that strengthens its attachment to microtubule tips. Strikingly, Stu2 can either stabilize or destabilize kinetochore attachments, depending on the level of kinetochore tension and whether the microtubule tip is assembling or disassembling. These dichotomous effects of Stu2 are independent of its previously studied regulation of microtubule dynamics. Altogether, our results demonstrate how a kinetochore-associated factor can confer opposing, tension-dependent effects to selectively stabilize tension-bearing attachments, providing mechanistic insight into the basis for accuracy during chromosome segregation.

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Introduction

Cellular and organismal fitness requires proper partitioning of genetic material during cell division. Failure to accurately segregate chromosomes causes aneuploidy, the most prevalent genetic alteration in tumor cells and a potential factor in the evolution of cancer (reviewed in Gordon et al., 2012). Chromosome segregation is driven by microtubule-based forces, which are generated at kinetochores. The kinetochores must stay bound to microtubule ‘plus ends’, where tubulin subunits are added and lost at a high rate and where the microtubule filaments switch stochastically between phases of assembly and disassembly (Mitchison and Kirschner, 1984).

Kinetochores are conserved macromolecular complexes containing multiple copies of various subcomplexes that assemble onto centromeric DNA (reviewed in Cheeseman, 2014). The major microtubule binding activity within the kinetochore is attributed to the conserved Ndc80 complex (the Ndc80 protein is termed Hec1 in humans) because knockdowns in vivo cause severe defects in kinetochore-microtubule attachment (Cheeseman et al., 2006; DeLuca et al., 2005; McCleland et al., 2004; Wigge and Kilmartin, 2001). However, additional complexes interact with the Ndc80 complex and contribute to attachments, such as the yeast Dam1 complex and its putative functional ortholog, the human Ska complex (Cheeseman et al., 2001; Hanisch et al., 2006; Welburn et al., 2009). While much is understood about how these subcomplexes function alone, it is not known how the activities of these various complexes are coordinated within the larger kinetochore structure. In addition, the extent to which additional kinetochore components contribute to kinetochore-microtubule attachment remains unclear.

To ensure accurate chromosome segregation, sister kinetochores must ‘biorient’, attaching to microtubules from opposite poles, prior to anaphase. Once kinetochores biorient, they come under tension from opposing microtubule pulling forces. Pioneering work showed that incorrect kinetochore attachments are unstable due to the absence of tension (Dietz, 1958; Nicklas and Koch, 1969). The selective release of attachments lacking tension gives the cell another chance to establish proper attachments. While this error correction process relies partly on the Aurora B kinase, which phosphorylates Ndc80 and other kinetochore proteins (reviewed in Carmena et al., 2012; Krenn and Musacchio, 2015), kinetochore-microtubule attachments also possess an intrinsic tension selectivity. Tension directly stabilizes attachments independently of the Aurora B error correction system (Akiyoshi et al., 2010) via two inter-related properties: First, kinetochores bind more stably to assembling tips than to disassembling tips. Second, tension promotes microtubule assembly, which therefore reinforces kinetochore-microtubule attachments at higher forces. Although these properties are sufficient to explain the stabilization of kinetochore-microtubule attachments by tension, specific factors that mediate this activity have not yet been identified.

One conserved family of proteins that localizes to kinetochores and microtubule tips and could therefore contribute to the tension-dependent stabilization of attachments is the XMAP215 family (ch-TOG in humans and Stu2 in budding yeast) (Gard and Kirschner, 1987; He et al., 2001; Hsu and Toda, 2011; Kalantzaki et al., 2015; Ohkura et al., 1988; Tanaka et al., 2005; Tang et al., 2013; Wang and Huffaker, 1997). These proteins generally function as microtubule polymerases by accelerating growth and inhibiting catastrophe (Al-Bassam et al., 2006, 2012; Brouhard et al., 2008; Podolski et al., 2014; Widlund et al., 2011), although microtubule-destabilizing activity has also been reported in some contexts (van Breugel et al., 2003; Shirasu-Hiza et al., 2003). They are large proteins that contain highly conserved tumor over-expressed gene (TOG) domain arrays that bind curved tubulin dimers and are thought to accelerate growth by increasing the effective concentration of tubulin subunits near the microtubule plus end (Ayaz et al., 2012, 2014; Fox et al., 2014). They also contain additional functional domains (reviewed in Al-Bassam and Chang, 2011), such as a basic linker that promotes binding to the microtubule lattice. Nearly all XMAP215 orthologs are essential for viability and localize to a variety of microtubule-related structures (reviewed in Al-Bassam and Chang, 2011). Intriguingly, in both yeast and human cells, loss of the XMAP215 family member leads to chromosome alignment defects and to the appearance of detached kinetochores, suggesting a role in attaching kinetochores to microtubules (Gandhi et al., 2011; Gergely et al., 2003; Gillett et al., 2004; Kitamura et al., 2010; Kosco et al., 2001; Marco et al., 2013; Meraldi et al., 2004; Severin et al., 2001). In fission yeast, the XMAP215 homologs bind to the kinetochore and are implicated in regulating microtubule attachments (Hsu and Toda, 2011; Tang et al., 2013). However, these phenotypes are generally assumed to arise indirectly due to their effects on microtubule dynamics. Whether this protein family also participates more directly in kinetochore-microtubule attachment remains uncertain.

Here, we use a reconstitution system to uncover a direct role for the XMAP215 family in kinetochore-microtubule coupling. We show that a conserved interaction between Stu2 and the Ndc80 complex strengthens kinetochore- and Ndc80-based tip attachments in vitro. Surprisingly, this function of kinetochore-associated Stu2 does not require its polymerase activity. Instead, we find that the presence of Stu2 on kinetochores directly stabilizes their attachments to assembling microtubules while destabilizing their attachments to disassembling microtubules and, furthermore, that these activities are force-dependent. These force- and microtubule assembly state-dependent activities of Stu2 impart tension selectivity to the kinetochore, enabling it to remain tip-attached for longer durations when tension is increased. Together, our findings suggest that kinetochore-associated Stu2 activity is critical for tuning kinetochore function to make proper microtubule attachments, providing mechanistic insight into the manner in which tension promotes accurate chromosome segregation.

Results

Stu2 kinetochore association depends on an interaction with the Ndc80 complex

We previously detected Stu2 co-purifying with native yeast kinetochores by mass spectrometry (Akiyoshi et al., 2010), suggesting it might contribute to the activity of reconstituted kinetochore-microtubule interactions in vitro. To begin analyzing this, we first confirmed that Stu2 is present on isolated kinetochore particles. Native kinetochores are isolated from budding yeast cells via single-step immunoprecipitation of the Mis12/MIND/Mtw1 complex component Dsn1-His-Flag (Akiyoshi et al., 2010). These kinetochore particles contain the “core” kinetochore components but lack tubulin and some other transiently associated factors (Akiyoshi et al., 2010). We confirmed that Stu2 is present on isolated kinetochores by immunoblotting (Figure 1A).

Figure 1. Stu2 is a core kinetochore component that associates with the Ndc80 complex.

Figure 1

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A) Protein lysates were prepared from Stu2-13Myc (SBY2861), Dsn1-6His-3Flag (SBY8253) or Stu2-13Myc Dsn1-6His-3Flag (SBY10343) yeast strains. Kinetochore particles were purified by α-Flag immunoprecipitation (IP) and analyzed by immunoblotting. Ctf19 is an inner kinetochore component shown as a control.

B) Protein lysates prepared from cultures shifted to 37°C (for 2 hours) containing Stu2-13Myc (SBY2861), Dsn1-6His-3Flag (SBY8253), Stu2-13Myc Dsn1-6His-3Flag (SBY10343) or Stu2-13Myc Dsn1-6His-3Flag in combination with the temperature sensitive alleles dad1-1 (SBY10345), ndc80-1 (SBY10434) or spc105-15 (SBY10438). Kinetochore particles were purified by α-Flag IP and analyzed by immunoblotting.

C) Protein lysates prepared from strains containing Stu2-3V5 (SBY11709) and Spc24-6His-3Flag Spc105-AID (SBY14022), respectively. Immobilized Ndc80c-beads were incubated with Stu2-3V5, washed and eluted with Flag peptide. Control beads lacking Ndc80c were incubated with untagged lysate (from SBY3) prior to incubating with Stu2-3V5. Ndc80c-bound proteins were analyzed by silver stained SDS-PAGE.

To understand how Stu2 localizes to kinetochores in the absence of microtubules, we identified the subcomplex required for Stu2-kinetochore association. Kinetochore particles were purified from cells carrying temperature sensitive alleles of a component of the Dam1 (dad1-1), Ndc80 (ndc80-1) or KNL1Spc105 (spc105-15) complexes. The Stu2 kinetochore-association was disrupted only in cells carrying an ndc80-1 allele (Figure 1B), in agreement with previously reported chromatin immunoprecipitation data (He et al., 2001; Ma et al., 2007). We further confirmed this by isolating kinetochores from cells carrying an ndc80-AID (auxin inducible degron; Nishimura et al., 2009) allele indicating there is a kinetochore-bound pool of Stu2 that requires the Ndc80 complex for its association (Figure S1A).

Stu2 binds directly to the Ndc80 complex in vitro

Although the fission yeast Stu2 homologs interact with the Ndc80 complex (Hsu and Toda, 2011; Tang et al., 2013), the budding yeast Stu2 and Ndc80 proteins were reported to not interact in a yeast two-hybrid assay (Maure et al., 2011). We therefore directly tested whether Stu2 and Ndc80 complex (Ndc80c) associate. We independently isolated them via single-step immunoprecipitation of Stu2-V5 or an Ndc80c component, Spc24-His-Flag, followed by high salt washes to remove co-purifying proteins (Figure 1C & S1B). These conditions result in the isolation of the heterotetrameric Ndc80c or Stu2 to high purity (Figure 1C). We then incubated immobilized Ndc80c with purified Stu2-V5 and detected a specific interaction (Figure 1C & S1D), suggesting that Stu2 associates with kinetochores via Ndc80c.

Kinetochore-associated Stu2 makes a major contribution to attachment strength

To analyze the function of Stu2 at kinetochores, we generated cells containing a stu2-AID allele at the endogenous locus that targets the protein for degradation when the TIR1 F-box protein and the hormone auxin are present (Nishimura et al., 2009). Under these conditions, the Stu2-AID protein is rapidly degraded (Figure S2A) and the cells are inviable (Figure 2A). To determine whether these Stu2-depleted cells display a defect in kinetochore-microtubule attachments in vivo, as previously observed in various stu2 mutants (Gandhi et al., 2011; Gillett et al., 2004; Kitamura et al., 2010; Kosco et al., 2001; Marco et al., 2013; Severin et al., 2001), we examined spindle morphology and kinetochore distribution by fluorescence microscopy. In budding yeast, properly attached bioriented kinetochores cluster and exhibit a characteristic bi-lobed distribution at metaphase when they come under tension (Goshima and Yanagida, 2000; He et al., 2000; Pearson et al., 2001). As expected, when cells were arrested in metaphase (using a cdc20-AID strain) nearly all had a bipolar spindle and bi-lobed kinetochore foci (Figure 2B-C; 98±1%). In contrast, cells depleted of Stu2 (cdc20-AID stu2-AID) arrested with an abnormally short bipolar spindle and three classes of kinetochore configurations (Figure 2B & Kosco et al., 2001; Marco et al., 2013; Pearson et al., 2003; Severin et al., 2001). 55±2% of the cells had normal bi-lobed kinetochore foci, albeit less discrete due to the dramatically shorter spindle. However, 29±1% arrested with a single kinetochore focus and 16±1% showed clear kinetochore-microtubule attachment defects judged by a kinetochore signal off the spindle axis (Figure 2B-C).

Figure 2. Cells lacking Stu2 have kinetochore-microtubule attachment defects.

Figure 2

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A) Wild-type (SBY3), stu2-AID (SBY13772) and stu2-AID cells expressing STU2-3V5 from an ectopic locus (SBY13901) were serially diluted (5-fold) and spotted on yeast extract peptone plates containing either DMSO or auxin.

B) Exponentially growing cdc20-AID MTW1-3GFP TUB1-CFP (SBY15985) cultures, or also containing stu2-AID (SBY15986), were treated with auxin for 2.5h, then fixed and analyzed for Mtw1-3GFP (kinetochore) localization and spindle morphology (Tub1-CFP). Representative images of cdc20-AID (wild-type) and cdc20-AID stu2-AID cells arrested in metaphase (cdc20-AID was used as a control to ensure all strains arrested in metaphase). Mtw1 (green) localization was categorized as bi-lobed, mono-lobed or unattached (off the spindle axis; white arrow head). DAPI-stained DNA shown in blue, Tub1-CFP shown in red. Boxed regions are magnified and shown in the rightmost columns. White bars for (B) and (D) represent 2 µm.

C) Quantification of Mtw1 localization from (B). Error bars represent standard deviation of three independent experiments; n=200 cells for each experiment.

D) Exponentially growing mad2? (SBY468) or mad2? stu2-AID (SBY16236) cells containing fluorescently labeled chromosome IV were released from a G1 arrest into auxin containing media. Representative images of mad2? (correct segregation) and mad2? stu2-AID (missegregated) cells are shown. DAPI-stained DNA shown in blue, LacI-GFP (marking chromosome IV) shown in green.

E) Quantification of chromosome segregation in anaphase from (D). Error bars represent standard deviation of three independent experiments; n=200 cells for each experiment.

To monitor chromosome segregation after Stu2 depletion, we fluorescently marked chromosome IV (Straight et al., 1996) and also deleted a component of the spindle checkpoint to allow stu2 mutants to progress into anaphase (Figure S2B & Severin et al., 2001). Nearly all cells containing Stu2 function (mad2?) properly segregated a copy of chromosome IV to each daughter nucleus (Figure 2D-E; 95±1%). However, cells depleted of Stu2 (stu2-AID mad2?) displayed high rates of chromosome missegregation, with 56±1% of anaphase cells containing GFP signal in only one of two nuclei (Figure 2D-E). Together, these data confirm that cells lacking Stu2 have defective kinetochore-microtubule interactions.

To determine whether the population of Stu2 specifically associated with the kinetochore mediates microtubule attachment, we analyzed the attachment strength of kinetochore particles in vitro. Kinetochores purified from Stu2-depleted cells (Stu2-AID) lacked Stu2, but otherwise appeared intact as judged by overall protein composition (Figure 3A-B). To measure kinetochore strength, we used an optical trapping-based ‘force-ramp’ technique, where kinetochores were linked to beads and then attached to growing microtubule ends using the laser trap (Figure 3C & reviewed in Franck et al., 2010). The instrument was then programmed to increase force across the kinetochore-microtubule interface until the attachment ruptured (Figure 3D). For consistency, rupture force measurements were always made from assembling microtubule tips at kinetochore concentrations we previously showed monitored single kinetochore-microtubule attachments (Akiyoshi et al., 2010). Individual wild-type kinetochores ruptured at an average of 9.1±0.5 pN, equivalent to the previous measurements of wild-type particles (Akiyoshi et al., 2010). In contrast, Stu2-depleted kinetochores were significantly weaker, rupturing at an average of 4.3±0.3 pN (Figure 3E). This decrease is similar to that observed for kinetochores lacking the Dam1 complex, which rupture at 2.8±0.2 pN on average (Figure S3; Akiyoshi et al., 2010). Thus, the absence of Stu2 weakens kinetochore attachments nearly as much as the loss of the Dam1 complex, which is widely considered to be a crucial microtubule attachment factor (reviewed in Nogales and Ramey, 2009).

Figure 3. Kinetochore-associated Stu2 significantly contributes to the attachment strength of purified kinetochore particles.

Figure 3

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A) Protein lysates prepared from Dsn1-6His-3Flag (SBY8253) or Dsn1-6His-3Flag Stu2-AID (SBY13772) strains. SBY13772 was treated with auxin for 30 min prior to harvesting cells. Kinetochore particles were purified by α-Flag immunoprecipitation and analyzed by SDS-PAGE and silver stain analysis. Note: co-purified Stu2 is not visible by silver stain analysis at this concentration.

B) Exponentially growing Stu2-AID (SBY11856) or Stu2-AID lacking TIR1 (SBY11858) cultures that also contained Dsn1-6His-3Flag and Dam1-9Myc were treated with auxin. Protein lysates were prepared 0, 30 or 60 min post auxin addition and kinetochore particles were purified by α-Flag immunoprecipitation (IP) and analyzed by immunoblotting.

C) Schematic of optical trap assay. Dynamic microtubules are grown from coverslip-anchored seeds. Purified kinetochores are linked to beads via Dsn1 and are manipulated using an optical trap to exert applied force across the kinetochore-microtubule interface.

D) Representative records of applied force versus time for wild-type (black) or Stu2-AID (red) kinetochore particles bound to assembling microtubule tips. Applied force was increased at a rate of 0.25 pN s−1 until attachment rupture (marked by arrows). Gray points show raw data. Colored traces show the same data after smoothing with a 500 ms sliding boxcar average.

E) Left: Mean rupture forces for wild-type or Stu2-AID kinetochore particles either untreated or pre-incubated with Stu2-Flag. Error bars in E and F represent standard error of the mean (SEM; n = 22–46 events). p-values in E and F determined using a two-tailed unpaired t test (n.s.= not significant; ****= p<0.0001). Right: Attachment survival probability versus force for the same data.

F) Left: Mean rupture forces for Ndc80c-linked beads untreated or incubated with Stu2-3V5. Right: Attachment survival probability versus force for the same data.

To determine whether the reduced strength is due solely to the loss of Stu2, we tested whether the addition of purified Stu2 could reconstitute microtubule attachment strength. First, we confirmed that purified Stu2 binds kinetochore particles in an Ndc80c-dependent manner (Figure S4A-B). Next, we measured rupture force distributions for wild-type and Stu2-depleted kinetochore particles pre-incubated with purified Stu2-Flag (Figure S1C). The addition of Stu2 completely reconstituted the attachment strength of Stu2-depleted kinetochore particles (9.2±0.7 pN) while not affecting the rupture force of wild-type particles containing endogenous Stu2 (9.6±0.6 pN; Figure 3E). Together, these results show that kinetochore-bound Stu2 significantly contributes to the overall attachment strength of purified kinetochore particles.

Stu2 directly strengthens Ndc80-based attachments

If Stu2 strengthens kinetochores via its association with Ndc80c, we reasoned that it might also strengthen tip attachments formed by Ndc80c alone. When Ndc80c (described in Figure 1C) was bound to polystyrene beads at sufficiently high density, such that multiple complexes could engage simultaneously with the microtubule tip, it maintained attachments to growing microtubule tips with an average rupture strength of 3.7±0.3 pN as previously seen (Figure 3F; Powers et al., 2009). The addition of purified Stu2 increased the rupture strength of these Ndc80c-coated beads dramatically, to an average of 10.6±0.6 pN (Figure 3F). We observed similar results using recombinant Ndc80c instead of native Ndc80c purified from yeast (data not shown). The Xenopus Stu2 family member XMAP215 alone forms load-bearing attachments to dynamic microtubule tips (Trushko et al., 2013), suggesting that Stu2 by itself might also possess an inherent tip-coupling activity. To test this, we measured rupture force distributions for Stu2-decorated beads and found an average strength of 3.8±0.6 pN (Figure S4C-D). Together, these results demonstrate that Stu2 binding to Ndc80c enhances tip coupling, possibly through the addition of its own inherent microtubule binding activity.

Conserved enhancement of Ndc80c activity via an interaction with ch-TOG

To examine whether the orthologous human proteins also interact, we incubated recombinant Hec1/Ndc80c with immobilized recombinant ch-TOG and found a specific association (Figure 4A & S5). To test whether ch-TOG affects the strength of Hec1/Ndc80c-based attachments, we linked purified Hec1/Ndc80 complex to polystyrene beads and measured rupture force distributions in the presence or absence of ch-TOG. Human Hec1/Ndc80c forms a significantly stronger microtubule attachment relative to the yeast Ndc80c under these conditions (average rupture strength of 12.7±1.3 pN for human vs. ~4 pN for yeast), for reasons that are unknown. Nevertheless, the addition of purified ch-TOG led to a statistically significant increase in rupture strength, to an average of 16.3±1.0 pN (Figure 4B). Thus, the association with kinetochores and the strengthening of kinetochore-tip attachments appear to be conserved activities shared by the yeast and human orthologs, Stu2 and ch-TOG.

Figure 4. ch-TOG contributes to the attachment strength of purified Hec1/Ndc80c.

Figure 4

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A) GFP-tagged ch-TOG was immobilized by α-GFP IP. Immobilized ch-TOG-beads were incubated with Hec1/Ndc80c, washed and eluted in sample buffer. ch-TOG-bound proteins were analyzed by immunoblotting.

B) Left: Mean rupture forces for Hec1/Ndc80 complex-linked beads untreated or incubated with ch-TOG. Error bars in represent SEM (n = 27–37 events). p-value determined using a two-tailed unpaired t test (p=0.024). Right: Attachment survival probability versus force for the same data.

Stu2 did not contribute to tension-induced changes in microtubule dynamics

Because Stu2 is a microtubule polymerase (Podolski et al., 2014), its role at the kinetochore might be mediated through changes in the dynamics of kinetochore-attached microtubules, as previously proposed (Hsu and Toda, 2011; Tang et al., 2013). To address this, we used an optical trapping-based ‘force-clamp’ assay. As before, bead-bound kinetochores were attached to microtubule tips using an optical trap. However, rather than gradually increasing the force, we instead applied fixed levels of tension in the direction of microtubule growth. In this assay, kinetochores track continuously with tip growth and shortening, allowing us to monitor the dynamic instability of kinetochore-attached microtubules with high spatiotemporal resolution (Figure 5A-B; Akiyoshi et al., 2010). We examined a range of forces (from 1-5 pN) and compared growth and shortening speeds as well as switch rates (catastrophe and rescue frequencies) for microtubule tips attached to kinetochores that either contained or lacked Stu2 (wild-type or Stu2-AID kinetochores, respectively). For microtubules attached to wild-type kinetochores, the growth speeds and rescue rates increased with tension, while the shortening speeds and catastrophe rates decreased with tension, consistent with our previous observations using recombinant components or native kinetochores (Akiyoshi et al., 2010; Franck et al., 2007). Surprisingly, however, when attached to kinetochores lacking Stu2, the microtubules behaved indistinguishably from those attached to kinetochores that retained Stu2. We found no clear differences in any of the four dynamic rate parameters for microtubules attached to Stu2-AID kinetochores versus wild-type kinetochores (Figure 5C-D), measured over the full range of experimentally accessible forces. Thus the kinetochore-bound pool of Stu2 does not contribute to the tension-induced changes in microtubule dynamics that we observe in vitro.

Figure 5. Kinetochore-associated Stu2 affects attachment stability without altering microtubule dynamics.

Figure 5

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A) Representative record of position versus time for wild-type (SBY8253) kinetochore particles subjected continuously to 1.0±0.1 pN of force. Increasing position represents movement coupled to microtubule tip assembly. Decreasing position represents movement driven by tip disassembly. Arrows indicate catastrophes (↓) and rescues (↑). Green circles indicate detachment of the bead from the microtubule tip. Inset shows detachment event at higher resolution, illustrating that it occurred during tip disassembly.

B) Schematic of two-state model with detachment during assembly and disassembly (rates k3 and k4, respectively), and the interconversion between the assembly and disassembly states (k1 and k2).

C - F) Measured rates of microtubule assembly and disassembly (growth and shortening, C), microtubule catastrophe and rescue (k1 and k2, D), kinetochore detachment during microtubule assembly (k3, E), and kinetochore detachment during microtubule disassembly (k4, F), for wild-type (black) and Stu2-AID (red) kinetochore particles subjected continuously to indicated amount of force. For C-D, exponential fits shown are for wild-type kinetochore particles. For E-F, exponential fits for both wild-type and Stu2-AID are shown. Error bars represent counting uncertainty (n = 5-92 events). Wild-type data were combined with wild-type data from (Akiyoshi et al., 2010) (see Figure S6).

Stu2 has dichotomous affects on attachment stability

Although kinetochore-attached microtubule dynamics were unaffected by the absence of Stu2, the stability of kinetochore-microtubule coupling clearly was affected. Kinetochore particles lacking Stu2 detached more frequently from assembling tips than wild-type particles at all forces examined (Figure 5E), consistent with our rupture force experiments (Figure 3E-F). Furthermore, this difference was magnified at higher forces indicating that, during tip growth, the contribution of Stu2 to attachment stability was enhanced by tension. During tip shortening, the effect of Stu2 was also force-dependent but, remarkably, its contribution was reversed: When examined at low tension (≤ 2 pN), kinetochores lacking Stu2 detached less frequently from disassembling tips than wild-type particles, indicating that the presence of Stu2 can destabilize attachments specifically during tip shortening (Figure 5F). This Stu2-dependent destabilization during tip shortening was suppressed by tension. Together, these results show that Stu2 affects kinetochore-microtubule attachment stability in a manner that depends on the state of the microtubule tip and on the level of kinetochore tension.

Stu2 underlies selective stabilization of tension-bearing kinetochore attachments

We previously showed that the overall lifetime of reconstituted kinetochore-microtubule attachments varies biphasically with tension, initially increasing with force, reaching an optimum at ~5 pN, and then decreasing as the force is raised further (Akiyoshi et al., 2010). This intrinsic selectivity for tension-bearing attachments occurs because tension inhibits microtubule disassembly (mainly by suppressing catastrophes), and because the kinetochores detach far less frequently from assembling than from disassembling tips. Our discovery that Stu2 alters the tension-dependence of detachment frequencies (Figure 5E-F) suggested that it might also contribute to the intrinsic tension selectivity of kinetochores. We therefore examined how the presence or absence of kinetochore-associated Stu2 affects the overall attachment lifetime-vs-force relationship. Consistent with our previous observations (Akiyoshi et al., 2010), increasing tension from 1 to 5 pN increased mean attachment lifetimes for wild-type kinetochores 3-fold, from 20 ± 4 to 58 ± 16 min (Figure 6A-B & S6). Strikingly, this tension-dependent stabilization was completely abolished for kinetochores lacking Stu2. Their mean attachment lifetimes decreased monotonically with increasing force, from 34 ± 11 min at 0.9 pN down to 8.5 ± 1.3 min at 2.8 pN (Figure 6B), indicating that Stu2 is essential for the tension selectivity of reconstituted kinetochore-microtubule attachments.

Figure 6. Stu2 mediates tension dependent stabilization of kinetochore-microtubule interactions.

Figure 6

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A) Representative records of position versus time for wild-type (black, SBY8253) and Stu2-AID (red, SBY11860) kinetochore particles subjected continuously to 1.0±0.1 pN or 2.8±0.1 pN of force (as described in Figure 5A). Insets show detachment events at higher resolution. Attachment duration increases with force for wild-type but decreases with force for Stu2-AID kinetochores. For clarity, traces are offset vertically.

B) Measured attachment lifetimes for wild-type (black) and Stu2-AID (red) kinetochore particles subjected continuously to indicated amount of force. Curves show prediction of the two-state model (see text). Error bars represent counting uncertainty (n = 5-92 events).

To understand why Stu2 depletion abolishes tension selectivity, we analyzed the force-clamp data using a simple two-state kinetic model (Akiyoshi et al., 2010). The model predicts mean attachment lifetimes given catastrophe and rescue frequencies for a kinetochore-attached microtubule (i.e., rates k1 and k2, respectively; Figure 5B & 5D) and given kinetochore detachment frequencies during tip growth and shortening (k3 and k4; Figure 5E-F). We fit the force-dependence of all four rates with exponential curves and found that the detachment rates (k3 or k4) for Stu2-depleted kinetochores were more force-sensitive (i.e., fit by steeper curves, with more positive slopes in Figure 5E-F), and the unloaded detachment rate during disassembly was also significantly reduced (i.e., lower y-intercept in Figure 5F) relative to wild-type. We found that by changing only the detachment rates (k3 or k4) of our model, the predicted lifetime-vs-force curve decayed monotonically with force, providing an excellent fit to the measured lifetimes (see red curve, Figure 6B). Thus, the Stu2-dependent changes in detachment rates alone are sufficient to explain the tension selectivity observed with wild-type kinetochores.

Discussion

The faithful execution of chromosome segregation is an essential event during cell division and requires the tension-dependent stabilization of properly bioriented kinetochore-microtubule attachments prior to anaphase. Here, we report a previously unknown function for the conserved Stu2 protein in directly regulating kinetochore-microtubule attachments. Remarkably, the kinetochore-associated function of Stu2 is force-dependent and serves to selectively stabilize tension-bearing attachments. Together, our data identify kinetochore-associated Stu2 as a state-sensitive attachment factor that underlies kinetochore mechano-sensitivity to ensure accurate chromosome segregation.

XMAP215 homologs contribute directly to kinetochore-microtubule coupling

The major function ascribed to XMAP215 family members is promoting microtubule assembly via polymerase activity (reviewed in Al-Bassam and Chang, 2011). Although cells lacking Stu2 or ch-TOG display chromosome alignment defects and unattached kinetochores (Gandhi et al., 2011; Gergely et al., 2003; Gillett et al., 2004; Kosco et al., 2001; Marco et al., 2013; Meraldi et al., 2004; Severin et al., 2001), it has not been clear whether these phenotypes are consequences of general defects in microtubule dynamics or whether they might reflect a more specific function at the kinetochore. Here, by reconstituting kinetochore-microtubule interactions in vitro, we specifically investigated the role of kinetochore-associated Stu2. We find that Stu2 makes a large, direct contribution to the strength of kinetochore-microtubule coupling, and furthermore, that this previously uncharacterized function of Stu2 is likely conserved and separable from its role in regulating microtubule dynamics. These data suggest that a fraction of the cellular pool of Stu2 behaves as a “core” kinetochore component that associates stably with the Ndc80 complex to strengthen kinetochore attachments to dynamic microtubule tips, consistent with previous work which detected Stu2 in close proximity and at near-stoichiometric levels with the Ndc80 complex (Aravamudhan et al., 2014).

There are a number of possible mechanisms that could explain how Stu2 strengthens kinetochore-microtubule attachments. Purified Stu2 alone can couple beads to dynamic microtubule tips in vitro, similar to the family member XMAP215 (Trushko et al., 2013), suggesting that these proteins might bring additional direct microtubule binding activity to the kinetochore. Alternatively, Stu2 might alter Ndc80c function, either by allosterically promoting the interaction of Ndc80c with the microtubule or by influencing microtubule tip structure in a way that enhances Ndc80c attachment. Regardless of the underlying mechanism, our work shows that the contribution of Stu2 to kinetochore attachment strength is significant, and similar to that of the Dam1 complex, which is widely considered to be a major kinetochore-microtubule coupling factor (reviewed in Nogales and Ramey, 2009). These results could explain why mutations in Ndc80c cause severe kinetochore-microtubule attachment defects in vivo even though the in vitro microtubule-binding activity of purified Ndc80c alone is relatively weak. Because Ndc80c recruits both Stu2 and Dam1c to the kinetochore, its phenotypes in vivo reflect the mislocalization of multiple microtubule couplers.

The microtubule polymerization rates in our assays were unaffected by kinetochore-associated Stu2, even though it was in the vicinity of the microtubule tips. It is possible that the effective concentration of Stu2 at kinetochores might be below what is required to promote microtubule assembly, or that the orientation of kinetochore-associated Stu2 is incompatible with its polymerase function. Alternatively, the use of mammalian tubulin, which is a poorer substrate for Stu2’s microtubule polymerase function (Podolski et al., 2014), might have masked this activity. An important, technically challenging goal for the future will be to measure how kinetochore-associated Stu2 affects the dynamics of microtubules grown from conspecific yeast tubulin. Nevertheless, we have observed dramatic changes in the stability of kinetochore-microtubule attachments under conditions where no detectable changes in microtubule dynamics occurred. Therefore, our work shows that Stu2 plays a critical and hitherto underappreciated role in regulating kinetochore-microtubule attachments.

The effects of kinetochore-associated Stu2 on attachment stability are regulated by tension

Stu2 confers opposite effects on kinetochore-microtubule attachment stability depending on the level of tension and on the state of the microtubule tip. It is not yet possible to assign these Stu2-dependent effects to established structural features of the kinetochore-microtubule interface, because kinetochore-microtubule coupling remains poorly understood in mechanistic detail. However, some candidate mechanisms are suggested by the selective binding of Stu2 and Ndc80c to curved and straight conformations of tubulin, respectively (Alushin et al., 2010; Ayaz et al., 2012, 2014), and by the presumed arrangements of these tubulin conformations at the assembling and disassembling tips of kinetochore-attached microtubules. We speculate that by selectively binding curved tubulins at the tip, kinetochore-associated Stu2 might form microtubule links that do not interfere with Ndc80c, which binds straight tubulins that are presumably located within the microtubule lattice (Figure 7A). Faster tip growth at higher tension could increase the number of curved tubulins at the tip, thereby enhancing the contribution of Stu2 to kinetochore attachment stability. During tip disassembly at low tension, the kinetochore-associated Stu2 has a destabilizing effect on attachment. Under these conditions it may directly compete with or occlude the microtubule-binding activities of other kinetochore components (such as Ndc80c, Figure 7B), or it may alter the structure of the disassembling tip in a manner that inhibits their binding. In any case, the interference by Stu2 is relieved as tension is increased. A speculative explanation is that tension-dependent stretching of the kinetochore structure itself might relieve this inhibition by spatially separating Stu2 from the other microtubule-binding kinetochore elements (Figure 7B). Regardless of the mechanism, our observation that Stu2 affects attachment stability in a direct and tension-dependent manner implicates it as a mechanically regulated element of the kinetochore-microtubule interface.

Figure 7. Model of Stu2’s role in selectively stabilizing tension-bearing kinetochore-microtubule attachments.

Figure 7

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Stu2’s kinetochore function is directly modulated both by tension and the assembly state of the microtubule tip. These activities impart mechano-sensitivity to the kinetochore, which results in the direct stabilization of tension-bearing attachments.

A) (Left panel) Kinetochore-associated Stu2 might specifically bind to curved tubulin subunits at the assembling tip to form additional microtubule links that do not interfere with the Ndc80c (which binds straight tubulins within the microtubule lattice). Alternately, Stu2 could allosterically enhance the ability of the Ndc80c to bind to the microtubule tip or strengthen the attachment indirectly by altering the microtubule tip structure to promote kinetochore binding. (Right panel) The faster growth at high levels of tension might bring more curved tubulin dimers to the growing tip, thereby allowing more Stu2 molecules to engage. A hypothetical arrangement of kinetochore-bound Stu2 is shown based on data from (Aravamudhan et al., 2014). For simplicity, only Ndc80c and Stu2 are depicted.

B) (Left panel) At low levels of tension, Stu2 impedes kinetochore attachment to disassembling microtubule tips, perhaps by occluding the Ndc80c from microtubule binding. (Right panel) At high levels of tension, the increased force across the kinetochore-microtubule interface may alter the kinetochore and/or straighten protofilaments at the microtubule tip. Under these conditions, the destabilizing activity of Stu2 is suppressed.

Stu2 function underlies the intrinsic selectivity of kinetochores for tension-bearing attachments

Although tension-dependent stabilization is widely accepted as the basis for mitotic accuracy, how tension stabilizes kinetochore-microtubule attachments remains unclear. The Aurora B kinase promotes the release of erroneous attachments through phosphorylation of various kinetochore components (reviewed in Carmena et al., 2012; Krenn and Musacchio, 2015). However, we previously discovered that kinetochores exhibit an intrinsic selectivity for tension-bearing attachments that is independent of Aurora B (Akiyoshi et al., 2010). Our current results now show that kinetochore-associated Stu2 is a key component of this direct mechano-sensitivity. By preventing detachment specifically during microtubule assembly, Stu2 enables long-lived kinetochore attachments, especially when tension is high. Conversely, by promoting detachment during disassembly at low force, Stu2 helps to ensure that relaxed kinetochore attachments are short-lived. Both of these effects together result in the selective stabilization of tension-bearing attachments. In the future, it will be critical to learn how tension regulates these Stu2 activities, and how the intrinsic tension-sensitivity that they create is integrated with the error correction activity of Aurora B. It will also be important to determine how these activities change at anaphase, where kinetochores stay attached to disassembling tips at low tension. Our observation that removing Stu2 function from kinetochores dramatically improves the attachment duration on disassembling microtubules suggests that Stu2 may be inhibited or released at anaphase onset to maintain kinetochore attachments during prolonged microtubule disassembly. Intriguingly, Stu2 family members exhibit regulated changes in localization during the cell cycle that are required for accurate chromosome segregation (Aoki et al., 2006; Aravamudhan et al., 2014).

Concluding remarks

Our findings reveal an uncharacterized function of Stu2 that is regulated mechanically, such that kinetochore-microtubule attachments are intrinsically stabilized by tension, implicating it in the correction of erroneous kinetochore-microtubule attachments. Stu2’s association with kinetochores and its ability to strengthen kinetochore-tip attachments are properties shared by its human ortholog, ch-TOG. Chromosome segregation errors are the most prevalent genetic alteration in tumor cells and have been proposed to be a major factor in the evolution of cancer (reviewed in Gordon et al., 2012). Because ch-TOG is overexpressed in various tumor types (named for colonic and hepatic tumor overexpressed gene; Charrasse et al., 1995, 1998), it will be important to determine whether its function at kinetochores contributes to tumorigenesis and, ultimately, whether kinetochore-associated ch-TOG might be a useful therapeutic target.

Experimental Procedures

Strain Construction and Microbial Techniques

Standard media and microbial techniques were used (Sherman et al., 1974). Yeast strains were constructed by standard genetic techniques. Specific plasmid construction and yeast strains used in this study are described in Supplemental Experimental Procedures and Table S1. The auxin inducible degron (AID) system was used as described in (Nishimura et al., 2009). 100-500 µM IAA (indole-3-acetic acid; auxin) was added to media to induce degradation of the AID-tagged protein. To monitor chromosome segregation, cells carrying a tandem array of lacO sequences integrated at TRP1 (~12kb from CENIV) and a LacI-GFP fusion (Straight et al., 1996) were arrested in G1 with α-factor. Cells were released into medium containing auxin, lacking α-factor pheromone, and chromosome segregation was determined in binucleate cells. To examine kinetochore localization and spindle morphology, exponentially growing cultures were treated with auxin for 2.5h, fixed and analyzed for Mtw1-3GFP localization and Tub1-CFP spindle morphology. Cells were imaged using a Nikon E600 microscope with a 60X objective (NA=1.40), equipped with a Photometrics Cascade 512B digital camera. Seven Z-stacks (0.2 micron apart) were acquired and all frames with nuclear signal in focus were maximally projected. See Supplemental Experimental Procedures for further details.

Protein Biochemistry

Native kinetochore particles, Ndc80c and Stu2 were purified from asynchronously growing ?S. cerevisiae cells as in (Akiyoshi et al., 2010) or as described in Supplemental Experimental Procedures. Standard procedures for sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblotting were followed. For in vitro binding assays, purified native Ndc80c or kinetochores were immobilized and incubated with 30-90 ng of purified Stu2-3V5 for 30 min at room temperature with gentile agitation. Associated proteins were eluted by peptide elution and analyzed by silver stained SDS-PAGE or immunoblotting. Immobilized GFP-tagged ch-TOG was incubated with 100 nM of Hec1/Ndc80c (prepared as in Umbreit et al., 2012) for 40 min at room temperature with gentile agitation. Associated proteins were eluted by boiling in sample buffer and analyzed by immunoblotting. For more details, see Supplemental Experimental Procedures.

Optical Trap Assays

Optical trap-based bead motility assays were performed as in (Akiyoshi et al., 2010; Umbreit et al., 2012). Streptavidin-coated beads were functionalized with biotinylated anti-penta-His antibody and decorated with purified kinetochores (via Dsn1-6His-3Flag), purified native Ndc80 complex (via Spc24-6His-3Flag), purified Hec1/Ndc80 complex (via Spc24-6His), or purified native Stu2 (via Stu2-6His-3Flag). Protein-coated beads were bound to dynamic microtubule tips and an optical trap was used to apply a defined amount of force in the direction of microtubule assembly. See Supplemental Experimental Procedures and Tables S2-3 for more details.

Supplementary Material

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NIHMS778545-supplement-6.pdf (1,005.3KB, pdf)
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Highlights.

  • ch-TOG and Stu2 exhibit a conserved interaction with the Ndc80 kinetochore complex

  • Kinetochore-bound Stu2 directly contributes to microtubule attachment stability

  • Stu2’s kinetochore function is force- and microtubule growth state-dependent

  • Stu2 selectively stabilizes tension-bearing kinetochore attachments

A protein involved in attachment of spindle microtubules to the kinetochore during chromosome segregation selectively stabilizes tension-bearing attachments because its functional output is context dependent: it can either stabilize or destabilize attachments depending on the level of kinetochore tension and the state of the microtubule tip.

Acknowledgements

We are grateful to Dan Gestaut and Trisha Davis for kindly providing purified Hec1/Ndc80 complex, Shih-Chieh Ti and Tarun Kapoor for generously providing purified ch-TOG, Arshad Desai for providing antibodies, Angelika Amon, Leon Chan, Eris Duro and Adèle Marston for reagents, Geert Kops for generating an spc105-AID allele, and Neil Umbreit and Krishna Sarangpani for technical assistance. We thank Bungo Akiyoshi, Trisha Davis, Geert Kops, Luke Rice, and members of the Biggins lab for their critical reading of this manuscript. M.P.M. is an HHMI Fellow of the Damon Runyon Cancer Research Foundation. This work was supported by a Packard Fellowship 2006-30521 (to C.L.A.), NIH grants R01GM079373 (to C.L.A) and R01GM064386 (to S.B.).

S.B. is also an investigator of the Howard Hughes Medical Institute.

Footnotes

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Author Contributions

M.P.M. conceptually designed and performed experiments, analyzed the data and wrote the manuscript; C.L.A. and S.B. conceptually designed experiments, analyzed the data and wrote the manuscript with M.P.M.

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NIHMS778545-supplement-1.docx (30.4KB, docx)
2
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NIHMS778545-supplement-3.xlsx (52.6KB, xlsx)
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