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In mitotic spindles, each sister chromatid is directly attached to a spindle pole through microtubule bundles known as kinetochore fibres. Microspherule protein 1 (MCRS1) is now shown to support spindle assembly by localizing to the minus ends of kinetochore fibres and protecting them from depolymerization.
The mitotic spindle, a dynamic ensemble of microtubules, microtubule-associated proteins and motor proteins, aligns sister chromatids in the middle of the cell before segregating them to opposite poles1. Mitotic spindle assembly is initiated by microtubule polymerization at chromosomes through a RanGTP gradient, and at centrosomes when present1. An important component of this machinery is the kinetochore fibres (K-fibres), stable microtubule bundles that connect the kinetochore on each sister chromatid with one of the two spindle poles. The aligned sister chromatids separate at the onset of anaphase and K-fibres decrease in length while maintaining their attachment to the kinetochore, thereby moving each sister chromatid to opposite poles. Considerable work has been devoted to understanding how microtubule plus ends attach to kinetochores and how their dynamics are regulated2. However, much less is known about the minus ends of K-fibres, the dynamics of which are likely to be subject to complex regulation. In this issue, Meunier and Vernos now show that MCRS1 plays an important role in mitotic spindle formation by localizing to K-fibre minus ends to regulate their stability3.
MCRS1 is known to be involved in the activation of ribosomal RNA transcription4, the regulation of RNA polymerase-II-dependent transcription5 and the inhibition of telomerase activity6 (Fig. 1a). Based on its reported localization to nucleoli4 and centrosomes7, Meunier and Vernos postulated that MCRS1 could be a RanGTP-regulated spindle-assembly factor. Investigation of MCRS1 localization led to the striking observation that during mitosis MCRS1 is localized to the minus-end region of K-fibres, but not to other spindle microtubules. MCRS1 depletion by RNA interference (RNAi) resulted in a striking mitotic phenotype: spindle assembly was significantly delayed and cells remained in mitosis for more than 8 hours, in contrast to control cells, which completed cell division in 1 hour. During this time, MCRS1-depleted spindles cycled through phases of collapse and reassembly while displaying an activated spindle checkpoint, a safety mechanism that prevents chromosome missegregation and aberrant mitotic progression.
To investigate the mechanism by which MCRS1 contributes to spindle assembly, the authors treated mitotic cells with the microtubule-depolymerizing drug nocodazole and studied microtubule regrowth into astral microtubule arrays, termed asters, following drug washout. The results showed a marked defect in microtubule assembly around chromatin but not centrosomes, a phenotype that resembles the loss of function of TPX2, a factor that promotes microtubule growth around chromosomes8. Consistent with this phenotype, MCRS1 was localized within the centre of chromosomal microtubule asters shortly after nocodazole washout, but was absent from microtubule asters at centrosomes. In contrast, microtubule nucleating and stabilizing proteins, such as γ-tubulin and TPX2, localize to both centrosomal and chromosomal asters in similar experiments8. These data suggest that MCRS1 is specifically involved in chromosomal microtubule assembly.
MCRS1 could promote the growth of chromosomal asters either by nucleating new microtubules or by stabilizing pre-existing ones. Several MCRS1-depletion phenotypes support the latter mechanism and indicate that MCRS1 specifically stabilizes microtubule minus ends. The authors observed that in the absence of MCRS1, K-fibres could still form in monopolar spindles (where they could be most clearly visualized), but were 40% shorter than K-fibres of control cells. In addition, when exposed to cold treatment, which depolymerizes microtubules, the shorter K-fibres of MCRS1-depleted cells depolymerized more rapidly than those of wild-type cells, suggesting that they were less stable. When assessing pole-ward flux, a hallmark of mitotic and meiotic spindles resulting from microtubule minus-end depolymerization at the poles coupled with plus-end polymerization9, the authors discovered that in the absence of MCRS1, tubulin subunits in spindle microtubules fluxed more rapidly towards the spindle poles. Moreover, MCRS1-depleted cells had shorter spindles and increased distances between kinetochore pairs, indicating an increase in pulling forces on K-fibres10. Taking into account prior work on kinetochore microtubules11, the faster pole-ward flux, a shorter spindle and increased kinetochore distance all point to faster minus-end depolymerization in the absence of MCRS1. These results, combined with the localization of MCRS1 at K-fibres, imply that MCRS1 is protecting K-fibre minus ends against microtubule depolymerization.
How might MCRS1 protect the minus ends of K-fibres? The kinesin-13 MCAK is a microtubule depolymerase that is important for the control of microtubule stability, the rate of poleward flux, and overall spindle length12. Patronin, a microtubule minus-end capping protein, is known to block the depolymerizing action of kinesin-13 depolymerases13, and Meunier and Vernos suggest a similar mechanism for the function of MCRS1 at K-fibres. The authors demonstrated that as was previously shown for Patronin, double RNAi depletion of kinesin-13 and MCRS1 could rescue at least a subset of the MCRS1 defects in vivo. Furthermore, kinesin-13 reduced centrosome-nucleated microtubules in Xenopus egg-extract experiments, but was unable to do so in the presence of MCRS1. In addition, although MCRS1 and kinesin-13 did not interact in vitro, purified MCRS1 could displace kinesin-13 from microtubules in a microtubule-pelleting assay. Based on these observations the authors proposed that MCRS1 helps to stabilize K-fibres by suppressing the activity of kinesin-13 at microtubule minus ends (Fig. 1b).
Finally, Meunier and Vernos investigated whether MCRS1 is regulated by RanGTP, which was the question that prompted the study in the first place. A well-studied system for examining the effects of RanGTP is Xenopus egg extract arrested in meiosis II (ref. 14). Addition of a constitutively active Ran mutant induces chromatin-mediated generation of microtubules by releasing spindle assembly factors, such as TPX2, from importin-β (ref. 15). Similar to TPX2, Meunier and Vernos showed that MCRS1 was associated with importin-β and could be released from this complex by RanGTP in Xenopus egg extracts and in vitro, indicating that it is a RanGTP-regulated spindle-assembly factor (Fig. 1b).
In identifying MCRS1 as a K-fibre regulator, this work brings a previously unrecognized protein to the spotlight in the mitosis field. This protein seems to share a similar, but not identical, mitotic phenotype to Patronin. Both molecules regulate spindle length by controlling the pole-ward flux of microtubules, a phenomenon that involves the depolymerization of microtubule minus ends by kinesin-13. Simultaneous depletion of kinesin-13 rescues the RNAi depletion phenotypes of both Patronin13 and MCRS1, as demonstrated in the present study. However, the two proteins are not redundant in function, as depletion of either one alone generates a marked phenotype. Although these phenotypes have not yet been rigorously examined side-by-side in the same cell type, this raises questions as to why the cell has two minus-end protectors and what may be the distinct roles of Patronin and MCRS1. Furthermore, the exact mechanism of MCRS1 function at K-fibre minus ends remains to be determined. Patronin has been shown to bind very specifically to microtubule minus ends to block kinesin-13-induced depolymerization. Thus far, MCRS1 seems to bind along the length of microtubules in vitro, raising questions as to how it stabilizes microtubule ends against the action of kinesin-13 and how it concentrates towards the poleward ends of K-fibres. Moreover, the present work does not demonstrate protection against kinesin-13-induced depolymerization with purified MCRS1, and thus whether MCRS1 alone is sufficient for this effect remains unknown. Additionally, the ability of MRCS1-coated beads to promote microtubule assembly in Xenopus egg extracts suggests that it can recruit microtubule growth promoters. However, whether this biochemical activity reflects a second role of MRCS1 in spindle formation, in addition to its better-characterized effects on stabilizing microtubule minus ends against kinesin-13, is unclear from the present work.
Although much remains to be understood about the regulation and transitions of MRCS1 between its very different functions in mitosis and interphase, the work of Meunier and Vernos adds to our knowledge of mitotic spindle assembly and provides a basis for interesting future studies.
COMPETING FINANCIAL INTERESTS
The authors declare no competing financial interests.