Assembling the spindle midzone in the right place at the right time

Zentrum für Molekulare Biologie der Universität Heidelberg, Heidelberg, Germany.
Cell cycle (Georgetown, Tex.) (Impact Factor: 4.57). 03/2008; 7(3):283-6. DOI: 10.4161/cc.7.3.5349
Source: PubMed


During anaphase the mitotic spindle extends dramatically, promoting the segregation of the chromosomes into the two daughter cells. The spindle midzone, assembled at the onset of anaphase, is critical for this extension. How this assembly is linked to progression through the cell cycle is not fully understood. Our data show that in budding yeast the conserved phosphatase Cdc14, activated in early anaphase, regulates the formation of the spindle midzone. Cdc14 dephosphorylates residues of a core midzone component, the conserved microtubule bundling factor Ase1, that were previously phosphorylated by the cyclin-dependent kinase complex. In addition, Cdc14 activation is also indirectly responsible for midzone localization of the separase-Slk19 complex. This dual control of midzone assembly by Cdc14 is necessary for the formation of the focused and centered spindle midzone that drives the continuous and full elongation of the anaphase spindle. The identification of Ase1 as a key Cdc14 substrate elucidates how spindle midzone assembly is coordinated with the metaphase to anaphase transition.

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    • "Therefore, a second possibility is that discrete CPCs carry out different mitotic functions, some of which may not require the catalytic activity of Aurora B. To further elucidate the mitotic roles and mechanisms of CPC function, we focus on the role of CPCs on the anaphase spindle. Mutant analysis in budding yeast suggests that CPCs play a role in spindle midzone organization (Mackay et al., 1998; Khmelinskii and Schiebel, 2008). In budding yeast, the spindle midzone is a poorly defined region of overlapping interpolar microtubules where multiple microtubule-associated proteins (MAPs) contribute to stability and outward spindle forces. "
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    ABSTRACT: During mitosis, chromosome passenger complexes (CPCs) exhibit a well-conserved association with the anaphase spindle and have been implicated in spindle stability. However, their precise effect on the spindle is not clear. In this paper, we show, in budding yeast, that a CPC consisting of CBF3, Bir1, and Sli15, but not Ipl1, is required for normal spindle elongation. CPC mutants slow spindle elongation through the action of the bipolar kinesins Cin8 and Kip1. The same CPC mutants that slow spindle elongation also result in the enrichment of Cin8 and Kip1 at the spindle midzone. Together, these findings argue that CPCs function to organize the spindle midzone and potentially switch motors between force generators and molecular brakes. We also find that slowing spindle elongation delays the mitotic exit network (MEN)-dependent release of Cdc14, thus delaying spindle breakdown until a minimal spindle size is reached. We propose that these CPC- and MEN-dependent mechanisms are important for coordinating chromosome segregation with spindle breakdown and mitotic exit.
    Full-text · Article · Apr 2011 · The Journal of Cell Biology
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    • "A nonphosphorylatable form of Ase1p (one that does not require Cdc14p to be dephosphorylated ) can rescue the midzone assembly defects of slk19D mutants, but not their spindle stability defects. These findings suggested to Schiebel and colleagues that the midzone assembly defects of SLK19 mutants are due to loss of FEAR (Khmelinskii and Schiebel 2008) and raised the possibility that the spindle stability defects of SLK19 mutants are due to loss of a FEAR-independent function. This work tests these ideas. "
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    ABSTRACT: Slk19p is a member of the Cdc-14 early anaphase release (FEAR) pathway, a signaling network that is responsible for activation of the cell-cycle regulator Cdc14p in Saccharomyces cerevisiae. Disruption of the FEAR pathway results in defects in anaphase, including alterations in the assembly and behavior of the anaphase spindle. Many phenotypes of slk19Δ mutants are consistent with a loss of FEAR signaling, but other phenotypes suggest that Slk19p may have FEAR-independent roles in modulating the behavior of microtubules in anaphase. Here, a series of SLK19 in-frame deletion mutations were used to test whether Slk19p has distinct roles in anaphase that can be ascribed to specific regions of the protein. Separation-of-function alleles were identified that are defective for either FEAR signaling or aspects of anaphase spindle function. The data suggest that in early anaphase one region of Slk19p is essential for FEAR signaling, while later in anaphase another region is critical for maintaining the coordination between spindle elongation and the growth of interpolar microtubules.
    Full-text · Article · Oct 2010 · Genetics
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    • "Spindles consist of two antiparallel arrays of microtubules that overlap in the spindle center, an organization that ensures spindle function. At anaphase onset, the central spindle reorganizes to form the midzone, a dense specialized array of antiparallel microtubules (Glotzer, 2009; Khmelinskii and Schiebel, 2008). The midzone size in metazoans is typically 2–3 mm in length (Mastronarde et al., 1993). "
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    ABSTRACT: During cell division, microtubules are arranged in a large bipolar structure, the mitotic spindle, to segregate the duplicated chromosomes. Antiparallel microtubule overlaps in the spindle center are essential for establishing bipolarity and maintaining spindle stability throughout mitosis. In anaphase, this antiparallel microtubule array is tightly bundled forming the midzone, which serves as a hub for the recruitment of proteins essential for late mitotic events. The molecular mechanism of midzone formation and the control of its size are not understood. Using an in vitro reconstitution approach, we show here that PRC1 autonomously bundles antiparallel microtubules and recruits Xklp1, a kinesin-4, selectively to overlapping antiparallel microtubules. The processive motor Xklp1 controls overlap size by overlap length-dependent microtubule growth inhibition. Our results mechanistically explain how the two conserved, essential midzone proteins PRC1 and Xklp1 cooperate to constitute a minimal protein module capable of dynamically organizing the core structure of the central anaphase spindle.
    Full-text · Article · Aug 2010 · Cell
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