Academic journal article Genetics

Escape from Mitotic Arrest: An Unexpected Connection between Microtubule Dynamics and Epigenetic Regulation of Centromeric Chromatin in Schizosaccharomyces Pombe

Academic journal article Genetics

Escape from Mitotic Arrest: An Unexpected Connection between Microtubule Dynamics and Epigenetic Regulation of Centromeric Chromatin in Schizosaccharomyces Pombe

Article excerpt

ACCURATE chromosome segregation relies on precise assembly of the multiprotein kinetochore complex, which mediates the link between chromosomes and the mitotic spindle (Przewloka and Glover 2009). The histone H3 variantCENP-A,alongwithepigeneticmarksoncanonicalhistones, defines centromeric chromatin, which forms the template for kinetochore assembly (Allshire and Karpen 2008; Verdaasdonk and Bloom 2011). Kinetochores serve as the site of spindle microtubule attachment (Santaguida and Musacchio 2009) and, along with the centromere, translate microtubule attachment status to mediate force balance, tension sensing, and spindle checkpoint control (Bloom 2014). When associations between chromosomes and the spindle are incorrect, the spindle assembly checkpoint (SAC) delays progression through mitosis by preventing the onset of anaphase (Musacchio and Salmon 2007; Foley and Kapoor 2013).

Fission yeast centromeric chromatin (Allshire and Ekwall 2015) consists of a central core (cnt) and internal inverted repeats (imr), defined by the presence of CENP-A and methylated histone H3 K4 chromatin, flanked by heterochromatic outer repeats (otr) marked by methylated histone H3 K9 (Partridge et al. 2000; Takahashi et al. 2000; Kniola et al. 2001). The constitutive centromere-associated network (CCAN) forms the inner kinetochore and consists of more than a dozen conserved proteins, including fission yeast Mis6 (CENP-I in mammalian cells) that promotes incorporation of CENP-A at the centromere (McAinsh and Meraldi 2011; Westhorpe and Straight 2015). The KMN network is composed of the tethering protein Knl1, the Mis12 complex, and the NDC80 complex, a heterotetramer consisting of Ndc80, Nuf2, Spc24, and Spc25, that links microtubules to the kinetochore (Petrovic et al. 2014). The Ndc80 protein has a loop domain that associates with the evolutionarily conserved microtubule-associated protein (MAP) Dis1; mutations in Ndc80 that eliminate Dis1 binding mimic loss of Dis1 function (Hsu and Toda 2011). As a member of the XMAP215/TOG1 family of proteins that modulate microtubule dynamics (Ohkura et al. 2001; Brouhard et al. 2008), Dis1 may serve to modify the behavior of spindle microtubules and/or the Ndc80 complex to stabilize kinetochore-bound microtubules (Cheerambathur and Desai 2014).

The XMAP215/TOG family of proteins mediates microtubule polymer assembly, spindle formation, kinetochore function, and cell morphogenesis (Slep 2010; Al-Bassam and Chang 2011). The gene encoding Dis1 was first identified in fission yeast in a cytologic screen of cold-sensitive (cs)mutants:whiledis1 is not essential for cell division at typical growth temperatures, loss of dis1 function due to deletion (dis1D) or a point mutation (dis1-288)conferscold sensitivity (Ohkura et al. 1988; Nabeshima et al. 1995). At low temperature, dis1 mutants exhibit the dis (defect in sister-chromatid disjoining) phenotype: mitotic arrest with hypercondensed chromosomes that fail to separate. The association of Dis1 with Ndc80 (Hsu and Toda 2011) suggests a model whereby Dis1 localization to kinetochores allows it to influence the polymerization of kinetochore microtubules. The alp14 gene encodes a second XMAP215/TOG family protein in fission yeast (Garcia et al. 2001). While mutation or disruption of dis1 resultsincoldsensitivity (Ohkura et al. 1988), disruption of alp14 results in temperature sensitivity, and deletion of both leads to nonviability at temperatures permissive for each of the single mutants (Garcia et al. 2001).

We have reported previously that deletion of the gene encoding fission yeast Msc1 restores viability to cells lacking dis1 function (Qiu et al. 2010). Msc1 is required for chromosome stability and was first isolated as a multicopy suppressor of loss of Chk1 function (Ahmed et al. 2004), a conserved protein kinase important for the DNA damage-induced cellcycle checkpoint response (Bartek and Lukas 2003). Msc1 influences centromere-kinetochore function because it exhibits genetic interactions with the centromere-specifi c histone H3 variant CENP-A (encoded by cnp1), and the kinetochore components Mis6, a component of the CCAN, and Mis12, a component of the KMN network (Ahmed et al. …

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