Bub1 Kinase and Sgo1 Modulate Pericentric Chromatin in Response to Altered Microtubule Dynamics

Abstract

Background Tension sensing of bi-oriented chromosomes is essential for the fidelity of chromosome segregation. The spindle assembly checkpoint (SAC) conveys lack of tension or attachment to the anaphase promoting complex. Components of the SAC (Bub1) phosphorylate histone H2A (S121) and recruit the protector of cohesin, Shugoshin (Sgo1) to the inner centromere. How the chromatin structural modifications of the inner centromere are integrated into the tension sensing mechanisms and the checkpoint are not known. Results We have identified a Bub1/Sgo1 dependent structural change in the geometry and dynamics of kinetochores and the pericentric chromatin upon reduction of microtubule dynamics. The cluster of inner kinetochores contract while the pericentric chromatin and cohesin that encircle spindle microtubules undergo a radial expansion. Despite its increased spatial distribution, the pericentric chromatin is less dynamic. The change in dynamics is due to histone H2A phosphorylation and Sgo1 recruitment to the pericentric chromatin, rather than microtubule dynamics. Conclusions Bub1 and Sgo1 act as a rheostat to regulate the chromatin spring and maintain force balance. Through Histone H2A S121 phosphorylation and recruitment of Sgo1, Bub1 kinase softens the chromatin spring in response to changes in microtubule dynamics. The geometric alteration of all 16 kinetochores and pericentric chromatin reflect global changes in the pericentromeric region and provide mechanisms for mechanically amplifying damage at a single kinetochore microtubule.

Kinetochore Function and Chromosome Segregation Rely on Critical Residues in Histones H3 and H4 in Budding Yeast

“…An attractive hypothesis is that the DNA at the entry/exit site of the nucleosome may come under tension when kinetochores biorient, thus signaling to the cell that the kinetochores have achieved biorientation. Consistent with this, tension-dependent changes in budding yeast pericentromeric chromatin structure have been observed by microscopy (Haase et al 2012;Verdaasdonk et al 2012). The localization of Sgo1 to this region may therefore be coupled to its ability to trigger the spindle checkpoint when the pericentromeric chromatin is not under tension.…”

“…While budding yeast pericentromeres lack heterochromatin, a conserved feature is the enrichment of cohesin and Sgo1 to promote kinetochore biorientation (Blat and Kleckner 1999;Tanaka et al 1999;Kiburz et al 2005Kiburz et al , 2008Eckert et al 2007). In addition, evidence suggests that the pericentromeric chromatin adopts a specialized intramolecular structure that is organized by Sgo1 and facilitates biorientation in budding yeast (Yeh et al 2008;Haase et al 2012). Consistent with this, changes in pericentromeric chromatin composition lead to defects in the organization of inner kinetochore proteins and chromosome segregation (Chambers et al 2012;Verdaasdonk et al 2012).…”

“…The localization of Sgo1 to this region may therefore be coupled to its ability to trigger the spindle checkpoint when the pericentromeric chromatin is not under tension. Sgo1 and Bub1 modulate pericentromeric chromatin structure in response to microtubule dynamics (Haase et al 2012), so it is possible that the histone mutations that we have identified alter a specific structural property associated with pericentromeres. We attempted to analyze chromatin structure in the pericentromere region in the absence of tension and Sgo1, but the resolution of the assay that we used was not sensitive enough to detect any changes (data not shown).…”

The Composition, Functions, and Regulation of the Budding Yeast Kinetochore

“…At least one function of pericentric cohesion is to facilitate kinetochore biorientation by resisting the pulling forces of microtubules and/or by promoting the architecture of sister kinetochores (Eckert et al 2007;Fernius and Marston 2009;Ng et al 2009;Bloom and Joglekar 2010). Consistent with this, the geometry and elasticity of the pericentromere and inner kinetochore can change in response to alterations in microtubule dynamics (Haase et al 2012;Stephens et al 2013). These properties are regulated by the Bub1 and Sgo1 proteins as well as various chromatin-remodeling complexes (Haase et al 2012;Verdaasdonk et al 2012).…”

“…Consistent with this, the geometry and elasticity of the pericentromere and inner kinetochore can change in response to alterations in microtubule dynamics (Haase et al 2012;Stephens et al 2013). These properties are regulated by the Bub1 and Sgo1 proteins as well as various chromatin-remodeling complexes (Haase et al 2012;Verdaasdonk et al 2012). While heterochromatin recruits pericentric cohesin in some organisms (Bernard et al 2001;Fukagawa et al 2004), components of the kinetochore itself direct cohesion enrichment in budding yeast (Megee et al 1999;Tanaka et al 1999;Weber et al 2004;Eckert et al 2007;Fernius and Marston 2009;Ng et al 2009;Fernius et al 2013).…”