The signaling network that silences the spindle assembly checkpoint upon the establishment of chromosome bipolar attachment

Abstract

Improper kinetochore attachments activate the spindle assembly checkpoint (SAC) to prevent anaphase onset, but it is poorly understood how this checkpoint is silenced to allow anaphase onset. Chromosome bipolar attachment applies tension on sister kinetochores, and the lack of tension delays anaphase onset. In budding yeast, the delay induced by tension defects depends on the intact SAC as well as increase in ploidy (Ipl1)/Aurora kinase and a centromere-associated protein ShuGOshin (Sgo1). Here we provide evidence indicating that Ipl1-dependent phosphorylation of the kinetochore protein Duo1 and Mps1 interacting (Dam1) prevents SAC silencing when tension is absent. The nonphosphorylatable dam1 mutant cells, as well as sgo1 mutant cells, are competent in SAC activation but unable to prevent SAC silencing in response to tension defects. We further found that phosphomimetic dam1 mutants exhibited delayed anaphase onset mainly due to the failure in SAC silencing, but destabilized kinetochore attachment likely plays a minor role in this delay. Because the tension resulting from bipolar attachment triggers the dephosphorylation of Dam1 by protein phosphatase 1, this dephosphorylation likely coordinates SAC silencing with chromosome bipolar attachment. Therefore, Sgo1, Ipl1 kinase, Dam1, and protein phosphatase 1 comprise the SAC silencing network that ensures the correct timing for anaphase onset.

Premature Silencing of the Spindle Assembly Checkpoint Is Prevented by the Bub1-H2A-Sgo1-PP2A Axis in Saccharomyces cerevisiae

“…We showed that Ipl1 kinase and Sgo1 are required to prevent SAC silencing in response to syntelic attachment induced by CIK1-CC overexpression (Jin et al 2012;Jin and Wang 2013). Because the centromere localization of Sgo1 depends on H2A phosphorylation by Bub1 kinase (Fernius and Hardwick 2007;Kawashima et al 2010;Nerusheva et al 2014), Bub1 kinase activity might be required to prevent SAC silencing in response to syntelic attachment, although this kinase activity is dispensable for SAC activation (Fernius and Hardwick 2007).…”

“…The anaphase entry delay in dam1-3D mutants is alleviated by bub1-DK The phosphorylation of kinetochore protein Dam1 by Ipl1 kinase destabilizes kinetochore-microtubule attachment to facilitate correction (Pinsky et al 2006;Tien et al 2010), but the delayed anaphase entry in phospho-mimetic mutant dam1-3D is primarily attributed to its inability to silence the SAC (Jin and Wang 2013). Thus, we tested if the anaphase entry delay in dam1-3D requires the kinase activity of Bub1.…”

“…phosphorylation, indicating compromised SAC silencing (Jin and Wang 2013). We compared Mad1 dephosphorylation kinetics in synchronized dam1-3D and dam1-3D bub1-DK cells.…”

“…We found that phospho-deficient dam1 mutants showed premature SAC silencing, while the phospho-mimetic dam1 mutants exhibited delayed anaphase entry. This delay is mainly attributed to the failure of SAC silencing, whereas defective kinetochore attachment only plays a minor role (Jin and Wang 2013). Because Dam1 dephosphorylation is triggered by tension at kinetochores (Keating et al 2009), Ipl1-dependent Dam1 phosphorylation likely prevents SAC silencing until chromosome bipolar attachment generates tension at kinetochores.…”

“…Therefore, this mechanism links bipolar attachment and SAC silencing (Wang et al 2014). SGO1 encodes a centromere-binding protein (Indjeian et al 2005), and SGO1 deletion also leads to premature SAC silencing (Jin et al 2012;Jin and Wang 2013). However, it is largely unknown how Sgo1 regulates SAC silencing at the molecular level.…”