A Novel Protein Phosphatase 1-Dependent Spindle Checkpoint Silencing Mechanism

Abstract

SummaryThe spindle checkpoint is a surveillance system acting in mitosis to delay anaphase onset until all chromosomes are properly attached to the mitotic spindle [1, 2]. When the checkpoint is activated, the Mad2 and Mad3 proteins directly bind and inhibit Cdc20, which is an essential activator of an E3 ubiquitin ligase known as the anaphase-promoting complex (APC) [3]. When the checkpoint is satisfied, Cdc20-APC is activated and polyubiquitinates securin and cyclin, leading to the dissolution of sister chromatid cohesion and mitotic progression. Several protein kinases play critical roles in spindle checkpoint signaling, but the mechanism (or mechanisms) by which they inhibit mitotic progression remains unclear [4]. Furthermore, it is not known whether their activity needs to be reversed by protein phosphatases before anaphase onset can occur. Here we employ fission yeast to show that Aurora (Ark1) kinase activity is directly required to maintain spindle checkpoint arrest, even in the presence of many unattached kinetochores. Upon Ark1 inhibition, checkpoint complexes are disassembled and cyclin B is rapidly degraded. Importantly, checkpoint silencing and cyclin B degradation require the kinetochore-localized isoform of protein phosphatase 1 (PP1Dis2). We propose that PP1Dis2-mediated dephosphorylation of checkpoint components forms a novel spindle checkpoint silencing mechanism.

Collaborative Control of Cell Cycle Progression by the RNA Exonuclease Dis3 and Ras Is Conserved Across Species

“…Here, we draw a parallel to dis2/PP1, a gene that displays similar cell cycle phenotypes to dis3 in S. pombe and flies (Ohkura et al 1988;Axton et al 1990). During mitosis, Dis2/PP1 activity is thought to prevent chromosome separation until all chromosome pairs are bioriented on the mitotic spindle (Vanoosthuyse and Hardwick 2009;Meadows et al, 2011); once this criterion is met, its activity is thought to be directly Figure 7 dis3 and ras cooperate to accelerate cells through the G2/M phase. Graph shows the relative percentage of cells of the indicated genotype in the G1, S, and G2/M phases of the cell cycle.…”

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

“…Just as phosphorylation is required to engage the checkpoint, dephosphorylation via protein phosphatase 1 (PP1) is required to silence the checkpoint (Pinsky et al 2009;Vanoosthuyse and Hardwick 2009). PP1 reverses the Mps1-mediated phosphorylation on Spc105, and it likely has additional key substrates (Pinsky et al 2009;London et al 2012).…”

The Spindle Assembly Checkpoint: Clock or Domino?

“…Specifically, it phosphorylates the microtubule-binding KMN network at kinetochores in a proximity-dependent manner if kinetochores are not stretched apart through a bipolar attachment Welburn et al 2010). But it has also been implicated directly in the spindle checkpoint signaling, as it contributes to the loading of Mad2 to kinetochores in human cells and is essential for the spindle checkpoint in S. pombe (Johnson et al 2004;Vanoosthuyse and Hardwick 2009). Given that Ipl1/Ark1 is essential in fungi, it is not considered a checkpoint protein.…”

“…However, experimental evidence suggest that these proteins cannot be the only silencing system, as cells lacking Spindly still silence the checkpoint upon chromosome alignment (Gassmann et al 2010). Studies both in S. cerevisiae and S. pombe demonstrate that checkpoint inactivation requires the protein phosphatase 1, indicating the need for a counterbalance to the checkpoint kinases, in particular to the Aurora B type of protein kinases (Pinsky et al 2009;Vanoosthuyse and Hardwick 2009). Another study in S. cerevisiae proposed that, while the spindle assembly checkpoint inhibits APC/C Cdc20 activity, the checkpoint protein Mps1 is in turn a target of the APC/C, indicating that these activities counteract one each other in a double negative feedback loop (Palframan et al 2006).…”