Publications

Abstract

During mitosis, a cell divides its duplicated genome into two identical daughter cells. This process must occur without errors to prevent prolifera- tive diseases (e.g., cancer). A key mechanism controlling mitosis is the precise temporal addition and removal of over 32,000 phosphorylation events by a network of kinases and counterbalancing phosphatases. The identity, magnitude, and temporal regulation of these phosphorylation events has emerged recently, largely from advances in mass spectrometry (Sharma et al., 2014; Kuilman et al., 2015; McCloy et al., 2015). Here, we show phosphoevents currently believed to be key regulators of mitosis. These events have been mapped using Minardo (Ma et al., 2013), a layout in which time flows around a schematic ?circtanglar? cell. Phosphory- lation and dephosphorylation events are depicted with solid and dashed arrows, respectively. Each arrow stem indicates a kinase or phosphatase, and arrowheads indicate phosphosites. Residue numbering on these phosphosites refer to human proteins. Kinase and phosphatase proteins that orchestrate multiple phosphoevents are represented as white tracks. We have estimated temporal positions for each phosphoevent by reviewing previous literature (cited below and in the online version). In the online version, hovering the mouse over a track highlights all related events, while clicking on a phosphosite opens a popup detailing the role of this site in regulating mitosis. The SnapShot begins in late G2 phase, with the activation of the central driver of mitosis, cyclin dependent kinase 1 (CDK1) at the centrosome. As cells enter prophase, AURKA, PLK1, and the NEK family of kinases drive centrosome separation and maturation. At the same time, AURKB and GSG2 (also known as Haspin) drive chromosome condensation (green arrows). Prophase culminates in nuclear envelope breakdown (NEBD), releasing the nuclear contents into the cytosol. NEBD is driven by phosphorylation of nuclear pore and lamin proteins, combined with physical disruption by the growing mitotic spindle. PP1 and PP2A phosphatase activity is then suppressed by CDK1 and MASTL respectively, helping ensure that phosphosites remain phosphorylated, thus preventing mitotic collapse (Álvarez-Fernández et al., 2013). During pro-metaphase, the Spindle Assembly Checkpoint (SAC, orange arrows), prevents mitotic exit until chromosomes correctly attached to the mitotic spindle. The dynamically growing mitotic spindle attaches to chromosomes at the kinetochore, a network of scaffolding and signalling proteins located at the centromere of each chromosome (burgundy arrows). Correct bipolar attachment of the mitotic spindle helps position chromosomes at the center of the cell (metaphase plate), generating equal tension across the kinetochore, satisfying the SAC. This releases the activity of the E3 ubiquitin ligase APC/CCDC20 resulting in destruction of many proteins, including cyclin B1. Declining cyclin B levels (marked by X) reduce CDK1 activity, which further removes inhibition on the APC/C creating a negative feedback loop. Reduced CDK1 activity allows PP1 to reactivate, which then dephosphorylates MASTL (Rogers et al., 2016) leading to the reactivation of PP2A-B55 and PP2A-B56 (Grallert et al., 2014). The specific dephosphorylation of key phosphosites drives the early events of mitotic exit (Cundell et al., 2016). In parallel, APC/CCDC20 degrades securin, releasing the protease separase, which cleaves the Scc1/cohesin ring complex, resulting in separa- tion of chromatids towards the opposing poles. The chromatids also begin to decondense and are engulfed by the reforming nuclear envelope. Dephosphorylation of the nuclear pore complex (NUPs) proteins drives their sequential recruitment into the reforming envelope, resulting in the resumption of normal nuclear-cytoplasmic protein transport (animated). The mitotic spindle begins to retract to the poles, as the central spindle emerges between the separating chromatids (magenta arrows). This newly formed anti-parallel microtubule structure attracts AURKB, and PLK1, which then promote microtubule bundling and provide positional cues for RhoA-mediated membrane ingression during telophase, and subse- quent cell cleavage during cytokinesis. This current SnapShot summarises and helps to clarify ongoing research efforts into understanding the phosphoregulation of mitosis. Specifi- cally, future experiments that provide greater temporal resolution of phosphosite dynamics and more detailed understanding of the phosphatase complexes regulating critical phosphosites will be especially insightful. For an animated version of this SnapShot, please see http://www.cell.com/cell/enhanced/odonoghue.