Phosphorylation regulates the p31Comet-mitotic arrest-deficient 2 (Mad2) interaction to promote spindle assembly checkpoint (SAC) activity

p31Comet Splice Variants Induce Distinct Spindle Assembly Checkpoint Dynamics due to Their Unique N-Termini

The role of p31Comet in deactivating the spindle assembly checkpoint is well described in the literature; however, the data are all completed using Variant 2 of p31Comet. p31Comet is known to be expressed as two different splice variants: Variant 1 and Variant 2. Variant 1 contains an additional 32 N-terminal residues compared to Variant 2. We report that Variant 1 exhibits a reduced ability to bind to MAD2 and thus a reduced ability to induce mitotic progression. Additionally, we show that Variant 1 exhibits reduced stability compared to Variant 2. We further show that Variant 1 is uniquely expressed in the Testes, indicating a potentially unique role of Variant 1 in that organ. Overall, we demonstrate the N-terminus of p31Comet is capable of modulating p31Comet activity in mitosis.

Principles and dynamics of spindle assembly checkpoint signalling

The transmission of a complete set of chromosomes to daughter cells during cell division is vital for development and tissue homeostasis. The spindle assembly checkpoint (SAC) ensures correct segregation by informing the cell cycle machinery of potential errors in the interactions of chromosomes with spindle microtubules prior to anaphase. To do so, the SAC monitors microtubule engagement by specialized structures known as kinetochores and integrates local mechanical and chemical cues such that it can signal in a sensitive, responsive and robust manner. In this Review, we discuss how SAC proteins interact to allow production of the mitotic checkpoint complex (MCC) that halts anaphase progression by inhibiting the anaphase-promoting complex/cyclosome (APC/C). We highlight recent advances aimed at understanding the dynamic signalling properties of the SAC and how it interprets various naturally occurring intermediate attachment states. Further, we discuss SAC signalling in the context of the mammalian multisite kinetochore and address the impact of the fibrous corona. We also identify current challenges in understanding how the SAC ensures high-fidelity chromosome segregation.

Kinetochore‐catalyzed MCC formation: A structural perspective

The spindle assembly checkpoint (SAC) is a cellular surveillance mechanism that functions to ensure accurate chromosome segregation during mitosis. Macromolecular complexes known as kinetochores, act as the interface of sister chromatid attachment to spindle microtubules. In response to unattached kinetochores, the SAC activates its effector, the mitotic checkpoint complex (MCC), which delays mitotic exit until all sister chromatid pairs have achieved successful attachment to the bipolar mitotic spindle. Formation of the MCC (composed of Mad2, BubR1, Bub3 and Cdc20) is regulated by an Mps1 kinase-dependent phosphorylation signaling cascade which assembles and repositions components of the MCC onto a catalytic scaffold. This scaffold functions to catalyze the conversion of the HORMA-domain protein Mad2 from an "inactive" open-state (O-Mad2) into an "active" closed-Mad2 (C-Mad2), and simultaneous Cdc20 binding. Here, our current understanding of the molecular mechanisms underlying the kinetic barrier to C-Mad2:Cdc20 formation will be reviewed. Recent progress in elucidating the precise molecular choreography orchestrated by the catalytic scaffold to rapidly assemble the MCC will be examined, and unresolved questions will be highlighted. Ultimately, understanding how the SAC rapidly activates the checkpoint not only provides insights into how cells maintain genomic integrity during mitosis, but also provides a paradigm for how cells can utilize molecular switches, including other HORMA domain-containing proteins, to make rapid changes to a cell's physiological state.

Inhibition of MAD2L1 Mediates Pulmonary Fibrosis through Impairment of Mitochondrial Function and Induction of Cell Senescence

Idiopathic pulmonary fibrosis (IPF) is a chronic, irreversible, and progressive interstitial lung disease characterized by recurrent alveolar epithelial cell injury, fibroblast hyperproliferation, and cumulative deposition of extracellular matrix leading to alveolar destruction in the lungs. Mitotic arrest deficient 2 like 1 (MAD2L1) is a component of the mitotic spindle assembly checkpoint that prevents the onset of anaphase until all chromosomes are properly aligned at metaphase and is a potential therapeutic target in cancers. However, the role of MAD2L1 in pulmonary fibrosis has not been explored. We analyzed the expression of MAD2L1 in lung tissues from control subjects, IPF patients, and mice with bleomycin-induced fibrosis via IHC, qRT-PCR, and Western blot analysis. We examined the roles of MAD2L1 in ROS production, mitochondrial function, cell senescence, and the establishment of a profibrotic microenvironment. We found that MAD2L1 was highly upregulated in alveolar epithelial cells in fibrotic lung tissues from both patients with IPF and mice with bleomycin-induced fibrosis. Loss of MAD2L1 expression or activity led to decreases of cell viability and proliferation in A549 cells. Subsequent mechanistic investigation demonstrated that inhibition of MAD2L1 damaged mitochondria, which led to augmented ROS production and cellular senescence, and thus promoted the establishment of a profibrotic microenvironment. Taken together, these results reveal that alleviation of alveolar epithelial cell mitochondrial damage arising from augmentation of MAD2L1 may be a novel therapeutic strategy for mitigating pulmonary fibrosis.

Evolutionary Dynamics and Molecular Mechanisms of HORMA Domain Protein Signaling

Controlled assembly and disassembly of multi-protein complexes is central to cellular signaling. Proteins of the widespread and functionally diverse HORMA family nucleate assembly of signaling complexes by binding short peptide motifs through a distinctive safety-belt mechanism. HORMA proteins are now understood as key signaling proteins across kingdoms, serving as infection sensors in a bacterial immune system and playing central roles in eukaryotic cell cycle, genome stability, sexual reproduction, and cellular homeostasis pathways. Here, we describe how HORMA proteins' unique ability to adopt multiple conformational states underlies their functions in these diverse contexts. We also outline how a dedicated AAA+ ATPase regulator, Pch2/TRIP13, manipulates HORMA proteins' conformational states to activate or inactivate signaling in different cellular contexts. The emergence of Pch2/TRIP13 as a lynchpin for HORMA protein action in multiple genome-maintenance pathways accounts for its frequent misregulation in human cancers and highlights TRIP13 as a novel therapeutic target. Expected final online publication date for the Annual Review of Biochemistry, Volume 91 is June 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

The Mad2-Binding Protein p31comet as a Potential Target for Human Cancer Therapy

The spindle assembly checkpoint (SAC) is a surveillance mechanism that prevents mitotic exit at the metaphase-to-anaphase transition until all chromosomes have established correct bipolar attachment to spindle microtubules. Activation of SAC relies on the assembly of the mitotic checkpoint complex (MCC), which requires conformational change from inactive open Mad2 (OMad2) to the active closed Mad2 (C-Mad2) at unattached kinetochores. The Mad2-binding protein p31^comet plays a key role in controlling timely mitotic exit by promoting SAC silencing, through preventing Mad2 activation and promoting MCC disassembly. Besides, increasing evidences highlight the p31^comet potential as target for cancer therapy. Here, we provide an updated overview of the functional significance of p31^comet in mitotic progression, and discuss the potential of deregulated expression of p31^comet in cancer and in therapeutic strategies.

p31comet promotes homologous recombination by inactivating REV7 through the TRIP13 ATPase

The repair of DNA double strand breaks (DSBs) that arise from external mutagenic agents and routine cellular processes is essential for life. DSBs are repaired by two major pathways, homologous recombination (HR) and classical nonhomologous end joining (C-NHEJ). DSB repair pathway choice is largely dictated at the step of 5'-3' DNA end resection, which is promoted during S phase, in part by BRCA1. Opposing end resection is the 53BP1 protein, which recruits the ssDNA-binding REV7-Shieldin complex to favor C-NHEJ repair. We recently identified TRIP13 as a proresection factor that remodels REV7, causing its dissociation from the Shieldin subunit SHLD3. Here, we identify p31^comet, a negative regulator of MAD2 and the spindle assembly checkpoint, as an important mediator of the TRIP13-REV7 interaction. p31^comet binds to the REV7-Shieldin complex in cells, promotes REV7 inactivation, and causes PARP inhibitor resistance. p31^comet also participates in the extraction of REV7 from the chromatin. Furthermore, p31^comet can counteract REV7 function in translesion synthesis (TLS) by releasing it from REV3 in the Pol ζ complex. Finally, p31^comet, like TRIP13, is overexpressed in many cancers and this correlates with poor prognosis. Thus, we reveal a key player in the regulation of HR and TLS with significant clinical implications.

The conserved AAA-ATPase PCH-2 TRIP13 regulates spindle checkpoint strength

Spindle checkpoint strength is dictated by the number of unattached kinetochores, cell volume, and cell fate. We show that the conserved AAA-ATPase PCH-2/TRIP13, which remodels the checkpoint effector Mad2 from an active conformation to an inactive one, controls checkpoint strength in Caenorhabditis elegans. Having previously established that this function is required for spindle checkpoint activation, we demonstrate that in cells genetically manipulated to decrease in cell volume, PCH-2 is no longer required for the spindle checkpoint or recruitment of Mad2 at unattached kinetochores. This role is not limited to large cells: the stronger checkpoint in germline precursor cells also depends on PCH-2. PCH-2 is enriched in germline precursor cells, and this enrichment relies on conserved factors that induce asymmetry in the early embryo. Finally, the stronger checkpoint in germline precursor cells is regulated by CMT-1, the ortholog of p31^comet, which is required for both PCH-2′s localization to unattached kinetochores and its enrichment in germline precursor cells. Thus, PCH-2, likely by regulating the availability of inactive Mad2 at and near unattached kinetochores, governs checkpoint strength. This requirement may be particularly relevant in oocytes and early embryos enlarged for developmental competence, cells that divide in syncytial tissues, and immortal germline cells.

Deregulation of Chromosome Segregation and Cancer

The mitotic spindle assembly checkpoint (SAC) is an intricate cell signaling system that ensures the high fidelity and timely segregation of chromosomes during cell division. Mistakes in this process can lead to the loss, gain, or rearrangement of the genetic material. Gross chromosomal aberrations are usually lethal but can cause birth and development defects as well as cancer. Despite advances in the identification of SAC protein components, important details of the interactions underpinning chromosome segregation regulation remain to be established. This review discusses the current understanding of the function, structure, mode of regulation, and dynamics of the assembly and disassembly of SAC subcomplexes, which ultimately safeguard the accurate transmission of a stable genome to descendants. We also discuss how diverse oncoviruses take control of human cell division by exploiting the SAC and the potential of this signaling circuitry as a pool of drug targets to develop effective cancer therapies.

Interplay between Phosphatases and the Anaphase-Promoting Complex/Cyclosome in Mitosis

Accurate division of cells into two daughters is a process that is vital to propagation of life. Protein phosphorylation and selective degradation have emerged as two important mechanisms safeguarding the delicate choreography of mitosis. Protein phosphatases catalyze dephosphorylation of thousands of sites on proteins, steering the cells through establishment of the mitotic phase and exit from it. A large E3 ubiquitin ligase, the anaphase-promoting complex/cyclosome (APC/C) becomes active during latter stages of mitosis through G1 and marks hundreds of proteins for destruction. Recent studies have revealed the complex interregulation between these two classes of enzymes. In this review, we highlight the direct and indirect mechanisms by which phosphatases and the APC/C mutually influence each other to ensure accurate spatiotemporal and orderly progression through mitosis, with a particular focus on recent insights and conceptual advances.

Role of Polo-like kinase 1 in the regulation of the action of p31comet in the disassembly of mitotic checkpoint complexes

The Mad2-binding protein p31comet has important roles in the inactivation of the mitotic checkpoint system, which delays anaphase until chromosomes attach correctly to the mitotic spindle. The activation of the checkpoint promotes the assembly of a Mitotic Checkpoint Complex (MCC), which inhibits the action of the ubiquitin ligase APC/C (Anaphase-Promoting Complex/Cyclosome) to degrade inhibitors of anaphase initiation. The inactivation of the mitotic checkpoint requires the disassembly of MCC. p31comet promotes the disassembly of mitotic checkpoint complexes by liberating their Mad2 component in a joint action with the ATPase TRIP13. Here, we investigated the regulation of p31comet action. The release of Mad2 from checkpoint complexes in extracts from nocodazole-arrested HeLa cells was inhibited by Polo-like kinase 1 (Plk1), as suggested by the effects of selective inhibitors of Plk1. Purified Plk1 bound to p31comet and phosphorylated it, resulting in the suppression of its activity (with TRIP13) to disassemble checkpoint complexes. Plk1 phosphorylated p31comet on S102, as suggested by the prevention of the phosphorylation of this residue in checkpoint extracts by the selective Plk1 inhibitor BI-2536 and by the phosphorylation of S102 with purified Plk1. An S102A mutant of p31comet had a greatly decreased sensitivity to inhibition by Plk1 of its action to disassemble mitotic checkpoint complexes. We propose that the phosphorylation of p31comet by Plk1 prevents a futile cycle of MCC assembly and disassembly during the active mitotic checkpoint.

MAD2-p31comet axis deficiency reduces cell proliferation, migration and sensitivity of microtubule-interfering agents in glioma

Mitotic arrest deficient-like-1 (MAD2, also known as MAD2L1) is thought to be an important spindle assembly checkpoint protein, which ensures accurate chromosome segregation and is closely associated with poor prognosis in many cancer. As a MAD2 binding protein, p31^comet counteracts the function of MAD2 and leads to mitotic checkpoint silence. In this study, we explore the function of MAD2-p31^comet axis in malignant glioma cells. Our results showed that disruption of MAD2-p31^comet axis by MAD2 knockdown or p31^comet overexpression suppressed cell proliferation, survival and migration of glioma, indicating that MAD2-p31^comet axis is required for maintaining glioma cells malignancy. It is noted that MAD2 depletion or p31^comet overexpression reduced the sensitivity of glioma cells to microtubule-interfering agents paclitaxel and vinblastine, providing clinical guidance for application of such drugs. Taken together, our findings suggest that MAD2-p31^comet axis may serve as a potential therapeutic target for glioma.

Phosphorylation of Xenopus p31(comet) potentiates mitotic checkpoint exit

p31^comet plays an important role in spindle assembly checkpoint (SAC) silencing. However, how p31^comet's activity is regulated remains unclear. Here we show that the timing of M-phase exit in Xenopus egg extracts (XEEs) depends upon SAC activity, even under conditions that are permissive for spindle assembly. p31^comet antagonizes the SAC, promoting XEE progression into anaphase after spindles are fully formed. We further show that mitotic p31^comet phosphorylation by Inhibitor of nuclear factor κ-B kinase-β (IKK-β) enhances this role in SAC silencing. Together, our findings implicate IKK-β in the control of anaphase timing in XEE through p31^comet activation and SAC downregulation.

The multifaceted roles of the HORMA domain in cellular signaling

The HORMA domain is a multifunctional protein–protein interaction module found in diverse eukaryotic signaling pathways including the spindle assembly checkpoint, numerous DNA recombination/repair pathways, and the initiation of autophagy. In all of these pathways, HORMA domain proteins occupy key signaling junctures and function through the controlled assembly and disassembly of signaling complexes using a stereotypical “safety belt” peptide interaction mechanism. A recent explosion of structural and functional work has shed new light on these proteins, illustrating how strikingly similar structural mechanisms give rise to radically different functional outcomes in each family of HORMA domain proteins.

An epigenome-wide association meta-analysis of prenatal maternal stress in neonates: A model approach for replication

Prenatal maternal stress exposure has been associated with neonatal differential DNA methylation. However, the available evidence in humans is largely based on candidate gene methylation studies, where only a few CpG sites were evaluated. The aim of this study was to examine the association between prenatal exposure to maternal stress and offspring genome-wide cord blood methylation using different methods. First, we conducted a meta-analysis and follow-up pathway analyses. Second, we used novel region discovery methods [i.e., differentially methylated regions (DMRs) analyses]. To this end, we used data from two independent population-based studies, the Generation R Study (n = 912) and the Avon Longitudinal Study of Parents and Children (ALSPAC, n = 828), to (i) measure genome-wide DNA methylation in cord blood and (ii) extract a prenatal maternal stress composite. The meta-analysis (ntotal = 1,740) revealed no epigenome-wide (meta P <1.00e-07) associations of prenatal maternal stress exposure with neonatal differential DNA methylation. Follow-up analyses of the top hits derived from our epigenome-wide meta-analysis (meta P <1.00e-04) indicated an over-representation of the methyltransferase activity pathway. We identified no Bonferroni-corrected (P <1.00e-06) DMRs associated with prenatal maternal stress exposure. Combining data from two independent population-based samples in an epigenome-wide meta-analysis, the current study indicates that there are no large effects of prenatal maternal stress exposure on neonatal DNA methylation. Such replication efforts are essential in the search for robust associations, whether derived from candidate gene methylation or epigenome-wide studies.

Phosphoproteomic Profiling Reveals Epstein-Barr Virus Protein Kinase Integration of DNA Damage Response and Mitotic Signaling

Epstein-Barr virus (EBV) is etiologically linked to infectious mononucleosis and several human cancers. EBV encodes a conserved protein kinase BGLF4 that plays a key role in the viral life cycle. To provide new insight into the host proteins regulated by BGLF4, we utilized stable isotope labeling by amino acids in cell culture (SILAC)-based quantitative proteomics to compare site-specific phosphorylation in BGLF4-expressing Akata B cells. Our analysis revealed BGLF4-mediated hyperphosphorylation of 3,046 unique sites corresponding to 1,328 proteins. Frequency analysis of these phosphosites revealed a proline-rich motif signature downstream of BGLF4, indicating a broader substrate recognition for BGLF4 than its cellular ortholog cyclin-dependent kinase 1 (CDK1). Further, motif analysis of the hyperphosphorylated sites revealed enrichment in ATM, ATR and Aurora kinase substrates while functional analyses revealed significant enrichment of pathways related to the DNA damage response (DDR), mitosis and cell cycle. Phosphorylation of proteins associated with the mitotic spindle assembly checkpoint (SAC) indicated checkpoint activation, an event that inactivates the anaphase promoting complex/cyclosome, APC/C. Furthermore, we demonstrated that BGLF4 binds to and directly phosphorylates the key cellular proteins PP1, MPS1 and CDC20 that lie upstream of SAC activation and APC/C inhibition. Consistent with APC/C inactivation, we found that BGLF4 stabilizes the expression of many known APC/C substrates. We also noted hyperphosphorylation of 22 proteins associated the nuclear pore complex, which may contribute to nuclear pore disassembly and SAC activation. A drug that inhibits mitotic checkpoint activation also suppressed the accumulation of extracellular EBV virus. Taken together, our data reveal that, in addition to the DDR, manipulation of mitotic kinase signaling and SAC activation are mechanisms associated with lytic EBV replication. All MS data have been deposited in the ProteomeXchange with identifier PXD002411 (http://proteomecentral.proteomexchange.org/dataset/PXD002411).

Epstein-Barr virus (EBV) is a herpesvirus that is associated with B cell and epithelial human cancers. Herpesviruses encode a protein kinase which is an important regulator of lytic virus replication and is consequently a target for anti-viral drug development. The EBV genome encodes for a serine/threonine protein kinase called BGLF4. Previous work on BGLF4 has largely focused on its cyclin-dependent kinase 1 (CDK1)-like activity. The range of BGLF4 cellular substrates and the full impact of BGLF4 on the intracellular microenvironment still remain to be elucidated. Here, we utilized unbiased quantitative phosphoproteomic approach to dissect the changes in the cellular phosphoproteome that are mediated by BGLF4. Our MS analyses revealed extensive hyperphosphorylation of substrates that are normally targeted by CDK1, Ataxia telangiectasia mutated (ATM), Ataxia telangiectasia and Rad3-related (ATR) proteins and Aurora kinases. The up-regulated phosphoproteins were functionally linked to the DNA damage response, mitosis and cell cycle pathways. Our data demonstrate widespread changes in the cellular phosphoproteome that occur upon BGLF4 expression and suggest that manipulation of the DNA damage and mitotic kinase signaling pathways are central to efficient EBV lytic replication.

p31comet-Induced Cell Death Is Mediated by Binding and Inactivation of Mad2

Mad2, a key component of the spindle checkpoint, is closely associated with chromosomal instability and poor prognosis in cancer. p31^comet is a Mad2-interacting protein that serves as a spindle checkpoint silencer at mitosis. In this study, we showed that p31^comet-induced apoptosis and senescence occur via counteraction of Mad2 activity. Upon retroviral transduction of p31^comet, the majority of human cancer cell lines tested lost the ability to form colonies in a low-density seeding assay. Cancer cells with p31^comet overexpression underwent distinct apoptosis and/or senescence, irrespective of p53 status, confirming the cytotoxicity of p31^comet. Interestingly, both cytotoxic and Mad2 binding activities were eliminated upon deletion of the C-terminal 30 amino acids of p31^comet. Point mutation or deletion of the region affecting Mad2 binding additionally abolished cytotoxic activity. Consistently, wild-type Mad2 interacting with p31^comet, but not its non-binding mutant, inhibited cell death, indicating that the mechanism of p31^comet-induced cell death involves Mad2 inactivation. Our results clearly suggest that the regions of p31^comet affecting interactions with Mad2, including the C-terminus, are essential for induction of cell death. The finding that p31^comet-induced cell death is mediated by interactions with Mad2 that lead to its inactivation is potentially applicable in anticancer therapy.

TRIP13PCH-2 promotes Mad2 localization to unattached kinetochores in the spindle checkpoint response

The ability of the conserved ATPase TRIP13^PCH-2 to disassemble a Mad2-containing complex is critical to promote the spindle checkpoint response by contributing to the robust localization of Mad2 to unattached kinetochores.

The spindle checkpoint acts during cell division to prevent aneuploidy, a hallmark of cancer. During checkpoint activation, Mad1 recruits Mad2 to kinetochores to generate a signal that delays anaphase onset. Yet, whether additional factors contribute to Mad2’s kinetochore localization remains unclear. Here, we report that the conserved AAA+ ATPase TRIP13^PCH-2 localizes to unattached kinetochores and is required for spindle checkpoint activation in Caenorhabditis elegans. pch-2 mutants effectively localized Mad1 to unattached kinetochores, but Mad2 recruitment was significantly reduced. Furthermore, we show that the C. elegans orthologue of the Mad2 inhibitor p31(comet)^CMT-1 interacts with TRIP13^PCH-2 and is required for its localization to unattached kinetochores. These factors also genetically interact, as loss of p31(comet)^CMT-1 partially suppressed the requirement for TRIP13^PCH-2 in Mad2 localization and spindle checkpoint signaling. These data support a model in which the ability of TRIP13^PCH-2 to disassemble a p31(comet)/Mad2 complex, which has been well characterized in the context of checkpoint silencing, is also critical for spindle checkpoint activation.

TRIP13 is a protein-remodeling AAA+ ATPase that catalyzes MAD2 conformation switching

The AAA+ family ATPase TRIP13 is a key regulator of meiotic recombination and the spindle assembly checkpoint, acting on signaling proteins of the conserved HORMA domain family. Here we present the structure of the Caenorhabditis elegans TRIP13 ortholog PCH-2, revealing a new family of AAA+ ATPase protein remodelers. PCH-2 possesses a substrate-recognition domain related to those of the protein remodelers NSF and p97, while its overall hexameric architecture and likely structural mechanism bear close similarities to the bacterial protein unfoldase ClpX. We find that TRIP13, aided by the adapter protein p31(comet), converts the HORMA-family spindle checkpoint protein MAD2 from a signaling-active ‘closed’ conformer to an inactive ‘open’ conformer. We propose that TRIP13 and p31(comet) collaborate to inactivate the spindle assembly checkpoint through MAD2 conformational conversion and disassembly of mitotic checkpoint complexes. A parallel HORMA protein disassembly activity likely underlies TRIP13's critical regulatory functions in meiotic chromosome structure and recombination.

DOI:http://dx.doi.org/10.7554/eLife.07367.001

The genetic material inside human and other animal cells is made of DNA and is packaged in structures called chromosomes. Before a cell divides, the entire set of chromosomes is copied so that each chromosome is now made of two identical sister ‘chromatids’.

Next, the chromosomes line up on a structure called the spindle, which is made of filaments called microtubules. Cells have a surveillance system known as the spindle assembly checkpoint that halts cell division until every chromosome is correctly aligned on the spindle. Once the chromosomes are in place, the checkpoint is turned off and the spindle pulls the chromatids apart so that each daughter cell receives a complete set of chromosomes.

A protein called MAD2 plays an important role in the spindle assembly checkpoint. It can adopt two distinct shapes: in the ‘closed’ shape it is active and halts cell division, but in the ‘open’ shape it is inactive and allows cell division to proceed. Another protein called TRIP13 can help turn off the checkpoint, but it is not clear how this works or whether TRIP13 acts on MAD2 directly.

Here, Ye et al. studied these proteins using a technique called X-ray crystallography and several biochemical techniques. The experiments show that TRIP13 belongs to a family of proteins known as ‘AAA-ATPases’, which can unfold proteins to alter their activity. Ye et al. found that TRIP13 binds to an adaptor protein that allows it to bind to the closed form of MAD2. TRIP13 then unfolds a part of the MAD2 protein, converting MAD2 into the open shape.

Ye et al. propose that, once all chromosomes are lined up on the spindle, TRIP13 turns off the spindle assembly checkpoint by converting closed MAD2 to open MAD2. Also, when cells are not undergoing cell division, TRIP13 may maintain MAD2 in the open shape to prevent cells from turning on the spindle assembly checkpoint at the wrong time. Further work will be needed to show how TRIP13 recognizes the closed form of MAD2, and whether it can act in a similar way on other proteins in the cell.

DOI:http://dx.doi.org/10.7554/eLife.07367.002