Closed MAD2 (C-MAD2) is selectively incorporated into the mitotic checkpoint complex (MCC)

TRIP13 regulates progression of gastric cancer through stabilising the expression of DDX21

GC (Gastric cancer) is one of the most common malignant tumours, with over 95% of gastric cancer patients being adenocarcinoma and most gastric cancer patients having no apparent symptoms in the early stages. Finding biomarkers for early screening of gastric cancer and exploring new targets for gastric cancer treatment are urgent problems to be solved in the treatment of gastric cancer, with significant clinical outcomes for the survival rate of gastric cancer patients. The AAA+ family ATPase thyroid hormone receptor-interacting protein 13 (TRIP13) has been reported to play an essential role in developing various tumours. However, the biological function and molecular mechanism of TRIP13 in gastric cancer remain unclear. This study confirms that TRIP13 is highly expressed in gastric cancer tissue samples and that TRIP13 participates in the proliferation, migration, invasion in vitro, and tumourigenesis and metastasis in vivo of gastric cancer cells. Mechanistically, this study confirms that TRIP13 directly interacts with DDX21 and stabilises its expression by restraining its ubiquitination degradation, thereby promoting gastric cancer progression. Additionally, histone deacetylase 1 (HDAC1) is an upstream factor of TRIP13, which could target the TRIP13 promoter region to promote the proliferation, migration, and invasion of gastric cancer cells. These results indicate that TRIP13 serve is a promising biomarker for the treating of gastric cancer patients, and the HDAC1-TRIP13/DDX21 axis might provide a solid theoretical basis for clinical treatment of gastric cancer patients.

CAMK2D serves as a molecular scaffold for RNF8-MAD2 complex to induce mitotic checkpoint in glioma

MAD2 is a spindle assembly checkpoint protein that participates in the formation of mitotic checkpoint complex, which blocks mitotic progression. RNF8, an established DNA damage response protein, has been implicated in mitotic checkpoint regulation but its exact role remains poorly understood. Here, RNF8 proximity proteomics uncovered a role of RNF8-MAD2 in generating the mitotic checkpoint signal. Specifically, RNF8 competes with a small pool of p31comet for binding to the closed conformer of MAD2 via its RING domain, while CAMK2D serves as a molecular scaffold to concentrate the RNF8-MAD2 complex via transient/weak interactions between its p-Thr287 and RNF8's FHA domain. Accordingly, RNF8 overexpression impairs glioma stem cell (GSC) mitotic progression in a FHA- and RING-dependent manner. Importantly, low RNF8 expression correlates with inferior glioma outcome and RNF8 overexpression impedes GSC tumorigenicity. Last, we identify PLK1 inhibitor that mimics RNF8 overexpression using a chemical biology approach, and demonstrate a PLK1/HSP90 inhibitor combination that synergistically reduces GSC proliferation and stemness. Thus, our study has unveiled a previously unrecognized CAMK2D-RNF8-MAD2 complex in regulating mitotic checkpoint with relevance to gliomas, which is therapeutically targetable.

Anticancer Effects and Molecular Action of 7-α-Hydroxyfrullanolide in G2/M-Phase Arrest and Apoptosis in Triple Negative Breast Cancer Cells

Triple negative breast cancer (TNBC) is a breast cancer subtype characterized by the absence of estrogen receptor, progesterone receptor and human epidermal growth factor receptor 2 expression. TNBC cells respond poorly to targeted chemotherapies currently in use and the mortality rate of TNBC remains high. Therefore, it is necessary to identify new chemotherapeutic agents for TNBC. In this study, the anti-cancer effects of 7-α-hydroxyfrullanolide (7HF), derived from Grangea maderaspatana, on MCF-7, MDA-MB-231 and MDA-MB-468 breast cancer cells were assessed using MTT assay. The mode of action of 7HF in TNBC cells treated with 6, 12 and 24 µM of 7HF was determined by flow cytometry and propidium iodide (PI) staining for cell cycle analysis and annexin V/fluorescein isothiocyanate + PI staining for detecting apoptosis. The molecular mechanism of action of 7HF in TNBC cells was investigated by evaluating protein expression using proteomic techniques and western blotting. Subsequently, 7HF exhibited the strongest anti-TNBC activity toward MDA-MB-468 cells and a concomitantly weak toxicity toward normal breast cells. The molecular mechanism of action of low-dose 7HF in TNBC cells primarily involved G2/M-phase arrest through upregulation of the expression of Bub3, cyclin B1, phosphorylated Cdk1 (Tyr 15) and p53-independent p21. Contrastingly, the upregulation of PP2A-A subunit expression may have modulated the suppression of various cell survival proteins such as p-Akt (Ser 473), FoxO3a and β-catenin. The concurrent apoptotic effect of 7HF on the treated cells was mediated via both intrinsic and extrinsic modes through the upregulation of Bax and active cleaved caspase-7-9 expression and downregulation of Bcl-2 and full-length caspase-7-9 expression. Notably, the proteomic approach revealed the upregulation of the expression of pivotal protein clusters associated with G1/S-phase arrest, G2/M-phase transition and apoptosis. Thus, 7HF exhibits promising anti-TNBC activity and at a low dose, it modulates signal transduction associated with G2/M-phase arrest and apoptosis.

The cytochrome P450 enzyme CYP24A1 increases proliferation of mutant KRAS-dependent lung adenocarcinoma independent of its catalytic activity

We previously reported that overexpression of cytochrome P450 family 24 subfamily A member 1 (CYP24A1) increases lung cancer cell proliferation by activating RAS signaling and that CYP24A1 knockdown inhibits tumor growth. However, the mechanism of CYP24A1-mediated cancer cell proliferation remains unclear. Here, we conducted cell synchronization and biochemical experiments in lung adenocarcinoma cells, revealing a link between CYP24A1 and anaphase-promoting complex (APC), a key cell cycle regulator. We demonstrate that CYP24A1 expression is cell cycle-dependent; it was higher in the G₂-M phase and diminished upon G₁ entry. CYP24A1 has a functional destruction box (D-box) motif that allows binding with two APC adaptors, CDC20-homologue 1 (CDH1) and cell division cycle 20 (CDC20). Unlike other APC substrates, however, CYP24A1 acted as a pseudo-substrate, inhibiting CDH1 activity and promoting mitotic progression. Conversely, overexpression of a CYP24A1 D-box mutant compromised CDH1 binding, allowing CDH1 hyperactivation, thereby hastening degradation of its substrates cyclin B1 and CDC20, and accumulation of the CDC20 substrate p21, prolonging mitotic exit. These activities also occurred with a CYP24A1 isoform 2 lacking the catalytic cysteine (Cys-462), suggesting that CYP24A1's oncogenic potential is independent of its catalytic activity. CYP24A1 degradation reduced clonogenic survival of mutant KRAS-driven lung cancer cells, and calcitriol treatment increased CYP24A1 levels and tumor burden in Lsl-KRAS^G12D mice. These results disclose a catalytic activity-independent growth-promoting role of CYP24A1 in mutant KRAS-driven lung cancer. This suggests that CYP24A1 could be therapeutically targeted in lung cancers in which its expression is high.

Survivin regulates chromosome segregation by modulating the phosphorylation of Aurora B during porcine oocyte meiosis

SURVIVIN is an essential chromosomal passenger complex (CPC) subunit and participates in cell division. In this study, we used porcine oocyte as a model to investigate the roles of Survivin during porcine oocyte maturation. Survivin was highly expressed in germinal vesicle (GV) and germinal vesicle breakdown (GVBD) stages oocytes, mainly localized in the GV at GV stage and on the chromosomes after GVBD. We have used RNA interference to specifically deplete Survivin in oocytes during in vitro maturation (IVM). Immunofluorescence assay showed that Survivin-depleted oocytes failed to produce polar body in meiosisⅠ (failed to complete cytokinesis), and they were arrested in metaphaseⅠwith misaligned chromosomes. The homologous chromosomes in Survivin-depleted oocytes could not be separated normally. Moreover, both the phosphorylation levels of Aurora B and the mRNA level of Mad2L1 related to spindle assembly checkpoint (SAC) was decreased in Survivin-depleted oocytes, which thus inhibited the degradation of Cyclin B1 (CCNB1) to complete meiosis. Taken together, we conclude that Survivin is an important mediator of centromere and midbody docking of Aurora-B as well as its activity and regulates SAC and MPF activity during meiosis in porcine oocytes.

MAD1: Kinetochore Receptors and Catalytic Mechanisms

The mitotic checkpoint monitors kinetochore-microtubule attachment, delays anaphase onset and prevents aneuploidy when unattached or tensionless kinetochores are present in cells. Mitotic arrest deficiency 1 (MAD1) is one of the evolutionarily conserved core mitotic checkpoint proteins. MAD1 forms a cell cycle independent complex with MAD2 through its MAD2 interaction motif (MIM) in the middle region. Such a complex is enriched at unattached kinetochores and functions as an unusual catalyst to promote conformational change of additional MAD2 molecules, constituting a crucial signal amplifying mechanism for the mitotic checkpoint. Only MAD2 in its active conformation can be assembled with BUBR1 and CDC20 to form the Mitotic Checkpoint Complex (MCC), which is a potent inhibitor of anaphase onset. Recent research has shed light on how MAD1 is recruited to unattached kinetochores, and how it carries out its catalytic activity. Here we review these advances and discuss their implications for future research.

USP9X Limits Mitotic Checkpoint Complex Turnover to Strengthen the Spindle Assembly Checkpoint and Guard against Chromosomal Instability

Faithful chromosome segregation during mitosis depends on the spindle assembly checkpoint (SAC), which delays progression through mitosis until every chromosome has stably attached to spindle microtubules via the kinetochore. We show here that the deubiquitinase USP9X strengthens the SAC by antagonizing the turnover of the mitotic checkpoint complex produced at unattached kinetochores. USP9X thereby opposes activation of anaphase-promoting complex/cyclosome (APC/C) and specifically inhibits the mitotic degradation of SAC-controlled APC/C substrates. We demonstrate that depletion or loss of USP9X reduces the effectiveness of the SAC, elevates chromosome segregation defects, and enhances chromosomal instability (CIN). These findings provide a rationale to explain why loss of USP9X could be either pro- or anti-tumorigenic depending on the existing level of CIN.

Direct interactions of mitotic arrest deficient 1 (MAD1) domains with each other and MAD2 conformers are required for mitotic checkpoint signaling

As a sensitive signaling system, the mitotic checkpoint ensures faithful chromosome segregation by delaying anaphase onset even when a single kinetochore is unattached to mitotic spindle microtubules. The key signal amplification reaction for the checkpoint is the conformational conversion of "open" mitotic arrest deficient 2 (O-MAD2) into "closed" MAD2 (C-MAD2). The reaction has been suggested to be catalyzed by an unusual catalyst, a MAD1:C-MAD2 tetramer, but how the catalysis is executed and regulated remains elusive. Here, we report that in addition to the well-characterized middle region of MAD1 containing the MAD2-interaction motif (MIM), both N- and C-terminal domains (NTD and CTD) of MAD1 also contribute to mitotic checkpoint signaling. Unlike the MIM, which stably associated only with C-MAD2, the NTD and CTD in MAD1 surprisingly bound both O- and C-MAD2, suggesting that these two domains interact with both substrates and products of the O-to-C conversion. MAD1^NTD and MAD1^CTD also interacted with each other and with the MPS1 protein kinase, which phosphorylated both NTD and CTD. This phosphorylation decreased the NTD:CTD interaction and also CTD's interaction with MPS1. Of note, mutating the phosphorylation sites in the MAD1^CTD, including Thr-716, compromised MAD2 binding and the checkpoint responses. We further noted that Ser-610 and Tyr-634 also contribute to the mitotic checkpoint signaling. Our results have uncovered that the MAD1^NTD and MAD1^CTD directly interact with each other and with MAD2 conformers and are regulated by MPS1 kinase, providing critical insights into mitotic checkpoint signaling.

Function and regulation mechanism of Chk1 during meiotic maturation in porcine oocytes

Checkpoint 1 (Chk1), as an important member of DNA replication checkpoint and DNA damage response, has an important role during the G2/M stage of mitosis. In this study, we used porcine oocyte as a model to investigate the function of Chk1 during porcine oocyte maturation. Chk1 was expressed from germinal vesicle (GV) to metaphase II (MII) stages, mainly localized in the cytoplasm at GV stage and moved to the spindle after germinal vesicle breakdown (GVBD). Chk1 depletion not only induced oocytes to be arrested at MI stage with abnormal chromosomes arrangement, but also inhibited the degradation of Cyclin B1 and decreased the expression of Mitotic Arrest Deficient 2-Like 1 (Mad2L1), one of spindle assembly checkpoint (SAC) proteins, and cadherin 1 (Cdh1), one of coactivation for anaphase-promoting complex/cyclosome (APC/C). Moreover, Chk1 overexpression delayed GVBD. These results demonstrated that Chk1 facilitated the timely degradation of Cyclin B1 at anaphase I (AI) and maintained the expression of Mad2L1 and Cdh1, which ensured that all chromosomes were accurately located in a line, and then oocytes passed metaphase I (MI) and AI and exited from the first meiotic division successfully. In addition, we proved that Chk1 had not function on GVBD of porcine oocytes, which suggested that maturation of porcine oocytes did not need the DNA damage checkpoint, which was different from the mouse oocyte maturation.

[Study on the expression of TRIP13 mRNA in chronic lymphocytic leukemia B lymphocyte and the molecular mechanism of TRIP13 mediated JVM-2 cell proliferation and apoptosis]

Objective: To investigate the clinical significance of expression level of thyroid hormone receptor interactors 13 (TRIP13) gene to probe its function and downstream molecular mechanism in chronic lymphocytic leukemia (CLL) . Methods: Real-time quantitative PCR method was used to detect the expression levels of TRIP13 mRNA of CD19(+) B lymphocytes in 30 cases of patients with CLL and 12 cases of peripheral blood hematopoietic stem cell donors (normal control group) . Lentivirus mediated shRNA was used to interference the mRNA and TRIP13 protein in CLL cells JVM-2. Scramble sequence was used as control. Methyl thiazolyl tetrazolium colorimetric assay (MTT) and flow cytometry was used to detect the cell proliferation and apoptosis in TRIP13 knocked-down and negative control JVM-2 cells. Results: TRIP13 mRNA level was significantly higher in 30 cases of CLL patients (2(-△Ct)= 0.014 89) compared with 12 healthy donors (2(-△Ct)= 0.000 19) (P<0.001) . Validated TRIP13 shRNA target was achieved in JVM2 cell. Compared with the control group, down-regulation of TRIP13 expression could significantly inhibit the proliferation of JVM-2 cells and induce apoptosis. The expressions of Myc and Bcl-2 protein in JVM-2 cells decreased significantly after interference with TRIP13 (P<0.001) , and the expressions of Bax, caspase 3 and Bad protein increased significantly (P<0.001) . Conclusion: TRIP13 mRNA significantly over-expressed in CLL patients CD19(+) B lymphocytes. TRIP13 could influence JVM2 cell proliferation and apoptosis through proliferation- and apoptosis-related proteins.

TRIP13 is expressed in colorectal cancer and promotes cancer cell invasion

Thyroid hormone receptor interactor 13 (TRIP13) is a member of the ATPases associated with various cellular activities family of proteins and is highly conserved in a wide range of species. Recent studies have demonstrated that TRIP13 is critical for the inactivation of the spindle assembly checkpoint and is associated with the progression of certain cancers. In the present study, the role of TRIP13 in colorectal cancer (CRC) was examined. Reverse transcription-quantitative polymerase chain reaction analysis revealed that TRIP13 messenger RNA was highly expressed in multiple CRC tissues. The depletion of TRIP13 in CRC cells suppressed cell proliferation, migration and invasion. To determine whether the catalytic activity of TRIP13 was critical for cancer progression, an inactive mutant of TRIP13 was expressed in CRC cells. The invasion of cancer cells that expressed the mutant TRIP13 was significantly reduced compared with that of the wild type TRIP13-expressing cancer cells. These results indicate that TRIP13 could be a potential target for CRC treatment.

The mitotic checkpoint complex (MCC): looking back and forth after 15 years

The mitotic checkpoint is a specialized signal transduction pathway that contributes to the fidelity of chromosome segregation. The signaling of the checkpoint originates from defective kinetochore-microtubule interactions and leads to formation of the mitotic checkpoint complex (MCC), a highly potent inhibitor of the Anaphase Promoting Complex/Cyclosome (APC/C)—the E3 ubiquitin ligase essential for anaphase onset. Many important questions concerning the MCC and its interaction with APC/C have been intensively investigated and debated in the past 15 years, such as the exact composition of the MCC, how it is assembled during a cell cycle, how it inhibits APC/C, and how the MCC is disassembled to allow APC/C activation. These efforts have culminated in recently reported structure models for human MCC:APC/C supra-complexes at near-atomic resolution that shed light on multiple aspects of the mitotic checkpoint mechanisms. However, confusing statements regarding the MCC are still scattered in the literature, making it difficult for students and scientists alike to obtain a clear picture of MCC composition, structure, function and dynamics. This review will comb through some of the most popular concepts or misconceptions about the MCC, discuss our current understandings, present a synthesized model on regulation of CDC20 ubiquitination, and suggest a few future endeavors and cautions for next phase of MCC research.

OTSSP167 Abrogates Mitotic Checkpoint through Inhibiting Multiple Mitotic Kinases

OTSSP167 was recently characterized as a potent inhibitor for maternal embryonic leucine zipper kinase (MELK) and is currently tested in Phase I clinical trials for solid tumors that have not responded to other treatment. Here we report that OTSSP167 abrogates the mitotic checkpoint at concentrations used to inhibit MELK. The abrogation is not recapitulated by RNAi mediated silencing of MELK in cells. Although OTSSP167 indeed inhibits MELK, it exhibits off-target activity against Aurora B kinase in vitro and in cells. Furthermore, OTSSP167 inhibits BUB1 and Haspin kinases, reducing phosphorylation at histones H2AT120 and H3T3 and causing mislocalization of Aurora B and associated chromosomal passenger complex from the centromere/kinetochore. The results suggest that OTSSP167 may have additional mechanisms of action for cancer cell killing and caution the use of OTSSP167 as a MELK specific kinase inhibitor in biochemical and cellular assays.

Stable folding intermediates prevent fast interconversion between the closed and open states of Mad2 through its denatured state

Different states of metamorphic proteins can interconvert under physiological conditions to realize corresponding functions. The mechanism behind the conversion is critical for understanding how these proteins work. We report a combined thermodynamic and kinetic study on the folding/unfolding process of the open and closed conformers of mitotic arrest deficient protein 2 (Mad2), a metamorphic protein. It has been observed that open Mad2 (O-Mad2) can convert to closed Mad2 (C-Mad2). Our results show that O-Mad2 and C-Mad2 have similar thermodynamic stability, which explains the presence of metamorphosis. The folding/unfolding kinetics suggest that the conversion between O-Mad2 and C-Mad2 would be much faster than that reported previously if this conversion goes through the denatured state (U) directly, i.e. through an O-Mad2-denatured state (U)-C-Mad2 (O-U-C) pathway. This inconsistency implies that there exist stable intermediates in between the native and denatured states of Mad2, which would either slow down the O-U-C interconversion or prevent it going through the denatured state.

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

Thyroid hormone receptor interacting protein 13 (TRIP13) AAA-ATPase is a novel mitotic checkpoint-silencing protein

The mitotic checkpoint (or spindle assembly checkpoint) is a fail-safe mechanism to prevent chromosome missegregation by delaying anaphase onset in the presence of defective kinetochore-microtubule attachment. The target of the checkpoint is the E3 ubiquitin ligase anaphase-promoting complex/cyclosome. Once all chromosomes are properly attached and bioriented at the metaphase plate, the checkpoint needs to be silenced. Previously, we and others have reported that TRIP13 AAA-ATPase binds to the mitotic checkpoint-silencing protein p31(comet). Here we show that endogenous TRIP13 localizes to kinetochores. TRIP13 knockdown delays metaphase-to-anaphase transition. The delay is caused by prolonged presence of the effector for the checkpoint, the mitotic checkpoint complex, and its association and inhibition of the anaphase-promoting complex/cyclosome. These results suggest that TRIP13 is a novel mitotic checkpoint-silencing protein. The ATPase activity of TRIP13 is essential for its checkpoint function, and interference with TRIP13 abolished p31(comet)-mediated mitotic checkpoint silencing. TRIP13 overexpression is a hallmark of cancer cells showing chromosomal instability, particularly in certain breast cancers with poor prognosis. We suggest that premature mitotic checkpoint silencing triggered by TRIP13 overexpression may promote cancer development.

Spindle assembly checkpoint robustness requires Tpr-mediated regulation of Mad1/Mad2 proteostasis

Tpr is a kinetochore-independent, rate-limiting factor required to mount and sustain a robust spindle assembly checkpoint response by stabilizing Mad1 and Mad2 before mitosis.

Tpr is a conserved nuclear pore complex (NPC) protein implicated in the spindle assembly checkpoint (SAC) by an unknown mechanism. Here, we show that Tpr is required for normal SAC response by stabilizing Mad1 and Mad2 before mitosis. Tpr coimmunoprecipitated with Mad1 and Mad2 (hereafter designated as Tpr/Mad1/Mad2 or TM2 complex) during interphase and mitosis, and is required for Mad1–c-Mad2 recruitment to NPCs. Interestingly, Tpr was normally undetectable at kinetochores and dispensable for Mad1, but not for Mad2, kinetochore localization, which suggests that SAC robustness depends on Mad2 levels at kinetochores. Protein half-life measurements demonstrate that Tpr stabilizes Mad1 and Mad2, ensuring normal Mad1–c-Mad2 production in an mRNA- and kinetochore-independent manner. Overexpression of GFP-Mad2 restored normal SAC response and Mad2 kinetochore levels in Tpr-depleted cells. Mechanistically, we provide evidence that Tpr might spatially regulate SAC proteostasis through the SUMO-isopeptidases SENP1 and SENP2 at NPCs. Thus, Tpr is a kinetochore-independent, rate-limiting factor required to mount and sustain a robust SAC response.

Monopolar spindle 1 (MPS1) kinase promotes production of closed MAD2 (C-MAD2) conformer and assembly of the mitotic checkpoint complex

MPS1 kinase is an essential component of the spindle assembly checkpoint (SAC), but its functioning mechanisms are not fully understood. We have shown recently that direct interaction between BUBR1 and MAD2 is critical for assembly and function of the human mitotic checkpoint complex (MCC), the SAC effector. Here we report that inhibition of MPS1 kinase activity by reversine disrupts BUBR1-MAD2 as well as CDC20-MAD2 interactions, causing premature activation of the anaphase-promoting complex/cyclosome. The effect of MPS1 inhibition is likely due to reduction of closed MAD2 (C-MAD2), as expressing a MAD2 mutant (MAD2(L13A)) that is locked in the C conformation rescued the checkpoint defects. In the presence of reversine, exogenous C-MAD2 does not localize to unattached kinetochores but is still incorporated into the MCC. Contrary to a previous report, we found that sustained MPS1 activity is required for maintaining both the MAD1·C-MAD2 complex and open MAD2 (O-MAD2) at unattached kinetochores to facilitate C-MAD2 production. Additionally, mitotic phosphorylation of BUBR1 is also affected by MPS1 inhibition but seems dispensable for MCC assembly. Our results support the notion that MPS1 kinase promotes C-MAD2 production and subsequent MCC assembly to activate the SAC.

The end of mitosis from a phosphatase perspective

Transition through mitosis, the cell division cycle phase deputed to segregate replicated chromosomes, requires a wave of protein phosphorylation. While in the past decades a wealth of information has been gathered on the major kinase activities responsible for the onset of mitosis, only recently has a picture emerged of how their effects are reversed by protein phosphatases at the end of mitosis. Here, we summarized some recent data on the relevance for protein phosphatases in the reversal of mitotic phosphorylation required to complete mitosis in vertebrate cells.

Identification of novel mitosis regulators through data mining with human centromere/kinetochore proteins as group queries

Background

Proteins functioning in the same biological pathway tend to be transcriptionally co-regulated or form protein-protein interactions (PPI). Multiple spatially and temporally regulated events are coordinated during mitosis to achieve faithful chromosome segregation. The molecular players participating in mitosis regulation are still being unravelled experimentally or using in silico methods.

Results

An extensive literature review has led to a compilation of 196 human centromere/kinetochore proteins, all with experimental evidence supporting the subcellular localization. Sixty-four were designated as “core” centromere/kinetochore components based on peak expression and/or well-characterized functions during mitosis. By interrogating and integrating online resources, we have mined for genes/proteins that display transcriptional co-expression or PPI with the core centromere/kinetochore components. Top-ranked hubs in either co-expression or PPI network are not only enriched with known mitosis regulators, but also contain candidates whose mitotic functions are not yet established. Experimental validation found that KIAA1377 is a novel centrosomal protein that also associates with microtubules and midbody; while TRIP13 is a novel kinetochore protein and directly interacts with mitotic checkpoint silencing protein p31^comet.

Conclusions

Transcriptional co-expression and PPI network analyses with known human centromere/kinetochore proteins as a query group help identify novel potential mitosis regulators.