Protein architecture of the human kinetochore microtubule attachment site.

Chromosome segregation requires assembly of kinetochores on centromeric chromatin to mediate interactions with spindle microtubules and control cell-cycle progression. To elucidate the protein architecture of human kinetochores, we developed a two-color fluorescence light microscopy method that measures average label separation, Delta, at <5 nm accuracy. Delta analysis of 16 proteins representing core structural complexes spanning the centromeric chromatin-microtubule interface, when correlated with mechanical states of spindle-attached kinetochores, provided a nanometer-scale map of protein position and mechanical properties of protein linkages. Treatment with taxol, which suppresses microtubule dynamics and activates the spindle checkpoint, revealed a specific switch in kinetochore architecture. Cumulatively, Delta analysis revealed that compliant linkages are restricted to the proximity of chromatin, suggested a model for how the KMN (KNL1/Mis12 complex/Ndc80 complex) network provides microtubule attachment and generates pulling forces from depolymerization, and identified an intrakinetochore molecular switch that may function in controlling checkpoint activity.

[Cross-sectional analysis of libido status and risk factors in male patients with chronic headache].

OBJECTIVE: Exploring the libido status of male chronic headache patients and analyzing its relationship with headache symptoms, sleep, anxiety, and depression, providing reference for the comprehensive treatment of male chronic headache. METHODS: 179 patients with chronic headache who visited the Third Affiliated Hospital of Qiqihar Medical College from January 2022 to February 2023 were selected. The male Self Rated Libido Scale , Visual Analog Scale for Pain, Migraine Disability Assessment Scale, Pittsburgh Sleep Quality Index, Generalized Anxiety Disorder Scale-7, and Patient Health Questionnaire-9 were used to evaluate the libido status, headache severity, disability level, sleep quality, anxiety, and depression of the research subjects, respectively. RESULTS: Among 179 male chronic headache patients, 97 were chronic migraine (CM) patients and 82 were chronic tension type (CTT) patients, and 47 were screened for low libido. The influencing factors of libido in male chronic headache patients include age, smoking, frequency of exercise, course of disease, severity of pain, frequency of headache, disability score, sleep quality, anxiety and depression (all P<0.05). Compared with male CTT patients, male CM patients have higher pain severity, headache frequency, disability score, and anxiety score, while lower libido score (all P<0.05). The results of multivariate analysis showed that age, frequency of exercise, course of disease, severity of pain, frequency of headache, degree of disability, sleep quality, anxiety, and depression were the influencing factors for the decline of libido in male chronic headache patients. CONCLUSION: It is common for male chronic headache patients to experience decreased libido, with male chronic migraine (CM) patients exhibiting more severe reductions. Advanced age, decreased physical activity, longer disease duration, severe pain intensity, frequent headaches, higher disability levels, poor sleep, anxiety, and depression are risk factors for decreased libido in male chronic headache patients.

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. MAD1NTD and MAD1CTD 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 MAD1CTD, 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 MAD1NTD and MAD1CTD directly interact with each other and with MAD2 conformers and are regulated by MPS1 kinase, providing critical insights into mitotic checkpoint signaling.

Disassembly of mitotic checkpoint complexes by the joint action of the AAA-ATPase TRIP13 and p31(comet).

The mitotic (or spindle assembly) checkpoint system delays anaphase until all chromosomes are correctly attached to the mitotic spindle. When the checkpoint is active, a Mitotic Checkpoint Complex (MCC) assembles and inhibits the ubiquitin ligase Anaphase-Promoting Complex/Cyclosome (APC/C). MCC is composed of the checkpoint proteins Mad2, BubR1, and Bub3 associated with the APC/C activator Cdc20. When the checkpoint signal is turned off, MCC is disassembled and the checkpoint is inactivated. The mechanisms of the disassembly of MCC are not sufficiently understood. We have previously observed that ATP hydrolysis is required for the action of the Mad2-binding protein p31(comet) to disassemble MCC. We now show that HeLa cell extracts contain a factor that promotes ATP- and p31(comet)-dependent disassembly of a Cdc20-Mad2 subcomplex and identify it as Thyroid Receptor Interacting Protein 13 (TRIP13), an AAA-ATPase known to interact with p31(comet). The joint action of TRIP13 and p31(comet) also promotes the release of Mad2 from MCC, participates in the complete disassembly of MCC and abrogates checkpoint inhibition of APC/C. We propose that TRIP13 plays centrally important roles in the sequence of events leading to MCC disassembly and checkpoint inactivation.

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.

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.

Structural and functional analysis of the related transcriptional enhancer factor-1 and NF-κB interaction.

The related transcriptional enhancer factor-1 (RTEF-1) increases gene transcription of hypoxia-inducible factor 1α (HIF-1α) and enhances angiogenesis in endothelium. Both hypoxia and inflammatory factor TNF-α regulate gene expression of HIF-1α, but how RTEF-1 and TNF-α coordinately regulate HIF-1α gene transcription is unclear. Here, we found that RTEF-1 interacts with p65 subunit of NF-κB, a primary mediator of TNF-α. RTEF-1 increased HIF-1α promoter activity, whereas expression of p65 subunit inhibited the stimulatory effect. By contrast, knockdown of p65 markedly enhanced RTEF-1 stimulation on the HIF-1α promoter activity (7-fold). A physical interaction between RTEF-1 and p65 was confirmed by coimmunoprecipitation experiments in cells and glutathione S-transferase (GST)-pull-down assays. A computational analysis of RTEF-1 crystal structures revealed that a conserved surface of RTEF-1 potentially interacts with p65 via four amino acid residues located at T347, Y349, R351, and Y352. We performed site-directed mutagenesis and GST-pull-down assays and demonstrated that Tyr352 (Y352) in RTEF-1 is a key site for the formation of RTEF-1 and p65-NF-κB complex. An alanine mutation at Y352 of RTEF-1 disrupted the interaction of RTEF-1 with p65. Moreover, expression of RTEF-1 decreased TNF-α-induced HIF-1α promoter activity, IL-1ß, and IL-6 mRNA levels in cells; however, the effect of RTEF-1 was largely lost when Y352 was mutated to alanine. These results indicate that RTEF-1 interacts with p65-NF-κB through Y352 and that they antagonize each other for HIF-1α transcriptional activation, suggesting a novel mechanism by which RTEF-1 regulates gene expression, linking hypoxia to inflammation.

Explainable machine learning for early predicting treatment failure risk among patients with TB-diabetes comorbidity.

The present study aims to assess the treatment outcome of patients with diabetes and tuberculosis (TB-DM) at an early stage using machine learning (ML) based on electronic medical records (EMRs). A total of 429 patients were included at Chongqing Public Health Medical Center. The random-forest-based Boruta algorithm was employed to select the essential variables, and four models with a fivefold cross-validation scheme were used for modeling and model evaluation. Furthermore, we adopted SHapley additive explanations to interpret results from the tree-based model. 9 features out of 69 candidate features were chosen as predictors. Among these predictors, the type of resistance was the most important feature, followed by activated partial throm-boplastic time (APTT), thrombin time (TT), platelet distribution width (PDW), and prothrombin time (PT). All the models we established performed above an AUC 0.7 with good predictive performance. XGBoost, the optimal performing model, predicts the risk of treatment failure in the test set with an AUC 0.9281. This study suggests that machine learning approach (XGBoost) presented in this study identifies patients with TB-DM at higher risk of treatment failure at an early stage based on EMRs. The application of a convenient and economy EMRs based on machine learning provides new insight into TB-DM treatment strategies in low and middle-income countries.

[Health Benefit Assessment of Coal-to-electricity Policy on PM2.5 Pollution in Beijing-Tianjin-Hebei Region].

To accurately assess the health benefits of the coal-to-electricity policy during the heating period in the Beijing-Tianjin-Hebei(BTH) Region, the premature deaths caused by PM2.5 before and after the implementation of the coal-to-electricity policy during the heating period in each district and county of the BTH Region were estimated, and the corresponding health loss values were calculated using the willingness to pay method. The results showed that the implementation of the coal-to-electricity policy in the BTH Region brought 1745 cases(95% CI:1443-1907) of health benefits and 2.38 billion yuan(95% CI:1.45-3.06) in economic benefits. In Beijing, Tianjin, and Hebei there were 495 cases(95% CI:436-554), 296 cases(95% CI:238-354), and 954 cases(95% CI:693-1076) of health benefits, respectively. The economic benefits were 0.35 billion yuan(95% CI:0.30-0.39), 0.33 billion yuan(95% CI:0.27-0.40), and 1.70 billion yuan(95% CI:0.88-2.28), respectively, accounting for 0.01%, 0.02%, and 0.04% of GDP in each region. The number of premature deaths due to COPD, LC, ALRI, IHD, and STROKE decreased by 187 cases(95% CI:165-224), 318 cases(95% CI:178-458), 193 cases(95% CI:115-204), 506 cases(95% CI:232-780), and 542 cases(95% CI:463-621), respectively. Areas with relatively high environmental PM2.5 concentrations and concentrated population-intensive pollution emissions can achieve significant health and economic benefits.

BUBR1 and closed MAD2 (C-MAD2) interact directly to assemble a functional mitotic checkpoint complex.

The mitotic checkpoint maintains genomic stability by ensuring that chromosomes are accurately segregated during mitosis. When the checkpoint is activated, the mitotic checkpoint complex (MCC), assembled from BUBR1, BUB3, CDC20, and MAD2, directly binds and inhibits the anaphase-promoting complex/cyclosome (APC/C) until all chromosomes are properly attached and aligned. The mechanisms underlying MCC assembly and MCC-APC/C interaction are not well characterized. Here, we show that a novel interaction between BUBR1 and closed MAD2 (C-MAD2) is essential for MCC-mediated inhibition of APC/C. Intriguingly, Arg(133) and Gln(134) in C-MAD2 are required for BUBR1 interaction. The same residues are also critical for MAD2 dimerization and MAD2 binding to p31(comet), a mitotic checkpoint silencing protein. Along with previously characterized BUBR1-CDC20 and C-MAD2-CDC20 interactions, our results underscore the integrity of the MCC for its activity and suggest the fundamental importance of the MAD2 αC helix in modulating mitotic checkpoint activation and silencing.

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.

Tripin/hSgo2 recruits MCAK to the inner centromere to correct defective kinetochore attachments.

hSgo2 (previously annotated as Tripin) was recently reported to be a new inner centromere protein that is essential for centromere cohesion (Kitajima et al., 2006). In this study, we show that hSgo2 exhibits a dynamic distribution pattern, and that its localization depends on the BUB1 and Aurora B kinases. hSgo2 is concentrated at the inner centromere of unattached kinetochores, but extends toward the kinetochores that are under tension. This localization pattern is reminiscent of MCAK, which is a microtubule depolymerase that is believed to be a key component of the error correction mechanism at kinetochores. Indeed, we found that hSgo2 is essential for MCAK to localize to the centromere. Delocalization of MCAK accounts for why cells depleted of hSgo2 exhibit kinetochore attachment defects that go uncorrected, despite a transient delay in the onset of anaphase. Consequently, these cells exhibit a high frequency of lagging chromosomes when they enter anaphase. We confirmed that hSgo2 is associated with PP2A, and we propose that it contributes to the spatial regulation of MCAK activity within inner centromere and kinetochore.

Human CENP-I specifies localization of CENP-F, MAD1 and MAD2 to kinetochores and is essential for mitosis.

The kinetochore, a macromolecular complex located at the centromere of chromosomes, provides essential functions for accurate chromosome segregation. Kinetochores contain checkpoint proteins that monitor attachments between the kinetochore and microtubules to ensure that cells do not exit mitosis in the presence of unaligned chromosomes. Here we report that human CENP-I, a constitutive protein of the kinetochore that shares limited similarity with Mis6 of Schizosaccharomyces pombe, is required for the localization of CENP-F and the checkpoint proteins MAD1 and MAD2 to kinetochores. Depletion of CENP-I from kinetochores causes the cell cycle to delay in G2. Although monopolar chromosomes in CENP-I-depleted cells fail to establish bipolar connections, the cells are unable to arrest in mitosis. These cells are transiently delayed in mitosis in a MAD2-dependent manner, even though their kinetochores are depleted of MAD2. The delay is extended considerably when the number of unattached kinetochores is increased. This suggests that no single unattached kinetochore in CENP-I-depleted cells can arrest mitosis. The collective output from many unattached kinetochores is required to reach a threshold signal of 'wait for anaphase' to sustain a prolonged mitotic arrest.

Mapping the assembly pathways that specify formation of the trilaminar kinetochore plates in human cells.

We report the interactions amongst 20 proteins that specify their assembly to the centromere-kinetochore complex in human cells. Centromere protein (CENP)-A is at the top of a hierarchy that directs three major pathways, which are specified by CENP-C, -I, and Aurora B. Each pathway consists of branches that intersect to form nodes that may coordinate the assembly process. Complementary EM studies found that the formation of kinetochore trilaminar plates depends on the CENP-I/NUF2 branch, whereas CENP-C and Aurora B affect the size, shape, and structural integrity of the plates. We found that hMis12 is not constitutively localized at kinetochores, and that it is not essential for recruiting CENP-I. Our studies also revealed that kinetochores in HeLa cells contain an excess of CENP-A, of which approximately 10% is sufficient to promote the assembly of normal levels of kinetochore proteins. We elaborate on a previous model that suggested kinetochores are assembled from repetitive modules (Zinkowski, R.P., J. Meyne, and B.R. Brinkley. 1991. J. Cell Biol. 113:1091-110).

Kinetochore structure and function.

The vertebrate kinetochore is a complex structure that specifies the attachments between the chromosomes and microtubules of the spindle and is thus essential for accurate chromosome segregation. Kinetochores are assembled on centromeric chromatin through complex pathways that are coordinated with the cell cycle. In the light of recent discoveries on how proteins assemble onto kinetochores and interact with each other, we review these findings in this article (which is part of the Chromosome Segregation and Aneuploidy series), and discuss their implications for the current mitotic checkpoint models - the template model and the two-step model. The template model proposes that Mad1-Mad2 at kinetochores acts as a template to change the conformation of another binding molecule of Mad2. This templated change in conformation is postulated as a mechanism for the amplification of the 'anaphase wait' signal. The two-step model proposes that the mitotic checkpoint complex (MCC) is the kinetochore-independent anaphase inhibitor, and the role of the unaligned kinetochore is to sensitize the anaphase-promoting complex/cyclosome (APC/C) to MCC-mediated inhibition.

HP1 proteins are essential for a dynamic nuclear response that rescues the function of perturbed heterochromatin in primary human cells.

Cellular information is encoded genetically in the DNA nucleotide sequence and epigenetically by the "histone code," DNA methylation, and higher-order packaging of DNA into chromatin. Cells possess intricate mechanisms to sense and repair damage to DNA and the genetic code. However, nothing is known of the mechanisms, if any, that repair and/or compensate for damage to epigenetically encoded information, predicted to result from perturbation of DNA and histone modifications or other changes in chromatin structure. Here we show that primary human cells respond to a variety of small molecules that perturb DNA and histone modifications by recruiting HP1 proteins to sites of altered pericentromeric heterochromatin. This response is essential to maintain the HP1-binding kinetochore protein hMis12 at kinetochores and to suppress catastrophic mitotic defects. Recruitment of HP1 proteins to pericentromeres depends on histone H3.3 variant deposition, mediated by the HIRA histone chaperone. These data indicate that defects in pericentromeric epigenetic heterochromatin modifications initiate a dynamic HP1-dependent response that rescues pericentromeric heterochromatin function and is essential for viable progression through mitosis.

Cell division symmetry control and cancer stem cells.

Stem cells including cancer stem cells (CSC) divide symmetrically or asymmetrically. Usually symmetric cell division makes two daughter cells of the same fate, either as stem cells or more differentiated progenies; while asymmetric cell division (ACD) produces daughter cells of different fates. In this review, we first provide an overview of ACD, and then discuss more molecular details of ACD using the well-characterized Drosophila neuroblast system as an example. Aiming to explore the connections between cell heterogeneity in cancers and the critical need of ACD for self-renewal and generating cell diversity, we then examine how cell division symmetry control impacts common features associated with CSCs, including niche competition, cancer dormancy, drug resistance, epithelial-mesenchymal transition (EMT) and its reverse process mesenchymal-epithelial transition (MET), and cancer stem cell plasticity. As CSC may underlie resistance to therapy and cancer metastasis, understanding how cell division mode is selected and executed in these cells will provide possible strategies to target CSC.

[Water Sources and Factors Controlling Hydro-chemical Compositions in the Yiluo River Basin].

An important tributary in the middle stream of the Yellow River, the Yiluo River consists of the Luohe River and Yihe River, which converge at Yanshi City. Mining activities were widely distributed in the upstream of the Yiluo River Basin (YRB), while residential areas concentrated in the downstream were coupled with extensively industrial and agricultural activities. To illustrate the influences of variable anthropogenic activities on the hydro-chemical composition of river water of the YRB, water samples from the main stream and tributaries were collected in the flood season (August) and normal season (December), respectively. The hydrogen and oxygen isotope values coupled with cation and anion content were analyzed. Temporal and spatial variations of hydrogen and oxygen isotopes and ion content were utilized to elucidate the sources and factors controlling the hydro-chemical composition of the river water, and to illustrate the pathways of human effects. The results demonstrated that:① Average hydrogen and oxygen isotope values (δD and δ18O) of Luo River water were -56‰ and -7.9‰, and -55‰ and -8.1‰ in the flood season and normal season, respectively. Mean δD and δ18O values of Yi River water were -49‰ and -6.9‰, and -53‰ and -7.8‰ in the flood season and normal season, respectively. These temporal variations indicated that river water was mainly recharged by local atmospheric precipitation. ② The dominant water hydro-chemical type was HCO3-SO4-Ca-Mg in the main stream of the YRB, and the ratios of Ca2+ and HCO3- molar equivalent concentrations in the flood season were lower than those in the normal season, while the ratios of SO42- molar equivalent concentrations were higher than those in the normal season, indicating more sulfate dissolved in the river water in the flood season. ③ Carbonic acid and sulfuric acid simultaneously reacted with carbonate and silicate rocks, and in the Luo River more carbonate rocks were weathered, while in the Yi River more silicate rocks were weathered. ④ Human effects on river water were mainly concentrated in the upstream where wastewater input was derived from mining activities, while in the downstream pollution of the river was due to industrial wastewater and sewage input. ⑤ Spatial variations of sulfate sulfur isotope values were mostly due to differences between anthropogenic activities in the upstream and downstream of the Yiluo River. Negative sulfur isotope values in the upstream river water confirmed dissolved sulfate from sulfide mineral oxidation, which also indirectly verified the rock chemical weathering by sulfuric acid in this area. Positive sulfur isotope values in downstream river water were connected with industrial wastewater and sewage.

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.