Phosphorylation of mammalian Sgo2 by Aurora B recruits PP2A and MCAK to centromeres

An Aurora kinase A-BOD1L1-PP2A B56 Axis promotes chromosome segregation fidelity

Cancer cells are often aneuploid and frequently display elevated rates of chromosome missegregation in a phenomenon called chromosomal instability (CIN). CIN is commonly caused by hyperstable kinetochore-microtubule (K-MT) attachments that reduces the efficiency of correction of erroneous K-MT attachments. We recently showed that UMK57, a chemical agonist of MCAK (alias KIF2C) improves chromosome segregation fidelity in CIN cancer cells although cells rapidly develop adaptive resistance. To determine the mechanism of resistance we performed unbiased proteomic screens which revealed increased phosphorylation in cells adapted to UMK57 at two Aurora kinase A phosphoacceptor sites on BOD1L1 (alias FAM44A). BOD1L1 depletion or Aurora kinase A inhibition eliminated resistance to UMK57 in CIN cancer cells. BOD1L1 localizes to spindles/kinetochores during mitosis, interacts with the PP2A phosphatase, and regulates phosphorylation levels of kinetochore proteins, chromosome alignment, mitotic progression and fidelity. Moreover, the BOD1L1 gene is mutated in a subset of human cancers, and BOD1L1 depletion reduces cell growth in combination with clinically relevant doses of taxol or Aurora kinase A inhibitor. Thus, an Aurora kinase A -BOD1L1-PP2A axis promotes faithful chromosome segregation during mitosis.

Exploring Plant Meiosis: Insights from the Kinetochore Perspective

The central player for chromosome segregation in both mitosis and meiosis is the macromolecular kinetochore structure, which is assembled by >100 structural and regulatory proteins on centromere DNA. Kinetochores play a crucial role in cell division by connecting chromosomal DNA and microtubule polymers. This connection helps in the proper segregation and alignment of chromosomes. Additionally, kinetochores can act as a signaling hub, regulating the start of anaphase through the spindle assembly checkpoint, and controlling the movement of chromosomes during anaphase. However, the role of various kinetochore proteins in plant meiosis has only been recently elucidated, and these proteins differ in their functionality from those found in animals. In this review, our current knowledge of the functioning of plant kinetochore proteins in meiosis will be summarized. In addition, the functional similarities and differences of core kinetochore proteins in meiosis between plants and other species are discussed, and the potential applications of manipulating certain kinetochore genes in meiosis for breeding purposes are explored.

Kinase PLK1 regulates the disassembly of the lateral elements and the assembly of the inner centromere during the diakinesis/metaphase I transition in male mouse meiosis

PLK1 is a serine/threonine kinase with crucial roles during mitosis. However, its involvement during mammalian male meiosis remains largely unexplored. By inhibiting the kinase activity of PLK1 using BI 2536 on organotypic cultures of seminiferous tubules, we found that the disassembly of SYCP3 and HORMAD1 from the lateral elements of the synaptonemal complex during diakinesis is impeded. We also found that the normal recruitment of SYCP3 and HORMAD1 to the inner centromere in prometaphase I spermatocytes did not occur. Additionally, we analyzed the participation of PLK1 in the assembly of the inner centromere by studying its implication in the Bub1-H2AT120ph-dependent recruitment of shugoshin SGO2, and the Haspin-H3T3ph-dependent recruitment of Aurora B/C and Borealin. Our results indicated that both pathways are regulated by PLK1. Altogether, our results demonstrate that PLK1 is a master regulator of the late prophase I/metaphase I transition in mouse spermatocytes.

Preparation of Mitotic Cells for Fluorescence Microscopy

Cell cycle-dependent regulation of chromosome is a dynamic event. After replication in S phase, sister chromatids show dynamic behavior including condensation, alignment, and segregation in M phase. These beautiful behaviors of chromosomes observed through the microscope have fascinated people since more than 100 years ago, and now we can sketch the dynamics of regulatory proteins and their posttranscriptional modifications through the fluorescent microscope. The purpose of this chapter is describing the basic methods of immunofluorescence analysis of mitotic cells and chromosomes. Besides, the key ideas for improving the preparation of the specimen are also described. Because the characteristic of the proteins of your interest differs one by one, modifying the method might cause the crucial improvement in the observation.

Multi-omics analysis of kinesin family member 2C in human tumors: novel prognostic biomarker and tumor microenvironment regulator

Kinesin family member 2C (KIF2C) is the best-characterized member of the kinesin-13 family and is involved in accurately fine-tuned dynamics of mitotic spindles. As KIF2C is involved in both spindle formation and regulation of DNA double-strand breaks, precise regulation of KIF2C is essential to prevent malignant transformation associated with gains and losses of DNA content. In the present study, we initially reviewed The Cancer Genome Atlas database and observed that KIF2C is abundantly expressed in most tumor types. We then analyzed the gene alteration profile, protein expression, prognosis, and immune reactivities of KIF2C in more than 10,000 samples from several well-established databases. In addition, we conducted a gene enrichment set analysis to investigate the potential mechanisms underlying the role of KIF2C in tumorigenesis. Multi-omics analysis of KIF2C demonstrated significant statistical correlations between KIF2C expression and clinical prognosis, oncogenic signature gene sets, myeloid-derived suppressor cell infiltration, ImmunoScore, immune checkpoints, microsatellite instability, and tumor mutational burden across multiple tumors. Single-cell data showed that KIF2C is abundantly expressed in malignant cells. The experimental validation demonstrated that KIF2C is highly expressed in gastric cancer cell lines, gastric adenocarcinoma, and hepatocelluar carcinoma. The findings of this study provide important insight for understanding the role and mechanisms of KIF2C in tumorigenesis and immunotherapy in a variety of cancers.

Cohesin is required for meiotic spindle assembly independent of its role in cohesion in C. elegans

Accurate chromosome segregation requires a cohesin-mediated physical attachment between chromosomes that are to be segregated apart, and a bipolar spindle with microtubule plus ends emanating from exactly two poles toward the paired chromosomes. We asked whether the striking bipolar structure of C. elegans meiotic chromosomes is required for bipolarity of acentriolar female meiotic spindles by time-lapse imaging of mutants that lack cohesion between chromosomes. Both a spo-11 rec-8 coh-4 coh-3 quadruple mutant and a spo-11 rec-8 double mutant entered M phase with separated sister chromatids lacking any cohesion. However, the quadruple mutant formed an apolar spindle whereas the double mutant formed a bipolar spindle that segregated chromatids into two roughly equal masses. Residual non-cohesive COH-3/4-dependent cohesin on separated sister chromatids of the double mutant was sufficient to recruit haspin-dependent Aurora B kinase, which mediated bipolar spindle assembly in the apparent absence of chromosomal bipolarity. We hypothesized that cohesin-dependent Aurora B might activate or inhibit spindle assembly factors in a manner that would affect their localization on chromosomes and found that the chromosomal localization patterns of KLP-7 and CLS-2 correlated with Aurora B loading on chromosomes. These results demonstrate that cohesin is essential for spindle assembly and chromosome segregation independent of its role in sister chromatid cohesion.

Evolution of eukaryotic centromeres by drive and suppression of selfish genetic elements

Despite the universal requirement for faithful chromosome segregation, eukaryotic centromeres are rapidly evolving. It is hypothesized that rapid centromere evolution represents an evolutionary arms race between selfish genetic elements that drive, or propagate at the expense of organismal fitness, and mechanisms that suppress fitness costs. Selfish centromere DNA achieves preferential inheritance in female meiosis by recruiting more effector proteins that alter spindle microtubule interaction dynamics. Parallel pathways for effector recruitment are adaptively evolved to suppress functional differences between centromeres. Opportunities to drive are not limited to female meiosis, and selfish transposons, plasmids and B chromosomes also benefit by maximizing their inheritance. Rapid evolution of selfish genetic elements can diversify suppressor mechanisms in different species that may cause hybrid incompatibility.

Haspin participates in AURKB recruitment to centromeres and contributes to chromosome congression in male mouse meiosis

Chromosome segregation requires that centromeres properly attach to spindle microtubules. This essential step regulates the accuracy of cell division and therefore must be precisely regulated. One of the main centromeric regulatory signaling pathways is the Haspin-H3T3ph-chromosomal passenger complex (CPC) cascade, which is responsible for the recruitment of the CPC to the centromeres. In mitosis, Haspin kinase phosphorylates histone H3 at threonine 3 (H3T3ph), an essential epigenetic mark that recruits the CPC, whose catalytic component is Aurora B kinase. However, the centromeric Haspin-H3T3ph-CPC pathway remains largely uncharacterized in mammalian male meiosis. We have analyzed Haspin functions by either its chemical inhibition in cultured spermatocytes using LDN-192960, or the ablation of Haspin gene in Haspin-/-. Our studies suggest that Haspin kinase activity is required for proper chromosome congression during both meiotic divisions and for the recruitment of Aurora B and kinesin MCAK to meiotic centromeres. However, the absence of H3T3ph histone mark does not alter Borealin and SGO2 centromeric localization. These results add new and relevant information regarding the regulation of the Haspin-H3T3ph-CPC pathway and centromere function during meiosis.

CCAR2 controls mitotic progression through spatiotemporal regulation of Aurora B

CCAR2 (cell cycle and apoptosis regulator 2) is a multifaceted protein involved in cell survival and death following cytotoxic stress. However, little is known about the physiological functions of CCAR2 in regulating cell proliferation in the absence of external stimuli. The present study shows that CCAR2-deficient cells possess multilobulated nuclei, suggesting a defect in cell division. In particular, the duration of mitotic phase was perturbed. This disturbance of mitotic progression resulted from premature loss of cohesion with the centromere, and inactivation of the spindle assembly checkpoint during prometaphase and metaphase. It resulted in the formation of lagging chromosomes during anaphase, leading ultimately to the activation of the abscission checkpoint to halt cytokinesis. The CCAR2-dependent mitotic progression was related to spatiotemporal regulation of active Aurora B. In conclusion, the results suggest that CCAR2 governs mitotic events, including proper chromosome segregation and cytokinetic division, to maintain chromosomal stability.

Dicer promotes genome stability via the bromodomain transcriptional co-activator BRD4

RNA interference is required for post-transcriptional silencing, but also has additional roles in transcriptional silencing of centromeres and genome stability. However, these roles have been controversial in mammals. Strikingly, we found that Dicer-deficient embryonic stem cells have strong proliferation and chromosome segregation defects as well as increased transcription of centromeric satellite repeats, which triggers the interferon response. We conducted a CRISPR-Cas9 genetic screen to restore viability and identified transcriptional activators, histone H3K9 methyltransferases, and chromosome segregation factors as suppressors, resembling Dicer suppressors identified in independent screens in fission yeast. The strongest suppressors were mutations in the transcriptional co-activator Brd4, which reversed the strand-specific transcription of major satellite repeats suppressing the interferon response, and in the histone acetyltransferase Elp3. We show that identical mutations in the second bromodomain of Brd4 rescue Dicer-dependent silencing and chromosome segregation defects in both mammalian cells and fission yeast. This remarkable conservation demonstrates that RNA interference has an ancient role in transcriptional silencing and in particular of satellite repeats, which is essential for cell cycle progression and proper chromosome segregation. Our results have pharmacological implications for cancer and autoimmune diseases characterized by unregulated transcription of satellite repeats.

While RNA interference is conserved across species, small RNA pathways are very diverse. In this study, Gutbrod et al. find that non-canonical roles of Dicer in genome stability are in fact deeply conserved from yeast to humans.

The Role of Mitotic Kinases and the RZZ Complex in Kinetochore-Microtubule Attachments: Doing the Right Link

During mitosis, the interaction of kinetochores (KTs) with microtubules (MTs) drives chromosome congression to the spindle equator and supports the segregation of sister chromatids. Faithful genome partition critically relies on the ability of chromosomes to establish and maintain proper amphitelic end-on attachments, a configuration in which sister KTs are connected to robust MT fibers emanating from opposite spindle poles. Because the capture of spindle MTs by KTs is error prone, cells use mechanisms that sense and correct inaccurate KT-MT interactions before committing to segregate sister chromatids in anaphase. If left unresolved, these errors can result in the unequal distribution of chromosomes and lead to aneuploidy, a hallmark of cancer. In this review, we provide an overview of the molecular strategies that monitor the formation and fine-tuning of KT-MT attachments. We describe the complex network of proteins that operates at the KT-MT interface and discuss how AURORA B and PLK1 coordinate several concurrent events so that the stability of KT-MT attachments is precisely modulated throughout mitotic progression. We also outline updated knowledge on how the RZZ complex is regulated to ensure the formation of end-on attachments and the fidelity of mitosis.

Changing places: Chromosomal Passenger Complex relocation in early anaphase

The Chromosomal Passenger Complex (CPC) regulates a plethora of processes during multiple stages of nuclear and cytoplasmic division. Early during mitosis, the CPC is recruited to centromeres and kinetochores, and ensures that the duplicated chromosomes become properly connected to microtubules from opposite poles of the mitotic spindle. Progression into anaphase is accompanied by a striking relocation of the CPC from centromeres to the antiparallel microtubule overlaps of the anaphase spindle and to the equatorial cortex. This translocation requires direct interactions of the CPC with the kinesin-6 family member MKLP2/KIF20A, and the inactivation of cyclin B-cyclin-dependent kinase-1 (CDK1). Here, we review recent progress in the regulation of this relocation event. Furthermore, we discuss why the CPC must be relocated during early anaphase in light of recent advances in the functions of the CPC post metaphase.

Shugosin 2 is a biomarker for pathological grading and survival prediction in patients with gliomas

Glioblastomas are the most common type of adult primary brain neoplasms. Clinically, it is helpful to identify biomarkers to predict the survival of patients with gliomas due to its poor outcome. Shugoshin 2 (SGO2) is critical in cell division and cell cycle progression in eukaryotes. However, the association of SGO2 with pathological grading and survival in patients with gliomas remains unclear. We analyzed the association between SGO2 expression and clinical outcomes from Gene Expression Omnibus (GEO) dataset profiles, The Cancer Genome Atlas (TCGA), and Chinese Glioma Genome Atlas (CGGA). SGO2 mRNA and protein expression in normal brain tissue and glioma cell lines were investigated via quantitative RT-PCR, Western blot, and IHC staining. The roles of SGO2 in proliferation, migration, and apoptosis of GBM cells were studied with wound-healing assay, BrdU assay, cell cycle analysis, and JC-1 assay. The protein–protein interaction (PPI) was analyzed via Search Tool for the Retrieval of Interacting Genes/Proteins (STRING). SGO2 mRNA expression predicted higher grade gliomas than non-tumor brain tissues. Kaplan–Meier survival analysis showed that patients with high-grade gliomas with a higher SGO2 expression had worse survival outcomes. SGO2 mRNA and protein expression were upper regulated in gliomas than in normal brain tissue. Inhibition of SGO2 suppressed cell proliferation and migration. Also, PPI result showed SGO2 to be a potential hub protein, which was related to the expression of AURKB and FOXM1. SGO2 expression positively correlates with WHO pathological grading and patient survival, suggesting that SGO2 is a biomarker that is predictive of disease progression in patients with gliomas.

Selfish centromeres, selfless heterochromatin

Centromeres are specialized regions on chromosomes recruiting a set of proteins required for faithful chromosome segregation. Differences in centromere strength can potentially bias chromosome segregation toward one of the daughter cells during division. Kumon et al. propose a new model of evolutionary impact on the balance of centromere strength.

The right place at the right time: Aurora B kinase localization to centromeres and kinetochores

The fidelity of chromosome segregation during mitosis is intimately linked to the function of kinetochores, which are large protein complexes assembled at sites of centromeric heterochromatin on mitotic chromosomes. These key "orchestrators" of mitosis physically connect chromosomes to spindle microtubules and transduce forces through these connections to congress chromosomes and silence the spindle assembly checkpoint. Kinetochore-microtubule attachments are highly regulated to ensure that incorrect attachments are not prematurely stabilized, but instead released and corrected. The kinase activity of the centromeric protein Aurora B is required for kinetochore-microtubule destabilization during mitosis, but how the kinase acts on outer kinetochore substrates to selectively destabilize immature and erroneous attachments remains debated. Here, we review recent literature that sheds light on how Aurora B kinase is recruited to both centromeres and kinetochores and discuss possible mechanisms for how kinase interactions with substrates at distinct regions of mitotic chromosomes are regulated.

Gene Activation as a Cell Protection Mechanism Against Gamma-Ray radiation

Introduction: Gamma radiation is accompanied by prominent biological effects and damages. Cell proliferation and tumorigenesis are highlighted as the main resulted effects of gamma radiation on cultured cells. This study aims to assess the dysregulated mode of gene function after gamma radiation in human Jurkat cells. Methods: Six gene expression profiles from Gene Expression Omnibus (GEO) were analyzed by GEO2R to find the significant differentially expressed genes (DEGs) via gamma radiation. Action map analysis was applied to screen the query DEGs. Results: Among 108 study genes, 20 critical DEGs including AURKA, AURKB, BORA, CCNB1, CCNB2, CCNF, CDC20, CDCA8, CENPA, CENPE, CENPF, KIF18A, KIF20A, KIF23, BUB1, DLGAP5, ECT2, PLK1, SGO2, and TPX2 were introduced as down-regulated genes by the gamma ray. Conclusion: Activators of the introduced critical genes may be the cell protector against gamma radiation.

Untangling the contribution of Haspin and Bub1 to Aurora B function during mitosis

Aurora B kinase is essential for faithful chromosome segregation during mitosis. During (pro)metaphase, Aurora B is concentrated at the inner centromere by the kinases Haspin and Bub1. However, how Haspin and Bub1 collaborate to control Aurora B activity at centromeres remains unclear. Here, we show that either Haspin or Bub1 activity is sufficient to recruit Aurora B to a distinct chromosomal locus. Moreover, we identified a small, Bub1 kinase–dependent Aurora B pool that supported faithful chromosome segregation in otherwise unchallenged cells. Joined inhibition of Haspin and Bub1 activities fully abolished Aurora B accumulation at centromeres. While this impaired the correction of erroneous KT–MT attachments, it did not compromise the mitotic checkpoint, nor the phosphorylation of the Aurora B kinetochore substrates Hec1, Dsn1, and Knl1. This suggests that Aurora B substrates at the kinetochore are not phosphorylated by centromere-localized pools of Aurora B, and calls for a reevaluation of the current spatial models for how tension affects Aurora B–dependent kinetochore phosphorylation.

A Condensed View of the Chromosome Passenger Complex

The inner centromere is a region on the mitotic chromosome that serves as a platform for mitotic signaling and possesses unique biophysical properties that enable it to withstand relatively large pulling forces that are generated by kinetochores (KTs) during chromosome segregation. The chromosomal passenger complex (CPC) localizes to and is the key regulator of inner centromere organization and function during mitosis. Recently, we demonstrated that in addition to its kinase and histone code-reading activities, the CPC also can undergo liquid-liquid phase separation (LLPS) and proposed that the inner centromere is a membraneless organelle scaffolded by the CPC. In this perspective, we explore mechanisms that can allow the formation and dissolution of this membraneless body. The cell-cycle-regulated spatially defined assembly and disassembly of the CPC condensate at the inner centromere can reveal general principles about how histone modifications control chromatin-bound membraneless organelles. We further explore how the ability of the CPC to undergo LLPS may contribute to the organization and function of the inner centromere during mitosis.

Phospho-regulation of the Shugoshin - Condensin interaction at the centromere in budding yeast

Correct bioriented attachment of sister chromatids to the mitotic spindle is essential for chromosome segregation. In budding yeast, the conserved protein shugoshin (Sgo1) contributes to biorientation by recruiting the protein phosphatase PP2A-Rts1 and the condensin complex to centromeres. Using peptide prints, we identified a Serine-Rich Motif (SRM) of Sgo1 that mediates the interaction with condensin and is essential for centromeric condensin recruitment and the establishment of biorientation. We show that the interaction is regulated via phosphorylation within the SRM and we determined the phospho-sites using mass spectrometry. Analysis of the phosphomimic and phosphoresistant mutants revealed that SRM phosphorylation disrupts the shugoshin–condensin interaction. We present evidence that Mps1, a central kinase in the spindle assembly checkpoint, directly phosphorylates Sgo1 within the SRM to regulate the interaction with condensin and thereby condensin localization to centromeres. Our findings identify novel mechanisms that control shugoshin activity at the centromere in budding yeast.

Proper chromosome segregation in eukaryotes is ensured through correct attachment of the spindle microtubules to the centromeric chromosomal regions. The attachment is mediated via the multimolecular proteinaceous complex called the kinetochore. This enables the establishment of bioirentation, when each sister chromatid is attached to microtubules emanating from opposite spindle poles. Shugoshin (Sgo1) is a conserved centromeric protein that facilitates biorientation through its interactions with the protein phosphatase PP2A-Rts1, chromosome passenger complex and centromeric condensin. Here, we identified a serine-rich motif that is required for the interaction of shugoshin with the condensin complex. We show that loss of this region impairs condensin enrichment at the centromere, chromosome biorientation, segregation as well as the function of the chromosome passenger complex in the error correction. Moreover, the interaction is phosphoregulated, as phosphorylation of the serine-rich motif on Sgo1 disrupts its interaction with condensin. Finally, we show that the conserved spindle assembly checkpoint kinase Mps1 is responsible for this phosphorylation. Our findings uncover novel regulatory mechanisms that facilitate proper chromosome segregation.

The Borealin dimerization domain interacts with Sgo1 to drive Aurora B-mediated spindle assembly

The chromosomal passenger complex (CPC), which includes the kinase Aurora B, is a master regulator of meiotic and mitotic processes that ensure the equal segregation of chromosomes. Sgo1 is thought to play a major role in the recruitment of the CPC to chromosomes, but the molecular mechanism and contribution of Sgo1-dependent CPC recruitment is currently unclear. Using Xenopus egg extracts and biochemical reconstitution, we found that Sgo1 interacts directly with the dimerization domain of the CPC subunit Borealin. Borealin and the PP2A phosphatase complex can bind simultaneously to the coiled-coil domain of Sgo1, suggesting that Sgo1 can integrate Aurora B and PP2A activities to modulate Aurora B substrate phosphorylation. A Borealin mutant that specifically disrupts the Sgo1–Borealin interaction results in defects in CPC chromosomal recruitment and Aurora B–dependent spindle assembly, but not in spindle assembly checkpoint signaling at unattached kinetochores. These findings establish a direct molecular connection between Sgo1 and the CPC and have major implications for the different functions of Aurora B, which promote the proper interaction between spindle microtubules and chromosomes.

CDK1-PLK1/SGOL2/ANLN pathway mediating abnormal cell division in cell cycle may be a critical process in hepatocellular carcinoma

This study aims to investigate the potential mechanisms and identify core biomarkers of Hepatocellular carcinoma (HCC). The profile GSE113850 was downloaded to analyze the differentially expressed genes. Gene ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG), and protein-protein interaction network analysis were used to reveal the main signal pathways of the differentially expressed genes (DEGs) and hub genes. The correlation between core gene expression and pathological stages, the disease-free survival analysis, the overall survival analysis were analyzed by Gene Expression Profiling Interactive Analysis. Furthermore, we reidentified the expression level of core genes of carcinoma tissues and para-carcinoma tissues from 14 HCC patients with real-time reverse transcription-polymerase chain reaction analysis (RT-PCR) and western blotting. After SK-Hep1 cell was treated with cyclin-dependent kinase 1 (CDK1) siRNA for 72 h, we detected the expression of the core genes and fluorescence-activated cell sorting analysis. A total of 378 DEGs were found. GO and KEGG analysis revealed that the DEGs were mainly enriched in the cell cycle. There were positive correlations among CDK1, polo-like kinase 1, shugoshin2 and anillin actin-binding protein. Moreover, the expression levels of four core genes were related to the HCC occurrence, pathological stages, and survivorship curve. The clinical HCC specimens verified the higher expression level of core genes by real-time RT-PCR. The transfection of siCDK1 in SK-Hep1 resulted in a disordered cell cycle. Furthermore, CDK1 knockdown suppressed the expression of PLK1, ANLN, and SGOL2. The CDK1-PLK1/SGOL2/ANLN pathway mediating abnormal cell division in the cell cycle might be a critical process in HCC.

Meiosis: How Gambling Chromosomes Beat the Rules

Selfish centromeres exploit asymmetric female meiosis to drive non-Mendelian segregation in their favor. Using inherent differences in drive propensity between mouse chromosomes, a new study reveals how proteins that modify chromatin states and microtubule stability enable this selfish behavior.

Aurora B kinase activity is regulated by SET/TAF1 on Sgo2 at the inner centromere

Phosphorylation of kinetochore proteins destabilizes improper kinetochore–microtubule attachments. Asai et al. find that SET/TAF1, an inhibitor of the PP2A phosphatase, binds shugoshin 2 and corrects erroneous kinetochore–microtubule attachment by maintaining Aurora B kinase activity. Therefore, SET has a key role in establishing chromosome bi-orientation by balancing Aurora B and PP2A activity.

The accurate regulation of phosphorylation at the kinetochore is essential for establishing chromosome bi-orientation. Phosphorylation of kinetochore proteins by the Aurora B kinase destabilizes improper kinetochore–microtubule attachments, whereas the phosphatase PP2A has a counteracting role. Imbalanced phosphoregulation leads to error-prone chromosome segregation and aneuploidy, a hallmark of cancer cells. However, little is known about the molecular events that control the balance of phosphorylation at the kinetochore. Here, we show that localization of SET/TAF1, an oncogene product, to centromeres maintains Aurora B kinase activity by inhibiting PP2A, thereby correcting erroneous kinetochore–microtubule attachment. SET localizes at the inner centromere by interacting directly with shugoshin 2, with SET levels declining at increased distances between kinetochore pairs, leading to establishment of chromosome bi-orientation. Moreover, SET overexpression induces chromosomal instability by disrupting kinetochore–microtubule attachment. Thus, our findings reveal the novel role of SET in fine-tuning the phosphorylation level at the kinetochore by balancing the activities of Aurora B and PP2A.

Orchestration of the spindle assembly checkpoint by CDK1-cyclin B1

In mitosis, the spindle assembly checkpoint (SAC) monitors the formation of microtubule-kinetochore attachments during capture of chromosomes by the mitotic spindle. Spindle assembly is complete once there are no longer any unattached kinetochores. Here, we will discuss the mechanism and key components of spindle checkpoint signalling. Unattached kinetochores bind the principal spindle checkpoint kinase monopolar spindle 1 (MPS1). MPS1 triggers the recruitment of other spindle checkpoint proteins and the formation of a soluble inhibitor of anaphase, thus preventing exit from mitosis. On microtubule attachment, kinetochores become checkpoint silent due to the actions of PP2A-B56 and PP1. This SAC responsive period has to be coordinated with mitotic spindle formation to ensure timely mitotic exit and accurate chromosome segregation. We focus on the molecular mechanisms by which the SAC permissive state is created, describing a central role for CDK1-cyclin B1 and its counteracting phosphatase PP2A-B55. Furthermore, we discuss how CDK1-cyclin B1, through its interaction with MAD1, acts as an integral component of the SAC, and actively orchestrates checkpoint signalling and thus contributes to the faithful execution of mitosis.

SET binding to Sgo1 inhibits Sgo1-cohesin interactions and promotes chromosome segregation

At anaphase onset, Sgo1 function of cohesion protection must be disabled to allow timely chromosome segregation, but how this is achieved is not fully understood. Here, we show that SET, a known PP2A inhibitor, directly binds to a domain in Sgo1 in close proximity to the cohesin-binding motif. The Sgo1-cohesin binding can be disrupted by SET in a dose-dependent manner in vitro as well as by SET overexpression in cells, suggesting that SET is also an inhibitor to the Sgo1-cohesin binding. Furthermore, the SET binding-deficient Sgo1 mutant fully supports centromeric cohesion protection but delays chromosome segregation, suggesting that the SET-Sgo1 binding is required for timely chromosome segregation. Moreover, overexpression of SET WT, not the Sgo1 binding-deficient mutant, exacerbates the occurrence of cohesion fatigue in MG132-arrested cells. Conversely, SET depletion delays it. Thus, we propose that a major function of SET during mitosis is to disrupt the Sgo1-cohesin interaction, thereby promoting centromeric cohesion de-protection and timely chromosome segregation at anaphase onset.

The Centrosome and the Primary Cilium: The Yin and Yang of a Hybrid Organelle

Centrosomes and primary cilia are usually considered as distinct organelles, although both are assembled with the same evolutionary conserved, microtubule-based templates, the centrioles. Centrosomes serve as major microtubule- and actin cytoskeleton-organizing centers and are involved in a variety of intracellular processes, whereas primary cilia receive and transduce environmental signals to elicit cellular and organismal responses. Understanding the functional relationship between centrosomes and primary cilia is important because defects in both structures have been implicated in various diseases, including cancer. Here, we discuss evidence that the animal centrosome evolved, with the transition to complex multicellularity, as a hybrid organelle comprised of the two distinct, but intertwined, structural-functional modules: the centriole/primary cilium module and the pericentriolar material/centrosome module. The evolution of the former module may have been caused by the expanding cellular diversification and intercommunication, whereas that of the latter module may have been driven by the increasing complexity of mitosis and the requirement for maintaining cell polarity, individuation, and adhesion. Through its unique ability to serve both as a plasma membrane-associated primary cilium organizer and a juxtanuclear microtubule-organizing center, the animal centrosome has become an ideal integrator of extracellular and intracellular signals with the cytoskeleton and a switch between the non-cell autonomous and the cell-autonomous signaling modes. In light of this hypothesis, we discuss centrosome dynamics during cell proliferation, migration, and differentiation and propose a model of centrosome-driven microtubule assembly in mitotic and interphase cells. In addition, we outline the evolutionary benefits of the animal centrosome and highlight the hierarchy and modularity of the centrosome biogenesis networks.

Centromeric Cohesin: Molecular Glue and Much More

Sister chromatid cohesion, mediated by the cohesin complex, is a prerequisite for faithful chromosome segregation during mitosis. Premature release of sister chromatid cohesion leads to random segregation of the genetic material and consequent aneuploidy. Multiple regulatory mechanisms ensure proper timing for cohesion establishment, concomitant with DNA replication, and cohesion release during the subsequent mitosis. Here we summarize the most important phases of the cohesin cycle and the coordination of cohesion release with the progression through mitosis. We further discuss recent evidence that has revealed additional functions for centromeric localization of cohesin in the fidelity of mitosis in metazoans. Beyond its well-established role as "molecular glue", centromeric cohesin complexes are now emerging as a scaffold for multiple fundamental processes during mitosis, including the formation of correct chromosome and kinetochore architecture, force balance with the mitotic spindle, and the association with key molecules that regulate mitotic fidelity, particularly at the chromosomal inner centromere. Centromeric chromatin may be thus seen as a dynamic place where cohesin ensures mitotic fidelity by multiple means.

Shugoshin protects centromere pairing and promotes segregation of nonexchange partner chromosomes in meiosis

Faithful chromosome segregation during meiosis I depends upon the formation of connections between homologous chromosomes. Crossovers between homologs connect the partners, allowing them to attach to the meiotic spindle as a unit, such that they migrate away from one another at anaphase I. Homologous partners also become connected by pairing of their centromeres in meiotic prophase. This centromere pairing can promote proper segregation at anaphase I of partners that have failed to become joined by a crossover. Centromere pairing is mediated by synaptonemal complex (SC) proteins that persist at the centromere when the SC disassembles. Here, using mouse spermatocyte and yeast model systems, we tested the role of shugoshin in promoting meiotic centromere pairing by protecting centromeric synaptonemal components from disassembly. The results show that shugoshin protects the centromeric SC in meiotic prophase and, in anaphase, promotes the proper segregation of partner chromosomes that are not linked by a crossover.

Division of labour between PP2A-B56 isoforms at the centromere and kinetochore

PP2A-B56 is a serine/threonine phosphatase complex that regulates several major mitotic processes, including sister chromatid cohesion, kinetochore-microtubule attachment and the spindle assembly checkpoint. We show here that these key functions are divided between different B56 isoforms that localise to either the centromere or kinetochore. The centromeric isoforms rely on a specific interaction with Sgo2, whereas the kinetochore isoforms bind preferentially to BubR1 and other proteins containing an LxxIxE motif. In addition to these selective binding partners, Sgo1 helps to anchor PP2A-B56 at both locations: it collaborates with BubR1 to maintain B56 at the kinetochore and it helps to preserve the Sgo2/B56 complex at the centromere. A series of chimaeras were generated to map the critical region in B56 down to a small C-terminal loop that regulates the key interactions and defines B56 localisation. Together, this study describes how different PP2A-B56 complexes utilise isoform-specific interactions to control distinct processes during mitosis.

Phosphatases in Mitosis: Roles and Regulation

Mitosis requires extensive rearrangement of cellular architecture and of subcellular structures so that replicated chromosomes can bind correctly to spindle microtubules and segregate towards opposite poles. This process originates two new daughter nuclei with equal genetic content and relies on highly-dynamic and tightly regulated phosphorylation of numerous cell cycle proteins. A burst in protein phosphorylation orchestrated by several conserved kinases occurs as cells go into and progress through mitosis. The opposing dephosphorylation events are catalyzed by a small set of protein phosphatases, whose importance for the accuracy of mitosis is becoming increasingly appreciated. This review will focus on the established and emerging roles of mitotic phosphatases, describe their structural and biochemical properties, and discuss recent advances in understanding the regulation of phosphatase activity and function.

A positive feedback mechanism ensures proper assembly of the functional inner centromere during mitosis in human cells

The inner centromere region of a mitotic chromosome critically regulates sister chromatid cohesion and kinetochore-microtubule attachments. However, the molecular mechanism underlying inner centromere assembly remains elusive. Here, using CRISPR/Cas9-based gene editing in HeLa cells, we disrupted the interaction of Shugoshin 1 (Sgo1) with histone H2A phosphorylated on Thr-120 (H2ApT120) to selectively release Sgo1 from mitotic centromeres. Interestingly, cells expressing the H2ApT120-binding defective mutant of Sgo1 have an elevated rate of chromosome missegregation accompanied by weakened centromeric cohesion and decreased centromere accumulation of the chromosomal passenger complex (CPC), an integral part of the inner centromere and a key player in the correction of erroneous kinetochore-microtubule attachments. When artificially tethered to centromeres, a Sgo1 mutant defective in binding protein phosphatase 2A (PP2A) is not able to support proper centromeric cohesion and CPC accumulation, indicating that the Sgo1-PP2A interaction is essential for the integrity of mitotic centromeres. We further provide evidence indicating that Sgo1 protects centromeric cohesin to create a binding site for the histone H3-associated protein kinase Haspin, which not only inhibits the cohesin release factor Wapl and thereby strengthens centromeric cohesion but also phosphorylates histone H3 at Thr-3 to position CPC at inner centromeres. Taken together, our findings reveal a positive feedback-based mechanism that ensures proper assembly of the functional inner centromere during mitosis. They further suggest a causal link between centromeric cohesion defects and chromosomal instability in cancer cells.

The depolymerase activity of MCAK shows a graded response to Aurora B kinase phosphorylation through allosteric regulation

Kinesin-13 motors regulate precise microtubule dynamics and limit microtubule length throughout metazoans by depolymerizing microtubule ends. Recently, the kinesin-13 motor family member MCAK (also known Kif2C) has been proposed to undergo large conformational changes during its catalytic cycle, as it switches from being in solution to being bound to microtubules. Here, we reveal that MCAK has a compact conformation in solution through crosslinking and electron microscopy experiments. When MCAK is bound to the microtubule ends, it adopts an extended conformation with the N-terminus and neck region of MCAK interacting with the microtubule. Interestingly, the region of MCAK that interacts with the microtubule is the region phosphorylated by Aurora B and contains an end binding (EB) protein-binding motif. The level of phosphorylation of the N-terminus results in a graded microtubule depolymerase activity. Here, we show that the N-terminus of MCAK forms a platform to integrate Aurora B kinase downstream signals and in response fine-tunes its depolymerase activity during mitosis. We propose that this allosteric control mechanism allows decoupling of the N-terminus from the motor domain of MCAK to allow MCAK depolymerase activity at kinetochores.

Summary: The kinesin-13 MCAK has a compact conformation in solution but is extended when bound to microtubules. Aurora B phosphorylation of MCAK inhibits depolymerase activity by disrupting its extended conformation.

The cohesin complex in mammalian meiosis

Cohesin is an evolutionary conserved multi-protein complex that plays a pivotal role in chromosome dynamics. It plays a role both in sister chromatid cohesion and in establishing higher order chromosome architecture, in somatic and germ cells. Notably, the cohesin complex in meiosis differs from that in mitosis. In mammalian meiosis, distinct types of cohesin complexes are produced by altering the combination of meiosis-specific subunits. The meiosis-specific subunits endow the cohesin complex with specific functions for numerous meiosis-associated chromosomal events, such as chromosome axis formation, homologue association, meiotic recombination and centromeric cohesion for sister kinetochore geometry. This review mainly focuses on the cohesin complex in mammalian meiosis, pointing out the differences in its roles from those in mitosis. Further, common and divergent aspects of the meiosis-specific cohesin complex between mammals and other organisms are discussed.

Aurora B kinase activity-dependent and -independent functions of the chromosomal passenger complex in regulating sister chromatid cohesion

The chromosomal passenger complex (CPC) is a master regulator of mitosis. CPC consists of inner centromere protein (INCENP), Survivin, Borealin, and the kinase Aurora B and plays key roles in regulating kinetochore-microtubule attachments and spindle assembly checkpoint signaling. However, the role of CPC in sister chromatid cohesion, mediated by the cohesin complex, remains incompletely understood. Here, we show that Aurora B kinase activity contributes to centromeric cohesion protection partly through promoting kinetochore localization of the kinase Bub1. Interestingly, disrupting the interaction of INCENP with heterochromatin protein 1 (HP1) in HeLa cells selectively weakens cohesion at mitotic centromeres without detectably reducing the kinase activity of Aurora B. Thus, through this INCENP-HP1 interaction, the CPC also protects centromeric cohesion independently of Aurora B kinase activity. Moreover, the requirement for the INCENP-HP1 interaction in centromeric cohesion protection can be bypassed by tethering HP1 to centromeres or by depleting the cohesin release factor Wapl. We provide further evidence suggesting that the INCENP-HP1 interaction protects centromeric cohesion by promoting the centromere localization of Haspin, a protein kinase that antagonizes Wapl activity at centromeres. Taken together, this study identifies Aurora B kinase activity-dependent and -independent roles for the CPC in regulating centromeric cohesion during mitosis in human cells.

Shugoshin Is Essential for Meiotic Prophase Checkpoints in C. elegans

The conserved factor Shugoshin is dispensable in C. elegans for the two-step loss of sister chromatid cohesion that directs the proper segregation of meiotic chromosomes. We show that the C. elegans ortholog of Shugoshin, SGO-1, is required for checkpoint activity in meiotic prophase. This role in checkpoint function is similar to that of conserved proteins that structure meiotic chromosome axes. Indeed, null sgo-1 mutants exhibit additional phenotypes similar to that of a partial loss-of-function allele of the axis component, HTP-3: premature synaptonemal complex disassembly, the activation of alternate DNA repair pathways, and an inability to recruit a conserved effector of the DNA damage pathway, HUS-1. SGO-1 localizes to pre-meiotic nuclei when HTP-3 is present but not yet loaded onto chromosome axes and genetically interacts with a central component of the cohesin complex, SMC-3, suggesting that it contributes to meiotic chromosome metabolism early in meiosis by regulating cohesin. We propose that SGO-1 acts during pre-meiotic replication to ensure fully functional meiotic chromosome architecture, rendering these chromosomes competent for checkpoint activity and normal progression of meiotic recombination. Given that most research on Shugoshin has focused on its regulation of sister chromatid cohesion during chromosome segregation, this novel role may be conserved but previously uncharacterized in other organisms. Further, our findings expand the repertoire of Shugoshin's functions beyond coordinating regulatory activities at the centromere.

Aurora-PLK1 cascades as key signaling modules in the regulation of mitosis

Mitosis is controlled by reversible protein phosphorylation involving specific kinases and phosphatases. A handful of major mitotic protein kinases, such as the cyclin B-CDK1 complex, the Aurora kinases, and Polo-like kinase 1 (PLK1), cooperatively regulate distinct mitotic processes. Research has identified proteins and mechanisms that integrate these kinases into signaling cascades that guide essential mitotic events. These findings have important implications for our understanding of the mechanisms of mitotic regulation and may advance the development of novel antimitotic drugs. We review collected evidence that in vertebrates, the Aurora kinases serve as catalytic subunits of distinct complexes formed with the four scaffold proteins Bora, CEP192, INCENP, and TPX2, which we deem "core" Aurora cofactors. These complexes and the Aurora-PLK1 cascades organized by Bora, CEP192, and INCENP control crucial aspects of mitosis and all pathways of spindle assembly. We compare the mechanisms of Aurora activation in relation to the different spindle assembly pathways and draw a functional analogy between the CEP192 complex and the chromosomal passenger complex that may reflect the coevolution of centrosomes, kinetochores, and the actomyosin cleavage apparatus. We also analyze the roles and mechanisms of Aurora-PLK1 signaling in the cell and centrosome cycles and in the DNA damage response.

The Ndc80 complex targets Bod1 to human mitotic kinetochores

Regulation of protein phosphatase activity by endogenous protein inhibitors is an important mechanism to control protein phosphorylation in cells. We recently identified Biorientation defective 1 (Bod1) as a small protein inhibitor of protein phosphatase 2A containing the B56 regulatory subunit (PP2A-B56). This phosphatase controls the amount of phosphorylation of several kinetochore proteins and thus the establishment of load-bearing chromosome-spindle attachments in time for accurate separation of sister chromatids in mitosis. Like PP2A-B56, Bod1 directly localizes to mitotic kinetochores and is required for correct segregation of mitotic chromosomes. In this report, we have probed the spatio-temporal regulation of Bod1 during mitotic progression. Kinetochore localization of Bod1 increases from nuclear envelope breakdown until metaphase. Phosphorylation of Bod1 at threonine 95 (T95), which increases Bod1's binding to and inhibition of PP2A-B56, peaks in prometaphase when PP2A-B56 localization to kinetochores is highest. We demonstrate here that kinetochore targeting of Bod1 depends on the outer kinetochore protein Ndc80 and not PP2A-B56. Crucially, Bod1 depletion functionally affects Ndc80 phosphorylation at the N-terminal serine 55 (S55), as well as a number of other phosphorylation sites within the outer kinetochore, including Knl1 at serine 24 and 60 (S24, S60), and threonine T943 and T1155 (T943, T1155). Therefore, Ndc80 recruits a phosphatase inhibitor to kinetochores which directly feeds forward to regulate Ndc80, and Knl1 phosphorylation, including sites that mediate the attachment of microtubules to kinetochores.

Aurora kinase B dependent phosphorylation of 53BP1 is required for resolving merotelic kinetochore-microtubule attachment errors during mitosis

Defects in resolving kinetochore-microtubule attachment mistakes during mitosis is linked to chromosome instability associated with carcinogenesis as well as resistance to cancer therapy. Here we report for the first time that tumor suppressor p53-binding protein 1 (53BP1) is phosphorylated at serine 1342 (S1342) by Aurora kinase B both in vitro and in human cells, which is required for optimal recruitment of 53BP1 at kinetochores. Furthermore, 53BP1 staining normally localized on the outer kinetochore, extended to the whole kinetochore when it is merotelically-attached, in concert with mitotic centromere-associated kinesin. Kinetochore-binding of pS1342-53BP1 is essential for efficient resolving of merotelic attachment, a spontaneous kinetochore-microtubule connection error that usually causes aneuploidy. Consistently, loss of 53BP1 results in significant increase in lagging chromosome events, micronuclei formation and aneuploidy, due to the unresolved merotely in both cancer and primary cells, which is prevented by ectopic wild type 53BP1 but not by the nonphophorylable S1342A mutant. We thus document a novel DNA damage-independent function of 53BP1 in maintaining faithful chromosome segregation during mitosis.

The Ins and Outs of Aurora B Inner Centromere Localization

Error-free chromosome segregation is essential for the maintenance of genomic integrity during cell division. Aurora B, the enzymatic subunit of the Chromosomal Passenger Complex (CPC), plays a crucial role in this process. In early mitosis Aurora B localizes predominantly to the inner centromere, a specialized region of chromatin that lies at the crossroads between the inter-kinetochore and inter-sister chromatid axes. Two evolutionarily conserved histone kinases, Haspin and Bub1, control the positioning of the CPC at the inner centromere and this location is thought to be crucial for the CPC to function. However, recent studies sketch a subtler picture, in which not all functions of the CPC require strict confinement to the inner centromere. In this review we discuss the molecular pathways that direct Aurora B to the inner centromere and deliberate if and why this specific localization is important for Aurora B function.

The E2F activators control multiple mitotic regulators and maintain genomic integrity through Sgo1 and BubR1

The E2F1, E2F2, and E2F3a transcriptional activators control proliferation. However, how the E2F activators regulate mitosis to maintain genomic integrity is unclear. Centrosome amplification (CA) and unregulated spindle assembly checkpoint (SAC) are major generators of aneuploidy and chromosome instability (CIN) in cancer. Previously, we showed that overexpression of single E2F activators induced CA and CIN in mammary epithelial cells, and here we show that combined overexpression of E2F activators did not enhance CA. Instead, the E2F activators elevated expression of multiple mitotic regulators, including Sgo1, Nek2, Hec1, BubR1, and Mps1/TTK. cBioPortal analyses of the TCGA database showed that E2F overexpression in lobular invasive breast tumors correlates with overexpression of multiple regulators of chromosome segregation, centrosome homeostasis, and the SAC. Kaplan-Meier plots identified correlations between individual or combined overexpression of E2F1, E2F3a, Mps1/TTK, Nek2, BubR1, or Hec1 and poor overall and relapse-free survival of breast cancer patients. In MCF10A normal mammary epithelial cells co-overexpressing E2Fs, transient Sgo1 knockdown induced CA, high percentages of premature sister chromatid separation, chromosome losses, increased apoptosis, and decreased cell clonogenicity. BubR1 silencing resulted in chromosome losses without CA, demonstrating that Sgo1 and BubR1 maintain genomic integrity through two distinct mechanisms. Our results suggest that deregulated activation of the E2Fs in mammary epithelial cells is counteracted by activation of a Sgo1-dependent mitotic checkpoint.

Phospho-H1 Decorates the Inter-chromatid Axis and Is Evicted along with Shugoshin by SET during Mitosis

Precise control of sister chromatid separation during mitosis is pivotal to maintaining genomic integrity. Yet, the regulatory mechanisms involved are not well understood. Remarkably, we discovered that linker histone H1 phosphorylated at S/T18 decorated the inter-chromatid axial DNA on mitotic chromosomes. Sister chromatid resolution during mitosis required the eviction of such H1S/T18ph by the chaperone SET, with this process being independent of and most likely downstream of arm-cohesin dissociation. SET also directed the disassembly of Shugoshins in a polo-like kinase 1-augmented manner, aiding centromere resolution. SET ablation compromised mitotic fidelity as evidenced by unresolved sister chromatids with marked accumulation of H1S/T18ph and centromeric Shugoshin. Thus, chaperone-assisted eviction of linker histones and Shugoshins is a fundamental step in mammalian mitotic progression. Our findings also elucidate the functional implications of the decades-old observation of mitotic linker histone phosphorylation, serving as a paradigm to explore the role of linker histones in bio-signaling processes.

Inner centromere localization of the CPC maintains centromere cohesion and allows mitotic checkpoint silencing

Faithful chromosome segregation during mitosis requires that the kinetochores of all sister chromatids become stably connected to microtubules derived from opposite spindle poles. How stable chromosome bi-orientation is accomplished and coordinated with anaphase onset remains incompletely understood. Here we show that stable chromosome bi-orientation requires inner centromere localization of the non-enzymatic subunits of the chromosomal passenger complex (CPC) to maintain centromeric cohesion. Precise inner centromere localization of the CPC appears less relevant for Aurora B-dependent resolution of erroneous kinetochore-microtubule (KT-MT) attachments and for the stabilization of bi-oriented KT-MT attachments once sister chromatid cohesion is preserved via knock-down of WAPL. However, Aurora B inner centromere localization is essential for mitotic checkpoint silencing to allow spatial separation from its kinetochore substrate KNL1. Our data infer that the CPC is localized at the inner centromere to sustain centromere cohesion on bi-oriented chromosomes and to coordinate mitotic checkpoint silencing with chromosome bi-orientation.

APC/CCdh1 Enables Removal of Shugoshin-2 from the Arms of Bivalent Chromosomes by Moderating Cyclin-Dependent Kinase Activity

In mammalian females, germ cells remain arrested as primordial follicles. Resumption of meiosis is heralded by germinal vesicle breakdown, condensation of chromosomes, and their eventual alignment on metaphase plates. At the first meiotic division, anaphase-promoting complex/cyclosome associated with Cdc20 (APC/CCdc20) activates separase and thereby destroys cohesion along chromosome arms. Because cohesion around centromeres is protected by shugoshin-2, sister chromatids remain attached through centromeric/pericentromeric cohesin. We show here that, by promoting proteolysis of cyclins and Cdc25B at the germinal vesicle (GV) stage, APC/C associated with the Cdh1 protein (APC/CCdh1) delays the increase in Cdk1 activity, leading to germinal vesicle breakdown (GVBD). More surprisingly, by moderating the rate at which Cdk1 is activated following GVBD, APC/CCdh1 creates conditions necessary for the removal of shugoshin-2 from chromosome arms by the Aurora B/C kinase, an event crucial for the efficient resolution of chiasmata.

Drosophila Dalmatian combines sororin and shugoshin roles in establishment and protection of cohesion

Sister chromatid cohesion is crucial to ensure chromosome bi-orientation and equal chromosome segregation. Cohesin removal via mitotic kinases and Wapl has to be prevented in pericentromeric regions in order to protect cohesion until metaphase, but the mechanisms of mitotic cohesion protection remain elusive in Drosophila Here, we show that dalmatian (Dmt), an ortholog of the vertebrate cohesin-associated protein sororin, is required for protection of mitotic cohesion in flies. Dmt is essential for cohesion establishment during interphase and is enriched on pericentromeric heterochromatin. Dmt is recruited through direct association with heterochromatin protein-1 (HP1), and this interaction is required for cohesion. During mitosis, Dmt interdependently recruits protein phosphatase 2A (PP2A) to pericentromeric regions, and PP2A binding is required for Dmt to protect cohesion. Intriguingly, Dmt is sufficient to protect cohesion upon heterologous expression in human cells. Our findings of a hybrid system, in which Dmt exerts both sororin-like establishment functions and shugoshin-like heterochromatin-based protection roles, provide clues to the evolutionary modulation of eukaryotic cohesion regulation systems.

Specialize and Divide (Twice): Functions of Three Aurora Kinase Homologs in Mammalian Oocyte Meiotic Maturation

The aurora kinases (AURKs) comprise an evolutionarily conserved family of serine/threonine kinases involved in mitosis and meiosis. While most mitotic cells express two AURK isoforms (AURKA and AURKB), mammalian germ cells also express a third, AURKC. Although much is known about the functions of the kinases in mitosis, less is known about how the three isoforms function to coordinate meiosis. This review is aimed at describing what is known about the three isoforms in female meiosis, the similarities and differences between kinase functions, and speculates as to why mammalian germ cells require expression of three AURKs instead of two.

Bioinformatical analysis of eukaryotic shugoshins reveals meiosis-specific features of vertebrate shugoshins

Background

Shugoshins (SGOs) are proteins that protect cohesins located at the centromeres of sister chromatids from their early cleavage during mitosis and meiosis in plants, fungi, and animals. Their function is to prevent premature sister-chromatid disjunction and segregation. The study focused on the structural differences among SGOs acting during mitosis and meiosis that cause differences in chromosome behavior in these two types of cell division in different organisms.

Methods

A bioinformatical analysis of protein domains, conserved amino acid motifs, and physicochemical properties of 32 proteins from 25 species of plants, fungi, and animals was performed.

Results

We identified a C-terminal amino acid motif that is highly evolutionarily conserved among the SGOs protecting centromere cohesion of sister chromatids in meiotic anaphase I, but not among mitotic SGOs. This meiotic motif is arginine-rich in vertebrates. SGOs differ in different eukaryotic kingdoms by the sets and locations of amino acid motifs and the number of α-helical regions in the protein molecule.

Discussion

These structural differences between meiotic and mitotic SGOs probably could be responsible for the prolonged SGOs resistance to degradation during meiotic metaphase I and anaphase I. We suggest that the “arginine comb” in C-end meiotic motifs is capable of interaction by hydrogen bonds with guanine bases in the minor groove of DNA helix, thus protecting SGOs from hydrolysis. Our findings support independent evolution of meiosis in different lineages of multicellular organisms.

Molecular Regulation of the Spindle Assembly Checkpoint by Kinases and Phosphatases

The spindle assembly checkpoint (SAC) is a surveillance mechanism contributing to the preservation of genomic stability by monitoring the microtubule attachment to, and/or the tension status of, each kinetochore during mitosis. The SAC halts metaphase to anaphase transition in the presence of unattached and/or untensed kinetochore(s) by releasing the mitotic checkpoint complex (MCC) from these improperly-oriented kinetochores to inhibit the anaphase-promoting complex/cyclosome (APC/C). The reversible phosphorylation of a variety of substrates at the kinetochore by antagonistic kinases and phosphatases is one major signaling mechanism for promptly turning on or turning off the SAC. In such a complex network, some kinases act at the apex of the SAC cascade by either generating (monopolar spindle 1, MPS1/TTK and likely polo-like kinase 1, PLK1), or contributing to generate (Aurora kinase B) kinetochore phospho-docking sites for the hierarchical recruitment of the SAC proteins. Aurora kinase B, MPS1 and budding uninhibited by benzimidazoles 1 (BUB1) also promote sister chromatid biorientation by modulating kinetochore microtubule stability. Moreover, MPS1, BUB1, and PLK1 seem to play key roles in APC/C inhibition by mechanisms dependent and/or independent on MCC assembly. The protein phosphatase 1 and 2A (PP1 and PP2A) are recruited to kinetochores to oppose kinase activity. These phosphatases reverse the phosphorylation of kinetochore targets promoting the microtubule attachment stabilization, sister kinetochore biorientation and SAC silencing. The kinase-phosphatase network is crucial as it renders the SAC a dynamic, graded-signaling, high responsive, and robust process thereby ensuring timely anaphase onset and preventing the generation of proneoplastic aneuploidy.

The Ska complex promotes Aurora B activity to ensure chromosome biorientation

Chromosome biorientation and accurate segregation rely on the plasticity of kinetochore-microtubule (KT-MT) attachments. Aurora B facilitates KT-MT dynamics by phosphorylating kinetochore proteins that are critical for KT-MT interactions. Among the substrates whose microtubule and kinetochore binding is curtailed by Aurora B is the spindle and kinetochore-associated (Ska) complex, a key factor for KT-MT stability. Here, we show that Ska is not only a substrate of Aurora B, but is also required for Aurora B activity. Ska-deficient cells fail to biorient and display chromosome segregation errors underlying suppressed KT-MT turnover. These defects coincide with KNL1-Mis12-Ndc80 network hypophosphorylation, reduced mitotic centromere-associated kinesin localization, and Aurora B T-loop phosphorylation at kinetochores. We further show that Ska requires its microtubule-binding capability to promote Aurora B activity in cells and stimulates Aurora B catalytic activity in vitro. Finally, we show that protein phosphatase 1 counteracts Aurora B activity to enable Ska kinetochore accumulation once biorientation is achieved. We propose that Ska promotes Aurora B activity to limit its own microtubule and kinetochore association and to ensure that KT-MT dynamics and stability fall within an optimal balance for biorientation.