Sli15 associates with the ipl1 protein kinase to promote proper chromosome segregation in Saccharomyces cerevisiae

Activation is only the beginning: mechanisms that tune kinase substrate specificity

Kinases are master coordinators of cellular processes, but to appropriately respond to the changing cellular environment, each kinase must recognize its substrates, target only those proteins on the correct amino acids, and in many cases, only phosphorylate a subset of potential substrates at any given time. Therefore, regulation of kinase substrate specificity is paramount to proper cellular function, and multiple mechanisms can be employed to achieve specificity. At the smallest scale, characteristics of the substrate such as its linear peptide motif and three-dimensional structure must be complementary to the substrate binding surface of the kinase. This surface is dynamically shaped by the activation loop and surrounding region of the substrate binding groove, which can adopt multiple conformations, often influenced by post-translational modifications. Domain-scale conformational changes can also occur, such as the interaction with pseudosubstrate domains or other regulatory domains in the kinase. Kinases may multimerize or form complexes with other proteins that influence their structure, function, and/or subcellular localization at different times and in response to different signals. This review will illustrate these mechanisms by examining recent work on four serine/threonine kinases: Aurora B, CaMKII, GSK3β, and CK1δ. We find that these mechanisms are often shared by this diverse set of kinases in diverse cellular contexts, so they may represent common strategies that cells use to regulate cell signaling, and it will be enlightening to continue to learn about the depth and robustness of kinase substrate specificity in additional systems.

Aurora B controls anaphase onset and error-free chromosome segregation in trypanosomes

Kinetochores form the interface between chromosomes and spindle microtubules and are thus under tight control by a complex regulatory circuitry. The Aurora B kinase plays a central role within this circuitry by destabilizing improper kinetochore-microtubule attachments and relaying the attachment status to the spindle assembly checkpoint. Intriguingly, Aurora B is conserved even in kinetoplastids, a group of early-branching eukaryotes which possess a unique set of kinetochore proteins. It remains unclear how their kinetochores are regulated to ensure faithful chromosome segregation. Here, we show in Trypanosoma brucei that Aurora B activity controls the metaphase-to-anaphase transition through phosphorylation of the divergent Bub1-like protein KKT14. Depletion of KKT14 overrides the metaphase arrest resulting from Aurora B inhibition, while expression of non-phosphorylatable KKT14 delays anaphase onset. Finally, we demonstrate that re-targeting Aurora B to the outer kinetochore suffices to promote mitotic exit but causes extensive chromosome missegregation in anaphase. Our results indicate that Aurora B and KKT14 are involved in an unconventional circuitry controlling cell cycle progression in trypanosomes.

Comparative genomics of the closely related fungal genera Cryptococcus and Kwoniella reveals karyotype dynamics and suggests evolutionary mechanisms of pathogenesis

In exploring the evolutionary trajectories of both pathogenesis and karyotype dynamics in fungi, we conducted a large-scale comparative genomic analysis spanning the Cryptococcus genus, encompassing both global human fungal pathogens and nonpathogenic species, and related species from the sister genus Kwoniella. Chromosome-level genome assemblies were generated for multiple species, covering virtually all known diversity within these genera. Although Cryptococcus and Kwoniella have comparable genome sizes (about 19.2 and 22.9 Mb) and similar gene content, hinting at preadaptive pathogenic potential, our analysis found evidence of gene gain (via horizontal gene transfer) and gene loss in pathogenic Cryptococcus species, which might represent evolutionary signatures of pathogenic development. Genome analysis also revealed a significant variation in chromosome number and structure between the 2 genera. By combining synteny analysis and experimental centromere validation, we found that most Cryptococcus species have 14 chromosomes, whereas most Kwoniella species have fewer (11, 8, 5, or even as few as 3). Reduced chromosome number in Kwoniella is associated with formation of giant chromosomes (up to 18 Mb) through repeated chromosome fusion events, each marked by a pericentric inversion and centromere loss. While similar chromosome inversion-fusion patterns were observed in all Kwoniella species with fewer than 14 chromosomes, no such pattern was detected in Cryptococcus. Instead, Cryptococcus species with less than 14 chromosomes showed reductions primarily through rearrangements associated with the loss of repeat-rich centromeres. Additionally, Cryptococcus genomes exhibited frequent interchromosomal translocations, including intercentromeric recombination facilitated by transposons shared between centromeres. Overall, our findings advance our understanding of genetic changes possibly associated with pathogenicity in Cryptococcus and provide a foundation to elucidate mechanisms of centromere loss and chromosome fusion driving distinct karyotypes in closely related fungal species, including prominent global human pathogens.

A stable microtubule bundle formed through an orchestrated multistep process controls quiescence exit

Cells fine-tune microtubule assembly in both space and time to give rise to distinct edifices with specific cellular functions. In proliferating cells, microtubules are highly dynamics, and proliferation cessation often leads to their stabilization. One of the most stable microtubule structures identified to date is the nuclear bundle assembled in quiescent yeast. In this article, we characterize the original multistep process driving the assembly of this structure. This Aurora B-dependent mechanism follows a precise temporality that relies on the sequential actions of kinesin-14, kinesin-5, and involves both microtubule-kinetochore and kinetochore-kinetochore interactions. Upon quiescence exit, the microtubule bundle is disassembled via a cooperative process involving kinesin-8 and its full disassembly is required prior to cells re-entry into proliferation. Overall, our study provides the first description, at the molecular scale, of the entire life cycle of a stable microtubule structure in vivo and sheds light on its physiological function.

An unconventional regulatory circuitry involving Aurora B controls anaphase onset and error-free chromosome segregation in trypanosomes

Accurate chromosome segregation during mitosis requires that all chromosomes establish stable bi-oriented attachments with the spindle apparatus. Kinetochores form the interface between chromosomes and spindle microtubules and as such are under tight control by complex regulatory circuitry. As part of the chromosomal passenger complex (CPC), the Aurora B kinase plays a central role within this circuitry by destabilizing improper kinetochore-microtubule attachments and relaying the attachment status to the spindle assembly checkpoint, a feedback control system that delays the onset of anaphase by inhibiting the anaphase-promoting complex/cyclosome. Intriguingly, Aurora B is conserved even in kinetoplastids, an evolutionarily divergent group of eukaryotes, whose kinetochores are composed of a unique set of structural and regulatory proteins. Kinetoplastids do not have a canonical spindle checkpoint and it remains unclear how their kinetochores are regulated to ensure the fidelity and timing of chromosome segregation. Here, we show in Trypanosoma brucei, the kinetoplastid parasite that causes African sleeping sickness, that inhibition of Aurora B using an analogue-sensitive approach arrests cells in metaphase, with a reduction in properly bi-oriented kinetochores. Aurora B phosphorylates several kinetochore proteins in vitro, including the N-terminal region of the divergent Bub1-like protein KKT14. Depletion of KKT14 partially overrides the cell cycle arrest caused by Aurora B inhibition, while overexpression of a non-phosphorylatable KKT14 protein results in a prominent delay in the metaphase-to-anaphase transition. Finally, we demonstrate using a nanobody-based system that re-targeting the catalytic module of the CPC to the outer kinetochore is sufficient to promote mitotic exit but causes massive chromosome mis-segregation in anaphase. Our results indicate that the CPC and KKT14 are involved in an unconventional pathway controlling mitotic exit and error-free chromosome segregation in trypanosomes.

Comparative genomics of Cryptococcus and Kwoniella reveals pathogenesis evolution and contrasting karyotype dynamics via intercentromeric recombination or chromosome fusion

A large-scale comparative genomic analysis was conducted for the global human fungal pathogens within the Cryptococcus genus, compared to non-pathogenic Cryptococcus species, and related species from the sister genus Kwoniella. Chromosome-level genome assemblies were generated for multiple species of both genera, resulting in a dataset encompassing virtually all of their known diversity. Although Cryptococcus and Kwoniella have comparable genome sizes (about 19.2 and 22.9 Mb) and similar gene content, hinting at pre-adaptive pathogenic potential, our analysis found evidence in pathogenic Cryptococcus species of specific examples of gene gain (via horizontal gene transfer) and gene loss, which might represent evolutionary signatures of pathogenic development. Genome analysis also revealed a significant variation in chromosome number and structure between the two genera. By combining synteny analysis and experimental centromere validation, we found that most Cryptococcus species have 14 chromosomes, whereas most Kwoniella species have fewer (11, 8, 5 or even as few as 3). Reduced chromosome number in Kwoniella is associated with formation of giant chromosomes (up to 18 Mb) through repeated chromosome fusion events, each marked by a pericentric inversion and centromere loss. While similar chromosome inversion-fusion patterns were observed in all Kwoniella species with fewer than 14 chromosomes, no such pattern was detected in Cryptococcus. Instead, Cryptococcus species with less than 14 chromosomes, underwent chromosome reductions primarily through rearrangements associated with the loss of repeat-rich centromeres. Additionally, Cryptococcus genomes exhibited frequent interchromosomal translocations, including intercentromeric recombination facilitated by transposons shared between centromeres. Taken together, our findings advance our understanding of genomic changes possibly associated with pathogenicity in Cryptococcus and provide a foundation to elucidate mechanisms of centromere loss and chromosome fusion driving distinct karyotypes in closely related fungal species, including prominent global human pathogens.

Increased Aurora B expression reduces substrate phosphorylation and induces chromosomal instability

Increased Aurora B protein expression, which is common in cancers, is expected to increase Aurora B kinase activity, yielding elevated phosphorylation of Aurora B substrates. In contrast, here we show that elevated expression of Aurora B reduces phosphorylation of six different Aurora B substrates across three species and causes defects consistent with Aurora B inhibition. Complexes of Aurora B and its binding partner INCENP autophosphorylate in trans to achieve full Aurora B activation. Increased expression of Aurora B mislocalizes INCENP, reducing the local concentration of Aurora B:INCENP complexes at the inner centromere/kinetochore. Co-expression of INCENP rescues Aurora B kinase activity and mitotic defects caused by elevated Aurora B. However, INCENP expression is not elevated in concert with Aurora B in breast cancer, and increased expression of Aurora B causes resistance rather than hypersensitivity to Aurora B inhibitors. Thus, increased Aurora B expression reduces, rather than increases, Aurora B kinase activity.

Yeast Fin1-PP1 dephosphorylates an Ipl1 substrate, Ndc80, to remove Bub1-Bub3 checkpoint proteins from the kinetochore during anaphase

The spindle assembly checkpoint (SAC) prevents anaphase onset in response to chromosome attachment defects, and SAC silencing is essential for anaphase onset. Following anaphase onset, activated Cdc14 phosphatase dephosphorylates the substrates of cyclin-dependent kinase to facilitate anaphase progression and mitotic exit. In budding yeast, Cdc14 dephosphorylates Fin1, a regulatory subunit of protein phosphatase 1 (PP1), to enable kinetochore localization of Fin1-PP1. We previously showed that kinetochore-localized Fin1-PP1 promotes the removal of the SAC protein Bub1 from the kinetochore during anaphase. We report here that Fin1-PP1 also promotes kinetochore removal of Bub3, the Bub1 partner, but has no effect on another SAC protein Mad1. Moreover, the kinetochore localization of Bub1-Bub3 during anaphase requires Aurora B/Ipl1 kinase activity. We further showed that Fin1-PP1 facilitates the dephosphorylation of kinetochore protein Ndc80, a known Ipl1 substrate. This dephosphorylation reduces kinetochore association of Bub1-Bub3 during anaphase. In addition, we found that untimely Ndc80 dephosphorylation causes viability loss in response to tensionless chromosome attachments. These results suggest that timely localization of Fin1-PP1 to the kinetochore controls the functional window of SAC and is therefore critical for faithful chromosome segregation.

A common molecular mechanism underlies the role of Mps1 in chromosome biorientation and the spindle assembly checkpoint

The Mps1 kinase corrects improper kinetochore-microtubule attachments, thereby ensuring chromosome biorientation. Yet, its critical phosphorylation targets in this process remain largely elusive. Mps1 also controls the spindle assembly checkpoint (SAC), which halts chromosome segregation until biorientation is attained. Its role in SAC activation is antagonised by the PP1 phosphatase and involves phosphorylation of the kinetochore scaffold Knl1/Spc105, which in turn recruits the Bub1 kinase to promote assembly of SAC effector complexes. A crucial question is whether error correction and SAC activation are part of a single or separable pathways. Here, we isolate and characterise a new yeast mutant, mps1-3, that is severely defective in chromosome biorientation and SAC signalling. Through an unbiased screen for extragenic suppressors, we found that mutations lowering PP1 levels at Spc105 or forced association of Bub1 with Spc105 reinstate both chromosome biorientation and SAC signalling in mps1-3 cells. Our data argue that a common mechanism based on Knl1/Spc105 phosphorylation is critical for Mps1 function in error correction and SAC signalling, thus supporting the idea that a single sensory apparatus simultaneously elicits both pathways.

Aurora kinase Ipl1 facilitates bilobed distribution of clustered kinetochores to ensure error-free chromosome segregation in Candida albicans

Candida albicans, an ascomycete, has an ability to switch to diverse morphological forms. While C. albicans is predominatly diploid, it can tolerate aneuploidy as a survival strategy under stress. Aurora kinase B homolog Ipl1 is a critical ploidy regulator that controls microtubule dynamics and chromosome segregation in Saccharomyces cerevisiae. In this study, we show that Ipl1 in C. albicans has a longer activation loop than that of the well-studied ascomycete S. cerevisiae. Ipl1 localizes to the kinetochores during the G1/S phase and associates with the spindle during mitosis. Ipl1 regulates cell morphogenesis and is required for cell viability. Ipl1 monitors microtubule dynamics which is mediated by separation of spindle pole bodies. While Ipl1 is dispensable for maintaining structural integrity and clustering of kinetochores in C. albicans, it is required for the maintenance of bilobed distribution of clustered kinetochores along the mitotic spindle. Depletion of Ipl1 results in erroneous kinetochore-microtubule attachments leading to aneuploidy due to which the organism can survive better in the presence of fluconazole. Taking together, we suggest that Ipl1 spatiotemporally ensures bilobed kinetochore distribution to facilitate bipolar spindle assembly crucial for ploidy maintenance in C. albicans.

Spatio-temporal regulation of nuclear division by Aurora B kinase Ipl1 in Cryptococcus neoformans

The nuclear division takes place in the daughter cell in the basidiomycetous budding yeast Cryptococcus neoformans. Unclustered kinetochores gradually cluster and the nucleus moves to the daughter bud as cells enter mitosis. Here, we show that the evolutionarily conserved Aurora B kinase Ipl1 localizes to the nucleus upon the breakdown of the nuclear envelope during mitosis in C. neoformans. Ipl1 is shown to be required for timely breakdown of the nuclear envelope as well. Ipl1 is essential for viability and regulates structural integrity of microtubules. The compromised stability of cytoplasmic microtubules upon Ipl1 depletion results in a significant delay in kinetochore clustering and nuclear migration. By generating an in silico model of mitosis, we previously proposed that cytoplasmic microtubules and cortical dyneins promote atypical nuclear division in C. neoformans. Improving the previous in silico model by introducing additional parameters, here we predict that an effective cortical bias generated by cytosolic Bim1 and dynein regulates dynamics of kinetochore clustering and nuclear migration. Indeed, in vivo alterations of Bim1 or dynein cellular levels delay nuclear migration. Results from in silico model and localization dynamics by live cell imaging suggests that Ipl1 spatio-temporally influences Bim1 or/and dynein activity along with microtubule stability to ensure timely onset of nuclear division. Together, we propose that the timely breakdown of the nuclear envelope by Ipl1 allows its own nuclear entry that helps in spatio-temporal regulation of nuclear division during semi-open mitosis in C. neoformans.

Unlike the model ascomycetous budding yeast Saccharomyces cerevisiae, microtubule organizing centers (MTOCs) coalesce to form the spindle pole body (SPB) in C. neoformans. This process also ensures unclustered kinetochores to gradually cluster in this organism. As C. neoformans cells enter mitosis, the nuclear envelope ruptures and the nucleus eventually moves to the daughter bud before division. Here, we combine cell and systems biology techniques to understand the key determinants of nuclear division in C. neoformans. We show that the evolutionarily conserved Aurora B kinase Ipl1 enters the nucleus during the mitotic phase as cells undergo semi-open mitosis. Ipl1 regulates dynamics of cytoplasmic microtubules, cytosolic proteins such as Bim1 and dynein-mediated cortical forces and integrity of the nuclear envelope to ensure timely kinetochore clustering and nuclear division in this medically relevant human pathogenic budding yeast.

Chromosome passenger complex is required for the survival of cells with ring chromosomes in fission yeast

Ring chromosomes are circular chromosomal abnormalities that have been reported in association with some genetic disorders and cancers. In Schizosaccharomyces pombe, lack of function of protection of telomere 1 (Pot1) or telomerase catalytic subunit (Trt1) results in survivors with circular chromosomes. Hitherto, it is poorly understood how cells with circular chromosomes survive and how circular chromosomes are maintained. Fission yeast Cut17/Bir1, Ark1, Pic1, and Nbl1 is a conserved chromosome passenger complex (CPC) functioning mainly throughout mitosis. Here, using a temperature-sensitive mutant of CPC subunits, we determined that CPC is synthetically lethal in combination with either Pot1 or Trt1. The pot1Δ pic1-T269 double mutant, which has circular chromosomes, showed a high percentage of chromosome mis-segregation and DNA damage foci at 33°C. We furthermore found that neither Shugoshin Sgo2 nor heterochromatin protein Swi6, which contribute to the centromeric localization of CPC, were required for the survival in the absence of Pot1. Both the pot1Δ sgo2Δ and pot1Δ swi6Δ double mutants displayed a high percentage of DNA damage foci, but a low percentage of chromosome mis-segregation, suggesting the link between the high percentage of chromosome mis-segregation and the lethality of the CPC pot1Δ double mutant. Our results suggest that CPC is required for the survival of cells with circular chromosomes and sheds light on the possible roles of CPC in the maintenance of circular chromosomes.

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.

Leishmania donovani Aurora kinase: A promising therapeutic target against visceral leishmaniasis

BACKGROUND

Aurora kinases are key mitotic kinases executing multiple aspects of eukaryotic cell-division. The apicomplexan homologs being essential for survival, suggest that the Leishmania homolog, annotated LdAIRK, may be equally important.

METHODS

Bioinformatics, stage-specific immunofluorescence microscopy, immunoblotting, RT-PCR, molecular docking, in-vitro kinase assay, anti-leishmanial activity assays, flow cytometry, fluorescence microscopy.

RESULTS

Ldairk expression is seen to vary as the cell-cycle progresses from G1 through S and finally G2M and cytokinesis. Kinetic studies demonstrate their enzymatic activity exhibiting a Km and Vmax of 6.12μM and 82.9pmoles·min(-1)mg(-1) respectively against ATP using recombinant Leishmania donovani H3, its physiological substrate. Due to the failure of LdAIRK-/+ knock-out parasites to survive, we adopted a chemical knock-down approach. Based on the conservation of key active site residues, three mammalian Aurora kinase inhibitors were investigated to evaluate their potential as inhibitors of LdAIRK activity. Interestingly, the cell-cycle progressed unhindered, despite treatment with GSK-1070916 or Barasertib, inhibitors with greater potencies for the ATP-binding pocket compared to Hesperadin, which at nanomolar concentrations, severely compromised viability at IC50s 105.9 and 36.4nM for promastigotes and amastigotes, respectively. Cell-cycle and morphological studies implicated their role in both mitosis and cytokinesis.

CONCLUSION

We identified an Aurora kinase homolog in L. donovani implicated in cell-cycle progression, whose inhibition led to aberrant changes in cell-cycle progression and reduced viability.

GENERAL SIGNIFICANCE

Human homologs being actively pursued drug targets and the observations with LdAIRK in both promastigotes and amastigotes suggest their potential as therapeutic-targets. Importantly, our results encourage the exploration of other proteins identified herein as potential novel drug targets.

Characterization of macrozoospermia-associated AURKC mutations in a mammalian meiotic system

Aneuploidy is the leading genetic abnormality that leads to miscarriage, and it is caused by a failure of accurate chromosome segregation during gametogenesis or early embryonic divisions. Aurora kinase C (AURKC) is essential for formation of euploid sperm in humans because mutations in AURKC are correlated with macrozoospermia and these sperm are tetraploid. These mutations are currently the most frequent mutations that cause macrozoospermia and result from an inability to complete meiosis I (MI). Three of these mutations AURKC c.144delC (AURKC p.L49Wfs22), AURKC c.686G > A (AURKC p.C229Y) and AURKC c.744C > G (AURKC p.Y248*) occur in the coding region of the gene and are the focus of this study. By expressing these alleles in oocytes isolated from Aurkc^-/- mice, we show that the mutations have different effects on AURKC function during MI. AURKC p.L49Wfs22 is a loss-of-function mutant that perturbs localization of the chromosomal passenger complex (CPC), AURKC p.C229Y is a hypomorph that cannot fully support cell-cycle progression, and AURKC p.Y248* fails to localize and function with the CPC to support chromosome segregation yet retains catalytic activity in the cytoplasm. Finally, we show that these variants of AURKC cause meiotic failure and polyploidy due to a failure in AURKC-CPC function that results in metaphase chromosome misalignment. This study is the first to assess the function of mutant alleles of AURKC that affect human fertility in a mammalian meiotic system.

The Aurora B kinase in Trypanosoma brucei undergoes post-translational modifications and is targeted to various subcellular locations through binding to TbCPC1

The chromosomal passenger complex (CPC) in animals, consisting of Aurora B kinase and three evolutionarily conserved proteins, plays crucial roles in mitosis and cytokinesis. However, Trypanosoma brucei expresses an unusual CPC consisting of an Aurora-like kinase, TbAUK1, and two kinetoplastid-specific proteins, TbCPC1 and TbCPC2. Despite their essential functions, little is known about the regulation of TbAUK1 and the roles of TbCPC1 and TbCPC2. Here, we investigate the effect of post-translational modification on the activity and spatiotemporal control of TbAUK1, and demonstrate that phosphorylation of two conserved threonine residues in the activation loop of the kinase domain contributes to TbAUK1 activation and function. TbAUK1 is SUMOylated in vivo, and mutation of the SUMO-conjugation site compromises TbAUK1 function. Degradation of TbAUK1 requires two destruction boxes and is mediated by the anaphase-promoting complex/cyclosome (APC/C), whereas degradation of TbCPC1 and TbCPC2 is not dependent on the predicted destruction boxes and is APC/C-independent. Moreover, we determine the domains in CPC subunits that mediate the pairwise interactions, and show that disruption of the interaction impairs the localization of TbAUK1 and TbCPC2 but not TbCPC1. Our results demonstrate the requirement of post-translational modifications for TbAUK1 function and a crucial role of TbCPC1 in mediating TbAUK1 localization.

S. cerevisiae chromosomes biorient via gradual resolution of syntely between S phase and anaphase

Following DNA replication, eukaryotic cells must biorient all sister chromatids prior to cohesion cleavage at anaphase. In animal cells, sister chromatids gradually biorient during prometaphase, but current models of mitosis in S. cerevisiae assume that biorientation is established shortly after S phase. This assumption is based on the observation of a bilobed distribution of yeast kinetochores early in mitosis and suggests fundamental differences between yeast mitosis and mitosis in animal cells. By applying super-resolution imaging methods, we show that yeast and animal cells share the key property of gradual and stochastic chromosome biorientation. The characteristic bilobed distribution of yeast kinetochores, hitherto considered synonymous for biorientation, arises from kinetochores in mixed attachment states to microtubules, the length of which discriminates bioriented from syntelic attachments. Our results offer a revised view of mitotic progression in S. cerevisiae that augments the relevance of mechanistic information obtained in this powerful genetic system for mammalian mitosis.

Tension sensing by Aurora B kinase is independent of survivin-based centromere localization

Accurate segregation of the replicated genome requires chromosome biorientation on the spindle. Biorientation is ensured by Aurora B kinase, a member of the 4-subunit chromosomal passenger complex (CPC)^1,2. Localization of the CPC to the inner centromere is central to the current model for how tension ensures chromosome biorientation—kinetochore-spindle attachments not under tension remain close to the inner centromere and are destabilized by Aurora B phosphorylation, whereas kinetochores under tension are pulled away from the influence of Aurora B, stabilizing their microtubule attachments^3–5. Here we show that an engineered truncation of the INCENP/Sli15 subunit of budding yeast CPC that eliminates association with the inner centromere nevertheless supports proper chromosome segregation during both mitosis and meiosis. Truncated INCENP/Sli15 suppresses the deletion phenotypes of the inner centromere-targeting proteins Survivin/Bir1, Borealin/Nbl1, Bub1 and Sgo1⁶. Unlike wildtype INCENP/Sli15, truncated INCENP/Sli15 localizes to pre-anaphase spindle microtubules. Premature targeting of full-length INCENP/Sli15 to microtubules by preventing Cdk1 phosphorylation also suppresses inviability of Survivin/Bir1 deletion. These results suggest that activation of Aurora B/Ipl1 by clustering either on chromatin or on microtubules is sufficient for chromosome biorientation.

A Whole Genome Screen for Minisatellite Stability Genes in Stationary-Phase Yeast Cells

Repetitive elements comprise a significant portion of most eukaryotic genomes. Minisatellites, a type of repetitive element composed of repeat units 15−100 bp in length, are stable in actively dividing cells but change in composition during meiosis and in stationary-phase cells. Alterations within minisatellite tracts have been correlated with the onset of a variety of diseases, including diabetes mellitus, myoclonus epilepsy, and several types of cancer. However, little is known about the factors preventing minisatellite alterations. Previously, our laboratory developed a color segregation assay in which a minisatellite was inserted into the ADE2 gene in the yeast Saccharomyces cerevisiae to monitor alteration events. We demonstrated that minisatellite alterations that occur in stationary-phase cells give rise to a specific colony morphology phenotype known as blebbing. Here, we performed a modified version of the synthetic genetic array analysis to screen for mutants that produce a blebbing phenotype. Screens were conducted using two distinctly different minisatellite tracts: the ade2-min3 construct consisting of three identical 20-bp repeats, and the ade2-h7.5 construct, consisting of seven-and-a-half 28-bp variable repeats. Mutations in 102 and 157 genes affect the stability of the ade2-min3 and ade2-h7.5 alleles, respectively. Only seven hits overlapped both screens, indicating that different factors regulate repeat stability depending upon minisatellite size and composition. Importantly, we demonstrate that mismatch repair influences the stability of the ade2-h7.5 allele, indicating that this type of DNA repair stabilizes complex minisatellites in stationary phase cells. Our work provides insight into the factors regulating minisatellite stability.

Co-segregation of yeast plasmid sisters under monopolin-directed mitosis suggests association of plasmid sisters with sister chromatids

The 2-micron plasmid, a high copy extrachromosomal element in Saccharomyces cerevisiae, propagates itself with nearly the same stability as the chromosomes of its host. Plasmid stability is conferred by a partitioning system consisting of the plasmid-coded proteins Rep1 and Rep2 and a cis-acting locus STB. Circumstantial evidence suggests that the partitioning system couples plasmid segregation to chromosome segregation during mitosis. However, the coupling mechanism has not been elucidated. In order to probe into this question more incisively, we have characterized the segregation of a single-copy STB reporter plasmid by manipulating mitosis to force sister chromatids to co-segregate either without mother-daughter bias or with a finite daughter bias. We find that the STB plasmid sisters are tightly correlated to sister chromatids in the extents of co-segregation as well as the bias in co-segregation under these conditions. Furthermore, this correlation is abolished by delaying spindle organization or preventing cohesin assembly during a cell cycle. Normal segregation of the 2-micron plasmid has been shown to require spindle integrity and the cohesin complex. Our results are accommodated by a model in which spindle- and cohesin-dependent association of plasmid sisters with sister chromatids promotes their segregation by a hitchhiking mechanism.

Suppressors of ipl1-2 in components of a Glc7 phosphatase complex, Cdc48 AAA ATPase, TORC1, and the kinetochore

Ipl1/Aurora B is the catalytic subunit of a protein kinase complex required for chromosome segregation and nuclear division. Before anaphase, Ipl1 is required to establish proper kinetochore-microtubule associations and to regulate the spindle assembly checkpoint (SAC). The phosphatase Glc7/PP1 opposes Ipl1 for these activities. To investigate Ipl1 and Glc7 regulation in more detail, we isolated and characterized mutations in the yeast Saccharomyces cerevisiae that raise the restrictive temperature of the ipl-2 mutant. These suppressors include three intragenic, second-site revertants in IPL1; 17 mutations in Glc7 phosphatase components (GLC7, SDS22, YPI1); two mutations in SHP1, encoding a regulator of the AAA ATPase Cdc48; and a mutation in TCO89, encoding a subunit of the TOR Complex 1. Two revertants contain missense mutations in microtubule binding components of the kinetochore. rev76 contains the missense mutation duo1-S115F, which alters an essential component of the DAM1/DASH complex. The mutant is cold sensitive and arrests in G2/M due to activation of the SAC. rev8 contains the missense mutation ndc80-K204E. K204 of Ndc80 corresponds to K166 of human Ndc80 and the human Ndc80 K166E variant was previously shown to be defective for microtubule binding in vitro. In a wild-type IPL1 background, ndc80-K204E cells grow slowly and the SAC is activated. The slow growth and cell cycle delay of ndc80-K204E cells are partially alleviated by the ipl1-2 mutation. These data provide biological confirmation of a biochemically based model for the effect of phosphorylation on Ndc80 function.

Karyotypic determinants of chromosome instability in aneuploid budding yeast

Recent studies in cancer cells and budding yeast demonstrated that aneuploidy, the state of having abnormal chromosome numbers, correlates with elevated chromosome instability (CIN), i.e. the propensity of gaining and losing chromosomes at a high frequency. Here we have investigated ploidy- and chromosome-specific determinants underlying aneuploidy-induced CIN by observing karyotype dynamics in fully isogenic aneuploid yeast strains with ploidies between 1N and 2N obtained through a random meiotic process. The aneuploid strains exhibited various levels of whole-chromosome instability (i.e. chromosome gains and losses). CIN correlates with cellular ploidy in an unexpected way: cells with a chromosomal content close to the haploid state are significantly more stable than cells displaying an apparent ploidy between 1.5 and 2N. We propose that the capacity for accurate chromosome segregation by the mitotic system does not scale continuously with an increasing number of chromosomes, but may occur via discrete steps each time a full set of chromosomes is added to the genome. On top of such general ploidy-related effect, CIN is also associated with the presence of specific aneuploid chromosomes as well as dosage imbalance between specific chromosome pairs. Our findings potentially help reconcile the divide between gene-centric versus genome-centric theories in cancer evolution.

Spatiotemporal regulation of Ipl1/Aurora activity by direct Cdk1 phosphorylation

Oscillating cyclin-dependent kinase 1 (Cdk1) activity is the major regulator of cell-cycle progression, whereas the Aurora B kinase, as part of the chromosome passenger complex (CPC), controls critical aspects of mitosis such as chromosome condensation and biorientation on the spindle. How these kinases mechanistically coordinate their important functions is only partially understood. Here, using budding yeast, we identify a regulatory mechanism by which the Cdk1 kinase Cdc28 directly controls the Aurora kinase Ipl1. We show that Cdk1 phosphorylates Ipl1 on two serine residues in the N-terminal domain, thereby suppressing its association with the microtubule plus-end tracking protein Bim1 until the onset of anaphase. Failure to phosphorylate Ipl1 leads to its premature targeting to the metaphase spindle and results in constitutive Bim1 phosphorylation, which is normally restricted to anaphase. Cells expressing an Ipl1-Sli15 complex that cannot be phosphorylated by Cdk1 display a severe growth defect. Our work shows that Ipl1/Aurora is not only the catalytic subunit of the CPC but also an important regulatory target that allows Cdk1 to coordinate chromosome biorientation with spindle morphogenesis.

Correction of microtubule-kinetochore attachment errors: mechanisms and role in tumor suppression

During mitosis, cells segregate duplicated chromosomes with high fidelity in order to maintain genome stability. Proper attachment of sister kinetochores to spindle microtubules is critical for accurate chromosome segregation and is driven by complex mechanisms that promote the capture of unattached kinetochores and the resolution of erroneously attached kinetochores. Defects in these surveillance systems promote chromosome segregation and aneuploidy and can contribute to neoplastic transformation. Understanding, how, at the molecular level, accurate chromosome segregation is achieved may be crucial for our understanding of how cancer cells develop genome instability.

Ipl1/Aurora-dependent phosphorylation of Sli15/INCENP regulates CPC-spindle interaction to ensure proper microtubule dynamics

Dynamic microtubules facilitate chromosome arrangement before anaphase, whereas during anaphase microtubule stability assists chromosome separation. Changes in microtubule dynamics at the metaphase-anaphase transition are regulated by Cdk1. Cdk1-mediated phosphorylation of Sli15/INCENP promotes preanaphase microtubule dynamics by preventing chromosomal passenger complex (CPC; Sli15/INCENP, Bir1/Survivin, Nbl1/Borealin, Ipl1/Aurora) association with spindles. However, whether Cdk1 has sole control over microtubule dynamics, and how CPC-microtubule association influences microtubule behavior, are unclear. Here, we show that Ipl1/Aurora-dependent phosphorylation of Sli15/INCENP modulates microtubule dynamics by preventing CPC binding to the preanaphase spindle and to the central spindle until late anaphase, facilitating spatiotemporal control of microtubule dynamics required for proper metaphase centromere positioning and anaphase spindle elongation. Decreased Ipl1-dependent Sli15 phosphorylation drives direct CPC binding to microtubules, revealing how the CPC influences microtubule dynamics. We propose that Cdk1 and Ipl1/Aurora cooperatively modulate microtubule dynamics and that Ipl1/Aurora-dependent phosphorylation of Sli15 controls spindle function by excluding the CPC from spindle regions engaged in microtubule polymerization.

Overlapping kinetochore targets of CK2 and Aurora B kinases in mitotic regulation

Protein kinase CK2 is one of the most conserved kinases in eukaryotic cells and plays essential roles in diverse processes. While we know that CK2 plays a role(s) in cell division, our understanding of how CK2 regulates cell cycle progression is limited. In this study, we revealed a regulatory role for CK2 in kinetochore function. The kinetochore is a multi-protein complex that assembles on the centromere of a chromosome and functions to attach chromosomes to spindle microtubules. To faithfully segregate chromosomes and maintain genomic integrity, the kinetochore is tightly regulated by multiple mechanisms, including phosphorylation by Aurora B kinase. We found that a loss of CK2 kinase activity inhibits anaphase spindle elongation and results in chromosome missegregation. Moreover, a lack of CK2 activates the spindle assembly checkpoint. We demonstrate that CK2 associates with Mif2, the Saccharomyces cerevisiae homologue of human CENP-C, which serves as an important link between the inner and outer kinetochore. Furthermore, we show Mif2 and the inner kinetochore protein Ndc10 are phosphorylated by CK2, and this phosphorylation plays antagonistic and synergistic roles with Aurora B phosphorylation of these targets, respectively.

Chromosome passenger complexes control anaphase duration and spindle elongation via a kinesin-5 brake

During mitosis, chromosome passenger complexes (CPCs) exhibit a well-conserved association with the anaphase spindle and have been implicated in spindle stability. However, their precise effect on the spindle is not clear. In this paper, we show, in budding yeast, that a CPC consisting of CBF3, Bir1, and Sli15, but not Ipl1, is required for normal spindle elongation. CPC mutants slow spindle elongation through the action of the bipolar kinesins Cin8 and Kip1. The same CPC mutants that slow spindle elongation also result in the enrichment of Cin8 and Kip1 at the spindle midzone. Together, these findings argue that CPCs function to organize the spindle midzone and potentially switch motors between force generators and molecular brakes. We also find that slowing spindle elongation delays the mitotic exit network (MEN)-dependent release of Cdc14, thus delaying spindle breakdown until a minimal spindle size is reached. We propose that these CPC- and MEN-dependent mechanisms are important for coordinating chromosome segregation with spindle breakdown and mitotic exit.

Bub1, Sgo1, and Mps1 mediate a distinct pathway for chromosome biorientation in budding yeast

The conserved mitotic kinase Bub1 performs multiple functions that are only partially characterized. Besides its role in the spindle assembly checkpoint and chromosome alignment, Bub1 is crucial for the kinetochore recruitment of multiple proteins, among them Sgo1. Both Bub1 and Sgo1 are dispensable for growth of haploid and diploid budding yeast, but they become essential in cells with higher ploidy. We find that overexpression of SGO1 partially corrects the chromosome segregation defect of bub1Δ haploid cells and restores viability to bub1Δ tetraploid cells. Using an unbiased high-copy suppressor screen, we identified two members of the chromosomal passenger complex (CPC), BIR1 (survivin) and SLI15 (INCENP, inner centromere protein), as suppressors of the growth defect of both bub1Δ and sgo1Δ tetraploids, suggesting that these mutants die due to defects in chromosome biorientation. Overexpression of BIR1 or SLI15 also complements the benomyl sensitivity of haploid bub1Δ and sgo1Δ cells. Mutants lacking SGO1 fail to biorient sister chromatids attached to the same spindle pole (syntelic attachment) after nocodazole treatment. Moreover, the sgo1Δ cells accumulate syntelic attachments in unperturbed mitoses, a defect that is partially corrected by BIR1 or SLI15 overexpression. We show that in budding yeast neither Bub1 nor Sgo1 is required for CPC localization or affects Aurora B activity. Instead we identify Sgo1 as a possible partner of Mps1, a mitotic kinase suggested to have an Aurora B-independent function in establishment of biorientation. We found that Sgo1 overexpression rescues defects caused by metaphase inactivation of Mps1 and that Mps1 is required for Sgo1 localization to the kinetochore. We propose that Bub1, Sgo1, and Mps1 facilitate chromosome biorientation independently of the Aurora B-mediated pathway at the budding yeast kinetochore and that both pathways are required for the efficient turnover of syntelic attachments.

Laterally attached kinetochores recruit the checkpoint protein Bub1, but satisfy the spindle checkpoint

Kinetochore attachment to the ends of dynamic microtubules is a conserved feature of mitotic spindle organization that is thought to be critical for proper chromosome segregation. Although kinetochores have been described to transition from lateral to end-on attachments, the phase of lateral attachment has been difficult to study in yeast due to its transient nature. We have previously described a kinetochore mutant, DAM1-765, which exhibits lateral attachments and misregulation of microtubule length. Here we show that the misregulation of microtubule length in DAM1-765 cells occurs despite localization of microtubule associated proteins Bik1, Stu2, Cin8, and Kip3 to microtubules. DAM1-765 kinetochores recruit the spindle checkpoint protein Bub1, however Bub1 localization to DAM1-765 kinetochores is not sufficient to cause a cell cycle arrest. Interestingly, the DAM1-765 mutation rescues the temperature sensitivity of a biorientation-deficient ipl1-321 mutant, and DAM1-765 chromosome loss rates are similar to wild-type cells. The spindle checkpoint in DAM1-765 cells responds properly to unattached kinetochores created by nocodazole treatment and loss of tension caused by a cohesin mutant. Progression of DAM1-765 cells through mitosis therefore suggests that satisfaction of the checkpoint depends more highly on biorientation of sister kinetochores than on achievement of a specific interaction between kinetochores and microtubule plus ends.

The Aurora Kinase in Trypanosoma brucei plays distinctive roles in metaphase-anaphase transition and cytokinetic initiation

Aurora B kinase is an essential regulator of chromosome segregation with the action well characterized in eukaryotes. It is also implicated in cytokinesis, but the detailed mechanism remains less clear, partly due to the difficulty in separating the latter from the former function in a growing cell. A chemical genetic approach with an inhibitor of the enzyme added to a synchronized cell population at different stages of the cell cycle would probably solve this problem. In the deeply branched parasitic protozoan Trypanosoma brucei, an Aurora B homolog, TbAUK1, was found to control both chromosome segregation and cytokinetic initiation by evidence from RNAi and dominant negative mutation. To clearly separate these two functions, VX-680, an inhibitor of TbAUK1, was added to a synchronized T. brucei procyclic cell population at different cell cycle stages. The unique trans-localization pattern of the chromosomal passenger complex (CPC), consisting of TbAUK1 and two novel proteins TbCPC1 and TbCPC2, was monitored during mitosis and cytokinesis by following the migration of the proteins tagged with enhanced yellow fluorescence protein in live cells with time-lapse video microscopy. Inhibition of TbAUK1 function in S-phase, prophase or metaphase invariably arrests the cells in the metaphase, suggesting an action of TbAUK1 in promoting metaphase-anaphase transition. TbAUK1 inhibition in anaphase does not affect mitotic exit, but prevents trans-localization of the CPC from the spindle midzone to the anterior tip of the new flagellum attachment zone for cytokinetic initiation. The CPC in the midzone is dispersed back to the two segregated nuclei, while cytokinesis is inhibited. In and beyond telophase, TbAUK1 inhibition has no effect on the progression of cytokinesis or the subsequent G1, S and G2 phases until a new metaphase is attained. There are thus two clearly distinct points of TbAUK1 action in T. brucei: the metaphase-anaphase transition and cytokinetic initiation. This is the first time to our knowledge that the dual functions of an Aurora B homolog is dissected and separated into two clearly distinct time frames in a cell cycle.

The chromosomal passenger complex (CPC) is essential for chromosome segregation and cytokinesis in eukaryotes, but the detailed mechanism of cytokinetic regulation remains less clear, partly due to the difficulty in separating the two functions in a growing cell. A chemical genetic approach by adding an inhibitor of the Aurora kinase in the CPC to a synchronized cell population at different cell cycle stages would probably solve this problem. The CPC in Trypanosoma brucei consists of an Aurora-like kinase (TbAUK1) and two novel proteins and bears little resemblance to the CPC in other eukaryotes. It moves from kinetochores to the spindle midzone during metaphase-anaphase transition, and then displays a unique trans-localization to the anterior end of the cell to initiate cytokinesis by moving from the anterior to the posterior end of the cell to separate it into two. To envision the role of TbAUK1 in driving this unusual process, we applied a chemical genetic approach and demonstrated that there are two distinct points of TbAUK1 action in T. brucei: the metaphase to anaphase transition and cytokinetic initiation. This is the first time to our knowledge that the dual functions of an Aurora B homolog is dissected and separated into two clearly distinct time frames in a cell cycle.

Efficient chromosome biorientation and the tension checkpoint in Saccharomyces cerevisiae both require Bir1

Accurate chromosome segregation requires the capture of sister kinetochores by microtubules from opposite spindle poles prior to the initiation of anaphase, a state termed chromosome biorientation. In the budding yeast Saccharomyces cerevisiae, the conserved protein kinase Ipl1 (Aurora B in metazoans) is critical for ensuring correct chromosomal alignment. Ipl1 associates with its activators Sli15 (INCENP), Nbl1 (Borealin), and Bir1 (Survivin), but while Sli15 clearly functions with Ipl1 to promote chromosome biorientation, the role of Bir1 has been uncertain. Using a temperature-sensitive bir1 mutant (bir1-17), we show that Bir1 is needed to permit efficient chromosome biorientation. However, once established, chromosome biorientation is maintained in bir1-17 cells at the restrictive temperature. Ipl1 is partially delocalized in bir1-17 cells, and its protein kinase activity is markedly reduced under nonpermissive conditions. bir1-17 cells arrest normally in response to microtubule depolymerization but fail to delay anaphase when sister kinetochore tension is reduced. Thus, Bir1 is required for the tension checkpoint. Despite their robust mitotic arrest in response to nocodazole, bir1-17 cells are hypersensitive to microtubule-depolymerizing drugs and show a more severe biorientation defect on recovery from nocodazole treatment. The role of Bir1 therefore may become more critical when spindle formation is delayed.

Nbl1p: a Borealin/Dasra/CSC-1-like protein essential for Aurora/Ipl1 complex function and integrity in Saccharomyces cerevisiae

The Aurora kinase complex, also called the chromosomal passenger complex (CPC), is essential for faithful chromosome segregation and completion of cell division. In Fungi and Animalia, this complex consists of the kinase Aurora B/AIR-2/Ipl1p, INCENP/ICP-1/Sli15p, and Survivin/BIR-1/Bir1p. A fourth subunit, Borealin/Dasra/CSC-1, is required for CPC targeting to centromeres and central spindles and has only been found in Animalia. Here we identified a new core component of the CPC in budding yeast, Nbl1p. NBL1 is essential for viability and nbl1 mutations cause chromosome missegregation and lagging chromosomes. Nbl1p colocalizes and copurifies with the CPC, and it is essential for CPC localization, stability, integrity, and function. Nbl1p is related to the N-terminus of Borealin/Dasra/CSC-1 and is similarly involved in connecting the other CPC subunits. Distant homology searching identified nearly 200, mostly unannotated, Borealin/Dasra/CSC-1-related proteins from nearly 150 species within Fungi and Animalia. Analysis of the sequence of these proteins, combined with comparative protein structure modeling of Bir1p-Nbl1p-Sli15p using the crystal structure of the human Survivin-Borealin-INCENP complex, revealed a striking structural conservation across a broad range of species. Our biological and computational analyses therefore establish that the fundamental design of the CPC is conserved from Fungi to Animalia.

Bir1 is required for the tension checkpoint

The Saccharomyces cerevisiae chromosomal passenger proteins Ipl1 (Aurora B) and Sli15 (INCENP) are required for the tension checkpoint, but the role of the third passenger, Bir1, is controversial. We have isolated a temperature-sensitive mutant (bir1-107) in the essential C-terminal region of Bir1 known to be required for binding to Sli15. This allele reveals a checkpoint function for Bir1. The mutant displays a biorientation defect, a defective checkpoint response to lack of tension, and an inability to detach mutant kinetochores. Ipl1 localizes to aberrant foci when Bir1 localization is disrupted in the bir1-107 mutant. Thus, one checkpoint role of Bir1 is to properly localize Ipl1 and allow detachment of kinetochores. Quantitative analysis indicates that the chromosomal passengers colocalize with kinetochores in G1 but localize between kinetochores that are under tension. Bir1 localization to kinetochores is maintained in an mcd1-1 mutant in the absence of tension. Our results suggest that the establishment of tension removes Ipl1, Bir1, and Sli15, and their kinetochore detachment activity, from the vicinity of kinetochores and allows cells to proceed through the tension checkpoint.

Regulation of Sli15/INCENP, kinetochore, and Cdc14 phosphatase functions by the ribosome biogenesis protein Utp7

The Sli15–Ipl1–Bir1 chromosomal passenger complex is essential for proper kinetochore–microtubule attachment and spindle stability in the budding yeast Saccharomyces cerevisiae. During early anaphase, release of the Cdc14 protein phosphatase from the nucleolus leads to the dephosphorylation of Sli15 and redistribution of this complex from kinetochores to the spindle. We show here that the predominantly nucleolar ribosome biogenesis protein Utp7 is also present at kinetochores and is required for normal organization of kinetochore proteins and proper chromosome segregation. Utp7 associates with and regulates the localization of Sli15 and Cdc14. Before anaphase onset, it prevents the premature nucleolar release of Cdc14 and the premature concentration of Sli15 on the spindle. Furthermore, Utp7 can regulate the localization and phosphorylation status of Sli15 independent of its effect on Cdc14 function. Thus, Utp7 is a multifunctional protein that plays essential roles in the vital cellular processes of ribosome biogenesis, chromosome segregation, and cell cycle control.

Identification of a novel chromosomal passenger complex and its unique localization during cytokinesis in Trypanosoma brucei

Aurora B kinase is a key component of the chromosomal passenger complex (CPC), which regulates chromosome segregation and cytokinesis. An ortholog of Aurora B was characterized in Trypanosoma brucei (TbAUK1), but other conserved components of the complex have not been found. Here we identified four novel TbAUK1 associated proteins by tandem affinity purification and mass spectrometry. Among these four proteins, TbKIN-A and TbKIN-B are novel kinesin homologs, whereas TbCPC1 and TbCPC2 are hypothetical proteins without any sequence similarity to those known CPC components from yeasts and metazoans. RNAi-mediated silencing of each of the four genes led to loss of spindle assembly, chromosome segregation and cytokinesis. TbKIN-A localizes to the mitotic spindle and TbKIN-B to the spindle midzone during mitosis, whereas TbCPC1, TbCPC2 and TbAUK1 display the dynamic localization pattern of a CPC. After mitosis, the CPC disappears from the central spindle and re-localizes at a dorsal mid-point of the mother cell, where the anterior tip of the daughter cell is tethered, to start cell division toward the posterior end, indicating a most unusual CPC-initiated cytokinesis in a eukaryote.

Dictyostelium Aurora kinase has properties of both Aurora A and Aurora B kinases

Aurora kinases are highly conserved proteins with important roles in mitosis. Metazoans contain two kinases, Aurora A and B, which contribute distinct functions at the spindle poles and the equatorial region respectively. It is not currently known whether the specialized functions of the two kinases arose after their duplication in animal cells or were already present in their ancestral kinase. We show that Dictyostelium discoideum contains a single Aurora kinase, DdAurora, that displays characteristics of both Aurora A and B. Like Aurora A, DdAurora has an extended N-terminal domain with an A-box sequence and localizes at the spindle poles during early mitosis. Like Aurora B, DdAurora binds to its partner DdINCENP and localizes on centromeres at metaphase, the central spindle during anaphase, and the cleavage furrow at the end of cytokinesis. DdAurora also has several unusual properties. DdAurora remains associated with centromeres in anaphase, and this association does not require an interaction with DdINCENP. DdAurora then localizes at the cleavage furrow, but only at the end of cytokinesis. This localization is dependent on DdINCENP and the motor proteins Kif12 and myosin II. Thus, DdAurora may represent the ancestral kinase that gave rise to the different Aurora kinases in animals and also those in other organisms.

Centromeric Aurora-B activation requires TD-60, microtubules, and substrate priming phosphorylation

The chromosome passenger complex (CPC) controls chromosome congression, kinetochore-microtubule attachments, and spindle checkpoint signaling during mitosis. Aurora-B kinase is the catalytic subunit of the CPC. To understand how a single kinase can regulate such diverse events, we have investigated the activation of Aurora-B and describe two distinct activation mechanisms. First, Aurora-B activation in vitro requires two cofactors, telophase disc-60kD (TD-60) and microtubules. TD-60 is critical to localize both the CPC and Haspin kinase activity to centromeres and thus regulates Aurora-B at several levels. Second, Aurora-B substrates can inhibit kinase activation, and this is relieved by phosphorylation of these substrates by the centromeric kinases Plk1 and Haspin. These regulatory mechanisms suggest models for phosphorylation by Aurora-B of centromeric substrates at unaligned chromosomes and merotelic attachments.

Chromosomal passengers: conducting cell division

Mitosis and meiosis are remarkable processes during which cells undergo profound changes in their structure and physiology. These events are orchestrated with a precision that is worthy of a classical symphony, with different activities being switched on and off at precise times and locations throughout the cell. One essential 'conductor' of this symphony is the chromosomal passenger complex (CPC), which comprises Aurora-B protein kinase, the inner centromere protein INCENP, survivin and borealin (also known as Dasra-B). Studies of the CPC are providing insights into its functions, which range from chromosome-microtubule interactions to sister chromatid cohesion to cytokinesis, and constitute one of the most dynamic areas of ongoing mitosis and meiosis research.

Aurora controls sister kinetochore mono-orientation and homolog bi-orientation in meiosis-I

Aurora-B kinases are important regulators of mitotic chromosome segregation, where they are required for the faithful bi-orientation of sister chromatids. In contrast to mitosis, sister chromatids have to be oriented toward the same spindle pole in meiosis-I, while homologous chromosomes are bi-oriented. We find that the fission yeast Aurora kinase Ark1 is required for the faithful bi-orientation of sister chromatids in mitosis and of homologous chromosomes in meiosis-I. Unexpectedly, Ark1 is also necessary for the faithful mono-orientation of sister chromatids in meiosis-I, even though the canonical mono-orientation pathway, which depends on Moa1 and Rec8, seems intact. Our data suggest that Ark1 prevents unified sister kinetochores during metaphase-I from merotelic attachment to both spindle poles and thus from being torn apart during anaphase-I, revealing a novel mechanism promoting monopolar attachment. Furthermore, our results provide an explanation for the previously enigmatic observation that fission yeast Shugoshin Sgo2, which assists in loading Aurora to centromeres, and its regulator Bub1 are required for the mono-orientation of sister chromatids in meiosis-I.

A Bir1p Sli15p kinetochore passenger complex regulates septin organization during anaphase

Kinetochore-passenger complexes in metazoans have been proposed to coordinate the segregation of chromosomes in anaphase with the induction of cytokinesis. Passenger protein homologues in the budding yeast Saccharomyces cerevisiae play a critical role early in mitosis, ensuring proper biorientation of kinetochore-microtubule attachments. Our recent work has implicated the passenger protein Bir1p (Survivin) and the inner kinetochore complex centromere binding factor 3 (CBF3) in the regulation of septin dynamics during anaphase. Here, we present data that is consistent with there being multiple passenger protein complexes. Our data show that Bir1p links together a large passenger complex containing Ndc10p, Sli15p (INCENP), and Ipl1p (Aurora B) and that the interaction between Bir1p and Sli15p is specifically involved in regulating septin dynamics during anaphase. Neither conditional alleles nor mutants of BIR1 that disrupt the interaction between Bir1p and Sli15p resulted in mono-attached kinetochores, suggesting that the Bir1p-Sli15p complex functions in anaphase and independently from Sli15p-Ipl1p complexes. We present a model for how discrete passenger complexes coordinate distinct aspects of mitosis.

The localization of inner centromeric protein (INCENP) at the cleavage furrow is dependent on Kif12 and involves interactions of the N terminus of INCENP with the actin cytoskeleton

The inner centromeric protein (INCENP) and other chromosomal passenger proteins are known to localize on the cleavage furrow and to play a role in cytokinesis. However, it is not known how INCENP localizes on the furrow or whether this localization is separable from that at the midbody. Here, we show that the association of Dictyostelium INCENP (DdINCENP) with the cortex of the cleavage furrow involves interactions with the actin cytoskeleton and depends on the presence of the kinesin-6-related protein Kif12. We found that Kif12 is found on the central spindle and the cleavage furrow during cytokinesis. Kif12 is not required for the redistribution of DdINCENP from centromeres to the central spindle. However, in the absence of Kif12, DdINCENP fails to localize on the cleavage furrow. Domain analysis indicates that the N terminus of DdINCENP is necessary and sufficient for furrow localization and that it binds directly to the actin cytoskeleton. Our data suggest that INCENP moves from the central spindle to the furrow of a dividing cell by a Kif12-dependent pathway. Once INCENP reaches the equatorial cortex, it associates with the actin cytoskeleton where it then concentrates toward the end of cytokinesis.

REKLES is an ARID3-restricted multifunctional domain

Bright/Dril1/ARID3a is a B cell-specific, matrix association (or attachment) region-binding transcriptional regulator of immunoglobulin heavy chain genes and of E2F1-dependent cell cycle progression. Bright contains a central DNA binding domain termed ARID (AT-rich interacting domain) and a C-terminal region termed REKLES (for a conserved amino acid motif). The ARID domain has been identified in seven highly conserved families of metazoan proteins (ARID1-5 and JARID1-2), whereas REKLES is found only in the ARID3 subfamily (composed of Bright/ARID3a, Bdp/ARID3b, and Bright-like/ARID3c). REKLES consists of two subdomains: a modestly conserved N-terminal REKLESalpha and a highly conserved (among ARID3 orthologous proteins) C-terminal REKLESbeta. Previously we showed that Bright undergoes nucleocytoplasmic shuttling and that REKLESalpha and -beta were required, respectively, for nuclear import and Crm1-dependent nuclear export. Here we show that Bright further requires REKLESbeta for self-association or paralogue association and for nuclear matrix targeting. REK-LES promotes and regulates the extent of Bright multimerization, which occurs in the absence or presence of target DNA and is necessary for specific DNA binding. REKLESbeta-mediated interaction of Bright with Bdp, which localizes strictly to the nucleus, traps Bright within the nucleus via neutralization of its nuclear export activity. These results identify REKLES as a multifunctional domain that has co-evolved with and regulates functional properties of the ARID3 DNA binding domain.

The enhancement of pericentromeric cohesin association by conserved kinetochore components promotes high-fidelity chromosome segregation and is sensitive to microtubule-based tension

Sister chromatid cohesion, conferred by the evolutionarily conserved cohesin complex, is essential for proper chromosome segregation. Cohesin binds to discrete sites along chromosome arms, and is especially enriched surrounding centromeres, but past studies have not clearly defined the roles of arm and pericentromeric cohesion in chromosome segregation. To address this issue, we developed a technique that specifically reduced pericentromeric cohesin association on a single chromosome without affecting arm cohesin binding. Under these conditions, we observed more extensive stretching of centromeric chromatin and elevated frequencies of chromosome loss, suggesting that pericentromeric cohesin enrichment is essential for high-fidelity chromosome transmission. The magnitude of pericentromeric cohesin association was negatively correlated with tension between sister kinetochores, with the highest levels of association in cells lacking kinetochore-microtubule attachments. Pericentromeric cohesin recruitment required evolutionarily conserved components of the inner and central kinetochore. Together, these observations suggest that pericentromeric cohesin levels reflect the balance of opposing forces: the kinetochore-mediated enhancement of cohesin binding and the disruption of binding by mechanical tension at kinetochores. The involvement of conserved kinetochore components suggests that this pathway for pericentromeric cohesin enrichment may have been retained in higher eukaryotes to promote chromosome biorientation and accurate sister chromatid segregation.

A Bir1-Sli15 complex connects centromeres to microtubules and is required to sense kinetochore tension

Proper connections between centromeres and spindle microtubules are of critical importance in ensuring accurate segregation of the genome during cell division. Using an in vitro approach based on the sequence-specific budding yeast centromere, we identified a complex of the chromosomal passenger proteins Bir1 and Sli15 (Survivin and INCENP) that links centromeres to microtubules. This linkage does not require Ipl1/Aurora B kinase, whose targeting and activation are controlled by Bir1 and Sli15. Ipl1 is the tension-dependent regulator of centromere-microtubule interactions that ensures chromosome biorientation on the spindle. Elimination of the linkage between centromeres and microtubules mediated by Bir1-Sli15 phenocopies mutations that selectively cripple Ipl1 kinase activation. These findings lead us to propose that the Bir1-Sli15-mediated linkage, which bridges centromeres and microtubules and includes the Aurora kinase-activating domain of INCENP family proteins, is the tension sensor that relays the mechanical state of centromere-microtubule attachments into local control of Ipl1 kinase activity.

The NoCut Pathway Links Completion of Cytokinesis to Spindle Midzone Function to Prevent Chromosome Breakage

During anaphase, spindle elongation pulls sister chromatids apart until each pair is fully separated. In turn, cytokinesis cleaves the cell between the separated chromosomes. What ensures that cytokinesis proceeds only after that all chromosome arms are pulled out of the cleavage plane was unknown. Here, we show that a signaling pathway, which we call NoCut, delays the completion of cytokinesis in cells with spindle-midzone defects. NoCut depends on the Aurora kinase Ipl1 and the anillin-related proteins Boi1 and Boi2, which localize to the site of cleavage in an Ipl1-dependent manner and act as abscission inhibitors. Inactivation of NoCut leads to premature abscission and chromosome breakage by the cytokinetic machinery and is lethal in cells with spindle-elongation defects. We propose that NoCut monitors clearance of chromatin from the midzone to ensure that cytokinesis completes only after all chromosomes have migrated to the poles.

Drosophila Incenp is required for cytokinesis and asymmetric cell division during development of the nervous system

The chromosomal passenger protein complex has emerged as a key player in mitosis, with important roles in chromatin modifications, kinetochore-microtubule interactions, chromosome bi-orientation and stability of the bipolar spindle, mitotic checkpoint function, assembly of the central spindle and cytokinesis. The inner centromere protein (Incenp; a subunit of this complex) is thought to regulate the Aurora B kinase and target it to its substrates. To explore the roles of the passenger complex in a developing multicellular organism, we have performed a genetic screen looking for new alleles and interactors of Drosophila Incenp. We have isolated a new null allele of Incenp that has allowed us for the first time to study the functions of the chromosomal passengers during development. Homozygous incenp(EC3747) embryos show absence of phosphorylation of histone H3 in mitosis, failure of cytokinesis and polyploidy, and defects in peripheral nervous system development. These defects are consistent with depletion of Aurora B kinase activity. In addition, the segregation of the cell-fate determinant Prospero in asymmetric neuroblast division is abnormal, suggesting a role for the chromosomal passenger complex in the regulation of this process.

Phosphorylation of the chromosomal passenger protein Bir1 is required for localization of Ndc10 to the spindle during anaphase and full spindle elongation

The Saccharomyces cerevisiae inhibitor of apoptosis (IAP) repeat protein Bir1 localizes as a chromosomal passenger. A deletion analysis of Bir1 identified two regions important for function. The C-terminal region is essential for growth, binds Sli15, and is necessary and sufficient for the localization of Bir1 as a chromosomal passenger. The middle region is not essential but is required to localize the inner kinetochore protein Ndc10 to the spindle during anaphase and to the midzone at telophase. In contrast, precise deletion of the highly conserved IAP repeats conferred no phenotype and did not alter the cell cycle delay caused by loss of cohesin. Bir1 is phosphorylated in a cell cycle-dependent manner. Mutation of all nine CDK consensus sites in the middle region of Bir1 significantly decreased the level of phosphorylation and blocked localization of Ndc10 to the spindle at anaphase. Moreover, immunoprecipitation of Ndc10 with Bir1 was dependent on phosphorylation. The loss of Ndc10 from the anaphase spindle prevented elongation of the spindle beyond 7 microm. We conclude that phosphorylation of the middle region of Bir1 is required to bring Ndc10 to the spindle at anaphase, which is required for full spindle elongation.

Contractile ring-independent localization of DdINCENP, a protein important for spindle stability and cytokinesis

Dictyostelium DdINCENP is a chromosomal passenger protein associated with centromeres, the spindle midzone, and poles during mitosis and the cleavage furrow during cytokinesis. Disruption of the single DdINCENP gene revealed important roles for this protein in mitosis and cytokinesis. DdINCENP null cells lack a robust spindle midzone and are hypersensitive to microtubule-depolymerizing drugs, suggesting that their spindles may not be stable. Furthermore DdCP224, a protein homologous to the microtubule-stabilizing protein TOGp/XMAP215, was absent from the spindle midzone of DdINCENP null cells. Overexpression of DdCP224 rescued the weak spindle midzone defect of DdINCENP null cells. Although not required for the localization of the myosin II contractile ring and subsequent formation of a cleavage furrow, DdINCENP is important for the abscission of daughter cells at the end of cytokinesis. Finally, we show that the localization of DdINCENP at the cleavage furrow is modulated by myosin II but it occurs by a mechanism different from that controlling the formation of the contractile ring.

A novel role for the CBF3 kinetochore-scaffold complex in regulating septin dynamics and cytokinesis

In budding yeast, the kinetochore scaffold complex centromere binding factor 3 (CBF3) is required to form kinetochores on centromere DNA and to allow proper chromosome segregation. We have previously shown that SKP1 and SGT1 balance the assembly and turnover of CBF3 complexes, a cycle that we suggest is independent of its role in chromosome segregation (Rodrigo-Brenni, M.C., S. Thomas, D.C. Bouck, and K.B. Kaplan. 2004. Mol. Biol. Cell. 15:3366-3378). We provide evidence that this cycle contributes to a second, kinetochore-independent function of CBF3. In this study, we show that inhibiting the assembly of CBF3 causes disorganized septins and defects in cell polarity that give rise to cytokinesis failures. Specifically, we show that septin ring separation and disassembly is delayed in anaphase, suggesting that CBF3 regulates septin dynamics. Only mutations that affect the CBF3 cycle, and not mutants in outer kinetochore subunits, cause defects in septins. These results demonstrate a novel role for CBF3 in regulating cytokinesis, a role that is reminiscent of passenger proteins. Consistent with this possibility, we find that CBF3 interacts with Bir1p, the homologue of the passenger protein Survivin. Mutants in Bir1p similarly affect septin organization, leading us to propose that CBF3 and Bir1p act as passenger proteins to coordinate chromosome segregation with cytokinesis.

The Ipl1-Aurora protein kinase activates the spindle checkpoint by creating unattached kinetochores

The spindle checkpoint ensures accurate chromosome segregation by delaying cell-cycle progression until all sister kinetochores capture microtubules from opposite poles and come under tension (for reviews, see refs 1, 2). Although the checkpoint is activated by either the lack of kinetochore-microtubule attachments or defects in the tension exerted by microtubule-generated forces, it is not clear whether these signals are linked. We investigated the connection between tension and attachment by studying the conserved budding yeast Ipl1Aurora protein kinase that is required for checkpoint activation in the absence of tension but not attachment. Here, we show that spindle-checkpoint activation in kinetochore mutants that seem to have unattached kinetochores depends on Ipl1 activity. When Ipl1 function was impaired in these kinetochore mutants, the attachments were restored and the checkpoint was turned off. These data indicate that Ipl1 activates the checkpoint in response to tension defects by creating unattached kinetochores. Moreover, although the Dam1 kinetochore complex has been implicated as a key downstream target, we found the existence of unidentified Ipl1 sites on Dam1 or additional important substrates that regulate both microtuble detachment and the checkpoint.