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

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.

The Opposing Functions of Protein Kinases and Phosphatases in Chromosome Bipolar Attachment

Accurate chromosome segregation during cell division is essential to maintain genome integrity in all eukaryotic cells, and chromosome missegregation leads to aneuploidy and therefore represents a hallmark of many cancers. Accurate segregation requires sister kinetochores to attach to microtubules emanating from opposite spindle poles, known as bipolar attachment or biorientation. Recent studies have uncovered several mechanisms critical to chromosome bipolar attachment. First, a mechanism exists to ensure that the conformation of sister centromeres is biased toward bipolar attachment. Second, the phosphorylation of some kinetochore proteins destabilizes kinetochore attachment to facilitate error correction, but a protein phosphatase reverses this phosphorylation. Moreover, the activity of the spindle assembly checkpoint is regulated by kinases and phosphatases at the kinetochore, and this checkpoint prevents anaphase entry in response to faulty kinetochore attachment. The fine-tuned kinase/phosphatase balance at kinetochores is crucial for faithful chromosome segregation during both mitosis and meiosis. Here, we discuss the function and regulation of protein phosphatases in the establishment of chromosome bipolar attachment with a focus on the model organism budding yeast.

Aurora B-INCENP Localization at Centromeres/Inner Kinetochores Is Required for Chromosome Bi-orientation in Budding Yeast

For proper chromosome segregation in mitosis, sister kinetochores must interact with microtubules from opposite spindle poles (chromosome bi-orientation) [1, 2]. To promote bi-orientation, Aurora B kinase disrupts aberrant kinetochore-microtubule interactions [3, 4, 5, 6]. It has long been debated how Aurora B halts this action when bi-orientation is established and tension is applied across sister kinetochores. A popular explanation for it is that, upon bi-orientation, sister kinetochores are pulled in opposite directions, stretching the outer kinetochores [7, 8] and moving Aurora B substrates away from Aurora-B-localizing sites at centromeres (spatial separation model) [3, 5, 9]. This model predicts that Aurora B localization at centromeres is required for bi-orientation. However, this notion was challenged by the observation that Bir1 (yeast survivin), which recruits Ipl1-Sli15 (yeast Aurora B-INCENP) to centromeres, can become dispensable for bi-orientation [10]. This raised the possibility that Aurora B localization at centromeres is dispensable for bi-orientation. Alternatively, there might be a Bir1-independent mechanism for recruiting Ipl1-Sli15 to centromeres or inner kinetochores [5, 9]. Here, we show that the COMA inner kinetochore sub-complex physically interacts with Sli15, recruits Ipl1-Sli15 to the inner kinetochore, and promotes chromosome bi-orientation, independently of Bir1, in budding yeast. Moreover, using an engineered recruitment of Ipl1-Sli15 to the inner kinetochore when both Bir1 and COMA are defective, we show that localization of Ipl1-Sli15 at centromeres or inner kinetochores is required for bi-orientation. Our results give important insight into how Aurora B disrupts kinetochore-microtubule interaction in a tension-dependent manner to promote chromosome bi-orientation.

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The COMA inner kinetochore sub-complex facilitates chromosome bi-orientation

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COMA physically interacts with Sli15 and recruits Ipl1-Sli15 to the inner kinetochore

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This function of COMA is independent of Bir1 and its role supporting robust cohesion

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Localizing Ipl1-Sli15 at centromeres/inner kinetochores is crucial for bi-orientation

Outer kinetochore protein Dam1 promotes centromere clustering in parallel with Slk19 in budding yeast

A higher order organization of the centromeres in the form of clustering of these DNA loci has been observed in many organisms. While centromere clustering is biologically significant to achieve faithful chromosome segregation, the underlying molecular mechanism is yet to be fully understood. In budding yeast, a kinetochore-associated protein Slk19 is shown to have a role in clustering in association with the microtubules whereas removal of either Slk19 or microtubules alone does not have any effect on the centromere clustering. Furthermore, Slk19 is non-essential for growth and becomes cleaved during anaphase whereas clustering being an essential event occurs throughout the cell cycle. Hence, we searched for an additional factor involved in the clustering and since the integrity of the kinetochore complex is shown to be crucial for centromere clustering, we restricted our search within the complex. We observed that the outermost kinetochore protein Dam1 promotes centromere clustering through stabilization of the kinetochore integrity. While in the absence of Dam1 we failed to detect Slk19 at the centromere, on the other hand, we found almost no Dam1 at the centromere in the absence of Slk19 and microtubules suggesting interdependency between these two pathways. Strikingly, we observed that overexpression of Dam1 or Slk19 could restore the centromere clustering largely in the cells devoid of Slk19 and microtubules or Dam1, respectively. Thus, we propose that in budding yeast, centromere clustering is achieved at least by two parallel pathways, through Dam1 and another via Slk19, in concert with the microtubules suggesting that having a dual mechanism may be crucial for ensuring microtubule capture by the point centromeres where each attaches to only one microtubule.

A comprehensive, mechanistically detailed, and executable model of the cell division cycle in Saccharomyces cerevisiae

Understanding how cellular functions emerge from the underlying molecular mechanisms is a key challenge in biology. This will require computational models, whose predictive power is expected to increase with coverage and precision of formulation. Genome-scale models revolutionised the metabolic field and made the first whole-cell model possible. However, the lack of genome-scale models of signalling networks blocks the development of eukaryotic whole-cell models. Here, we present a comprehensive mechanistic model of the molecular network that controls the cell division cycle in Saccharomyces cerevisiae. We use rxncon, the reaction-contingency language, to neutralise the scalability issues preventing formulation, visualisation and simulation of signalling networks at the genome-scale. We use parameter-free modelling to validate the network and to predict genotype-to-phenotype relationships down to residue resolution. This mechanistic genome-scale model offers a new perspective on eukaryotic cell cycle control, and opens up for similar models-and eventually whole-cell models-of human cells.

A role for CENP-A/Cse4 phosphorylation on serine 33 in deposition at the centromere

Centromeres are the sites of assembly of the kinetochore, which connect the chromatids to the microtubules for sister chromatid segregation during cell division. Centromeres are characterized by the presence of the histone H3 variant CENP-A (termed Cse4 in Saccharomyces cerevisiae). Here, we investigated the function of serine 33 phosphorylation of Cse4 (Cse4-S33ph) in S. cerevisiae, which lies within the essential N-terminal domain (END) of the extended Cse4 N-terminus. Significantly, we identified histone H4-K5, 8, 12R to cause a temperature-sensitive growth defect with mutations in Cse4-S33 and sensitivity to nocodazole and hydroxyurea. Furthermore, the absence of Cse4-S33ph reduced the levels of Cse4 at centromeric sequences, suggesting that Cse4 deposition is defective in the absence of S33 phosphorylation. We furthermore identified synthetic genetic interactions with histone H2A-E57A and H2A-L66A, which both cause a reduced interaction with the histone chaperone FACT and reduced H2A/H2B levels in chromatin, again supporting the notion that a combined defect of H2A/H2B and Cse4 deposition causes centromeric defects. Altogether, our data highlight the importance of correct histone deposition in building a functional centromeric nucleosome and suggests a role for Cse4-S33ph in this process.

Kinesins relocalize the chromosomal passenger complex to the midzone for spindle disassembly

Mitotic spindle disassembly after chromosome separation is as important as spindle assembly, yet the molecular mechanisms for spindle disassembly are unclear. In this study, we investigated how the chromosomal passenger complex (CPC), which contains the Aurora B kinase Ipl1, swiftly concentrates at the spindle midzone in late anaphase, and we researched the role of this dramatic relocalization during spindle disassembly. We showed that the kinesins Kip1 and Kip3 are essential for CPC relocalization. In cells lacking Kip1 and Kip3, spindle disassembly is severely delayed until after contraction of the cytokinetic ring. Purified Kip1 and Kip3 interact directly with the CPC and recruit it to microtubules in vitro, and single-molecule experiments showed that the CPC diffuses dynamically on microtubules but that diffusion stops when the CPC encounters a Kip1 molecule. We propose that Kip1 and Kip3 trap the CPC at the spindle midzone in late anaphase to ensure timely spindle disassembly.

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 phosphorylation of a kinetochore protein Dam1 by Aurora B/Ipl1 kinase promotes chromosome bipolar attachment in yeast

The interaction between chromosomes and spindle microtubules is essential for chromosome segregation. The kinetochore complex mediates this interaction. Previous studies indicate that the stability of kinetochore attachment is regulated by Aurora B/Ipl1 kinase and this regulation is conserved from yeast to mammalian cells. In budding yeast Saccharomyces cerevisiae, the ten-subunit Dam1/DASH complex bridges the interaction between kinetochores and microtubules, and some in vitro evidence indicates that the phosphorylation of Dam1 protein by Ipl1 kinase destabilizes this interaction. However, it is not clear if Dam1 phosphorylation is sufficient to regulate the stability of kinetochore attachment in vivo. Also, the significance of this regulation in response to chromosome detachment has not been fully investigated. Here we report that phospho-deficient dam1-3A mutants show stabilized kinetochore-microtubule attachment in vivo. This significantly delays the establishment of chromosome bipolar attachment after the disruption of kinetochore-microtubule interaction by a microtubule depolymerizing drug nocodazole. Moreover, dam1-3A cells show dramatic chromosome mis-segregation after treatment with nocodazole, presumably due to the combination of compromised bipolar attachment and premature spindle assembly checkpoint silencing in the mutant cells. Therefore, the regulation of Dam1 phosphorylation imposed by Ipl1 kinase is critical for faithful chromosome segregation.

A Functional Link Between Bir1 and the Saccharomyces cerevisiae Ctf19 Kinetochore Complex Revealed Through Quantitative Fitness Analysis

The chromosomal passenger complex (CPC) is a key regulator of eukaryotic cell division, consisting of the protein kinase Aurora B/Ipl1 in association with its activator (INCENP/Sli15) and two additional proteins (Survivin/Bir1 and Borealin/Nbl1). Here, we report a genome-wide genetic interaction screen in Saccharomyces cerevisiae using the bir1-17 mutant, identifying through quantitative fitness analysis deletion mutations that act as enhancers and suppressors. Gene knockouts affecting the Ctf19 kinetochore complex were identified as the strongest enhancers of bir1-17, while mutations affecting the large ribosomal subunit or the mRNA nonsense-mediated decay pathway caused strong phenotypic suppression. Thus, cells lacking a functional Ctf19 complex become highly dependent on Bir1 function and vice versa. The negative genetic interaction profiles of bir1-17 and the cohesin mutant mcd1-1 showed considerable overlap, underlining the strong functional connection between sister chromatid cohesion and chromosome biorientation. Loss of some Ctf19 components, such as Iml3 or Chl4, impacted differentially on bir1-17 compared with mutations affecting other CPC components: despite the synthetic lethality shown by either iml3∆ or chl4∆ in combination with bir1-17, neither gene knockout showed any genetic interaction with either ipl1-321 or sli15-3 Our data therefore imply a specific functional connection between the Ctf19 complex and Bir1 that is not shared with Ipl1.

Evolutionary dynamics of the kinetochore network in eukaryotes as revealed by comparative genomics

During eukaryotic cell division, the sister chromatids of duplicated chromosomes are pulled apart by microtubules, which connect via kinetochores. The kinetochore is a multiprotein structure that links centromeres to microtubules, and that emits molecular signals in order to safeguard the equal distribution of duplicated chromosomes over daughter cells. Although microtubule‐mediated chromosome segregation is evolutionary conserved, kinetochore compositions seem to have diverged. To systematically inventory kinetochore diversity and to reconstruct its evolution, we determined orthologs of 70 kinetochore proteins in 90 phylogenetically diverse eukaryotes. The resulting ortholog sets imply that the last eukaryotic common ancestor (LECA) possessed a complex kinetochore and highlight that current‐day kinetochores differ substantially. These kinetochores diverged through gene loss, duplication, and, less frequently, invention and displacement. Various kinetochore components co‐evolved with one another, albeit in different manners. These co‐evolutionary patterns improve our understanding of kinetochore function and evolution, which we illustrated with the RZZ complex, TRIP13, the MCC, and some nuclear pore proteins. The extensive diversity of kinetochore compositions in eukaryotes poses numerous questions regarding evolutionary flexibility of essential cellular functions.

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.

The composition, functions, and regulation of the budding yeast kinetochore

The propagation of all organisms depends on the accurate and orderly segregation of chromosomes in mitosis and meiosis. Budding yeast has long served as an outstanding model organism to identify the components and underlying mechanisms that regulate chromosome segregation. This review focuses on the kinetochore, the macromolecular protein complex that assembles on centromeric chromatin and maintains persistent load-bearing attachments to the dynamic tips of spindle microtubules. The kinetochore also serves as a regulatory hub for the spindle checkpoint, ensuring that cell cycle progression is coupled to the achievement of proper microtubule-kinetochore attachments. Progress in understanding the composition and overall architecture of the kinetochore, as well as its properties in making and regulating microtubule attachments and the spindle checkpoint, is discussed.

Phosphorylation of Sli15 by Ipl1 is important for proper CPC localization and chromosome stability in Saccharomyces cerevisiae

The chromosomal passenger complex (CPC) is a key regulator of eukaryotic cell division, consisting of the protein kinase Aurora B/Ipl1 in association with its activator (INCENP/Sli15) and two additional proteins (Survivin/Bir1 and Borealin/Nbl1). Here we have identified multiple sites of CPC autophosphorylation on yeast Sli15 that are located within its central microtubule-binding domain and examined the functional significance of their phosphorylation by Ipl1 through mutation of these sites, either to non-phosphorylatable alanine (sli15-20A) or to acidic residues to mimic constitutive phosphorylation (sli15-20D). Both mutant sli15 alleles confer chromosome instability, but this is mediated neither by changes in the capacity of Sli15 to activate Ipl1 kinase nor by decreased efficiency of chromosome biorientation, a key process in cell division that requires CPC function. Instead, we find that mimicking constitutive phosphorylation of Sli15 on the Ipl1 phosphorylation sites causes delocalization of the CPC in metaphase, whereas blocking phosphorylation of Sli15 on the Ipl1 sites drives excessive localization of Sli15 to the mitotic spindle in pre-anaphase cells. Consistent with these results, direct interaction of Sli15 with microtubules in vitro is greatly reduced either following phosphorylation by Ipl1 or when constitutive phosphorylation at the Ipl1-dependent phosphorylation sites is mimicked by aspartate or glutamate substitutions. Furthermore, we find that mimicking Ipl1 phosphorylation of Sli15 interferes with the ‘tension checkpoint’ – the CPC-dependent mechanism through which cells activate the spindle assembly checkpoint to delay anaphase in the absence of tension on kinetochore-microtubule attachments. Ipl1-dependent phosphorylation of Sli15 therefore inhibits its association with microtubules both in vivo and in vitro and may negatively regulate the tension checkpoint mechanism.

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.

Phosphorylation regulates kinase and microtubule binding activities of the budding yeast chromosomal passenger complex in vitro

The chromosomal passenger complex (CPC) is a key regulator of mitosis in eukaryotes. It comprises four essential and conserved proteins known in mammals/yeasts as Aurora B/Ipl1, INCENP/Sli15, Survivin/Bir1, and Borealin/Nbl1. These subunits act together in a highly controlled fashion. Regulation of Aurora B/Ipl1 kinase activity and localization is critical for CPC function. Although regulation of CPC localization and kinase activity in vivo has been investigated elsewhere, studies on the complete, four-subunit CPC and its basic biochemical properties are only beginning. Here we describe the biochemical characterization of purified and complete Saccharomyces cerevisiae four-subunit CPC. We determined the affinity of the CPC for microtubules and demonstrated that the binding of CPC to microtubules is primarily electrostatic in nature and depends on the acidic C-terminal tail (E-hook) of tubulin. Moreover, phosphorylation of INCENP/Sli15 on its microtubule binding region also negatively regulates CPC affinity for microtubules. Furthermore, we show that phosphorylation of INCENP/Sli15 is required for activation of the kinase Aurora B/Ipl1 and can occur in trans. Although phosphorylation of INCENP/Sli15 is essential for activation, we determined that a version of the CPC lacking the INCENP/Sli15 microtubule binding region (residues Glu-91 to Ile-631) is able to form an intact complex that retains microtubule binding activity. Thus, we conclude that this INCENP/Sli15 linker domain plays a largely regulatory function and is not essential for complex formation or microtubule binding.

The chromosomal passenger complex (CPC): from easy rider to the godfather of mitosis

Successful cell division requires the precise and timely coordination of chromosomal, cytoskeletal and membrane trafficking events. These processes are regulated by the competing actions of protein kinases and phosphatases. Aurora B is one of the most intensively studied kinases. In conjunction with inner centromere protein (INCENP), borealin (also known as Dasra) and survivin it forms the chromosomal passenger complex (CPC). This complex targets to different locations at differing times during mitosis, where it regulates key mitotic events: correction of chromosome-microtubule attachment errors; activation of the spindle assembly checkpoint; and construction and regulation of the contractile apparatus that drives cytokinesis. Our growing understanding of the CPC has seen it develop from a mere passenger riding on the chromosomes to one of the main controllers of mitosis.

Arabidopsis α Aurora kinases function in formative cell division plane orientation

To establish three-dimensional structures/organs, plant cells continuously have to adapt the orientation of their division plane in a highly regulated manner. However, mechanisms underlying switches in division plane orientation remain elusive. Here, we characterize a viable double knockdown mutant in Arabidopsis thaliana group α Aurora (AUR) kinases, AUR1 and AUR2, (aur1-2 aur2-2), with a primary defect in lateral root formation and outgrowth. Mutant analysis revealed that aur1-2 aur2-2 lateral root primordia are built from randomly oriented cell divisions instead of distinct cell layers. This phenotype could be traced back to cytokinesis defects and misoriented cell plates during the initial anticlinal pericycle cell divisions that give rise to lateral root primordia. Complementation assays showed that the Arabidopsis α group Aurora kinases are functionally divergent from the single β group member AUR3 and that AUR1 functions in division plane orientation prior to cytokinesis. In addition to defective lateral root patterning, aur1-2 aur2-2 plants also show defects in orienting formative divisions during embryogenesis, divisions surrounding the main root stem cell niche, and divisions surrounding stomata formation. Taken together, our results put forward a central role for α Aurora kinases in regulating formative division plane orientation throughout development.

Global analysis of core histones reveals nucleosomal surfaces required for chromosome bi-orientation

The attachment of sister kinetochores to microtubules from opposite spindle poles is essential for faithful chromosome segregation. Kinetochore assembly requires centromere-specific nucleosomes containing the histone H3 variant CenH3. However, the functional roles of the canonical histones (H2A, H2B, H3, and H4) in chromosome segregation remain elusive. Using a library of histone point mutants in Saccharomyces cerevisiae, 24 histone residues that conferred sensitivity to the microtubule-depolymerizing drugs thiabendazole (TBZ) and benomyl were identified. Twenty-three of these mutations were clustered at three spatially separated nucleosomal regions designated TBS-I, -II, and -III (TBZ/benomyl-sensitive regions I-III). Elevation of mono-polar attachment induced by prior nocodazole treatment was observed in H2A-I112A (TBS-I), H2A-E57A (TBS-II), and H4-L97A (TBS-III) cells. Severe impairment of the centromere localization of Sgo1, a key modulator of chromosome bi-orientation, occurred in H2A-I112A and H2A-E57A cells. In addition, the pericentromeric localization of Htz1, the histone H2A variant, was impaired in H4-L97A cells. These results suggest that the spatially separated nucleosomal regions, TBS-I and -II, are necessary for Sgo1-mediated chromosome bi-orientation and that TBS-III is required for Htz1 function.

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.

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 evolutionary landscape of the chromatin modification machinery reveals lineage specific gains, expansions, and losses

Model organisms such as yeast, fly, and worm have played a defining role in the study of many biological systems. A significant challenge remains in translating this information to humans. Of critical importance is the ability to differentiate those components where knowledge of function and interactions may be reliably inferred from those that represent lineage-specific innovations. To address this challenge, we use chromatin modification (CM) as a model system for exploring the evolutionary properties of their components in the context of their known functions and interactions. Collating previously identified components of CM from yeast, worm, fly, and human, we identified a "core" set of 50 CM genes displaying consistent orthologous relationships that likely retain their interactions and functions across taxa. In addition, we catalog many components that demonstrate lineage specific expansions and losses, highlighting much duplication within vertebrates that may reflect an expanded repertoire of regulatory mechanisms. Placed in the context of a high-quality protein-protein interaction network, we find, contrary to existing views of evolutionary modularity, that CM complex components display a mosaic of evolutionary histories: a core set of highly conserved genes, together with sets displaying lineage specific innovations. Although focused on CM, this study provides a template for differentiating those genes which are likely to retain their functions and interactions across species. As such, in addition to informing on the evolution of CM as a system, this study provides a set of comparative genomic approaches that can be generally applied to any biological systems.

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.

The life and miracles of kinetochores

Kinetochores are large protein assemblies built on chromosomal loci named centromeres. The main functions of kinetochores can be grouped under four modules. The first module, in the inner kinetochore, contributes a sturdy interface with centromeric chromatin. The second module, the outer kinetochore, contributes a microtubule-binding interface. The third module, the spindle assembly checkpoint, is a feedback control mechanism that monitors the state of kinetochore-microtubule attachment to control the progression of the cell cycle. The fourth module discerns correct from improper attachments, preventing the stabilization of the latter and allowing the selective stabilization of the former. In this review, we discuss how the molecular organization of the four modules allows a dynamic integration of kinetochore-microtubule attachment with the prevention of chromosome segregation errors and cell-cycle progression.

A link between aurora kinase and Clp1/Cdc14 regulation uncovered by the identification of a fission yeast borealin-like protein

The chromosomal passenger complex (CPC) regulates various events in cell division. This complex is composed of a catalytic subunit, Aurora B kinase, and three nonenzymatic subunits, INCENP, Survivin, and Borealin. Together, these four subunits interdependently regulate CPC function, and they are highly conserved among eukaryotes. However, a Borealin homologue has never been characterized in the fission yeast, Schizosaccharomyces pombe. Here, we isolate a previously uncharacterized S. pombe protein through association with the Cdc14 phosphatase homologue, Clp1/Flp1, and identify it as a Borealin-like member of the CPC. Nbl1 (novel Borealin-like 1) physically associates with known CPC components, affects the kinase activity and stability of the S. pombe Aurora B homologue, Ark1, colocalizes with known CPC subunits during mitosis, and shows sequence similarity to human Borealin. Further analysis of the Clp1-Nbl1 interaction indicates that Clp1 requires CPC activity for proper accumulation at the contractile ring (CR). Consistent with this, we describe negative genetic interactions between mutant alleles of CPC and CR components. Thus, this study characterizes a fission yeast Borealin homologue and reveals a previously unrecognized connection between the CPC and the process of cytokinesis in S. pombe.

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.