Loading of the 3F3/2 antigen onto kinetochores is dependent on the ordered assembly of the spindle checkpoint proteins

Kinetochore function is controlled by a phospho-dependent coexpansion of inner and outer components

It is widely accepted that the kinetochore is built on CENP-A-marked centromeric chromatin in a hierarchical order from inner to outer kinetochore. Recruitment of many kinetochore proteins depends on microtubule attachment status, but it remains unclear how their assembly/disassembly is orchestrated. Applying 3D structured illumination microscopy to Xenopus laevis egg extracts, here we reveal that in the absence of microtubule attachment, proteins responsible for lateral attachment and spindle checkpoint signaling expand to form micrometer-scale fibrous structures over CENP-A-free chromatin, whereas a core module responsible for end-on attachment (CENP-A, CENP-T, and Ndc80) does not. Both outer kinetochore proteins (Bub1, BubR1, Mad1, and CENP-E) and the inner kinetochore component CENP-C are integral components of the expandable module, whose assembly depends on multiple mitotic kinases (Aurora B, Mps1, and Plx1) and is suppressed by protein phosphatase 1. We propose that phospho-dependent coexpansion of CENP-C and outer kinetochore proteins promotes checkpoint signal amplification and lateral attachment, whereas their selective disassembly enables the transition to end-on attachment.

Spatial regulation of the spindle assembly checkpoint and anaphase-promoting complex in Aspergillus nidulans

The spindle assembly checkpoint (SAC) plays a critical role in preventing mitotic errors by inhibiting anaphase until all kinetochores are correctly attached to spindle microtubules. In spite of the economic and medical importance of filamentous fungi, relatively little is known about the behavior of SAC proteins in these organisms. In our efforts to understand the role of γ-tubulin in cell cycle regulation, we have created functional fluorescent protein fusions of four SAC proteins in Aspergillus nidulans, the homologs of Mad2, Mps1, Bub1/BubR1 and Bub3. Time-lapse imaging reveals that SAC proteins are in distinct compartments of the cell until early mitosis when they co-localize at the spindle pole body. SAC activity is, thus, spatially regulated in A. nidulans. Likewise, Cdc20, an activator of the anaphase-promoting complex/cyclosome, is excluded from interphase nuclei, but enters nuclei at mitotic onset and accumulates to a higher level in mitotic nuclei than in the surrounding nucleoplasm before leaving in anaphase/telophase. The activity of this critical cell cycle regulatory complex is likely regulated by the location of Cdc20. Finally, the γ-tubulin mutation mipAD159 causes a nuclear-specific failure of nuclear localization of Mps1 and Bub1/R1 but not of Cdc20, Bub3 or Mad2.

All together now: Polo joins the kinase network controlling the spindle assembly checkpoint in Drosophila

Maintenance of genomic stability during eukaryotic cell division relies on the Spindle Assembly Checkpoint (SAC), which has evolved as a surveillance mechanism that monitors kinetochore-microtubule attachment and prevents APC/C-mediated mitotic exit until all chromosomes are properly attached to the mitotic spindle. Reversible protein phosphorylation has long been accredited as a regulatory mechanism of the SAC. Nevertheless, knowledge of how several mitotic kinases act in concert within the signaling pathway to orchestrate SAC function is still emerging. In a recent study, we undertook a comprehensive dissection of the hierarchical framework controlling SAC function in Drosophila cells. We found that Polo lies at the top of the SAC pathway promoting the efficient recruitment of Mps1 to unattached kinetochores. This renders Mps1 fully active to control BubR1 phosphorylation that generates the 3F3/2 phosphoepitope at tensionless kinetochores. We have proposed that Polo is required for SAC function and that the molecular outcome of Mps1-dependent 3F3/2 formation is to promote the association of Cdc20 with BubR1 allowing proper kinetochore recruitment of Cdc20 and efficient assembly of the Mitotic Checkpoint Complex (MCC) required for a sustained SAC response.

Drosophila Polo regulates the spindle assembly checkpoint through Mps1-dependent BubR1 phosphorylation

Maintenance of genomic stability during eukaryotic cell division relies on the spindle assembly checkpoint (SAC) that prevents mitotic exit until all chromosomes are properly attached to the spindle. Polo is a mitotic kinase proposed to be involved in SAC function, but its role has remained elusive. We demonstrate that Polo and Aurora B functional interdependency comprises a positive feedback loop that promotes Mps1 kinetochore localization and activity. Expression of constitutively active Polo restores normal Mps1 kinetochore levels even after Aurora B inhibition, highlighting a role for Polo in Mps1 recruitment to unattached kinetochores downstream of Aurora B. We also show that Mps1 kinetochore localization is required for BubR1 hyperphosphorylation and formation of the 3F3/2 phosphoepitope. This is essential to allow recruitment of Cdc20 to unattached kinetochores and the assembly of anaphase-promoting complex/cyclosome-inhibitory complexes to levels that ensure long-term SAC activity. We propose a model in which Polo controls Mps1-dependent BubR1 phosphorylation to promote Cdc20 kinetochore recruitment and sustained SAC function.

Bub1 kinase activity drives error correction and mitotic checkpoint control but not tumor suppression

Mice expressing a version of Bub1 that lacks kinase activity have increased chromosome segregation errors and aneuploidy but not increased susceptibility to tumors.

The mitotic checkpoint protein Bub1 is essential for embryogenesis and survival of proliferating cells, and bidirectional deviations from its normal level of expression cause chromosome missegregation, aneuploidy, and cancer predisposition in mice. To provide insight into the physiological significance of this critical mitotic regulator at a modular level, we generated Bub1 mutant mice that lack kinase activity using a knockin gene-targeting approach that preserves normal protein abundance. In this paper, we uncover that Bub1 kinase activity integrates attachment error correction and mitotic checkpoint signaling by controlling the localization and activity of Aurora B kinase through phosphorylation of histone H2A at threonine 121. Strikingly, despite substantial chromosome segregation errors and aneuploidization, mice deficient for Bub1 kinase activity do not exhibit increased susceptibility to spontaneous or carcinogen-induced tumorigenesis. These findings provide a unique example of a modular mitotic activity orchestrating two distinct networks that safeguard against whole chromosome instability and reveal the differential importance of distinct aneuploidy-causing Bub1 defects in tumor suppression.

Spindle checkpoint-independent inhibition of mitotic chromosome segregation by Drosophila Mps1

Monopolar spindle 1 (Mps1) is essential for the spindle assembly checkpoint (SAC), which prevents anaphase onset in the presence of misaligned chromosomes. Moreover, Mps1 kinase contributes in a SAC-independent manner to the correction of erroneous initial attachments of chromosomes to the spindle. Our characterization of the Drosophila homologue reveals yet another SAC-independent role. As in yeast, modest overexpression of Drosophila Mps1 is sufficient to delay progression through mitosis during metaphase, even though chromosome congression and metaphase alignment do not appear to be affected. This delay in metaphase depends on the SAC component Mad2. Although Mps1 overexpression in mad2 mutants no longer causes a metaphase delay, it perturbs anaphase. Sister kinetochores barely move apart toward spindle poles. However, kinetochore movements can be restored experimentally by separase-independent resolution of sister chromatid cohesion. We propose therefore that Mps1 inhibits sister chromatid separation in a SAC-independent manner. Moreover, we report unexpected results concerning the requirement of Mps1 dimerization and kinase activity for its kinetochore localization in Drosophila. These findings further expand Mps1's significance for faithful mitotic chromosome segregation and emphasize the importance of its careful regulation.

Plo1 phosphorylates Dam1 to promote chromosome bi-orientation in fission yeast

The fungal-specific heterodecameric outer kinetochore DASH complex facilitates the interaction of kinetochores with spindle microtubules. In budding yeast, where kinetochores bind a single microtubule, the DASH complex is essential, and phosphorylation of Dam1 by the Aurora kinase homologue, Ipl1, causes detachment of kinetochores from spindle microtubules. We demonstrate that in the distantly related fission yeast, where the DASH complex is not essential for viability and kinetochores bind multiple microtubules, Dam1 is instead phosphorylated on serine 143 by the Polo kinase homologue, Plo1, during prometaphase and metaphase. This phosphorylation site is conserved in most fungal Dam1 proteins, including budding yeast Dam1. We show that Dam1 phosphorylation by Plo1 is dispensable for DASH assembly and chromosome retrieval but instead aids tension-dependent chromosome bi-orientation.

Sgo1 establishes the centromeric cohesion protection mechanism in G2 before subsequent Bub1-dependent recruitment in mitosis

Bub1 was one of the first protein kinases identified as a component of the spindle-assembly checkpoint, a surveillance mechanism that delays anaphase onset until all chromosomes are stably attached to spindle microtubules. Whereas the kinase activity of Bub1 is not required for checkpoint function in yeast, its requirement in mammalian cells is still unclear. Using a complementation assay with bona fide BUB1-null mouse embryonic fibroblasts, we show that the kinase activity of Bub1 is not required for checkpoint function or chromosome alignment. Its activity is, however, required for centromeric localisation of Sgo1, a known protector of centromeric cohesion. Despite the absence of Sgo1 from mitotic centromeres in cells devoid of Bub1 activity, centromeric cohesion is still maintained until anaphase. An explanation for this comes from observations showing that Sgo1 is first recruited to centromeric heterochromatin in G2, but then becomes diffusely localised throughout the nucleus in early prophase, before returning to centromeres later in prophase. Importantly, whereas centromeric localisation of Sgo1 in prophase is dependent on the kinase activity of Bub1, its recruitment to centromeric heterochromatin in G2 is not. Rather, the localisation of Sgo1 in G2 is abolished when heterochromatin protein 1 is not bound to centromeric heterochromatin. Thus, it seems that Sgo1 sets up the centromeric protection mechanism in G2, but that its Bub1-dependent localisation to centromeres during mitosis is not required to maintain cohesion.

The crystal structure of the N-terminal region of BUB1 provides insight into the mechanism of BUB1 recruitment to kinetochores

The interaction of the central mitotic checkpoint component BUB1 with the mitotic kinetochore protein Blinkin is required for the kinetochore localization and function of BUB1 in the mitotic spindle assembly checkpoint, the regulatory mechanism of the cell cycle that ensures the even distribution of chromosomes during the transition from metaphase to anaphase. Here, we report the 1.74 Å resolution crystal structure of the N-terminal region of BUB1. The structure is organized as a tandem arrangement of three divergent units of the tetratricopeptide motif. Functional assays in vivo of native and site-specific mutants identify the residues of human BUB1 important for the interaction with Blinkin and define one region of potential therapeutic interest. The structure provides insight into the molecular basis of Blinkin-specific recognition by BUB1 and, on a broader perspective, of the mechanism that mediates kinetochore localization of BUB1 in checkpoint-activated cells.

Requirements for protein phosphorylation and the kinase activity of polo-like kinase 1 (Plk1) for the kinetochore function of mitotic arrest deficiency protein 1 (Mad1)

Mitotic arrest deficiency protein 1 (Mad1) is associated with microtubule-unattached kinetochores in mitotic cells and is a component of the spindle assembly checkpoint (SAC). Here, we have studied the phosphorylation of Mad1 and mapped using liquid chromatography-tandem mass spectrometry several phosphorylated amino acids in this protein. One phosphorylated residue, Thr680, was characterized to be important for the kinetochore localization of Mad1 and its SAC function. We also found that in mitotic cells Mad1 co-immunoprecipitated with Plk1. Depletion of cellular Plk1 using small interfering RNAs and inhibition of the kinase activity of Plk1 using a kinase-dead mutant or a small molecule inhibitor attenuated Mad1 phosphorylation and its association with kinetochores. Collectively, these findings indicate mechanistic roles contributed by protein phosphorylation and Plk1 to the SAC activity of Mad1.

Mps1 kinase activity restrains anaphase during an unperturbed mitosis and targets Mad2 to kinetochores

Mps1 is an upstream component of the spindle assembly checkpoint, which, in human cells, is required for checkpoint activation in response to spindle damage but not apparently during an unperturbed mitosis. Mps1 also recruits Mad1 and Mad2 to kinetochores. However, whether the enzymatic activity of Mps1 is required for these processes is unclear. To address this question, we established an RNA interference (RNAi) complementation assay. Repression of Mps1 triggers premature anaphase, often with unaligned or maloriented chromosomes. This phenotype is rescued by an RNAi-resistant wild-type Mps1 transgene but not by a catalytically inactive mutant. An analogue-sensitive allele, Mps1^M602A, also rescues the RNAi-induced defect, but not when inhibited by the adenosine triphosphate analogue 1-NM-PP1. Thus, Mps1 activity does restrain anaphase during an unperturbed mitosis. Furthermore, although catalytically inactive Mps1 can restore kinetochore localization of Mad1, only the active kinase restores Mad2 localization. Thus, in human cells, Mps1 catalytic activity is required for spindle checkpoint function and recruitment of Mad2.

Cdk1 phosphorylation of BubR1 controls spindle checkpoint arrest and Plk1-mediated formation of the 3F3/2 epitope

Accurate chromosome segregation is controlled by the spindle checkpoint, which senses kinetochore- microtubule attachments and tension across sister kinetochores. An important step in the tension-signaling pathway involves the phosphorylation of an unknown protein by polo-like kinase 1/Xenopus laevis polo-like kinase 1 (Plx1) on kinetochores lacking tension to generate the 3F3/2 phosphoepitope. We report here that the checkpoint protein BubR1 interacts with Plx1 and that phosphorylation of BubR1 by Plx1 generates the 3F3/2 epitope. Formation of the BubR1 3F3/2 epitope by Plx1 requires a prior phosphorylation of BubR1 on Thr 605 by cyclin-dependant kinase 1 (Cdk1). This priming phosphorylation of BubR1 by Cdk1 is required for checkpoint-mediated mitotic arrest and for recruitment of Plx1 and the checkpoint protein Mad2 to unattached kinetochores. Biochemically, formation of the 3F3/2 phosphoepitope by Cdk1 and Plx1 greatly enhances the kinase activity of BubR1. Thus, Cdk1-mediated phosphorylation of BubR1 controls checkpoint arrest and promotes the formation of the kinetochore 3F3/2 epitope.

Tension-sensitive Plk1 phosphorylation on BubR1 regulates the stability of kinetochore microtubule interactions

Mitotic phosphorylation of the spindle checkpoint component BubR1 is highly conserved throughout evolution. Here, we demonstrate that BubR1 is phosphorylated on the Cdk1 site T620, which triggers the recruitment of Plk1 and phosphorylation of BubR1 by Plk1 both in vitro and in vivo. Phosphorylation does not appear to be required for spindle checkpoint function but instead is important for the stability of kinetochore-microtubule (KT-MT) interactions, timely mitotic progression, and chromosome alignment onto the metaphase plate. By quantitative mass spectrometry, we identify S676 as a Plk1-specific phosphorylation site on BubR1. Furthermore, using a phospho-specific antibody, we show that this site is phosphorylated during prometaphase, but dephosphorylated at metaphase upon establishment of tension between sister chromatids. These findings describe the first in vivo verified phosphorylation site for human BubR1, identify Plk1 as the kinase responsible for causing the characteristic mitotic BubR1 upshift, and attribute a KT-specific function to the hyperphosphorylated form of BubR1 in the stabilization of KT-MT interactions.

BubR1 and APC/EB1 cooperate to maintain metaphase chromosome alignment

The accurate segregation of chromosomes in mitosis requires the stable attachment of microtubules to kinetochores. The details of this complex and dynamic process are poorly understood. In this study, we report the interaction of a kinetochore-associated mitotic checkpoint kinase, BubR1, with two microtubule plus end–associated proteins, adenomatous polyposis coli (APC) and EB1, providing a potential link in stable kinetochore microtubule attachment. Using immunodepletion from and antibody addition to Xenopus laevis egg extracts, we show that BubR1 and its kinase activity are essential for positioning chromosomes at the metaphase plate. BubR1 associates with APC and EB1 in egg extracts, and the complex formation is necessary for metaphase chromosome alignment. Using purified components, BubR1 directly phosphorylates APC and forms a ternary complex with APC and microtubules. These findings support a model in which BubR1 kinase may directly regulate APC function involved in stable kinetochore microtubule attachment.

Bub1 is essential for assembly of the functional inner centromere

During mitosis, the inner centromeric region (ICR) recruits protein complexes that regulate sister chromatid cohesion, monitor tension, and modulate microtubule attachment. Biochemical pathways that govern formation of the inner centromere remain elusive. The kinetochore protein Bub1 was shown to promote assembly of the outer kinetochore components, such as BubR1 and CENP-F, on centromeres. Bub1 was also implicated in targeting of Shugoshin (Sgo) to the ICR. We show that Bub1 works as a master organizer of the ICR. Depletion of Bub1 from Xenopus laevis egg extract or from HeLa cells resulted in both destabilization and displacement of chromosomal passenger complex (CPC) from the ICR. Moreover, soluble Bub1 controls the binding of Sgo to chromatin, whereas the CPC restricts loading of Sgo specifically onto centromeres. We further provide evidence that Bub1 kinase activity is pivotal for recruitment of all of these components. Together, our findings demonstrate that Bub1 acts at multiple points to assure the correct kinetochore formation.