Analysis of mitotic phosphorylation of borealin

Small-molecule anticancer agents kill cancer cells by harnessing reactive oxygen species in an iron-dependent manner

In the course of generating a library of open-chain epothilones, we discovered a new class of small molecule anticancer agents that has no effect on tubulin but instead kills selected cancer cell lines by harnessing reactive oxygen species in an iron-dependent manner. Results of the preliminary studies are consistent with the recently described cell death mechanism ferroptosis. Studies are in progress to confirm ferroptosis as the cell death mechanism and to identify the specific molecular targets of these small molecule anticancer agents.

PKCɛ switches Aurora B specificity to exit the abscission checkpoint

The 'NoCut', or Aurora B abscission checkpoint can be activated if DNA is retained in the cleavage furrow after completion of anaphase. Checkpoint failure leads to incomplete abscission and a binucleate outcome. These phenotypes are also observed after loss of PKCɛ in transformed cell models. Here we show that PKCɛ directly modulates the Aurora B-dependent abscission checkpoint by phosphorylating Aurora B at S227. This phosphorylation invokes a switch in Aurora B specificity, with increased phosphorylation of a subset of target substrates, including the CPC subunit Borealin. This switch is essential for abscission checkpoint exit. Preventing the phosphorylation of Borealin leads to abscission failure, as does expression of a non-phosphorylatable Aurora B S227A mutant. Further, depletion of the ESCRT-III component and Aurora B substrate CHMP4C enables abscission, bypassing the PKCɛ-Aurora B exit pathway. Thus, we demonstrate that PKCɛ signals through Aurora B to exit the abscission checkpoint and complete cell division.

Borealin dimerization mediates optimal CPC checkpoint function by enhancing localization to centromeres and kinetochores

The Chromosomal Passenger Complex (CPC) localizes to centromeres where it activates the mitotic checkpoint in response to inappropriate inter-kinetochore tension. This error correction function is essential for proper chromosome segregation. Here we define several critical features of CPC localization and function. First, the Borealin dimerization domain suppresses dynamic exchange at the centromere to allow optimal CPC function. Second, Borealin dimerization is essential to target a subpopulation of CPC proximal to the kinetochore when the mitotic spindle is disrupted. This subpopulation is also needed for full CPC checkpoint function. The existence of a pool of CPC at the kinetochore suggests that error correction is more complicated than predicted from the Aurora B phosphorylation gradient model. Finally, Haspin kinase plays a key role in maintaining the slowly exchanging centromere Borealin pool, while Aurora B and Mps1 play minimal roles in maintaining CPC localization once cells are in mitosis.

A novel big protein TPRBK possessing 25 units of TPR motif is essential for the progress of mitosis and cytokinesis

Through the comprehensive analysis of the genomic DNA sequence of human chromosome 22, we identified a novel gene of 702 kb encoding a big protein of 2481 amino acid residues, and named it as TPRBK (TPR containing big gene cloned at Keio). A novel protein TPRBK possesses 25 units of the TPR motif, which has been known to associate with a diverse range of biological functions. Orthologous genes of human TPRBK were found widely in animal species, from insecta to mammal, but not found in plants, fungi and nematoda. Northern blotting and RT-PCR analyses revealed that TPRBK gene is expressed ubiquitously in the human and mouse fetal tissues and various cell lines of human, monkey and mouse. Immunofluorescent staining of the synchronized monkey COS-7 cells with several relevant antibodies indicated that TPRBK changes its subcellular localization during the cell cycle: at interphase TPRBK locates on the centrosomes, during mitosis it translocates from spindle poles to mitotic spindles then to spindle midzone, and through a period of cytokinesis it stays on the midbody. Co-immunoprecipitation assay and immunofluorescent staining with adequate antibodies revealed that TPRBK binds to Aurora B, and those proteins together translocate throughout mitosis and cytokinesis. Treatments of cells with two drugs (Blebbistatin and Y-27632), that are known to inhibit the contractility of actin-myosin, disturbed the proper intracellular localization of TPRBK. Moreover, the knockdown of TPRBK expression by small interfering RNA (siRNA) suppressed the bundling of spindle midzone microtubules and disrupted the midbody formation, arresting the cells at G(2)+M phase. These observations indicated that a novel big protein TPRBK is essential for the formation and integrity of the midbody, hence we postulated that TPRBK plays a critical role in the progress of mitosis and cytokinesis during mammalian cell cycle.

Effects of phosphatase and proteasome inhibitors on Borealin phosphorylation and degradation

The chromosomal passenger complex (CPC) senses tension defects at the kinetochore to activate the spindle assembly checkpoint, and helps to position the cleavage furrow. The CPC, consisting of INCENP, Survivin, Borealin and Aurora B localizes to the inner centromere at metaphase and re-localizes to the spindle midzone at anaphase; several CPC functions are regulated by post-translational modification. Borealin is phosphorylated at multiple sites and phosphorylation at S219 causes Borealin to migrate more slowly upon electrophoresis. Here we find that Cdk1 can induce a mobility shift of Borealin, suggesting that S219 phosphorylation is under Cdk1 control. However, Cdk1 is inefficient at phosphorylating purified Borealin in vitro. A yeast orthologue of Borealin, Npl1, is dephosphorylated by the phosphatase Cdc14. We find no difference in the mobility shift of Borealin in human cells lacking either Cdc14A or Cdc14B. In contrast, the phosphatase inhibitor okadaic acid does delay the dephosphorylation of Borealin as cells exit mitosis. The proteasome inhibitor MG132 reduces Borealin phosphorylation in mitosis and increases the steady-state level of Borealin, especially in mutants lacking the C-terminus. However, a second, structurally unrelated proteasome inhibitor, lactacystin did not up-regulate Borealin. These results suggest that the effect of MG132 on Borealin is due to the inhibition of an intracellular protease other than the proteasome.

[Structure, localizations and functions of chromosomal passenger complex]

Chromosomal passenger complex (CPC) is mainly composed of a protein kinase Aurora B, inner centromere protein, Survivin, and Borealin. As in different periods of cell division, CPC can localise correctly to relavent destinations in time and interact on its different substrates in the mitotic cell. CPC modulats phosphorylation of histone H3 in chromatin aggregation and controlls cohesion or segregation of sister chromatids. It is involved in assembly of a mitotic spindle and its chromosomes catched. Moreover, CPC corrects attachment errors between kinetochore and microtuble and gives faithfuly chromosomal segregation and promoting cytokinesis. Here, the structure compenants, localization on chromosomal arms, inner centromere and central spindle, and functions in defferent posisions of CPC were briefly descriped.

Regulation of Borealin by phosphorylation at serine 219

The chromosomal passenger complex consisting of Borealin, INCENP, Survivin, and Aurora B follows a dynamic pattern of localization to perform its role as a regulator of chromosome alignment, aspects of the spindle assembly checkpoint, and cytokinesis. Post-translational modifications of chromosomal passenger proteins play an important role in regulating the localization and function of the complex. Borealin displays a slower electrophoretic mobility during mitosis as a result of phosphorylation. Here we show that phosphorylation at S219 is responsible for this mobility shift. An S219A mutant of Borealin that cannot be phosphorylated at this site displays a defect in centromere localization that is evident in cells arrested in mitosis with nocodazole. Further, the S219A form of Borealin is unable to efficiently rescue mitotic defects that occur upon knock-down of the endogenous protein. These defects are correlated with a reduction in the intensity of Mad2 staining at kinetochores in cells expressing the S219A form of Borealin. These results highlight an important role for phosphorylation of Borealin at S219 in the proper progression through mitosis.

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.

Length of mitotic arrest induced by microtubule-stabilizing drugs determines cell death after mitotic exit

Cell death induced by agents that disrupt microtubules can kill cells by inducing a prolonged mitotic block. This mitotic block is dependent on the spindle assembly checkpoint, a surveillance system that ensures the bipolar attachment of chromosomes to the mitotic spindle before the onset of anaphase. Under some conditions, the spindle assembly checkpoint can become weakened, allowing cells to exit mitosis despite the presence of chromosomes that are not properly attached to the mitotic spindle. Here, we use an Aurora kinase inhibitor to drive mitotic exit and test the effect of mitotic arrest length on death in the subsequent interphase. Cells that are blocked in mitosis for >15 h die shortly after exiting from mitosis, whereas cells that exit after being blocked for <15 h show variable fates, with some living for days after exiting mitosis. Cells blocked in mitosis by either Taxol or epothilone B are acutely sensitive to the death ligand tumor necrosis factor-related apoptosis-inducing ligand, suggesting that prolonged mitosis allows the gradual accumulation of internal death signals, rendering cells hypersensitive to additional prodeath cues. Death under these conditions is initiated while cyclin B1 is still present, indicating that cells are in mitosis. Our experiments suggest that there is a point of no return during prolonged mitotic block after which mitotic exit can no longer block death.

Borealin is repressed in response to p53/Rb signaling

Rb/E2F regulates many genes that encode proteins required for the cell cycle. Using affymetrix microarrays we previously identified genes regulated by the Rb proteins p130 and p107, many of which are involved in the cell cycle. Several genes with unknown functions were also repressed by p130 and p107, of which some have recently been found to have various roles in mitosis, the spindle checkpoint and cytokinesis. This study focuses on the regulation of borealin/dasra/cdca8, which encodes a recently discovered member of the chromosomal passenger complex. It is recorded that borealin is a cell cycle regulator, down-regulated in response to p53/Rb-signaling, and up-regulated in many types of cancerous tissues.