The Aurora B kinase complex is a critical regulator of chromosome segregation and cytokinesis. In Caenorhabditis elegans, AIR-2 (Aurora B) function requires ICP-1 (Incenp see Drosophila Incenp) and BIR-1 (Survivin). In various systems, Aurora B binds to orthologues of these proteins. Through genetic analysis, a new subunit of the Aurora B kinase complex, CSC-1, an ortholog of Borealin/Dasra, has been identified. C. elegans embryos depleted of CSC-1, AIR-2, ICP-1, or BIR-1 have identical phenotypes. CSC-1, BIR-1, and ICP-1 are interdependent for their localization, and all are required for AIR-2 localization. In vitro, CSC-1 binds directly to BIR-1. The CSC-1/BIR-1 complex, but not the individual subunits, associates with ICP-1. CSC-1 associates with ICP-1, BIR-1, and AIR-2 in vivo. ICP-1 dramatically stimulates AIR-2 kinase activity. This activity is not stimulated by CSC-1/BIR-1, suggesting that these two subunits function as targeting subunits for AIR-2 kinase (Romano, 2003).

In cells lacking centrosomes, such as those found in female meiosis, chromosomes must nucleate and stabilize microtubules in order to form a bipolar spindle. This study reports the identification of Dasra A and Dasra B, two new components of the vertebrate chromosomal passenger complex containing Incenp, Survivin, and the kinase Aurora B; this complex is required for chromatin-induced microtubule stabilization and spindle formation. The failure of microtubule stabilization caused by depletion of the chromosomal passenger complex was rescued by codepletion of the microtubule-depolymerizing kinesin MCAK, whose activity is negatively regulated by Aurora B. By contrast, evidence is presented that the Ran-GTP pathway of chromatin-induced microtubule nucleation does not require the chromosomal passenger complex, indicating that the mechanisms of microtubule assembly by these two pathways are distinct. It is proposed that the chromosomal passenger complex regulates local MCAK activity to permit spindle formation via stabilization of chromatin-associated microtubules (Sampath, 2004).

The chromosomal passenger complex of Aurora B kinase, INCENP, and Survivin has essential regulatory roles at centromeres and the central spindle in mitosis. Borealin has been identified as a novel member of the complex. Approximately half of Aurora B in mitotic cells is complexed with INCENP, Borealin, and Survivin; and Borealin binds Survivin and INCENP in vitro. A second complex contains Aurora B and INCENP, but no Borealin or Survivin. Depletion of Borealin by RNA interference delays mitotic progression and results in kinetochore-spindle misattachments and an increase in bipolar spindles associated with ectopic asters. The extra poles, which apparently form after chromosomes achieve a bipolar orientation, severely disrupt the partitioning of chromosomes in anaphase. Borealin depletion has little effect on histone H3 serine10 phosphorylation. These results implicate the chromosomal passenger holocomplex in the maintenance of spindle integrity and suggest that histone H3 serine10 phosphorylation is performed by an Aurora B-INCENP subcomplex (Gassmann, 2004).

Survivin is a component of the chromosomal passenger complex (CPC) that plays a role in maintenance of an active spindle checkpoint and in cytokinesis. To study whether these different functions can be attributed to distinct domains within the Survivin protein, Survivin-depleted cells were complemented with a variety of point- and deletion-mutants of Survivin. An intact baculovirus IAP repeat (BIR) domain is required for proper spindle checkpoint functioning, but dispensable for cytokinesis. In line with this, mutants lacking an intact BIR domain localized normally to the central spindle, but their localization to inner centromeres was severely perturbed. Consequently, these mutants failed to recruit Aurora B, Borealin/Dasra B, and BubR1 to centromeres and kinetochores, but they had retained the ability to recruit Aurora B and Borealin/Dasra B to the midzone and midbody. Thus, the C terminus of Survivin is sufficient for central spindle localization and execution of cytokinesis, but the additional presence of a functional BIR domain is essential for centromere targeting and spindle checkpoint function. Importantly, the data show that the function of the CPC at the centromere can be separated from its function at the central spindle and that execution of cytokinesis does not require prior concentration of the CPC at centromeres (Lens, 2006).

As mitosis progresses, the chromosomal passenger proteins (CPPs) Survivin, Aurora B, INCENP and Borealin dynamically colocalize to mitotic structures. Chromosomal passenger proteins are already expressed during G2, whereas the nuclear envelope is only disassembled at the end of prophase. However, the mechanisms that modulate their nucleocytoplasmic localization before nuclear envelope breakdown (NEB) are poorly characterized. Using epitope-tagged proteins, this study shows that Aurora B, like Survivin, undergoes CRM1-mediated nucleocytoplasmic shuttling, although both proteins lack identifiable 'classical' nuclear transport signals. In contrast, INCENP resides more stably in the nucleus and contains multiple nuclear localization signals. Finally, Borealin was detected in the nucleolus and the cytoplasm, but its cytoplasmic localization is not directly regulated by CRM1. Coexpression experiments indicate that the nuclear localization of Aurora B, but not of Survivin, is modulated by INCENP and that Survivin prevents the nucleolar accumulation of Borealin. These data reveal that, in contrast to their closely related localization during mitosis, the nucleocytoplasmic localization of the CPPs before NEB is largely unrelated. Furthermore, the specific effect of INCENP and Survivin on the localization of Aurora B and Borealin, respectively, suggests that different complexes of CPPs may exist not only during mitosis, as recently reported, but also before NEB (Rodriguez, 2006).

The chromosomal passenger complex (CPC) coordinates chromosomal and cytoskeletal events of mitosis. The enzymatic core of this complex (Aurora-B) is guided through the mitotic cell by its companion chromosomal passenger proteins, inner centromere protein (INCENP), Survivin and Borealin/Dasra-B, thereby allowing it to act at the right place at the right time. This study addresses the individual contributions of INCENP, Survivin and Borealin to the proper functioning of this complex. INCENP has an important role in stabilizing the complex, and Borealin acts to promote binding of Survivin to INCENP. Importantly, when Survivin is directly fused to INCENP, this hybrid can restore CPC function at the centromeres and midbody, even in the absence of Borealin and the centromere-targeting domain of INCENP. Thus, Survivin is an important mediator of centromere and midbody docking of Aurora-B during mitosis (Vader, 2006).

Chromosomal passenger proteins including Aurora B, Survivin, and Borealin/Dasra B, also called CDCA8/FLJ10468, are known to play crucial roles during mitosis and cell division. Inappropriate chromosomal segregation and cell division may cause auneuploidy leading to cancer. However, it is still unclear how the expression of chromosomal passenger proteins may be linked to cancer. In this study, it has been demonstrated that Borealin is a cell cycle-regulated gene and is upregulated at G2-M phases of the cell cycle. Borealin interacts with Survivin but not with Aurora B. The interaction domain of Survivin in Borealin was mapped to the N-terminal 92 amino-acid residues of Borealin. To examine the linkage between expression of Borealin and cancer, immunohistochemistry analysis was performed using anti-Borealin specific antibody on the paraffin-embedded gastric cancer tissues. The results showed that Borealin expression is significantly correlated with Survivin (P = 0.003) and Ki67 (P = 0.007) in gastric cancer. Interestingly, an increased nuclear Borealin level reveals borderline association with a poor survival rate (P = 0.047). Taken together, these results demonstrated that Borealin is a cell cycle-regulated chromosomal passenger protein and its aberrant expression is linked to a poor prognosis for gastric cancer (Chang, 2006).

Structure of a Survivin-Borealin-INCENP core complex reveals how chromosomal passengers travel together

The chromosomal passenger complex (CPC) is a key regulator of chromosome segregation and cytokinesis. CPC functions are connected to its localization. The complex first localizes to centromeres and later associates with the central spindle and midbody. Survivin, Borealin, and INCENP are the three components of the CPC that regulate the activity and localization of its enzymatic component, the kinase Aurora B. This study determined the 1.4 Å resolution crystal structure of the regulatory core of the CPC, revealing that Borealin and INCENP associate with the helical domain of Survivin to form a tight three-helical bundle. siRNA rescue experiments with structure-based mutants were used to explore the requirements for CPC localization. Ihe intertwined structural interactions of the core components lead to functional interdependence. Association of the core 'passenger' proteins creates a single structural unit, whose composite molecular surface presents conserved residues essential for central spindle and midbody localization (Jeyaprakash, 2007).

The crystal structure of the regulatory core of the CPC presented in this study together with the structure of Aurora B in complex with unphosphorylated INCENP suggests a composite model for the functional holoCPC. The N-terminal domains of Borealin (15-76) and INCENP (1-46) together with Survivin form the core of the CPC. The CPC core (in the presence of full length INCENP) is sufficient to target to the central spindle and midbody, but centromere targeting requires also the C-terminal domain of Borealin. The data reported in this study suggest that there is no significant direct interaction of Aurora B with Survivin or Borealin, in line with the view that Aurora B is incorporated into the CPC via binding to the IN-box of INCENP. INCENP regulates the localization of Aurora B by interacting with Borealin and Survivin via its N-terminal domain. The role of the coiled-coil domain of INCENP that is predicted between the N and C terminus is not yet clear but might simply reside in connecting the two functional modules of the protein. The experiments reported in this study unambiguously show that the core of the CPC is formed by the molecular interaction between the CPC components in a 1:1:1 ratio. In light of the recent report on the activation of Aurora B by the chromosomal enrichment of CPC, it is possible that in the presence of appropriate interacting proteins at the centromeres and central spindle, the CPC components might assemble into a larger complex of oligomeric nature (Jeyaprakash, 2007 and references therein).

Previous attempts at dissecting the role of individual passenger proteins in regulating the Aurora B kinase suggested an extraordinary interdependence between the subunits. The structural basis for this interdependence becomes evident in light of the crystal structure of the core CPC. A complex network of intermolecular interactions observed within the passenger proteins stabilizes the core of the CPC and holds the subunits together. Although the function and regulation of Aurora B involves three different proteins, these operate as a single structural unit. Specific disruption of any single passenger protein results in the impairment of the structural unit and in the failure of CPC targeting. This unit forms a composite molecular surface that is required to localize the CPC to the central spindle and midbody. In future studies it will be interesting to explore the relationship between this composite surface and potential interaction partners of the CPC at the central spindle as well as the effect of phosphorylation and microtubules on the localization of Aurora B and INCENP (Jeyaprakash, 2007).

borealin-related: Biological Overview | Regulation | Developmental Biology | Effects of Mutation | References

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