Recombinant Chicken Borealin-2 (CDCA9)

What is Chicken Borealin-2 (CDCA9) and what role does it play in the Chromosomal Passenger Complex?

Chicken Borealin-2 (CDCA9) is a chromosomal passenger protein that forms part of the Chromosomal Passenger Complex (CPC), a key regulatory assembly essential for proper chromosome segregation and cytokinesis during cell division. Similar to Borealin in other species, Chicken Borealin-2 forms a complex with Aurora B kinase, INCENP (Inner Centromere Protein), and Survivin. Approximately half of Aurora B in mitotic cells is complexed with INCENP, Borealin, and Survivin, while a second complex contains only Aurora B and INCENP .

The CPC shows dynamic localization during mitosis, appearing on chromosome arms and inner centromeres during prophase, concentrating at inner centromeres during metaphase, and relocating to the spindle midzone at anaphase onset before remaining at the midbody throughout telophase and cytokinesis . This precise localization pattern is critical for its function in regulating various mitotic events.

What is the structural organization of Chicken Borealin-2 and how does it interact with other CPC components?

Chicken Borealin-2, like its homologs in other species, contains specific domains that mediate its interactions with other CPC components:

• N-terminal CPC interacting domain (approximately 109 amino acids): Essential for interaction with Survivin and INCENP

• Mid-region: Contains intrinsically disordered segments

Binding studies with Borealin have demonstrated that:

• The N-terminal half (residues 1-141) binds Survivin efficiently but not INCENP

• The C-terminal half (residues 142-280) shows only weak interaction with Survivin and does not bind INCENP

• Both N- and C-terminal halves can bind to Borealin itself, suggesting possible dimerization

This structural organization allows Borealin-2 to serve as a scaffolding protein that helps stabilize the entire CPC complex.

What is the expression pattern of Chicken Borealin-2 during the cell cycle?

Similar to other chromosomal passenger proteins (Aurora B, INCENP, and Survivin), Borealin-2 shows cell cycle-dependent expression with levels significantly increasing during mitosis . This temporal regulation ensures proper CPC function during the critical stages of cell division.

Analysis by immunoblotting of extracts from cells arrested in S-phase versus mitosis reveals that Borealin levels increase significantly during mitosis, with corresponding increases in mRNA levels . This upregulation coincides with the appearance of phosphorylated forms of INCENP, suggesting coordinated regulation of CPC components during mitotic progression.

How does depletion of Borealin-2 affect cellular processes?

Depletion of Borealin results in several defects that highlight its essential role in mitosis:

These phenotypes demonstrate the critical importance of Borealin-2 in multiple aspects of cell division and development.

What are the functional differences between the two CPC subcomplexes observed in mitotic cells?

Research has revealed that mitotic cells contain at least two distinct CPC subcomplexes:

• The holocomplex containing INCENP, Aurora B, Borealin, and Survivin

• A subcomplex containing only INCENP and Aurora B

Immunoprecipitation experiments show that about half of Aurora B and most of INCENP and Survivin co-precipitate with Borealin. The remaining soluble Aurora B and INCENP form a separate complex that does not contain Borealin, Survivin, or TD-60 .

These distinct subcomplexes may perform specialized functions during different stages of mitosis or at different cellular locations. The holocomplex likely functions at centromeres and the central spindle, while the INCENP-Aurora B subcomplex may have additional roles elsewhere in the cell.

How do mutations in the CPC interaction domain of Borealin-2 affect its function?

The N-terminal CPC interaction domain of Borealin-2 is critical for its function. Studies in zebrafish using CRISPR-Cas9 to target this region have generated truncation alleles that produce proteins less than 52 amino acids long (compared to the full-length 255 amino acids) .

These truncated variants fail to interact with the CPC and are considered null alleles. Phenotypically, embryos from homozygous cdca9 mutant females exhibit:

• 100% embryonic lethality

• Failure to establish cell boundaries

• Formation of irregular-sized nuclei

• Inability to establish a furrow during cell division

• Disorganized microtubule arrays

These findings highlight the essential nature of the CPC interaction domain for proper Borealin-2 function in development and cell division.

What is the relationship between Borealin-2 and microtubule dynamics during early embryonic development?

Borealin-2 plays a crucial role in regulating microtubule organization during early embryonic development, particularly during furrow formation:

• In wild-type embryos, microtubules align parallel to each other and perpendicular to the furrow to create the Furrow Microtubule Array (FMA), with a microtubule exclusion zone forming along the furrow site

• In cdca9 mutant embryos, the microtubule exclusion zone fails to establish, and the FMA does not form properly

• Instead of forming organized arrays, astral microtubules from both poles continue to grow into a disorganized mesh-like structure

Importantly, Cdca9 protein localizes to the tips of astral microtubules in distinct puncta, positioned between the microtubules and germ plasm RNP (gpRNP) aggregates . This localization pattern suggests that Borealin-2 may serve as a linker between microtubules and cortical structures during early embryonic divisions.

How does the localization pattern of Borealin-2 differ from other CPC components in specialized cellular contexts?

While CPC components typically show identical localization patterns during typical mitosis, interesting differences emerge in specialized contexts such as germ plasm RNP aggregation:

• Cdca9 (Borealin-2), INCENP, and Aurora B kinase all colocalize at the tips of astral microtubules during germ plasm RNP transport to forming furrows

• Intriguingly, Birc5b (Survivin) accumulates within the growing gpRNP aggregate prior to and during furrow formation, while other CPC components do not

• The association of Birc5b with germ plasm masses continues during their asymmetric segregation in cleavage stages

Intensity profile analysis reveals that when microtubules associate with gpRNP aggregates, Cdca9 positions between microtubules and gpRNPs, while Birc5b fully colocalizes with the gpRNP aggregate . This differential localization suggests specialized functions for individual CPC components beyond their canonical roles in the complex.

What expression systems are optimal for producing functional recombinant Chicken Borealin-2?

Based on successful approaches with Borealin from other species, the following expression systems would be recommended:

For optimal results, consider:

• Using N-terminal tags (His, GST) for purification

• Including protease inhibitors during extraction

• Sequential chromatography approaches (affinity followed by size exclusion)

• Testing both full-length protein and functional domains separately

What immunoprecipitation strategies are most effective for studying Borealin-2 interactions?

To investigate Borealin-2 interactions with other CPC components, the following strategy has proven effective:

• Cell preparation:

  • Synchronize cells to enrich for mitotic populations

  • Use gentle cell lysis conditions to preserve protein complexes

  • Include phosphatase inhibitors to maintain phosphorylation states

• Immunoprecipitation:

  • Use affinity-purified antibodies against Borealin-2

  • For sequential immunoprecipitation: first immunoprecipitate with anti-Borealin-2, then subject the supernatant to a second round with anti-Aurora B

• Analysis:

  • Western blotting for known CPC components

  • Examine stoichiometry of interactions

  • Probe for differential complex formation under varying conditions

This approach allows identification of distinct subcomplexes and can reveal the proportion of each CPC component involved in different complexes.

What are the most effective methods for visualizing Borealin-2 localization during cell division?

Several complementary approaches provide optimal visualization of Borealin-2 during cell division:

• Fixed-cell imaging:

  • Affinity-purified polyclonal antibodies against full-length protein provide specific detection

  • Co-staining with other CPC components (Aurora B) confirms proper localization

  • Counter-staining with DNA (DAPI) and tubulin enables precise staging of mitotic cells

• Live-cell imaging:

  • GFP-tagged Borealin-2 fusion proteins enable dynamic tracking

  • Shows localization pattern transitioning from chromosome arms and inner centromeres in prophase to centromeres in metaphase, then to the spindle midzone and midbody during anaphase/telophase

• Analysis of mutant phenotypes:

  • Examining effects of Borealin-2 depletion on other CPC components

  • Combining with microtubule visualization to assess spindle defects

  • Quantifying mitotic stage distribution to detect cell cycle progression defects

What approaches can be used to study Borealin-2's role in germ plasm RNP aggregation?

To investigate Borealin-2's role in specialized processes like germ plasm RNP aggregation, the following methods have proven informative:

• Live imaging of gpRNP aggregation:

  • Fluorescent labeling of gpRNP components (e.g., NMII-p)

  • Quantification of gpRNP cluster sizes and distribution

  • Comparison between wild-type and mutant embryos

• Colocalization analysis:

  • Double-labeling for Borealin-2 and gpRNP markers

  • Intensity profile tracing along microtubules into gpRNP aggregates

  • Calculation of distance between protein maxima and Pearson coefficient values

• Comparative analysis with other CPC components:

  • Simultaneous visualization of multiple CPC components

  • Z-axis fluorescent intensity scans of protein puncta

  • Assessment of differential localization patterns in specialized contexts

These approaches have revealed that while Borealin-2 and other CPC components colocalize at astral microtubule tips, only Survivin (Birc5b) accumulates within gpRNP aggregates, suggesting specialized functions for individual CPC components in this process.