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

Abstract

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.

FIGURE 1. Structure and regulation of the chromosomal passenger complex (CPC)

(A, Left) Diagram of the CPC, which is formed by Aurora B, INCENP, Survivin and Borealin. The diagram shows domains and functions for each CPC component. (Right, upper): the crystal structure of full length Aurora B complexed with the INCENP C-terminus (AA790-894) (adapted with permission from ref ). (Right, bottom): the crystal structure of the three-helix bundle of INCENP(AA1-58), Borealin (AA10-109) and full length Survivin (adapted with permission from ref ). (B) Phospho-regulation of the CPC showing phosphorylations (yellow spheres) that regulate CPC localisation and function throughout mitosis (pale yellow boxes). Multiple kinases (coloured spheres) phosphorylate the CPC (plain arrows) to regulate CPC function. Additionally, Aurora B activates its own kinase activity by phosphorylating its T-loop (T232) and the INCENP TSS-motif (dotted arrows). Note that some of the depicted phosphorylations are present throughout mitosis (AurB T232) while others are present at specific stages of mitosis (INC T388, T58).

FIGURE 2. Coupling of Aurora B kinase Activation to CPC formation and localisation

(A) Activation of Aurora B (red circle) requires binding to INCENP (yellow) and phosphorylation in a feedback loop. Both of these phosphorylations are catalysed in trans. (B) Aurora B activation is coupled to CPC localisation in vivo. The localisation module of INCENP, Survivin and Borealin targets the CPC to histones at the inner-centromere and microtubules at the spindle midzone during early and late mitosis, respectively. Enrichment of the CPC at these locations facilitates auto-phosphorylation in trans, leading to full Aurora B activation.

FIGURE 3. The CPC produces spatial gradients of Aurora B activity throughout mitosis

(left) At metaphase, Aurora B phosphorylation (detected with a FRET sensor probe on microtubules) is high throughout the spindle and does not form a gradient. Addition of low dose Aurora B inhibitors reveals a phosphorylation gradient where activity is highest around chromatin and decreases towards the spindle poles. During anaphase, an Aurora B phosphorylation gradient is centred at the spindle midzone. (middle) At metaphase, Aurora B activity is high along the chromosome (FRET sensor probe on chromatin). A transient pulse of high dose Aurora B inhibitors followed by washout leads to the production of a phosphorylation gradient emanating from the centromere and decreasing along the arms. This gradient rapidly disappears as Aurora B activity recovers. (right) Unattached kinetochores exhibit a gradient of Aurora B activity emanating from centromeric chromatin towards the kinetochore (FRET sensor probes at various kinetochore locations). Microtubules (red) attach to the kinetochore, generating tension that physically stretches the kinetochore (visualized by stretching of the white kinetochore springs). This change pulls substrates at the kinetochore away from centromeric chromatin, resulting in a decrease in Aurora B activity along the kinetochore. Sensors on centromeric chromatin do not change their relative position upon microtubule binding and remain highly phosphorylated.

FIGURE 4. Recruitment of the CPC to centromeres during early mitosis

(A) During interphase, the CPC is targeted to heterochromatin through the interaction of INCENP (yellow) with dimeric HP1 (purple). HP1 requires Mis14 (pink) for proper localisation. HP1 also recruits Shugoshin proteins (green) through a similar mechanism. At the beginning of prophase, active Aurora B phosphorylates histone 3 Ser10, displacing HP1 from the adjacent H3K9me mark. A series of kinases then phosphorylate the CPC and centromeric histone tails to recruit the CPC to the inner centromere by (pro)metaphase. The BIR-domain of Survivin (green) binds H3T3ph while Borealin that has been phosphorylated by CDK1 binds shugoshin proteins, which interact with H2A S121ph. (B) Overlap between H3T3ph (blue) and H2A S121ph (yellow) defines the inner centromere (green) and recruits the CPC. H3T3ph is deposited by Haspin kinase recruited to centromeric chromatin by cohesin/HP1 (Left). H2A S121ph is deposited by Bub1 kinase, which is recruited to the kinetochore (red) by KNL-1 (purple) phosphorylated by MPS1 kinase (Right).

FIGURE 5

CPC re-localisation and function in mitotic exit (A) At anaphase onset, the CPC relocalises from chromosomes to the spindle midzone where Aurora B activity promotes centralspindlin recruitment. This stabilizes the spindle midzone and recruits factors important for late telophase/cytokinesis such as the RhoGEF Ect2 (purple). Aurora B also phosphorylates the RhoGEF H1 (pink) to prevent its targeting to microtubules prior to telophase. Additionally, a small population of the CPC accumulates at the cell cortex where cleavage furrow ingression will occur. (B) During telophase and cytokinesis, Ect2 stimulates the conversion of inactive Rho-GDP to active Rho-GTP. Dephosphorylated Rho GEF now also activates Rho near microtubules. This promotes actin polymerization and contractile ring formation. Aurora B-mediated phosphorylation of Septin filaments (running out of the plane of the page in this diagram) may also be important in contractile ring formation. The CPC promotes disassembly of intermediate filaments (dark purple) that could otherwise obstruct cleavage furrow constriction. In the presence of lagging chromosomes, it also activates the abscission checkpoint to prevent the completion of cytokinesis. Checkpoint activation involves recruitment of the membrane fusion protein Shrb/CHMP4C (yellow square) to Borealin and inactivation of its membrane fusion activity by Aurora B-mediated phosphorylation.