Correcting aberrant kinetochore microtubule attachments: a hidden regulation of Aurora B on microtubules

Abstract

For equal chromosome segregation, a pair of kinetochores on each duplicated chromosome must attach to microtubules connecting to opposite poles. The protein kinase Aurora B plays a critical role in destabilizing microtubules attached in a wrong orientation through phosphorylating kinetochore proteins. The mechanism behind this selective destabilization of aberrant attachments remains elusive. While Aurora B is most enriched on the centromere from prophase to metaphase, emerging evidence suggests the importance of Aurora B on microtubules in this process. Here I discuss two hypothetical models that could explain the requirement of Aurora B on microtubules for selective destabilization of aberrant attachments; microtubule-induced substrate masking and treadmill-removal of Aurora B on microtubules proximal to polymerizing ends.

Figure 1.. Aurora B-dependent error corrections of aberrant attachments.

A. Normal (amphitelic) and aberrant (syntelic and merotelic) attachments. B. Aurora B-mediated microtubule dissociation from aberrant attachments. Generation of an unattached kinetochore (yellow) activates the spindle checkpoint. C. Kinetochore attached to lateral side of microtubules can slide by the CENP-E motor toward the spindle equator [55]. Lateral attachment does not silence the checkpoint (yellow) [56*]. D. Conversion of lateral attachment to end on attachment, and stabilization of amphitelic attachments, which silence the spindle checkpoint. Color brightness of each kinetochore indicates the level of Aurora B-dependent phosphorylation (or checkpoint strength).

Figure 2.. Basic mechanistic principles behind selective stabilization of amphitelic attachments.

A. Geometric constraint. A pair of kinetochores linked through a geometric constraint (right) that interferes with attachment to microtubules coming from distal poles. The black box indicates the constraint, which may be promoted by the CPC at the centromere. In the absence of a geometric constraint (right), such as on paired homologous chromosomes during meiosis I, attachment from distal poles is enabled, resulting in merotelic attachment (and also syntelic attachment). B. Spontaneous microtubule turnovers. Each microtubule attachment has limited lifetime, and is spontaneously replaced by new attachment. A few aberrant attachments can be spontaneously replaced by correct attachment as the geometric constraint interferes with aberrant attachment. The turnover can be enhanced by Aurora B. C. Tension-induced enhancement of kinetochore-microtubule attachment (catch-bond mechanism). At a kinetochore under reduced tension (left), intrinsic microtubule binding is weak. At a kinetochore under tension (right), intrinsic microtubule binding is enhanced. Aurora B can weaken the microtubule-binding strength.

Figure 3.. Members of the CPC; Aurora B, INCENP, Survivin and Borealin.

Aurora B interacts with INCENP at the IN-box domain, which allosterically activates Aurora B. Aurora B-dependent phosphorylation (yellow circles) at the Aurora B activation loop and on INCENP promotes full activation. The N-terminal CEN domain of INCENP interacts with Survivin and Borealin. This trimeric module targets the CPC to centromere-enriched histone phosphorylation marks; H3 phosphorylated at Thr3 recognized by the BIR domain of Survivin, and H2A phosphorylated at Thr120, which recruits Borealin through Sgo1 in a manner dependent on Cdk1-mediated phosphorylation at Borealin (orange circles). The SAH domain interacts with microtubules, while the PRD domain also supports microtubule binding, which is suppressed by Cdk1-dependent phosphorylation (orange circles). INCENP also interacts with HP1, which appears to facilitate substrate recognition.

Figure 4.. Models: substrate masking by microtubules and treadmill-removal of Aurora B

A. Unattached kinetochore. Centromeric CPC enrichment activates Aurora B through autophosphorylation. Diffusion allows activated Aurora B (red) to access kinetochore substrates (orange dotted arrow), while its reaction is counteracted by PP2A at the kinetochore. The positively charged N-terminal tail of the Ndc80/Hec1 subunit of the Ndc80 complex (green) is recognized by Aurora B and microtubules. In the absence microtubules, Aurora B activated at the centromere can phosphorylate Ndc80. B. Kinetochore attached to shrinking microtubule ends. The kinetochore is moving to a pole, and is under reduced tension. In the attached kinetochore, microtubules and Aurora B may compete to interact with the Ndc80 N-terminus, necessitating the microtubule-binding capacity of the CPC for effective Ndc80 phosphorylation (substrate masking by microtubules). The majority of the CPC with high Cdk1-dependent phosphorylation on INCENP (shown as orange circles) cannot bind to microtubules, but a small fraction of the CPC with reduced phosphorylation can bind to microtubules for a limited timeframe, during which Aurora B is activated. At the same duration, CPC with activated Aurora B comes closer to kinetochore substrates by treadmilling due to plus-end depolymerization, or stays close to the kinetochore (in the case of taxol-stabilized microtubules), facilitating accessibility of active Aurora B to its kinetochore substrates. C. Kinetochore attached to growing microtubule ends. The kinetochore is moving away from a pole, and is under tension. The CPC with inactive Aurora B can be activated upon microtubule binding, but by the time when Aurora B is activated, it will already have been treadmilled away from the kinetochore, reducing its accessibility to kinetochore substrates (treadmill-removal). The CPC activated by microtubules or the centromeric chromatin may diffuse into the cytoplasm, but the high Cdk1 activity limits the rebinding to microtubules within the kinetochore.