Regulation of kinetochore and inner-centromere structure and function by the Chromosome Passenger Complex during mitosis

Mitosis is an important stage in the cell cycle when the duplicated chromosomes are
segregated to the daughter cells. Errors in segregation of chromosomes can lead to
genomic instability that underlies multiple developmental diseases and cancer.
Aurora-B, which is a member of the Chromosome Passenger Complex (CPC), is a key
mitotic kinase and plays an important role in ensuring high fidelity mitosis by
phosphorylating numerous substrates in the mitotic spindle. Most of the CPC is
localized to the inner-centromere during pro-metaphase. This localization of the
CPC to the inner-centromere is important for the concentration-dependent autoactivation
of the CPC during mitosis. The inner-centromeric CPC also regulates
localization of multiple proteins to the inner-centromeres, which are important for
proper mitotic progression. How the CPC is maintained in the inner-centromere at
high concentration during pro-metaphase and the effect of this high concentration
of the CPC on the organization and composition of the inner-centromere are unclear.
In Chapter 2, I will show that the liquid-liquid phase separation driven by the
centromere-targeting region of the CPC is important for its inner-centromere
localization and function and may underlie the mesoscale organization of the innercentromere.
Once localized to the inner-centromere the key substrates that Aurora-
B phosphorylates to ensure error-free mitosis are often localized 100’s of nm away
from the site of peak kinase localization. It is unclear how the activity of the Aurora-
B kinase reaches its distant substrates. In Chapter 3, I will describe a mechanism
that enables the phosphorylation of distant outer kinetochore substrates by the CPC.
I will show that inner-centromeric and microtubule-bound non-inner centromeric
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CPC cooperate to ensure proper phosphorylation of the outer kinetochore
substrates, which is important for correction of improper kinetochore-microtubule
attachment. Apart from regulating kinetochore-microtubule attachment Aurora-B
also regulates the assembly of the outer kinetochore during mitosis. Aurora-B
activity at the kinetochore changes in response to the kinetochore-microtubule
attachment status and this change is important for proper mitosis. However, the
outer kinetochore organization is thought to remain unchanged before and after
kinetochore-microtubule attachment. It is thus unclear if the same interactions
underlie the organization of the core outer kinetochore before and after mature
kinetochore-microtubule attachment. In Chapter 4, I will present data that suggests
that different pathways play a role in the maintenance of the outer kinetochore
before and after mature kinetochore-microtubule attachment. I will show that the
outer kinetochore maintenance is dependent on the CPC and Plk1 activity before but
not after mature kinetochore-microtubule attachment.