The cell cycle is an ordered sequence of events culminating in the formation of
two identical daughter cells. Ensuring the order of the events is essential for
genomic integrity and cell proliferation. The sudden and synchronous splitting of
chromosomes during the metaphase to anaphase transition is one of the visually
most dramatic events of the cell cycle. The transition is driven by the activity of
the anaphase promoting complex/cyclosome (APC/C), an E3 ubiquitin ligase,
which initiates the destruction of its two essential targets, cyclin B and securin.
Cyclin B degradation inactivates the cyclin-dependent kinase 1 (CDK1) and
triggers a multitude of processes during mitotic exit. Degradation of securin
releases separase from its inhibition. Active separase subsequently triggers the
highly synchronous separation of sister chromatids. The separation is
irreversible and therefore needs to be highly accurate and tightly coordinated
with mitotic exit. Yet, little is known about the molecular events that determine
the timing of the single processes and coordinate the individual processes
relative to each other.
I have systematically studied the dynamics of the metaphase to anaphase
transition in the fission yeast Schizosaccharomyces pombe using live cell
imaging assays with high temporal resolution. My analysis shows that the
synchronicity of sister chromatid separation directly depends on the degradation
kinetic of its upstream regulator securin, which suggests the absence of
additional feedback regulation. Stochastic processes dominate the order in
which sister chromatids separate, but an intrinsic bias in chromosome
segregation exists, which is enhanced by decreased separase activity or securin
degradation rates.
Sister chromatid separation has to be tightly coordinated with the cyclin B
degradation-driven processes of mitotic exit. I find the temporal order of events
during the metaphase to anaphase transition to be remarkably robust against
changes in securin and cyclin B, even if the overall timing of the respective
events is severely altered. Competition of securin and cyclin B for the shared
degradation machinery as well as systematic variability in the protein thresholds
at which certain events occur contribute to the observed temporal robustness.
I further investigated the consequences of potential misregulation between
securin and cyclin B degradation-dependent events and show that high CDK1
activity at anaphase results in untimely destabilization of chromosome
attachment, activation of the mitotic checkpoint and inhibition of the APC/C. Yet,
we find that inhibition of the APC/C occurs with slow kinetics, which might
provide an additional buffer against the detrimental consequences of such a loss
in coordination
