Dynamics and regulation of Shugoshin and other pericentromeric proteins in budding yeast
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Date
28/11/2013Item status
Restricted AccessEmbargo end date
31/12/2100Author
Nerusheva, Olga
Metadata
Abstract
Accurate distribution of genetic material is critical for the formation of
functional cells and their proliferation. During cell division, sister chromatids
separate from each other and segregate to opposite poles. To ensure accurate
chromosome segregation all sister chromatids should be attached to microtubules
from opposite spindle poles, known as bi-orientation. Cohesin is a protein complex
that holds sister chromatids together from the time of its replication in S phase until
anaphase onset, and it is required for proper chromosome segregation both in mitosis
and in meiosis. It is distributed intermittently along the full length of chromosomes
with significant enrichment in the region surrounding the centromere, known as the
pericentromere. This chromosome domain was shown to be crucial for chromosome
bi-orientation.
In my PhD I studied how the establishment of tension between sister
chromatids in the process of bi-orientation affects the distribution of different
pericentromeric proteins on budding yeast chromosomes. It was known that levels of
cohesin at the pericentromere are decreased in response to the establishment of
tension. I demonstrate that other proteins, such as subunits of condensin, members of
the Chromosome Passenger Complex (CPC) and others, exhibit similar dynamics,
and suggest a model to explain this phenomenon.
Out of all studied proteins, Shugoshin (Sgo1) was the only one that was
completely removed from the pericentromere in response to spindle tension
establishment. There is evidence that Sgo1 plays a role in sensing spindle tension and
halting the cell cycle until this has been achieved but how it does so is not known.
Therefore, removal of Shugoshin from the pericentromere might be a signal for the
cell that bi-orientation occurred. I then found that spindle tension itself is not
sufficient for Sgo1 re-localization from the pericentromere, and there are other
factors that affect it. I showed that deletion of RTS1, a highly conserved regulatory
subunit of Protein Phosphatase 2A (PP2A), results in substantial enrichment of
Shugoshin at the pericentromere in the situation when spindle tension is absent. In
addition, Bub1 kinase, a protein that is required for Sgo1 localization, was found to
be removed from the centromere in response to spindle tension as well as Sgo1. The
role of Bub1 the in localization of Shugoshin is to phosphorylate histone H2A, which
then becomes a mark for Sgo1 loading. Therefore, we assume that Sgo1 dynamics
and, potentially, its role in sensing bi-orientation, are regulated through the array of
phosphorylation and de-phosphorylation events at the pericentromeric area.
Finally, I have also found that budding yeast Sgo1 undergoes the
posttranslational modification as sumoylation. I showed that sumoylation of
Shugoshin is not required for its removal from the pericentromere during biorientation.
However, it might be important for the regulation of Sgo1 degradation
and its role in the metaphase to anaphase transition in mitosis.