Mechanisms of cohesin protection and removal during meiosis in Saccharomyces cerevisiae
Barton, Rachael Emilia
Meiosis is a specialised form of cell division which results in the formation of haploid cells from a diploid progenitor, and thus meiosis halves the chromosome content of the parental cell. A tightly controlled sequence of chromosome segregation events is required to ensure that chromosome missegregation does not occur during meiosis. Chromosome missegregation causes aneuploidy, which in humans can result in the genetic disorder Down Syndrome, and is the leading cause of infertility and miscarriage. To avoid this, it is critical that in the first meiotic division homologous chromosomes are segregated, followed by sister chromatid segregation in meiosis II. Cohesin is a ring-shaped protein complex that holds that sister chromatids together during meiosis, and aids homologue pairing and recombination, as well as ensuring the correct timing of chromosome segregation. In meiosis I, cohesin must be cleaved by separase on the arms of the chromosomes, to allow homologue segregation, whilst at the centromere Sgo1-PP2A protects this pool of cohesin from cleavage and holds sister chromatids together. In meiosis II, the remaining centromeric cohesin is cleaved to allow segregation of sister chromatids. Therefore, correct regulation of Sgo1 and its interaction partners in meiosis is crucial to prevent aneuploidy. In this study I characterise the role of an Sgo1 binding partner, condensin, in meiotic chromosome segregation, and show that condensin is essential for faithful chromosome segregation in both meiosis I and II. Sgo1 is post-translationally modified in meiosis, and in this study Sgo1 phosphomutants were analysed, but were found to have no discernible effects on faithful chromosome segregation. However additional Sgo1 post-translational modifications were identified, leaving the regulation of Sgo1 by post-translational modification open to future study. Additional to the removal of cohesin by separase cleavage, there is a non-proteolytic pathway for cohesin removal. During mitotic prophase in higher eukaryotes, cohesin is destabilised from replicated sister chromatid arms through the action of Wapl. However, a subset of cohesin is protected from the destabilising effects of Wapl because acetylation of the Smc3 subunit of cohesin allows binding of sororin. Phosphorylation events prevent sororin association with chromosome arms, making cohesin susceptible to removal by Wapl. In contrast, at centromeres, shugoshin counteracts these phosphorylation events, thereby protecting centromeric cohesin from Wapl. During meiosis, Wapl is important in cohesin destabilisation in mouse, C. elegans and A. thaliana, and recent evidence suggests that this pathway is also active in budding yeast. However, it is unclear if the acetyltransferase, Eco1, and cohesin acetylation are important in the generation of cohesive cohesin in meiosis to allow faithful chromosome segregation. Additionally it is unclear if the destabilising activity of Wapl only contributes to cohesin removal on chromosome arms, or whether mechanisms are required to protect cohesin from destabilisation activity near centromeres. I aim to address these questions using budding yeast. Previous work showed that deletion of Wapl (Rad61) in meiosis prevents destabilisation of cohesin from the DNA prior to metaphase I. Consistently, I found Rad61 is regulated in meiosis, and has a role in promoting faithful chromosome segregation in tetranucleates. Additionally, Eco1 acetyltransferase is expressed during S phase of meiosis, and this expression coincides with Smc3 acetylation. In meiosis, disruption of Eco1 function and Smc3 acetylation has a detrimental impact on DNA segregation and cell viability. Further findings show that Sgo1 interacts with cohesin in budding yeast meiosis, and Sgo1 localisation to the chromatin is impacted in Eco1 mutants, but the precise interplay between Sgo1 and acetylated cohesin remains to be fully deduced.