Protein phosphorylation is an essential post-translational modification used in cells for regulating multiple biological processes in all organisms. Particularly, mitotic onset is regulated in all eukaryotes by an increase in cyclin-dependent kinase 1 (Cdk1) activity caused by the dephosphorylation of Cdk1 on a conserved tyrosine residue. PP2ARts1 is a phosphatase that participates in dephosphorylating the conserved tyrosine residue, tyrosine-19 (Y19). PP2ARts1 dephosphorylates phosphorylated serine and threonine residues. However, in vitro experiments suggest that in conjunction with the mammalian PP2A phosphatase activator (PTPA), PP2A gains phosphotyrosine specificity. My work indicates that Rrd1 and Rrd2 (the budding yeast homologs of PTPA) genetically interact with PP2ARts1 and the absence of these proteins cause a Swe1-dependent delay in mitosis. In parallel, utilizing a candidate approach to identify additional phosphatases specific to Cdk1-Y19, my work indicates that Ych1 and Arr2 act redundantly with Mih1 and Ptp1, and Ych1 may act downstream of PP2ARts1. In summation, my work provides the groundwork for how PP2ARts1 functions to dephosphorylate the conserved Y19 residue on Cdk1 and will lead to a better understanding of its role in regulating mitotic progression.

Meiosis is the process by which haploid gametes are produced from a diploid cell. It is a specialised form of cell division which involves one round of DNA replication followed by two rounds of chromosome segregation. Errors in the segregation process can give rise to aneuploidy, which can result in miscarriages and birth defects. In the first meiotic division homologous chromosomes are segregated, and sister chromatids are segregated in the second division. This is coordinated with two rounds of spindle microtubule assembly and disassembly. How these two processes are coordinated is unknown. In my PhD, I study the role of the protein phosphatase 2A (PP2A) regulatory subunit, Cdc55, in budding yeast meiosis. PP2A is a conserved heterotrimeric enzyme that has important roles in mitosis and meiosis. These roles are dictated by binding to either of its two regulatory subunits, Rts1 and Cdc55, in budding yeast . I show that Cdc55 is required for the proper assembly of a meiotic spindle in meiosis I, through the maintenance of the Cdc14 phosphatase in the nucleolus early in meiosis. In addition, Cdc55 is also required to limit the formation of PP2A complexes with the Rts1 regulatory subunit, and this is essential for the timely dissolution of sister chromatid cohesion. Thus, Cdc55 couples spindle assembly with chromosome segregation through its interactions with Cdc14 and PP2ARts1. Finally, I show some preliminary studies looking at the possible downstream effectors of Cdc14 that are important in this mechanism.

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