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.

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