SUMO Regulation of Bir1 in Mitotic Progression

Abstract

Organisms utilize many mechanisms in order to preserve their genomic integrity when faced with DNA damage or chromosomal instability. One such mechanism is protein sumoylation, in which E1, E2, and E3 ligases covalently link small ubiquitin‐like modifier (SUMO) peptides or chains to target proteins. The goal of this research is to examine the contribution of Bir1 sumoylation to genome stability in Saccharomyces cerevisiae. Bir1 is the target of one of the three S. cerevisiae E3 SUMO ligases, Mms21, and is a subunit of the chromosomal passenger complex (CPC). The CPC mediates the spindle assembly checkpoint (SAC) to establish chromosome bi‐orientation during mitosis. Therefore, it has previously been hypothesized that the sumoylation of Bir1 influences mitotic progression. In order to explore this hypothesis, BIR1 was genetically modified to mimic conditions in which Bir1 is constantly sumoylated (BIR1‐SuON) or cannot be sumoylated (BIR1‐SuOFF). The BIR1‐SuOFF mutants appeared to be more resistant to DNA damage caused by methyl methanesulfonate (MMS) and ultraviolet radiation than the BIR1‐SuON mutant, as well as the control strain (BIR1‐SuC). BIR1‐SuOff mutants phenocopied the temperature sensitive mutant of Ipl1, ipl1‐321, suggesting that mitotic checkpoint regulation may be affected in BIR1‐SuOff mutants. However, cell cycle analyses of BIR1‐SuOff mutants revealed that mitotic checkpoint activation was comparable to the control strain, at least in response to treatment with nocodazole, an agent that depolymerizes microtubules. We will further characterize the status of mitotic checkpoint activation in BIR1 mutants by examining Pds1 degradation and cell cycle analyses in the presence of other genotoxic agents.