Linking Sulfur Metabolism to the Cell Division Machinery in Yeast

The longstanding view has been that metabolism allows for cell division to take place, but that metabolic processes do not actively promote cell division. I have recently challenged this notion by identifying a unique gain-of-function metabolic mutant in the budding yeast Saccharomyces cerevisiae. Moderate over-expression of Abf2p, a conserved mitochondrial DNA (mtDNA) maintenance protein, increases the amount of mtDNA by 100-150%. I have shown that cells moderately over-expressing Abf2p can out-proliferate their wild type (WT) counterparts, initiate DNA replication sooner, and increase in size faster than WT cells. Yeast grown under certain conditions in continuous cultures become synchronized with respect to their oxygen consumption, displaying distinctive oxidative and reductive phases. In cells over-expressing Abf2p, the reductive phase is expanded compared to that of WT cells. Since glutathione, the cell?s main redox buffer and sulfur containing metabolite, peaks during this phase, I asked if sulfur metabolism was altered in cells with more mtDNA. Sulfur metabolite levels are increased ~40% in cells over-expressing Abf2p. Furthermore, exogenous addition of various sulfur containing compounds, which is known to increase sulfur metabolic flux, caused WT cells to increase in size faster and initiate DNA replication sooner, mimicking the phenotype seen in cells moderately overexpressing Abf2p. I then investigated possible interactions between sulfur metabolism enzymes and the yeast Cdk, Cdc28p. Performing co-immunoprecipitation experiments, two enzymes of the sulfur metabolic pathway were found to bind Cdc28p. One of these, Cys4p, lies at the critical junction point between the pathways leading to the formation of glutathione versus one carbon metabolism. The interaction of the enzymes with Cdc28p appears to be dependent on progression through the cell cycle, and preliminary evidence suggests that Cdc28p/Cys4p binding may peak at the G1/S transition of the cell cycle. In summary, I have identified a unique gain-of-function metabolic mutant in S. cerevisiae that leads to accelerated initiation of DNA replication. Sulfur metabolic flux is up-regulated in cells over-expressing Abf2p, and exogenous sulfur sources added to WT cultures phenocopied cells over-expressing Abf2p. Most importantly, I have shown a physical interaction between sulfur metabolic enzymes and the Cdk driving the cell cycle in yeast.