Entry into the cell cycle is regulated by nutrient availability such that cells do not divide when resources are limited. The Skp1-Cul1-F-box (SCF) ubiquitin ligase with the F-box protein Grr1 (SCF(Grr1)) controls the proteolytic turnover of regulators of cell-cycle entry and a glucose sensor, suggesting that it links the cell cycle with nutrient availability. Here, we show that SCF(Grr1) broadly regulates cellular metabolism. We have developed a proteomic screening method that uses high-throughput quantitative microscopy to comprehensively screen for ubiquitin-ligase substrates. Seven new metabolic targets of SCF(Grr1) were identified, including two regulators of glycolysis--the transcription factor Tye7 and Pfk27. The latter produces the second messenger fructose-2,6-bisphosphate that activates glycolysis and inhibits gluconeogenesis. We show that SCF(Grr1) targets Pfk27 and Tye7 in response to glucose removal. Moreover, Pfk27 is phosphorylated by the kinase Snf1, and unphosphorylatable Pfk27 is stable and inhibits growth in the absence of glucose. These results demonstrate a role for SCF(Grr1) in regulating the glycolytic-gluconeogenic switch.
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