SCFGrr1-Mediated Ubiquitination of Gis4 Modulates Glucose Response in Yeast

The F box protein Grr1 is the substrate specificity-determinant of the SCFGrr1 E3 ubiquitin ligase complex. Genetic analyses of Grr1 mutants have implicated Grr1 in glucose repression, specifically with regard to expression of the SUC2 transcript. To better understand Grr1, we screened for substrates using a mutant version of Grr1 that should not associate with the SCF complex. We identified Gis4 as a novel Grr1 substrate. Gis4 was originally isolated as a multi-copy suppressor of a Gal− phenotype in the triple mutant snf1 mig1 srb8. Here, we show that Gis4 binds Grr1 in vivo and that Grr1 protein levels positively affect the protein levels of Gis4. The Gis4 protein is stable in wild-type cells and in grr1Δ cells; however, Gis4 is ubiquitinated in a Grr1-dependent manner. Furthermore, we show that Gis4 interacts with Snf1 in a Grr1-dependent fashion, and that Gis4 is involved in de-repression of SUC2 and in transcription of other Snf1-dependent transcripts. Gis4 appears to connect the glucose repression and de-repression pathways. We suggest that Gis4 may explain the glucose repression defects in carbon source metabolism for the grr1 mutants.