Biochemical evidence for glucose-independent induction of HXT expression in Saccharomyces cerevisiae
The yeast glucose sensors Rgt2 and Snf3 generate a signal in response to glucose that leads to degradation of Mth1 and Std1, thereby relieving repression of Rgt1-repressed genes such as the glucose transporter genes (HXT). Mth1 and Std1 are degraded via the Yck1/2 kinase-SCFGrr1-26S proteasome pathway triggered by the glucose sensors. Here, we show that RGT2-1 promotes ubiquitination and subsequent degradation of Mth1 and Still regardless of the presence of glucose. Site-specific mutagenesis reveals that the conserved lysine residues of Mth1 and Std1 might serve as attachment sites for ubiquitin, and that the potential casein kinase (Yck1/2) sites of serine phosphorylation might control their ubiquitination. Finally, we show that active Snf1 protein kinase in high glucose prevents degradation of Mth1 and Std1. (C0 2007 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.