Grr1-dependent Inactivation of Mth1 Mediates Glucose-induced Dissociation of Rgt1 from HXT Gene Promoters

Abstract

In budding yeast, HXT genes encoding hexose permeases are induced by glucose via a mechanism in which the F box protein Grr1 antagonizes activity of the transcriptional repressor Rgt1. Neither the mechanism of Rgt1 inactivation nor the role of Grr1 in that process has been understood. We show that glucose promotes phosphorylation of Rgt1 and its dissociation from HXT gene promoters. This cascade of events is dependent upon the F-box protein Grr1. Inactivation of Rgt1 is sufficient to explain the requirement for Grr1 but does not involve Rgt1 proteolysis or ubiquitination. We show that inactivation of Mth1 and Std1, known negative regulators of HXT gene expression, leads to the hyperphosphorylation of Rgt1 and its dissociation from HXT promoters even in the absence of glucose. Furthermore, inactivation of Mth1 and Std1 bypasses the requirement for Grr1 for induction of these events, suggesting they are targets for inactivation by Grr1. Consistent with that proposal, Mth1 is rapidly eliminated in response to glucose via a mechanism that requires Grr1. Based upon these data, we propose that glucose acts via Grr1 to promote the degradation of Mth1. Degradation of Mth1 leads to phosphorylation and dissociation of Rgt1 from HXT promoters, thereby activating HXT gene expression.

Glucose sensing and signaling in Saccharomyces cerevisiae through the Rgt2 glucose sensor and casein kinase I

“…2C, lane 2), but Std1 protein is still observed after 1 h when the blot is overexposed to film (data not shown). Indeed, Mth1, but not Std1, degradation was observed in a recent study (3). We believe this difference is due to the different regulation of MTH1 and STD1 expression.…”

“…The glucose signal induces this expression by influencing the function of the Rgt1 transcriptional repressor. In the absence of glucose, Rgt1 is functional and binds to promoters of HXT genes and represses their expression (2,3). Glucose derepresses HXT gene expression by inhibiting Rgt1 repressor function.…”

“…Glucose derepresses HXT gene expression by inhibiting Rgt1 repressor function. This process requires the SCF Grr1 ubiquitin-protein ligase (3,4), suggesting that it involves protein ubiquitination and degradation. Repression of transcription by Rgt1 also requires the paralogous proteins Mth1 and Std1 (3,5).…”

“…This process requires the SCF Grr1 ubiquitin-protein ligase (3,4), suggesting that it involves protein ubiquitination and degradation. Repression of transcription by Rgt1 also requires the paralogous proteins Mth1 and Std1 (3,5). Because Mth1 and Std1 interact with both the Rgt1 repressor (5,6) and the Snf3 and Rgt2 glucose sensors (7,8), they are good candidates for the components of the signal transduction pathway that are inhibited by the glucose signal.…”

“…Because Mth1 and Std1 interact with both the Rgt1 repressor (5,6) and the Snf3 and Rgt2 glucose sensors (7,8), they are good candidates for the components of the signal transduction pathway that are inhibited by the glucose signal. Indeed, it was recently reported that Mth1 is degraded when glucose is added to cells by SCF Grr1 (3).…”