Two Glucose-sensing Pathways Converge on Rgt1 to Regulate Expression of Glucose Transporter Genes in Saccharomyces cerevisiae

Abstract

The yeast Saccharomyces cerevisiae deploys two different types of glucose sensors on its cell surface that operate in distinct glucose signaling pathways: the glucose transporter-like Snf3 and Rgt2 proteins and the Gpr1 receptor that is coupled to Gpa2, a G-protein ␣ subunit. The ultimate target of the Snf3/ Rgt2 pathway is Rgt1, a transcription factor that regulates expression of HXT genes encoding glucose transporters. We have found that the cAMP-dependent protein kinase A (PKA), which is activated by the Gpr1/Gpa2 glucose-sensing pathway and by a glucose-sensing pathway that works through Ras1 and Ras2, catalyzes phosphorylation of Rgt1 and regulates its function. Rgt1 is phosphorylated in vitro by all three isoforms of PKA, and this requires several serine residues located in PKA consensus sequences within Rgt1. PKA and the consensus serine residues of Rgt1 are required for glucose-induced removal of Rgt1 from the HXT promoters and for induction of HXT expression. Conversely, overexpression of the TPK genes led to constitutive expression of the HXT genes. The PKA consensus phosphorylation sites of Rgt1 are required for an intramolecular interaction that is thought to regulate its DNA binding activity. Thus, two different glucose signal transduction pathways converge on Rgt1 to regulate expression of glucose transporters.The budding yeast Saccharomyces cerevisiae prefers to ferment glucose even when oxygen is available (1-3). This specialized mode of metabolism yields only two ATPs per molecule of glucose fermented, requiring yeast cells to pump large amounts of glucose through glycolysis. They do this by enhancing the rate-limiting step of glucose metabolism, its transport into cells, by increasing expression of the HXT genes encoding glucose transporters (HXT). Glucose induction of HXT expression is achieved through the Snf3/Rgt2-Rgt1 signal transduction pathway, in which the glucose signal generated by the Snf3 and Rgt2 glucose sensors ultimately alters function of the Rgt1 transcription factor (4 -8).Rgt1 functions differently in cells exposed to different levels of glucose. In the absence of glucose, Rgt1 represses HXT expression in conjunction with Mth1 and Std1 (4) by binding to HXT gene promoters and recruiting the Ssn6 and Tup1 corepressors (4, 9). Induction of HXT gene expression is achieved by relieving Rgt1-mediated repression through glucose-induced degradation of Mth1 and Std1 (10 -12). Rgt1 also serves as a transcriptional activator that is required for full induction of HXT1 expression when glucose levels are high (4), although how it converts from a transcriptional repressor to an activator remains unclear. The level of glucose determines the phosphorylation state of Rgt1; it is hypophosphorylated in the absence of glucose and is hyperphosphorylated when glucose levels are high (9, 10, 13). It seems that glucose induces phosphorylation of Rgt1, which prevents it from binding to the HXT promoters and thus inhibits its repressor function (9, 10, 13).The cAMP-dependent protein kinase A (PKA) 2 i...

Endocytosis and Vacuolar Degradation of the Yeast Cell Surface Glucose Sensors Rgt2 and Snf3

“…Mth1 blocks PKA (cAMP-activated protein kinase A) phosphorylation of the Rgt1 repressor, enabling it to recruit Ssn6-Tup1 to the HXT promoters (11)(12)(13). Addition of glucose to glucosedepleted cells induces degradation of Mth1 (14 -18) and consequent phosphorylation of Rgt1 by PKA, leading to Rgt1 dissociation from DNA and thus to HXT gene expression (11,12). Hence, multiple mechanisms are involved for fine-tuned regulation of HXT gene expression (19).…”

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The Filamentous Growth MAPK Pathway Responds to Glucose Starvation Through the Mig1/2 Transcriptional Repressors in Saccharomyces cerevisiae

“…Gpr1 is another glucose sensor (Nakafuku et al 1988;Kraakman et al 1999;Harashima and Heitman 2002;Peeters et al 2007;Thevelein and Voordeckers 2009) that, together with the major nutrient regulatory GTPase Ras2 (Kataoka et al 1984), converges on adenylate cyclase to regulate cellular cAMP levels and PKA activity (Toda et al 1987;Robertson and Fink 1998;Pan and Heitman 1999;Robertson et al 2000). Communication and signal integration among the different pathways results in a unified response to fluctuating nutrient levels (Kaniak et al 2004;Kim and Johnston 2006).…”