Abstract
BackgroundIn yeast, glucose-dependent degradation of the Mth1 protein, a corepressor of the glucose transporter gene (HXT) repressor Rgt1, is a crucial event enabling expression of several HXT. This event occurs through a signaling pathway that involves the Rgt2 and Snf3 glucose sensors and yeast casein kinase 1 and 2 (Yck1/2). In this study, we examined whether the glucose sensors directly couple with Yck1/2 to convert glucose binding into an intracellular signal that leads to the degradation of Mth1.ResultsHigh levels of glucose induce degradation of Mth1 through the Rgt2/Snf3 glucose signaling pathway. Fluorescence microscopy analysis indicates that, under glucose-limited conditions, GFP-Mth1 is localized in the nucleus and does not shuttle between the nucleus and cytoplasm. If glucose-induced degradation is prevented due to disruption of the Rgt2/Snf3 pathway, GFP-Mth1 accumulates in the nucleus. When engineered to be localized to the cytoplasm, GFP-Mth1 is degraded regardless of the presence of glucose or the glucose sensors. In addition, removal of Grr1 from the nucleus prevents degradation of GFP-Mth1. These results suggest that glucose-induced, glucose sensor-dependent Mth1 degradation occurs in the nucleus. We also show that, like Yck2, Yck1 is localized to the plasma membrane via C-terminal palmitoylation mediated by the palmitoyl transferase Akr1. However, glucose-dependent degradation of Mth1 is not impaired in the absence of Akr1, suggesting that a direct interaction between the glucose sensors and Yck1/2 is not required for Mth1 degradation.ConclusionGlucose-induced, glucose sensor-regulated degradation of Mth1 occurs in the nucleus and does not require direct interaction of the glucose sensors with Yck1/2.
Highlights
In yeast, glucose-dependent degradation of the Mth1 protein, a corepressor of the glucose transporter gene (HXT) repressor Rgt1, is a crucial event enabling expression of several HXT
Glucose-induced degradation of Mth1 is not significantly impaired in the absence of Akr1 (Figure 5B). These results suggest that neither localization of yeast casein kinase 1 and 2 (Yck1/2) to the plasma membrane nor a direct interaction between Yck1/2 and the glucose sensors is required for degradation of Mth1
We provide several lines of evidence that Mth1 does not shuttle between the nucleus and cytoplasm and is degraded in the nucleus when glucose is present: 1) Mth1 is not excluded from the nucleus in response to glucose (Figure 2); 2) When engineered to be localized to the cytoplasm, Mth1 is degraded in the cytoplasm regardless of the presence of glucose and the glucose sensors (Figure 3); 3) Mth1 is not degraded when Grr1 is removed from the nucleus (Figure 4)
Summary
Glucose-dependent degradation of the Mth protein, a corepressor of the glucose transporter gene (HXT) repressor Rgt, is a crucial event enabling expression of several HXT. This event occurs through a signaling pathway that involves the Rgt and Snf glucose sensors and yeast casein kinase 1 and 2 (Yck1/2). Subsequent studies have shown that the plasma membrane-tethered casein kinases Yck1/2 phosphorylate Mth, triggering its ubiquitination and subsequent degradation [18] It has been shown by yeast-two-hybrid assay that Mth interacts with the Cterminal tails of the glucose sensors, suggesting that Mth is recruited to the plasma membrane [19,20,21]. The ubiquitinated Mth is targeted for degradation by the 26S proteasome [12,13,14,17]
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