Glucose Limitation Induces GCN4 Translation by Activation of Gcn2 Protein Kinase

Abstract

Phosphorylation of the ␣ subunit of eukaryotic initiation factor 2 (eIF-2␣) is a well-characterized mechanism regulating protein synthesis in response to environmental stresses. In the yeast Saccharomyces cerevisiae, starvation for amino acids induces phosphorylation of eIF-2␣ by Gcn2 protein kinase, leading to elevated translation of GCN4, a transcriptional activator of more than 50 genes. Uncharged tRNA that accumulates during amino acid limitation is proposed to activate Gcn2p by associating with Gcn2p sequences homologous to histidyl-tRNA synthetase (HisRS) enzymes. Given that eIF-2␣ phosphorylation in mammals is induced in response to both carbohydrate and amino acid limitations, we addressed whether activation of Gcn2p in yeast is also controlled by different nutrient deprivations. We found that starvation for glucose induces Gcn2p phosphorylation of eIF-2␣ and stimulates GCN4 translation. Induction of eIF-2␣ phosphorylation by Gcn2p during glucose limitation requires the function of the HisRS-related domain but is largely independent of the ribosome binding sequences of Gcn2p. Furthermore, Gcn20p, a factor required for Gcn2 protein kinase stimulation of GCN4 expression in response to amino acid starvation, is not essential for GCN4 translational control in response to limitation for carbohydrates. These results indicate there are differences between the mechanisms regulating Gcn2p activity in response to amino acid and carbohydrate deficiency. Gcn2p induction of GCN4 translation during carbohydrate limitation enhances storage of amino acids in the vacuoles and facilitates entry into exponential growth during a shift from low-glucose to high-glucose medium. Gcn2p function also contributes to maintenance of glycogen levels during prolonged glucose starvation, suggesting a linkage between amino acid control and glycogen metabolism.

Gcn2 Mediates Gcn4 Activation in Response to Glucose Stimulation or UV Radiation Not via GCN4 Translation

“…4A). An increase in GCN4-LacZ activity under glucose starvation was also observed recently by Yang et al (49). Namely, GCN4 translation increased under conditions that caused suppression of HIS4 transcription ( Figs.…”

Rationalizing translation attenuation in the network architecture of the unfolded protein response

“…Indeed the basic wiring of translational attenuation already exists in yeast in the GCN pathway, which becomes activated under amino acid and glucose starvation leading to transient Gcn2 kinase activation (3,23). Similar to the UPR, in the GCN pathway, a fast TA mechanism is combined with a slower transcriptional response upregulating amino acid synthesis.…”

Regulation of the small GTPase Rheb by amino acids

“…Our results do not exclude the possibility that the dual control mechanism originates from a single amino-acid sensor. A putative candidate could be GCN2, an eIF2a kinase, known to be activated by amino-acid starvation (Dever et al, 1992) and glucose starvation (Yang et al, 2000). Such an aminoacid sensor could, for example, directly phosphorylate TSC2 as well as proteins in the TOR complex.…”