Glucose limitation induces GCN4 translation by activation of gcn2 protein kinase

Phosphorylation of the alpha subunit of eukaryotic initiation factor 2 (eIF -2 alpha) 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 alpha by Gcn2 protein kinase, leading to elevated translation of GCN4, a transcriptional activator of more than 50 genes. Uncharged tRNA that accumulates during ami no acid limitation is proposed to activate Gcn2p by associating with Gcn2p sequences homologous to histidyl-tRNA synthetase (HisRS) enzymes. Given tha t eIF-2 alpha phosphorylation in mammals is induced in response to both car bohydrate and amino acid limitations, we addressed whether activation of Gc n2p in yeast is also controlled by different nutrient deprivations. We foun d that starvation for glucose induces Gcn2p phosphorylation of eIF-2 alpha and stimulates GCN4 translation. Induction of eIF-2 alpha phosphorylation b y Gcn2p during glucose limitation requires the function of the HisRS-relate d domain but is largely independent of the ribosome binding sequences of Gc n2p, Furthermore, Gcn20p, a factor required for Gcn2 protein kinase stimula tion of GCN4 expression in response to amino acid starvation, is not essent ial for GCN4 translational control in response to limitation for carbohydra tes. These results indicate there are differences between the mechanisms re gulating Gcn2p activity in response to amino acid and carbohydrate deficien cy. Gcn2p induction of GCN4 translation during carbohydrate limitation enha nces storage of amino acids in the vacuoles and facilitates entry into expo nential growth during a shift from low-glucose to high-glucose medium. Gcn2 p function also contributes to maintenance of glycogen levels during prolon ged glucose starvation, suggesting a linkage between amino acid control and glycogen metabolism.