Translational control by TOR and TAP42 through dephosphorylation of eIF2α kinase GCN2

Abstract

Yeast protein kinase GCN2 stimulates the translation of transcriptional activatorGCN4by phosphorylating eIF2α in response to amino acid starvation. Kinase activation requires binding of uncharged tRNA to a histidyl tRNA synthetase-related domain in GCN2. Phosphorylation of serine 577 (Ser 577) in GCN2 by another kinase in vivo inhibits GCN2 function in rich medium by reducing tRNA binding activity. We show that rapamycin stimulates eIF2α phosphorylation by GCN2, with attendant induction ofGCN4translation, while reducing Ser 577 phosphorylation in nonstarved cells. The alanine 577 (Ala 577) mutation in GCN2 (S577A) dampened the effects of rapamycin on eIF2α phosphorylation andGCN4translation, suggesting that GCN2 activation by rapamycin involves Ser 577 dephosphorylation. Rapamycin regulates the phosphorylation of Ser 577 and eIF2α by inhibiting the TOR pathway. Rapamycin-induced dephosphorylation of Ser 577, eIF2α phosphorylation, and induction ofGCN4all involve TAP42, a regulator of type 2A-related protein phosphatases. Our results add a new dimension to the regulation of protein synthesis by TOR proteins and demonstrate cross-talk between two major pathways for nutrient control of gene expression in yeast.