PKR and GCN2 Kinases and Guanine Nucleotide Exchange Factor Eukaryotic Translation Initiation Factor 2B (eIF2B) Recognize Overlapping Surfaces on eIF2α

Abstract

Four stress-responsive protein kinases, including GCN2 and PKR, phosphorylate eukaryotic translation initiation factor 2α (eIF2α) on Ser51 to regulate general and gene-specific protein synthesis. Phosphorylated eIF2 is an inhibitor of its guanine nucleotide exchange factor, eIF2B. Mutations that block translational regulation were isolated throughout the N-terminal OB-fold domain in Saccharomyces cerevisiae eIF2α, including those at residues flanking Ser51 and around 20 Å away in the conserved motif K₇₉GYID₈₃. Any mutation at Glu49 or Asp83 blocked translational regulation; however, only a subset of these mutations impaired Ser51 phosphorylation. Substitution of Ala for Asp83 eliminated phosphorylation by GCN2 and PKR both in vivo and in vitro, establishing the critical contributions of remote residues to kinase-substrate recognition. In contrast, mutations that blocked translational regulation but not Ser51 phosphorylation impaired the binding of eIF2B to phosphorylated eIF2α. Thus, two structurally distinct effectors of eIF2 function, eIF2α kinases and eIF2B, have evolved to recognize the same surface and overlapping determinants on eIF2α.