Protein synthesis is regulated in response to environmental stimuli by covalent modification, primarily phosphorylation, of components of the tranelational machinery. Phosphorylation of the α subunit of eIF‐2 is one of the best‐characterized mechanisms for down‐regulating protein synthesis in higher eukaryotes in response to various stress conditions. Three distinct protein kinases regulate protein synthesis in eukaryotic cells by phosphorylating the α subunit of eIF‐2 at serine‐51. There are two mammalian eIF‐2α kinases: the double‐stranded RNA‐dependent kinase (PKR) and heme‐regulated inhibitor kinase (HRI), and the yeast GCN2. The regulatory mechanisms and the molecular sizes of these eIF‐2α kinases are different. The expression of PKR is induced by interferon, and the kinase activity is stimulated by low concentrations of double‐stranded RNA. HRI is activated under heme‐defi‐cient conditions. Yeast GCN2 is activated by amino acid starvation. The phosphorylation of eIF‐2α results in the shutdown of protein synthesis. Nevertheless, the eIF‐2α kinases can regulate both global as well as specific mRNA translation. Inhibition of protein synthesis correlates with eIF‐2α phosphorylation in response to a wide variety of different stimuli, including heat shock, serum deprivation, glucose starvation, amino acid starvation, exposure to heavy metal ions, and viral infection. Finally, recent studies suggest a role for eIF‐2α phosphorylation in the control of cell growth and differentiation.—de Haro, C., Méndez, R., Santoyo, J. The eIF‐2α kinases and the control of protein synthesis. FASEB J. 10, 1378‐1387(1996)

In eukaryotic cells, protein synthesis is regulated in response to various environmental stresses by phosphorylating the a subunit of the eukaryotic initiation factor 2 (eIF2a). Three different eIF2a kinases have been identified in mammalian cells, the heme-regulated inhibitor (HRI), the interferon-inducible RNA-dependent kinase (PKR) and the endoplasmic reticulum-resident kinase (PERK). A fourth eIF2a kinase, termed GCN2, was previously characterized from Saccharomyces cerevisiae, Drosophila melanogaster and Neurospora crassa. Here we describe the cloning of a mouse GCN2 cDNA (MGCN2), which represents the first mammalian GCN2 homolog. MGCN2 has a conserved motif, N-terminal to the kinase subdomain V, and a large insert of 139 amino acids located between subdomains IV and V that are characteristic of the known eIF2a kinases. Furthermore, MGCN2 contains a class II aminoacyl-tRNA synthetase domain and a degenerate kinase segment, downstream and upstream of the eIF2a kinase domain, respectively, and both are singular features of GCN2 protein kinases. MGCN2 mRNA is expressed as a single message of < 5.5 kb in a wide range of different tissues, with the highest levels in the liver and the brain. Specific polyclonal anti-(MGCN2) immunoprecipitated an eIF2a kinase activity and recognized a 190 kDa phosphoprotein in Western blots from either mouse liver or MGCN2-transfected 293 cell extracts. Interestingly, serum starvation increased eIF2a phosphorylation in MGCN2-transfected human 293T cells. This finding provides evidence that GCN2 is the unique eIF2a kinase present in all eukaryotes from yeast to mammals and underscores the role of MGCN2 kinase in translational control and its potential physiological significance.