Characterization of the Initiation Factor eIF2B and Its Regulation in Drosophila melanogaster

Eukaryotic initiation factor (eIF) 2B catalyzes a key regulatory step in the initiation of mRNA translation. eIF2B is well characterized in mammals and in yeast, although little is known about it in other eukaryotes. eIF2B is a hetropentamer which mediates the exchange of GDP for GTP on eIF2. In mammals and yeast, its activity is regulated by phosphorylation of eIF2α. Here we have cloned Drosophila melanogaster cDNAs encoding polypeptides showing substantial similarity to eIF2B subunits from yeast and mammals. They also exhibit the other conserved features of these proteins. D. melanogaster eIF2Bα confers regulation of eIF2B function in yeast, while eIF2Bε shows guanine nucleotide exchange activity. In common with mammalian eIF2Bε, D. melanogaster eIF2Bε is phosphorylated by glycogen synthase kinase-3 and casein kinase II. Phosphorylation of partially purified D. melanogaster eIF2B by glycogen synthase kinase-3 inhibits its activity. Extracts of D. melanogaster S2 Schneider cells display eIF2B activity, which is inhibited by phosphorylation of eIF2α, showing the insect factor is regulated similarly to eIF2B from other species. In S2 cells, serum starvation increases eIF2α phosphorylation, which correlates with inhibition of eIF2B, and both effects are reversed by serum treatment. This shows that eIF2α phosphorylation and eIF2B activity are under dynamic regulation by serum. eIF2α phosphorylation is also increased by endoplasmic reticulum stress in S2 cells. These are the first data concerning the structure, function or control of eIF2B from D. melanogaster.