Characterization of the initiation factor eIF2B and its regulation in Drosophila melanogaster

Eukaryotic initiation factor (eIF) 2B catalyzes a key regulatory step in th e initiation of mRNA translation. eIF2B is well characterized in mammals an d 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 mam mals and yeast, its activity is regulated by phosphorylation of eIF2 alpha. Here we have cloned Drosophila melanogaster cDNAs encoding polypeptides sh owing substantial similarity to eIF2B subunits from yeast and mammals, They also exhibit the other conserved features of these proteins. D. melanogast er eIF2B alpha confers regulation of eIF2B function in yeast, while eIF2B e psilon shows guanine nucleotide exchange activity, In common with mammalian eIF2B epsilon, D. melanogaster eIF2B epsilon is phosphorylated by glycogen synthase kinase-3 and casein kinase II, Phosphorylation of partially purif ied D. melanogaster eIF2B by glycogen synthase kinase-3 inhibits its activi ty. Extracts of D. melanogaster S2 Schneider cells display eIF2B activity, which is inhibited by phosphorylation of eIF2 alpha; showing the insect fac tor is regulated similarly to eIF2B from other species. In S2 cells, serum starvation increases eIF2 alpha phosphorylation, which correlates with inhi bition of eIF2B, and both effects are reversed by serum treatment. This sho ws that eIF2 alpha phosphorylation and eIF2B activity are under dynamic reg ulation by serum. eIF2 alpha phosphorylation is also increased by endoplasm ic reticulum stress in S2 cells. These are the first data concerning the st ructure, function or control of eIF2B from D. melanogaster.