Crystal structure of eukaryotic translation initiation factor 2B

Eukaryotic cells restrict protein synthesis under various stress conditions, by inhibiting the eukaryotic translation initiation factor 2B (eIF2B)(1,2). eIF2B is the guanine nucleotide exchange factor for eIF2, a heterotrimeric G protein consisting of alpha-, beta- and gamma-subunits. eIF2B exchanges GDP for GTP on the.-subunit of eIF2 (eIF2.), and is inhibited by stress-induced phosphorylation of eIF2 alpha. eIF2B is a heterodecameric complex of two copies each of the alpha-, beta- and gamma-and e-subunits(3); its alpha-, beta- and delta-subunits constitute the regulatory subcomplex(4), while the gamma-and epsilon-subunits form the catalytic subcomplex(5). The three-dimensional structure of the entire eIF2B complex has not been determined. Here we present the crystal structure of Schizosaccharomyces pombe eIF2B with an unprecedented subunit arrangement, in which the alpha(2)beta(2)delta(2) hexameric regulatory subcomplex binds two.e dimeric catalytic subcomplexes on its opposite sides. A structure-based in vitro analysis by a surface-scanning site-directed photo-cross-linking method identified the eIF2a-binding and eIF2 gamma-binding interfaces, located far apart on the regulatory and catalytic subcomplexes, respectively. The eIF2 gamma-binding interface is located close to the conserved ` NF motif', which is important for nucleotide exchange. A structural model was constructed for the complex of eIF2B with phosphorylated eIF2 alpha, which binds to eIF2B more strongly than the unphosphorylated form. These results indicate that the eIF2 alpha phosphorylation generates the ` nonproductive' eIF2-eIF2B complex(5), which prevents nucleotide exchange on eIF2 gamma, and thus provide a structural framework for the eIF2B-mediated mechanism of stress-induced translational control.