Insights into the Architecture of the elF2Bα/β/δ Regulatory Subcomplex

Eukaryotic translation initiation factor 2B (eIF2B), the guanine nucleotide exchange factor for the G-protein eIF2, is one of the main targets for the regulation of protein synthesis. The eIF2B activity is inhibited in response to a wide range of stress factors and diseases, including viral infections, hypoxia, nutrient starvation, and heme deficiency, collectively known as the integrated stress response. eIF2B has five sub-units (alpha-epsilon). The alpha, beta, and delta subunits are homologous to each other and form the eIF2B regulatory subcomplex, which is believed to be a trimer consisting of monomeric alpha, beta, and delta subunits. Here we use a combination of biophysical methods, site-directed mutagenesis, and bioinformatics to show that the human eIF2B alpha subunit is in fact a homodimer, at odds with the current trimeric model for the eIF2B alpha/beta/delta regulatory complex. eIF2B alpha dimerizes using the same interface that is found in the homodimeric archaeal eIF2B alpha/beta/delta homolog aIF2B and related metabolic enzymes. We also present evidence that the eIF2B beta/delta binding interface is similar to that in the eIF2B alpha(2) homodimer. Mutations at the predicted eIF2B/beta/delta dimer interface cause genetic neurological disorders in humans. We propose that the eIF2B regulatory subcomplex is an alpha(2)beta(2)delta(2) hexamer, composed of one alpha(2) homodimer and two beta delta heterodimers. Our results offer novel insights into the architecture of eIF2B and its interactions with the G-protein eIF2.