eIF2B Mechanisms of Action and Regulation: A Thermodynamic View.

Eukaryotic translation initiation factor 2B (eIF2B) is the guanine nucleotide exchange factor of the GTPase eIF2, which brings the initiator Met-tRNAi to the ribosome in the form of the eIF2-GTP·Met-tRNAi ternary complex (TC). The activity of eIF2B is inhibited by phosphorylation of its substrate eIF2 by several stress-induced kinases, which triggers the integrated stress response (ISR). The ISR plays a central role in maintaining homeostasis in the cell under various stress conditions, and its dysregulation is a causative factor in the pathology of a number of neurodegenerative disorders. Over the past three decades, virtually every aspect of eIF2B function has been the subject of uncertainty or controversy: from the catalytic mechanism of nucleotide exchange, to whether eIF2B only catalyzes nucleotide exchange on eIF2 or also promotes binding of Met-tRNAi to eIF2-GTP to form the TC. Here, we provide the first complete thermodynamic analysis of the process of recycling of eIF2-GDP to the TC. The available evidence leads to the conclusion that eIF2 is channeled from the ribosome (as an eIF5·eIF2-GDP complex) to eIF2B, converted by eIF2B to the TC, which is then channeled back to eIF5 and the ribosome. The system has evolved to be regulated by multiple factors, including post-translational modifications of eIF2, eIF2B, and eIF5, as well as directly by the energy balance in the cell, through the GTP:GDP ratio.

Novel mechanisms of eIF2B action and regulation by eIF2α phosphorylation.

Eukaryotic translation initiation factor 2 (eIF2) is a heterotrimeric GTPase, which plays a critical role in protein synthesis regulation. eIF2-GTP binds Met-tRNAi to form the eIF2-GTP•Met-tRNAi ternary complex (TC), which is recruited to the 40S ribosomal subunit. Following GTP hydrolysis, eIF2-GDP is recycled back to TC by its guanine nucleotide exchange factor (GEF), eIF2B. Phosphorylation of the eIF2α subunit in response to various cellular stresses converts eIF2 into a competitive inhibitor of eIF2B, which triggers the integrated stress response (ISR). Dysregulation of eIF2B activity is associated with a number of pathologies, including neurodegenerative diseases, metabolic disorders, and cancer. However, despite decades of research, the underlying molecular mechanisms of eIF2B action and regulation remain unknown. Here we employ a combination of NMR, fluorescence spectroscopy, site-directed mutagenesis, and thermodynamics to elucidate the mechanisms of eIF2B action and its regulation by phosphorylation of the substrate eIF2. We present: (i) a novel mechanism for the inhibition of eIF2B activity, whereby eIF2α phosphorylation destabilizes an autoregulatory intramolecular interaction within eIF2α; and (ii) the first structural model for the complex of eIF2B with its substrate, eIF2-GDP, reaction intermediates, apo-eIF2 and eIF2-GTP, and product, TC, with direct implications for the eIF2B catalytic mechanism.

Insights into the architecture of the eIF2Bα/ß/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 subunits (α-ε). The α, ß, and δ subunits are homologous to each other and form the eIF2B regulatory subcomplex, which is believed to be a trimer consisting of monomeric α, ß, and δ subunits. Here we use a combination of biophysical methods, site-directed mutagenesis, and bioinformatics to show that the human eIF2Bα subunit is in fact a homodimer, at odds with the current trimeric model for the eIF2Bα/ß/δ regulatory complex. eIF2Bα dimerizes using the same interface that is found in the homodimeric archaeal eIF2Bα/ß/δ homolog aIF2B and related metabolic enzymes. We also present evidence that the eIF2Bß/δ binding interface is similar to that in the eIF2Bα2 homodimer. Mutations at the predicted eIF2Bß/δ dimer interface cause genetic neurological disorders in humans. We propose that the eIF2B regulatory subcomplex is an α2ß2δ2 hexamer, composed of one α2 homodimer and two ßδ heterodimers. Our results offer novel insights into the architecture of eIF2B and its interactions with the G-protein eIF2.