eIF2B Mechanisms of Action and Regulation: A Thermodynamic View

Abstract

Eukaryotic translation initiation factor 2B (eIF2B) is the guanine nucleotide exchange factor of the GTPase eIF2, which brings the initiator Met-tRNA to the ribosome in the form of the eIF2-GTP·Met-tRNA 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-tRNA 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.

Regulation of translation initiation factor eIF2B at the hub of the integrated stress response

“…GDP is more tightly bound to human eIF2 than the yeast complex and was not further stabilized by eIF5 in vitro (Sokabe, Fraser, & Hershey, 2012), but the DWEAR motif is conserved (Jennings & Pavitt, 2010b), and overexpression of eIF5 in human cells activates ATF4 reporter plasmids, in a manner similar to prior findings overexpressing eIF5 in yeast cells (Asano et al, 1999;Kozel et al, 2016;Singh et al, 2006). These data and recent modeling approaches suggest that eIF2-GDP/eIF5 interactions are critical for the yeast ISR and may be relevant to mammalian translation and the ISR (Bogorad, Lin, & Marintchev, 2018;Khan, Spurgeon, & von der Haar, 2018).…”

Translation Regulation by eIF2α Phosphorylation and mTORC1 Signaling Pathways in Non-Communicable Diseases (NCDs)

“…As mentioned, during translation initiation, eIF2 forms a ternary complex with Met-tRNA i and GTP, which binds the 40S subunit to form the 43S PIC ( Figure 2) [24]. Phosphorylation of eIF2α at Ser-51 dramatically increases the binding affinity of eIF2 for its guanine nucleotide exchange factor (GEF) known as eIF2B, resulting in sequestration that blocks eIF2-GDP to eIF2-GTP exchange activity and consequently inhibits translation [25,26]. Different stress conditions provoke eIF2α phosphorylation by any of four related protein kinases: the heme-regulated inhibitor (HRI) in red blood cells, the protein kinase RNA-activated (PKR) that responds to viral infection, the PKR-like endoplasmic reticulum (ER) kinase (PERK) that senses ER stress in the ISR/UPR pathway, and the general control non-depressible 2 kinase (GCN2) that responds to a deficit in levels of certain amino acids [27].…”

Structural Basis for eIF2B Inhibition in Integrated Stress Response

“…After GDP dissociates from eIF2γ, it is generally considered that GTP and Met-tRNAi binding enables the detachment of eIF2 from eIF2B, stimulated by another initiation factor, eIF5 (26,27). The structural alignment of eIF2γ in the productive complex and aIF2γ in the aIF2•GTP•Met-tRNAi ternary complex (aIF2-TC) structure (28) revealed that Met-tRNAi can access eIF2 in the productive complex, without clashing with most of eIF2B ( fig.…”