Mechanism of a memory-enhancing inhibitor of the integrated stress response
Cells maintain their proteins in a functional and balanced state by regulating protein synthesis, folding, trafficking and degradation. A central regulatory target in this process is the nucleotide exchange factor eIF2B. Under favorable conditions eIF2B acts as a biological catalyst, efficiently unloading GDP from translation initiation factor 2 (eIF2), a GTPase that is required for protein synthesis. Under conditions of stress such as viral infection or starvation, a conserved signaling pathway known as the integrated stress response (ISR) couples stress detection to the phosphorylation of eIF2. This phosphorylation event renders eIF2 a potent inhibitor of eIF2B to restrict protein synthesis. Relieved of a heavy translational burden, cells are afforded more time and resources to cope with stress.
Recently, a small molecule called ISRIB (integrated stress response inhibitor) was found to activate eIF2B. When systemically administered to mice, ISRIB enhances cognition, confers neuroprotection, and reduces inflammation. These cytoprotective effects highlight the importance of eIF2B in human health and the potential that this pathway offers for therapeutic intervention. To better understand eIF2B-mediated translational control and the physiological effects of ISRIB, we solved a 2.8Å cryo-EM structure of ISRIB-bound human eIF2B. The structure identified ISRIB’s binding site at the symmetric core of the eIF2B heterodecamer. Structural and biochemical analyses revealed that the fully active eIF2B heterodecamer depends on the assembly of two identical tetrameric subcomplexes, and that ISRIB promotes this step by cross-bridging the symmetry interface.
Additional cryoEM structures of eIF2 bound to eIF2B in the dephosphorylated state revealed the eIF2B decamer to be a static platform upon which one or two flexible eIF2 trimers bind and align with eIF2B’s bipartite catalytic centers to catalyze guanine nucleotide exchange. Phosphorylation refolds eIF2 alpha, allowing it to contact eIF2B at a different interface that is only present in the full decamer, and thereby sequesters eIF2B into a non-productive complex. Thus, regulation of eIF2B assembly emerges as a rheostat for eIF2B activity that tunes translation during the ISR and that can be further modulated by ISRIB.