Inhibitors of eIF4G1-eIF1 uncover its regulatory role of ER/UPR stress-response genes independent of eIF2a-phosphorylation

During translation initiation, eIF4G1 dynamically interacts with eIF4E and eIF1. While the role of eIF4E–eIF4G1 is well established, the regulatory functions of eIF4G1–eIF1 are poorly understood. Here, we report the identification of the eIF4G1–eIF1 inhibitors i14G1-10 and i14G1-12. i14G1s directly bind eIF4G1 and inhibit translation in vitro and in the cell, and their effects on translation are dependent on eIF4G1 levels. Translatome analyses revealed that i14G1s mimic eIF1 and eIF4G1 perturbations on the stringency of start codon selection and the opposing roles of eIF1–eIF4G1 in scanning-dependent and scanning-independent short 5′ untranslated region (UTR) translation. Remarkably, i14G1s activate ER/unfolded protein response (UPR) stress-response genes via enhanced ribosome loading, elevated 5′UTR translation at near-cognate AUGs, and unexpected concomitant up-regulation of coding-region translation. These effects are, at least in part, independent of eIF2α-phosphorylation. Interestingly, eIF4G1–eIF1 interaction itself is negatively regulated by ER stress and mTOR inhibition. Thus, i14G1s uncover an unknown mechanism of ER/UPR translational stress response and are valuable research tools and potential drugs against diseases exhibiting dysregulated translation.