Translation from the 5′ untranslated region shapes the integrated stress response

TLDR

Translated regions beyond annotated coding sequences, such as upstream open reading frames (uORFs) in ISR‑controlled mRNAs, are essential proteomic elements that can be translated preferentially even when eIF2·GTP·Met‑tRNA(i) is inhibited. The study seeks to trace uORF translation during the ISR and to show that persistent uORF translation shelters specific mRNAs from the ISR while generating peptides that could serve as MHC class I ligands. Using a tracing translation approach in T cells, the authors directly quantified uORF translation products during the ISR, revealing privileged initiation of uORFs despite eIF2·GTP·Met‑tRNA(i) inhibition. They discovered that BiP mRNA uORFs are translated without AUG initiation, require eIF2.

Abstract

Translated regions distinct from annotated coding sequences have emerged as essential elements of the proteome. This includes upstream open reading frames (uORFs) present in mRNAs controlled by the integrated stress response (ISR) that show "privileged" translation despite inhibited eukaryotic initiation factor 2-guanosine triphosphate-initiator methionyl transfer RNA (eIF2·GTP·Met-tRNA(i )(Met)). We developed tracing translation by T cells to directly measure the translation products of uORFs during the ISR. We identified signature translation events from uORFs in the 5' untranslated region of binding immunoglobulin protein (BiP) mRNA (also called heat shock 70-kilodalton protein 5 mRNA) that were not initiated at the start codon AUG. BiP expression during the ISR required both the alternative initiation factor eIF2A and non-AUG-initiated uORFs. We propose that persistent uORF translation, for a variety of chaperones, shelters select mRNAs from the ISR, while simultaneously generating peptides that could serve as major histocompatibility complex class I ligands, marking cells for recognition by the adaptive immune system.