Does eIF3 promote reinitiation after translation of short upstream ORFs also in mammalian cells?

Abstract

Reinitiation after translation of short upstream ORFs (uORFs) represents one of the means of regulation of gene expression on the mRNA-specific level in response to changing environmental conditions. Over the years it has been shown-mainly in budding yeast-that its efficiency depends on cis-acting features occurring in sequences flanking reinitiation-permissive uORFs, the nature of their coding sequences, as well as protein factors acting in trans. We earlier demonstrated that the first two uORFs from the reinitiation-regulated yeast GCN4 mRNA leader carry specific structural elements in their 5' sequences that interact with the translation initiation factor eIF3 to prevent full ribosomal recycling post their translation. Actually, this interaction turned out to be instrumental in stabilizing the mRNA·40S post-termination complex, which is thus capable to eventually resume scanning and reinitiate on the next AUG start site downstream. Recently, we also provided important in vivo evidence strongly supporting the long-standing idea that to stimulate reinitiation, eIF3 has to remain bound to ribosomes elongating these uORFs until their stop codon has been reached. Here we examined the importance of eIF3 and sequences flanking uORF1 of the human functional homolog of yeast GCN4, ATF4, in stimulation of efficient reinitiation. We revealed that the molecular basis of the reinitiation mechanism is conserved between yeasts and humans.

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

“…For GCN4, the best studied system, reinitiation-promoting sequence motifs have been identified upstream and downstream of uORFs 1 and 2 that help retain 40S ribosomes and eIF3 to facilitate control (Gunisova & Valasek, 2014;Mohammad, Munzarova Pondelickova, Zeman, Gunisova, & Valasek, 2017;Munzarova et al, 2011;Szamecz et al, 2008). ATF4 uORF1 also retains eIF3 and ribosomes to promote reinitiation (Hronova et al, 2017). Hence eIF3 retention signals are likely to be generally important for controlled reinitiation events following the translation of a short uORF.…”

Impact of RNA–Protein Interaction Modes on Translation Control: The Versatile Multidomain Protein Gemin5

“…Many RBPs have been reported to interact with UTRs, harboring structural regulatory motifs . However, in spite of the fundamental role of the RNA structure conformation of UTRs in translation control, the open reading frame (ORF) is also susceptible of containing regulatory regions that could be targets of RBPs, eIFs, and other cellular factors …”

Translation acrobatics: how cancer cells exploit alternate modes of translational initiation

“…This process of re‐initiation (Fig iii) has been known to exist for a while, but is only recently starting to be molecularly characterized . The canonical translation initiation factor eIF3h is involved in this process , as well as the non‐canonical initiation factors eIF2D, DENR, and MCTS1 . One key step in this process is likely the de novo recruitment of an initiator tRNA after termination of translation of the uORF, and these non‐canonical factors can recruit tRNA in an eIF2‐independent manner .…”