Translation initiation factors are involved in the mechanism of NMD resistance of mRNAs carrying a short open reading frame.

Nonsense-mediated mRNA decay (NMD) is a surveillance pathway that recognizes and rapidly degrades mRNAs containing premature termination codons (PTC). The unified model for NMD, proposes that the decision of NMD triggering is the outcome of the competition between the cytoplasmatic poly(A)-binding protein 1 (PABPC1) and the NMD effector UPF1 for the termination complex. Consequently, PTCs located far, in a linear sense, from the poly(A) tail and associated PABPC1, in mRNAs containing residual downstream exon junction complexes (EJCs) are expected to elicit NMD. Nevertheless, it has been reported that mRNAs containing PTCs in close proximity to the translation initiation codon (AUG-proximal PTCs) can substantially evade NMD. Here, we focused on the mechanistic basis for this NMD resistance, exemplified by human β-globin transcripts carrying an AUG-proximal PTC. We demonstrate that the establishment of an efficient translation termination reaction at the AUG-proximal PTC is dependent on PABPC1 interaction with the initiation factor eIF4G and with the release factor eRF3 at the terminating ribosome. The PABPC1-eIF4G interaction allows the mRNA to acquire a closed-loop conformation, which, in concert with PABPC1-eRF3 interaction underlies linkage of translation initiation with efficient termination at the AUG-proximal PTC. Thus, these interactions contribute to an NMD-resistant PTC definition at an early phase of translation elongation. Furthermore, we provide strong evidence that the eukaryotic initiation factor 3 is involved in delivering eIF4G-associated PABPC1 into the vicinity of the AUG-proximal PTC through eIF3h and eIF3f subunits. In fact, our preliminary data suggest that eIF3h and eIF3f depletion in HeLa cells affect the association of the eIF3 complex with the ribosome and eIF4G, respectively. These data corroborate a role for PABPC1 on NMD evasion of AUG-proximal transcripts and provides further insights into the mechanistic details of PTC definition and translation initiation