The interaction of cytoplasmic poly(A)-binding protein with eukaryotic initiation factor 4G suppresses nonsense-mediated mRNA decay

Abstract

Nonsense-mediated mRNA decay (NMD) eliminates different classes of mRNA substrates including transcripts with long 3 ′ UTRs. Current models of NMD suggest that the long physical distance between the poly(A) tail and the termination codon reduces the interaction between cytoplasmic poly(A)-binding protein (PABPC1) and the eukaryotic release factor 3a (eRF3a) during translation termination. In the absence of PABPC1 binding, eRF3a recruits the NMD factor UPF1 to the terminating ribosome, triggering mRNA degradation. Here, we have used the MS2 tethering system to investigate the suppression of NMD by PABPC1. We show that tethering of PABPC1 between the termination codon and a long 3 ′ UTR specifically inhibits NMDmediated mRNA degradation. Contrary to the current model, tethered PABPC1 mutants unable to interact with eRF3a still efficiently suppress NMD. We find that the interaction of PABPC1 with eukaryotic initiation factor 4G (eIF4G), which mediates the circularization of mRNAs, is essential for NMD inhibition by tethered PABPC1. Furthermore, recruiting either eRF3a or eIF4G in proximity to an upstream termination codon antagonizes NMD. While tethering of an eRF3a mutant unable to interact with PABPC1 fails to suppress NMD, tethered eIF4G inhibits NMD in a PABPC1-independent manner, indicating a sequential arrangement of NMD antagonizing factors. In conclusion, our results establish a previously unrecognized link between translation termination, mRNA circularization, and NMD suppression, thereby suggesting a revised model for the activation of NMD at termination codons upstream of long 3 ′ UTR.

Identification of elements in human long 3′ UTRs that inhibit nonsense-mediated decay

“…Previous studies indicate that translation termination at NMD-inducing termination codons is less efficient than translation termination occurring at normal termination codons (Amrani et al 2004;Peixeiro et al 2012). Thus, the cis elements might inhibit NMD by promoting efficient translation termination, as has been proposed for the poly(A) tail when in proximity of the termination codon (Amrani et al 2004;Eberle et al 2008;Ivanov et al 2008;Silva et al 2008;Singh et al 2008;Fatscher et al 2014). Alternatively, the cis elements could promote a level of termination codon readthrough, which has been previously associated with NMD evasion (Hogg and Goff 2010).…”

“…This observation raises the possibility that a trans factor with affinity for A/U-rich sequences may mediate the NMD-inhibitory effects of the cis elements. One candidate for this activity could be PABP, which has previously been found to show high affinity for A/U-rich sequences (Bollig et al 2003;Sladic et al 2004), and is a known antagonist of NMD (Amrani et al 2004;Eberle et al 2008;Ivanov et al 2008;Silva et al 2008;Singh et al 2008;Fatscher et al 2014). Alternatively, RNA binding proteins that either directly affect translation termination or NMD factor function, or that recruit other factors such as PABP, could be responsible (Fig.…”

“…A number of studies have demonstrated that artificial 3 ′ UTRs of 800-900 nt in length promote NMD and increase UPF1 assembly with targeted mRNAs (Eberle et al 2008;Singh et al 2008;Rebbapragada and Lykke-Andersen 2009;Hogg and Goff 2010;Huang et al 2011;Yepiskoposyan et al 2011;Hurt et al 2013). NMD can be prevented by placing cytoplasmic poly(A) binding protein (PABP) in proximity to the termination codon, suggesting that the increased distance between the translation termination event and cytoplasmic PABP that results from termination at a PTC contributes to NMD (Amrani et al 2004;Eberle et al 2008;Ivanov et al 2008;Silva et al 2008;Singh et al 2008;Fatscher et al 2014). The propensity of long 3 ′ UTRs to induce NMD is also supported by global studies showing enrichment of mRNAs containing long 3 ′ UTRs among endogenous NMD substrates (Mendell et al 2004;Bruno et al 2011;Yepiskoposyan et al 2011;Hurt et al 2013).…”

Nonsense-mediated mRNA decay in humans at a glance

“…For one, tethering PABPC1 or eIF4G, which interacts with PABPC1, to a position that resides between a termination codon and the 3′ poly(A) tail can inhibit NMD (Eberle et al, 2008;Fatscher et al, 2014;Joncourt et al, 2014;Peixeiro et al, 2012;Silva et al, 2008;Singh et al, 2008). Thus, NMD can be inhibited if PABPC1 is brought sufficiently close to the 3D environment of a termination codon, either by direct binding or through eIF4G-mediated recruitment, effectively shortening the 3′UTR length regardless of how many nucleotides constitute the 3′UTR.…”

Mechanism, factors, and physiological role of nonsense-mediated mRNA decay

“…Translation termination plays a pivotal role in the lifespan of an mRNA, because the position of the termination codon and efficiency of the termination process determines mRNA stability [1][2][3][4][5]. The termination process begins when a stop codon (UAA, UAG, or UGA) enters the A-site of a translating ribosome, causing it to stall.…”