Differential Regulation of the Three Eukaryotic mRNA Translation Initiation Factor (eIF) 4Gs by the Proteasome.

Amandine Alard,Corinne Bousquet,Stéphane Pyronnet,Frédéric Lopez,Christine Jean,Didier Poncet,Patrice Vende,Robert J Schneider,Catherine Marboeuf,Bertrand Fabre,Yvan Martineau

doi:10.3389/fgene.2019.00254

Abstract

The 4G family of eukaryotic mRNA translation initiation factors is composed of three members (eIF4GI, eIF4GII, and DAP5). Their specific roles in translation initiation are under intense investigations, but how their respective intracellular amounts are controlled remains poorly understood. Here we show that eIF4GI and eIF4GII exhibit much shorter half-lives than that of DAP5. Both eIF4GI and eIF4GII proteins, but not DAP5, contain computer-predicted PEST motifs in their N-termini conserved across the animal kingdom. They are both sensitive to degradation by the proteasome. Under normal conditions, eIF4GI and eIF4GII are protected from proteasomal destruction through binding to the detoxifying enzyme NQO1 [NAD(P)H:quinone oxidoreductase]. However, when cells are exposed to oxidative stress both eIF4GI and eIF4GII, but not DAP5, are degraded by the proteasome in an N-terminal-dependent manner, and cell viability is more compromised upon silencing of DAP5. These findings indicate that the three eIF4G proteins are differentially regulated by the proteasome and that persistent DAP5 plays a role in cell survival upon oxidative stress.

Highlights

In eukaryotes, most nuclear encoded mRNAs are modified at their 5 end with a cap-structure (m7GpppN, where N is any nucleotide)
NRF2 induces the transcriptional activation of genes capable of detoxifying intracellular ROS, including NQO1 which acts through its quinone oxidoreductase activity (Venugopal and Jaiswal, 1996)
This assumption was supported by the fact that oxidative stress is known to inhibit general cap-dependent translation initiation while DAP5 is believed to play a role in cap-independent translation under stress (Nevins et al, 2003), and that a cap-independent mode of NRF2 mRNA translation has been described upon oxidative stress

Summary

Introduction

Most nuclear encoded mRNAs are modified at their 5 end with a cap-structure (m7GpppN, where N is any nucleotide). Through multiple interactions, eIF4G plays a central role in capdependent translation initiation by bridging the mRNA 5 cap structure (via eIF4E) to the poly(A) tail (via PABP), and to the ribosome (via eIF3). Distinct phosphorylation sites targeted by different signaling pathways and with specific biological functions have been mapped in both amino-acid sequences. Both eIF4GI and eIF4GII interact with the MAPK-interacting protein kinases MNK1 (Pyronnet et al, 1999) or MNK2 (Scheper et al, 2001), but only eIF4GI has been described as an MNK1/2 substrate (Orton et al, 2004). How the steady state level of each protein is controlled and whether they can be differentially targeted to degradation upon stress remain poorly understood

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Conclusion