Mechanistic differences in eukaryotic initiation factor requirements for eIF4GI-driven cap-independent translation of structured mRNAs.

Protein translation is globally downregulated under stress conditions. Many proteins that are synthesized under stress conditions use a cap-independent translation initiation pathway. A subset of cellular mRNAs that encode for these proteins contain stable secondary structures within their 5'UTR, and initiate cap-independent translation using elements called cap-independent translation enhancers or internal ribosome entry sites within their 5'UTRs. The interaction among initiation factors such as eukaryotic initiation factor 4E (eIF4E), eIF4A, and eIF4GI, especially in regulating the eIF4F complex during noncanonical translation initiation of different 5'UTR mRNAs, is poorly understood. Here, equilibrium-binding assays, CD studies and in vitro translation assays were used to elucidate the recruitment of these initiation factors to the highly structured 5'UTRs of fibroblast-growth factor 9 (FGF-9) and hypoxia inducible factor 1 subunit alpha (HIF-1α) encoding mRNAs. We showed that eIF4A and eIF4E enhanced eIF4GI's binding affinity to the uncapped 5'UTR of HIF-1α mRNA, inducing conformational changes in the protein/RNA complex. In contrast, these factors have no effect on the binding of eIF4GI to the 5'UTR of FGF-9 mRNA. Recently, Izidoro et al. reported that the interaction of 42nt unstructured RNA to human eIF4F complex is dominated by eIF4E and ATP-bound state of eIF4A. Here, we show that structured 5'UTR mRNA binding mitigates this requirement. Based on these observations, we describe two possible cap-independent translation mechanisms for FGF-9 and HIF-1α encoding mRNAs used by cells to mitigate cellular stress conditions.