5′-UTR recruitment of the translation initiation factor eIF4GI or DAP5 drives cap-independent translation of a subset of human mRNAs

During unfavorable conditions (e.g.tumor hypoxia or viral infection), canonical, cap-dependent mRNA translation is suppressed in human cells. Nonetheless, a subset of physiologically important mRNAs (e.g.hypoxia-inducible factor 1 alpha [HIF-1 alpha], fibroblast growth factor 9 [FGF-9], and p53) is still translated by an unknown, cap-independent mechanism. Additionally, expression levels of eukaryotic translation initiation factor 4GI (eIF4GI) and of its homolog, death-associated protein 5 (DAP5), are elevated. By examining the 5 ' UTRs of HIF-1 alpha, FGF-9, and p53 mRNAs and using fluorescence anisotropy binding studies, luciferase reporter-basedin vitrotranslation assays, and mutational analyses, we demonstrate here that eIF4GI and DAP5 specifically bind to the 5 ' UTRs of these cap-independently translated mRNAs. Surprisingly, we found that the eIF4E-binding domain of eIF4GI increases not only the binding affinity but also the selectivity among these mRNAs. We further demonstrate that the affinities of eIF4GI and DAP5 binding to these 5 ' UTRs correlate with the efficiency with which these factors drive cap-independent translation of these mRNAs. Integrating the results of our binding and translation assays, we conclude that eIF4GI or DAP5 is critical for recruitment of a specific subset of mRNAs to the ribosome, providing mechanistic insight into their cap-independent translation.