elF4B and elF4G Jointly Stimulate elF4A ATPase and Unwinding Activities by Modulation of the elF4A Conformational Cycle

Eukaryotic translation initiation factor 4A (eIF4A) is a DEAD-box protein that participates in translation initiation. As an ATP-dependent RNA helicase, it is thought to resolve secondary structure elements from the 5 '-untranslated region of mRNAs to enable ribosome scanning. The RNA-stimulated ATPase and ATP-dependent helicase activities of elF4A are enhanced by auxiliary proteins, but the underlying mechanisms are still largely unknown. Here, we have dissected the effect of elF4B and elF4G on elF4A RNA-dependent ATPase- and RNA helicase activities and on elF4A conformation. We show for the first time that yeast elF4B, like its mammalian counterpart, can stimulate RNA unwinding by elF4A, although it does not affect the elF4A conformation. The elF4G middle domain enhances this stimulatory effect and promotes the formation of a closed elF4A conformation in the presence of ATP and RNA. The closed state of elF4A has been inferred but has not been observed experimentally before. elF4B and elF4G jointly stimulate ATP hydrolysis and RNA unwinding by elF4A and favor the formation of the closed elF4A conformer. Our results reveal distinct functions of elF4B and elF4G in synergistically stimulating the elF4A helicase activity in the mRNA scanning process. (C) 2013 Elsevier Ltd. All rights reserved.