The structure of a human translation initiation complex reveals two independent roles for the helicase eIF4A

Summary Initiation of mRNA translation is a key regulatory step in gene expression in all eukaryotes. Canonical initiation of translation in eukaryotes involves recruitment of the 43S preinitiation complex to the 5′ end of mRNA by the cap-binding complex eIF4F to form the 48S initiation complex (48S), followed by scanning along the mRNA until the start codon is selected.1–8 We have previously shown that eIF4F binds near the mRNA channel exit site of the 43S, leaving an open question about how mRNA secondary structure is removed as it enters the mRNA binding channel on the other side of the 40S subunit. 4 Here we describe a human 48S positioned at the start codon that shows that in addition to the eIF4A that is part of eIF4F, there is a second eIF4A helicase bound to the mRNA entry site. The entry channel bound eIF4A is positioned through interactions with eIF3 and the 40S subunit to enable its ATP-dependent helicase activity to directly unwind secondary structure located downstream of the scanning 48S complex. The structure also reveals universally conserved interactions between eIF4F and the 48S, likely explaining how this complex can promote mRNA recruitment in all eukaryotes. mRNA translation has emerged as an important tool for developing innovative therapies, yet several fundamental aspects of its regulation remain unknown. This work sheds light on the critical regulatory roles of eIF4A and eIF4F during the recruitment and scanning of the 5′ UTR of mRNA.