Structure of mammalian eIF3 in the context of the 43S preinitiation complex

During eukaryotic translation initiation, 43S complexes, comprising a 40S ribosomal subunit, initiator transfer RNA and initiation factors (eIF) 2, 3, 1 and 1A, attach to the 5′-terminal region of messenger RNA and scan along it to the initiation codon. Scanning on structured mRNAs also requires the DExH-box protein DHX29. Mammalian eIF3 contains 13 subunits and participates in nearly all steps of translation initiation. Eight subunits having PCI (proteasome, COP9 signalosome, eIF3) or MPN (Mpr1, Pad1, amino-terminal) domains constitute the structural core of eIF3, to which five peripheral subunits are flexibly linked. Here we present a cryo-electron microscopy structure of eIF3 in the context of the DHX29-bound 43S complex, showing the PCI/MPN core at ∼6 Å resolution. It reveals the organization of the individual subunits and their interactions with components of the 43S complex. We were able to build near-complete polyalanine-level models of the eIF3 PCI/MPN core and of two peripheral subunits. The implications for understanding mRNA ribosomal attachment and scanning are discussed. The cryo-electron microscopy structure of the eukaryotic initiation factor 3 (eIF3) within the larger 43S complex is determined; the improved resolution enables visualization of the secondary structures of the subunits, as well as the contacts between eIF3 and both eIF2 and DHX29. A close-up of the 43S preinitiation complex The eukaryotic initiation factor 3 (eIF3) is a thirteen-subunit accessory factor within the ribosomal 43S complex involved in binding of mRNAs and scanning to find the initiation codon. Yaser Hashem and colleagues have determined the cryo-electron microscopy structure of eIF3 within this larger complex. The improved resolution compared to previous structural determinations makes it possible to visualize the secondary structures of the subunits, as well as the contacts between eIF3 and both eIF2 and DXH29.