Molecular Landscape of the Ribosome Pre-initiation Complex during mRNA Scanning: Structural Role for eIF3c and Its Control by eIF5.

During eukaryotic translation initiation, eIF3 binds the solvent-accessible side of the 40S ribosome and recruits the gate-keeper protein eIF1 and eIF5 to the decoding center. This is largely mediated by the N-terminal domain (NTD) of eIF3c, which can be divided into three parts: 3c0, 3c1, and 3c2. The N-terminal part, 3c0, binds eIF5 strongly but only weakly to the ribosome-binding surface of eIF1, whereas 3c1 and 3c2 form a stoichiometric complex with eIF1. 3c1 contacts eIF1 through Arg-53 and Leu-96, while 3c2 faces 40S protein uS15/S13, to anchor eIF1 to the scanning pre-initiation complex (PIC). We propose that the 3c0:eIF1 interaction diminishes eIF1 binding to the 40S, whereas 3c0:eIF5 interaction stabilizes the scanning PIC by precluding this inhibitory interaction. Upon start codon recognition, interactions involving eIF5, and ultimately 3c0:eIF1 association, facilitate eIF1 release. Our results reveal intricate molecular interactions within the PIC, programmed for rapid scanning-arrest at the start codon.