Structural insights into the mammalian late-stage initiation complexes

In higher eukaryotes, the mRNA sequence in direct vicinity of the start codon, called the Kozak sequence (CRCCaugG, where R is a purine), is known to influence the rate of the initiation process. However, the molecular basis underlying its role remains poorly understood. Here, we present the cryo-electron microscopy (cryo-EM) structures of mammalian late-stage 48S initiation complexes (LS48S IC) in the presence of two different native mRNA sequences, {beta}-globin and histone 4 (H4) at overall resolution of 3[A] and 3.5[A], respectively. Our high-resolution structures unravel key interactions from the mRNA to eukaryotic initiation factors (eIF): 1A, 2, 3, 18S rRNA, and several 40S ribosomal proteins. In addition, we were able to study the structural role of ABCE1 in the formation of native 48S ICs. Our results reveal a comprehensive map of the ribosome/eIFs -mRNA and -tRNA interactions and suggest the impact of mRNA sequence on the structure of the LS48S IC.