Yeast Eukaryotic Initiation Factor 4B Remodels the MRNA Entry Site on the Small Ribosomal Subunit

Eukaryotic Initiation Factor 4 (eIF4) is a group of factors that activates mRNA for translation and recruit 43S preinitiation complex (PIC) to the mRNA 5' end, forming the 48S PIC. The eIF4 factors include mRNA 5' cap-binding protein eIF4E, ATP-dependent RNA helicase eIF4A, and scaffold protein eIF4G, which anchors eIF4A and eIF4E. Another eIF4 factor, eIF4B, stimulates the RNA helicase activity of eIF4A and facilitates mRNA recruitment. However, the mechanisms by which eIF4B binds the 40S ribosomal subunit and promotes mRNA recruitment remain poorly understood. Using cryo-Eletron Microscopy (cryo-EM), we obtained a map of the yeast 40S ribosomal subunit in a complex with eIF4B (40S-eIF4B complex). An extra density, tentatively assigned to yeast eIF4B, was observed near the mRNA entry channel of the 40S, contacting ribosomal proteins uS10, uS3, and eS10 as well as rRNA helix h16. Predictive modeling of the 40S-eIF4B complex suggests that the N-terminal domain of eIF4B binds near the mRNA entry channel, overlapping with the extra density observed in the 40S-eIF4B map. The partially open conformation of 40S in the 40S-eIF4B map is incompatible with eIF3j binding observed in the 48S PIC. Additionally, the extra density at the mRNA entry channel poses steric hindrance for eIF3g binding in the 48S PIC. Thus, structural insights suggest that eIF4B facilitates the release of eIF3j and the relocation of the eIF3b-g-i module during mRNA recruitment, thereby advancing our understanding of eIF4B's role in translation initiation.