eIF4G is the scaffold subunit of the eIF4F complex, whose binding domains for eIF4E and poly(A)-binding protein (PABP) are thought to enhance formation of activated eIF4F K mRNA K PABP complexes competent to recruit 43S pre-initiation complexes. We found that the RNA-binding region (RNA1) in the N-terminal domain (NTD) of yeast eIF4G1 can functionally substitute for the PABP-binding segment to rescue the function of an eIF4G1-459 mutant impaired for eIF4E binding. Assaying RNA-dependent PABP-eIF4G association in cell extracts suggests that RNA1, the PABP-binding domain, and two conserved elements (Box1 and Box2) between these segments have overlapping functions in forming native eIF4GKmRNAK PABP complexes. In vitro experiments confirm the role of RNA1 in stabilizing eIF4G-mRNA association, and further indicate that RNA1 and Box1 promote PABP binding, in addition to RNA binding, by the eIF4G1 NTD. Our findings indicate that PABP-eIF4G association is only one of several interactions that stabilize eIF4FKmRNA complexes, and emphasize that closed-loop mRNP formation via PABP-eIF4G interaction is non-essential in vivo. Interestingly, two other RNA-binding regions in eIF4G1 have critical functions downstream of eIF4F K mRNA assembly.

Background: Mammalian eIF4B stimulates eIF4A helicase activity, but its function in promoting translation initiation in yeast is unclear.Results: Yeast eIF4B enhances eIF4G⅐eIF4A association in vivo and in vitro. Conclusion: yeIF4B stimulates eIF4F assembly by promoting an eIF4G HEAT domain conformation conducive for binding eIF4A. Significance: A new function is established for eIF4B of supporting eIF4F assembly for mRNA activation.