Multiple elements in the eIF4G1 N-terminus promote assembly of eIF4G1•PABP mRNPs in vivo.

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•mRNA•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 eIF4G•mRNA•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 eIF4F•mRNA 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•mRNA assembly.