Physical association of eukaryotic initiation factor (eIF) 5 carboxyl-terminal domain with the lysine-rich eIF2β segment strongly enhances its binding to eIF3

The carboxyl-terminal domain (CTD) of eukaryotic initiation factor (eIF) 5 interacts with eIF1, eIF2β, and eIF3c, thereby mediating formation of the multifactor complex (MFC), an important intermediate for the 43 S preinitiation complex assembly. Here we demonstrate in vitro formation of a nearly stoichiometric quaternary complex containing eIF1 and the minimal segments of eIF2β, eIF3c, and eIF5. In vivo, overexpression of eIF2 and tRNA _i^Met suppresses the temperature-sensitive phenotype of tif5-7A altering eIF5-CTD by increasing interaction of the mutant eIF5 with eIF2 by mass action and restoring its defective interaction with eIF3. By contrast, overexpression of eIF1 exacerbated the tif5-7A phenotype because eIF1 forms unusual inhibitory complexes with a hyperstoichiometric amount of eIF1. Formation of such complexes leads to increased GCN4 translation, independent of eIF2 phosphorylation (general control derepressed or Gcd^- phenotype). We also provide biochemical evidence indicating that the association of eIF5-CTD with eIF2β strongly enhances its binding to eIF3c. Our results suggest strongly that MFC formation is an ordered event involving specific enhancement of eIF5-CTD binding to eIF3 on its binding to eIF2β. We propose that the primary function of eIF5-CTD is to serve as an assembly guide by rapidly promoting stoichiometric MFC assembly with the aid of eIF2 while excluding formation of nonfunctional complexes.