Eukaryotic initiation factors 4A (eIF4A) and 4G (eIF4G) mutually interact in a 1 : 1 ratio in vivo

mRNA translation in eukaryotic cells involves a set of proteins termed tran slation initiation factors (eIFs), several of which are involved in the bin ding of ribosomes to mRNA. These include eIF4G, a modular scaffolding prote in, and eIF4A, an RNA helicase, of which two closely related forms are know n in mammals, eIF4A(I) and eIF4A(II). In mammals, eIF4G possesses two indep endent sites for binding eIF4A, whereas in other eukaryotes (e.g. yeast) on ly one site appears to be present, thus raising the issue of the stoichiome try of eIF4G.eIF4A complexes in different eukaryotes. We show that in human embryonic kidney cells eIF4G is associated with eIF4A(I) or eIF4A(II) but not with both simultaneously, suggesting a stoichiometry of 1:1 rather than 1:2. To confirm this, eIF4A(I) or eIF4A(II) was expressed in a tagged form in these cells, and complexes with eIF4G were again isolated. Complexes co ntaining tagged eIF4A(I) or eIF4A(II) contained no endogenous eIF4A, suppor ting the notion that eIF4G binds only one molecule of eIF4A. Each binding s ite in eIF4G can bind either eIF4A(I) or eIF4A(II). The data imply that the second binding site in mammalian eIF4A does not bind an additional eIF4A m olecule and that initiation factor complexes in different eukaryotes contai n one eIF4A per eIF4G.