Structural and functional similarities between the central eukaryotic initiation factor (eIF)4A-binding domain of mammalian eIf4G and the eIf4A-binding domain of yeast eIF4G

The translation eukaryotic initiation factor (eIF)4G of the yeast Saccharom yces cerevisiae interacts with the RNA helicase eIF4A (a member of the DEAD -box protein family; where DEAD corresponds to Asp-Glu-Ala-Asp) through a C -terminal domain in eIF4G (amino acids 542-883). Mammalian eIF4G has two in teraction domains for eIF4A, a central domain and a domain close to the C-t erminus. This raises the question of whether eIF4A binding to eIF4G is cons erved between yeast and mammalian cells or whether it is different. We isol ated eIF4G1 mutants defective in eIF4A binding and showed that these mutant s are strongly impaired in translation and growth. Extracts from mutants di splaying a temperature-sensitive phenotype for growth have low in vitro tra nslation activity, which can be restored by addition of the purified eIF4G1 -eIF4E complex, but not by eIF4E alone. Analysis of mutant eIF4G(542-883) p roteins defective in eIF4A binding shows that the interaction of yeast eIF4 A with eIF4G1 depends on amino acid motifs that are conserved between the y east eIF4A-binding site and the central eIF4A-binding domain of mammalian e IF4G. We show that mammalian eIF4A binds tightly to yeast eIF4G1 and, furth ermore, that mutant yeast eIF4G(542-883) proteins, which do not bind yeast eIF4A, do not interact with mammalian eIF4A. Despite the conservation of th e eIF4A-binding site in eIF4G and the strong sequence conservation between yeast and mammalian eIF4A (66 degrees (degrees) identity; 82% similarity at the amino acid level) mammalian eIF4A does not substitute for the yeast fa ctor in vivo and is not functional in a yeast in vitro translation system.