Translational repression by human 4E-BP1 in yeast specifically requires human eIF4E as target

4E-binding proteins (4E-BPs) are believed to have important regulatory func tions in controlling the rate of translation initiation in mammalian cells. They do so by binding to the mRNA cap-binding protein, eIF4E, thereby inhi biting formation of the cap-binding complex, a process essential for cap-de pendent translation initiation. We have reproduced the translation-repressi ve function of human 4E-BP1 in yeast and find its activity to be dependent on substitution of human eIF4E for its yeast counterpart. Translation initi ation and growth are inhibited when human 4E-BP1 is expressed in a strain w ith the human eIF4E substitution, but not in an unmodified strain. We have compared the relative affinities of human 4E-BP1 for human and yeast eIF4E, both in vitro using an m(7)GTP cap-binding assay and in vivo using a yeast two-hybrid assay, and find that the affinity of human 4E-BP1 for human eIF 4E is markedly greater than for yeast eIF4E. Thus yeast eIF4E lacks structu ral features required for binding to human 4E-BP1. These results therefore demonstrate that the features of eIF4E required for binding to 4E-BP1 are d istinct from those required for cap-complex assembly.