BINDING OF EUKARYOTIC TRANSLATION INITIATION-FACTOR 4E (EIF4E) TO EIF4G REPRESSES TRANSLATION OF UNCAPPED MESSENGER-RNA

mRNA translation in crude extracts from the yeast Saccharomyces cerevi siae is stimulated by the cap structure and the poly(A) tail through t he binding of the cap-binding protein eukaryotic translation initiatio n factor 4E (eIF4E) and the poly(A) tail-binding protein Pab1p. These proteins also bind to the translation initiation factor eIF4G and ther eby link the mRNA to the general translational apparatus. In contrast, uncapped, poly(A)-deficient mRNA is translated poorly in yeast extrac ts, in part because of the absence of eIF4E and Pab1p binding sites on the mRNA. Here, we report that uncapped-mRNA translation is also repr essed in yeast extracts due to the binding of eIF4E to eIF4G, Specific ally, we find that mutations which weaken the eIF4E binding site on th e yeast eIF4G proteins Tif4631p and Tif4632p lead to temperature-sensi tive growth in vivo and the stimulation of uncapped-mRNA translation i n vitro. A mutation in eIF4E which disturbs its ability to interact wi th eIF4G also leads to a stimulation of uncapped-mRNA translation in v itro. Finally, overexpression of eIF4E in vivo or the addition of exce ss eIF4E in vitro reverses these effects of the mutations, These data support the hypothesis that the eIF4G protein can efficiently stimulat e translation of exogenous untapped mRNA in extracts but is prevented from doing so as a result of its association with eIF4E. They also sug gest that some mRNAs may be translationally regulated in vivo in respo nse to the amount of free eIF4G in the cell.