THE C-TERMINAL DOMAIN OF EUKARYOTIC PROTEIN-SYNTHESIS INITIATION-FACTOR (EIF) 4G IS SUFFICIENT TO SUPPORT CAP-INDEPENDENT TRANSLATION IN THE ABSENCE OF EIF4E

The foot and mouth disease virus, a picornavirus, encodes two forms of a cysteine proteinase (leader or L protease) that bisects the eIF4G p olypeptide of the initiation factor complex eIF4F into N-terminal (N-t ) and C-tenninal (C-t,) domains, Previously we showed that, although i rt vitro cleavage of the translation initiation factor, eIF4G, with L protease decreases cap-dependent translation, the cleavage products th emselves may directly promote cap-independent protein synthesis. We no w demonstrate that translation of uncapped mRNAs normally exhibits a s trong requirement for eIF4E. However, this dependence is abolished whe n eIF4G is cleaved, with the C-t domain capable of supporting translat ion in the absence of the N-t domain. In contrast, the efficient trans lation of the second cistron of bicistronic mRNAs, directed by two dis tinct Internal Ribosome Entry Segments (IRES), exhibits no requirement for eIF4E but is dependent upon either intact eIF4G or the C-t domain . These results demonstrate that: (i) the apparent requirement for eIF 4F for internal initiation on IRES-driven mRNAs can be fulfilled by th e C-t proteolytic cleavage product; (ii) when eIF4G is cleaved, the C- t domain can also support cap-independent translation of cellular mRNA s not possessing an IRES element, in the absence of eIF4E; and (iii) w hen eIF4G is intact, translation of cellular mRNAs, whether capped or uncapped, is strictly dependent upon eIF4E. These data complement rece nt work in other laboratories defining the binding sites for other ini tiation factors on the eIF4G molecule.