elF4G and its proteolytic cleavage products: effect on initiation of protein synthesis from capped, uncapped, and IRES-containing mRNAs.

Rhinovirus 2A and foot-and-mouth disease virus Lb proteinases stimulate the translation of uncapped messages and those carrying the rhinovirus and enterovirus Internal Ribosome Entry Segments (IRESes) by a mechanism involving the cleavage of host cell proteins. Here, we investigate this mechanism using an artificial dicistronic RNA containing the human rhinovirus IRES as intercistronic spacer. Because both proteinases cleave eukaryotic initiation factor 4G (eIF4G), we examined whether the cleavage products of eIF4G could stimulate uncapped or IRES-driven translation. Addition of intact eIF4F to translation extracts inhibited IRES-driven translation and reduced the translation stimulation observed in reactions pre-treated with Lb proteinase. Prolonged incubation of translation extracts with Lb proteinase removed all endogenous eIF4G and a substantial amount of the primary C- and N-terminal cleavage products. The translation of all mRNAs was reduced in such extracts. Capped mRNA translation was rescued by the addition of intact eIF4F. In contrast, addition of pre-cleaved eIF4F stimulated translation of uncapped or IRES-bearing messages to the levels seen upon proteinase addition. Furthermore, fractions containing the C-terminal, but not N-terminal, cleavage product of eIF4G stimulated translation moderately. These results demonstrate that the Lb and 2A proteinases stimulate translation of uncapped RNAs and those carrying IRESes by the production of cleavage products of eIF4G that enhance translation and by the removal of intact eIF4G that interferes with this stimulation.