EIF4G AND ITS PROTEOLYTIC CLEAVAGE PRODUCTS - EFFECT ON INITIATION OFPROTEIN-SYNTHESIS FROM CAPPED, UNCAPPED, AND IRES-CONTAINING MESSENGER-RNAS

Rhinovirus 2A and foot-and-mouth disease virus Lb proteinases stimulat e the translation of uncapped messages and those carrying the rhinovir us and enterovirus Internal Ribosome Entry Segments (IRESes) by a mech anism involving the cleavage of host cell proteins. Here, we investiga te this mechanism using an artificial dicistronic RNA containing the h uman rhinovirus IRES as intercistronic spacer. Because both proteinase s cleave eukaryotic initiation factor 4G (elF4G), we examined whether the cleavage products of elF4G could stimulate uncapped or IRES-driven translation. Addition of intact elF4F to translation extracts inhibit ed IRES-driven translation and reduced the translation stimulation obs erved in reactions pre-treated with Lb proteinase. Prolonged incubatio n of translation extracts with Lb proteinase removed all endogenous el F4G 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 elF4F. I n contrast, addition of pre-cleaved elF4F stimulated translation of un capped or IRES-bearing messages to the levels seen upon proteinase add ition. Furthermore, fractions containing the C-terminal, but not N-ter minal, cleavage product of elF4G stimulated translation moderately. Th ese results demonstrate that the Lb and 2A proteinases stimulate trans lation of uncapped RNAs and those carrying IRESes by the production of cleavage products of elF4G that enhance translation and by the remova l of intact elF4G that interferes with this stimulation.