Proteolytic cleavage of initiation factor eIF-4 gamma in the reticulocyte lysate inhibits translation of capped mRNAs but enhances that of uncapped mRNAs

Abstract

Infection of cells with the foot-and-mouth-disease virus, a member of the picornavirus family, results in the shut-off of host protein synthesis. A major contributory mechanism is the proteolytic destruction of the gamma subunit of the complex eIF-4, which functions in translation to promote the binding of the 43S ribosomal preinitiation complex to the 5' end of the cellular mRNA molecules bearing a 5' terminal cap structure. Picornavirus RNA molecules, which are uncapped, use a distinct mechanism for translational initiation, which can operate in the absence, or at low levels, of eIF-4. The proteolysis of eIF-4 gamma in cells infected by foot-and-mouth-disease virus results from expression of a virus-encoded cysteine proteinase known as Leader (or L) protease. We have used a transcription plasmid encoding this protease as a tool to deplete in vitro translation systems of eIF-4 gamma in order to elucidate in more detail the role of this polypeptide in the control of translation. Using in vitro transcribed mRNAs we have observed a marked contrast between capped and uncapped transcripts in the response of their translation to the proteolysis of eIF-4 gamma. Translation of capped mRNAs is, as expected, severely impaired, and is restored by addition of eIF-4 complex containing the intact gamma-subunit. On the other hand, translation of uncapped transcripts, normally inefficient, is substantially enhanced. The data suggest that the translation of uncapped mRNAs may be stimulated in this system by one or more of the proteolytic degradation products of eIF-4 gamma.