PROTEOLYTIC CLEAVAGE OF INITIATION-FACTOR EIF-4-GAMMA IN THE RETICULOCYTE LYSATE INHIBITS TRANSLATION OF CAPPED MESSENGER-RNAS BUT ENHANCESTHAT OF UNCAPPED MESSENGER-RNAS

Infection of cells with the foot-and-mouth-disease virus, a member of the picornavirus family, results in the shut-off of host protein synth esis. A major contributory mechanism is the proteolytic destruction of the gamma subunit of the complex eIF-4, which functions in translatio n to promote the binding of the 43S ribosomal preinitiation complex to the 5' end of cellular mRNA molecules bearing a 5' terminal cap struc ture. Picornavirus RNA molecules, which are uncapped, use a distinct m echanism for translational initiation, which can operate in the absenc e, 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 eluci date in more detail the role of this polypeptide in the control of tra nslation. Using in vitro transcribed mRNAs we have observed a marked c ontrast between capped and uncapped transcripts in the response of the ir translation to the proteolysis of eIF-4 gamma. Translation of cappe d mRNAs is, as expected, severely impaired, and is restored by additio n of eIF-4 complex containing the intact gamma-subunit. On the other h and, translation of uncapped transcripts, normally inefficient, is sub stantially enhanced. The data suggest that the translation of uncapped mRNAs may be stimulated in this system by one or more of the proteoly tic degradation products of eIF-4 gamma.