The proteolytic cleavage of eukaryotic initiation factor (eIF) 4G is prevented by eIF4E binding protein (PHAS-I; 4E-BP1) in the reticulocyte lysate.

A common feature of viral infection is the subversion of the host cell machinery towards the preferential translation of viral products. In some instances, this is partly mediated by the expression of virally encoded proteases which lead to the cleavage of initiation factor eIF4G. The foot-and-mouth disease virus encodes two forms of a cysteine proteinase (L protease) which bisects the eIF4G polypeptide into an N-terminal fragment containing the eIF4E binding site, and a C-terminal fragment which contains binding sites for eIF4A and eIF3 and which associates with the 40S ribosomal subunit. Previously, we have demonstrated that the cleavage of eIF4G by L protease stimulates the translation of uncapped transcripts encoding cellular proteins and supports internal initiation driven by picornavirus internal ribosome entry segment (IRES) elements. Use of reticulocyte lysates manipulated to deplete them of eIF4E and the N-terminal fragment suggests that the C-terminal fragment of eIF4G is responsible for these effects, and we have now confirmed this by purifying the C-terminal fragment and analysing its effects directly in the absence of L protease. Interestingly, we find that pre-incubation of reticulocyte lysates or ribosomal salt wash fractions with the specific eIF4E binding protein, PHAS-I (eIF4E-BP1), blocks the proteolytic cleavage of eIF4G by L protease. This effect can be reversed by addition of recombinant eIF4E. These data are consistent with a model whereby the L protease cleavage site in eIF4G is inaccessible until a change in conformation is induced by the binding of eIF4E. This may have implications for a role for eIF4E binding in triggering changes that expose other domains in the eIF4G molecule during initiation of translation.