We have isolated and characterized a novel homolog of eIF4G referred to as eIF4GII. eIF4GII is the second homolog of eIF4GI to be reported, the first being p97/NAT1/DAP-5. p97 differs from both eIF4GI and eIF4GII in that it does not interact with eIF4E, which is consistent with the lack of an eIF4E binding site (15
). As expected from the sequence homology, eIF4GII interacts with eIF4E, eIF4A, and eIF3. We have demonstrated that eIF4GII is a bona fide functional homolog of eIF4G by showing that eIF4GII in combination with eIF4E, but not alone, is capable of restoring cap-dependent translation in reticulocyte lysate treated with rhinovirus 2Apro
. In addition, eIF4GII is present in an eIF4F complex from HeLa cells, because it could be purified by m7
GDP affinity chromatography. We were not able to determine whether rabbit eIF4F preparations contain both eIF4GI and eIF4GII, because anti-eIF4GII antibody did not react with the rabbit eIF4F high-molecular-weight polypeptides (12a
Why have two different eIF4G proteins? One interesting possibility is that the two eIF4G proteins could be required for the translation of different classes of mRNAs. Although we found that eIF4GII can stimulate cap-dependent translation of CAT almost as efficiently as eIF4GI in a translation extract devoid of intact eIF4G, it is possible that eIF4GI and eIF4GII activities exhibit preferential mRNA binding. Alternatively, it is possible that both eIF4G forms are required for maximal translation of a given mRNA. Consistent with this, neither of the eIF4G forms could restore the translation of CAT mRNA in a 2Apro
-treated extract as efficiently as eIF4F (Fig. ). (We could not further address this issue because of our inability to achieve higher concentrations of the proteins for the limited volume of the in vitro translation system.) However, it is also possible that neither of the eIF4G isoforms was as active as endogenous eIF4G, because the recombinant eIF4GII and eIF4GI lack N-terminal sequences. It is of interest that there are also two forms of eIF4A, and both of these forms were shown to be associated with eIF4F, but in different amounts (8
). Since eIF4F might contain a mixture of eIF4GI and eIF4GII, it would be interesting to determine if each of the eIF4G polypeptides recognizes a different form of eIF4A.
Different functions for the two forms of eIF4G in yeast and plants have not been described. Recently, it was reported that the two eIF4G forms of yeast (TIF4631 and TIF4632 [12
]) could synergize with the poly(A) binding protein (Pabp) in stimulating cap-dependent translation (42
). Since eIF4G in yeast interacts directly with Pabp, it was of interest to determine if the two human eIF4G species could also interact with mammalian Pabp. We could not demonstrate binding of either of the human eIF4G species to Pabp (8a
). Perhaps there are other forms of eIF4G-like proteins that mediate the action of mammalian Pabp in translation initiation, or perhaps an extended N-terminal form of eIF4GI that is not yet available can bind to mammalian Pabp.
The finding of a new homolog of eIF4G may have important implications for the understanding of the shutoff of host protein synthesis following infection with several picornaviruses. The cleavage of eIF4G precedes the shutoff of host protein synthesis and is thought to be the major cause of the inhibition of translation (10
). However, under certain conditions, for example, when replication of poliovirus is inhibited with drugs, cleavage of eIF4GI occurs without complete shutoff of host protein synthesis (5
). These results led Bonneau and Sonenberg to conclude that cleavage of eIF4G (p220) is not sufficient for complete inhibition of host cell protein synthesis and that an additional event is required (5
). Pérez and Carrasco concluded, based on similar results, that eIF4G cleavage is not responsible for the shutoff of host protein synthesis (16
). However, the presence of a functional homolog of eIF4G raises the intriguing possibility that eIF4GII is more resistant to cleavage by 2Apro
than eIF4GI. Although we showed here that eIF4GII is degraded in infected cells, preliminary evidence suggests that it is less sensitive to cleavage, in that infection with poliovirus in the presence of drugs results in cleavage of eIF4GI, but not eIF4GII (12a
eIF4G is thought to play a major role as an adapter molecule in the assembly of the 43S ribosomal preinitiation complex (14
). The discovery of a functional homolog of eIF4G in this study, as well as a truncated eIF4G homolog, p97, earlier (15
), establishes the existence of an eIF4G family and raises the possibility that other eIF4G-like molecules could exist. It is possible that the various eIF4G adapters play important and specific roles in regulating translation in response to different stimuli or during different stages of development and differentiation.