Multiple isoforms of eukaryotic protein synthesis initiation factor 4E in Caenorhabditis elegans can distinguish between mono- and trimethylated mRNA cap structures

The rate-limiting step for cap dependent translation initiation in eukaryotes is recruitment of mRNA to the ribosome, An early event in this process is recognition of the m(7)GTP-containing cap structure at the 5'-end of the mRNA by initiation factor eIF4E, In the nematode Caenorhabditis elegans, mRNAs from 70% of the genes contain a different cap structure, m(3)(2,2,7)GTP. This cap structure is poorly recognized by mammalian elF4E, suggesting that C, elegans may possess a specialized form of elF4E that can recognize m(3)(2,2,7)GTP. Analysis of the C. elegans genomic sequence data base revealed the presence of three elF4E-like genes, here named ife-1, ife-2, and ife-3, cDNAs for these three eIF4E isoforms were cloned and sequenced, Isoform-specific antibodies were prepared from synthetic peptides based on nonhomologous regions of the three proteins. All three eIF4E isoforms were detected in extracts of C. elegans and were retained on m(7)GTP-Sepharose. One eIF4E isoform, IFE-1, was also retained on m,2 2 7GTP-Sepharose. Furthermore, binding of IFE-1 and IFE-2 to m(7)GTP-Sepharose was inhibited by m(3)(2,2,7)GTP. These results suggest that IFE-1 and IFE-2 bind both m(7)GTP- and m(3)(2,2,7)GTP-containing mRNA cap structures, although with different affinities. In conjunction with IFE-3, these eIF4E isoforms would permit cap-dependent recruitment of all C, elegans mRNAs to the ribosome.