MULTIPLE ISOFORMS OF EUKARYOTIC PROTEIN-SYNTHESIS INITIATION-FACTOR-4E IN CAENORHABDITIS-ELEGANS CAN DISTINGUISH BETWEEN MONOMETHYLATED ANDTRIMETHYLATED MESSENGER-RNA CAP STRUCTURES

The rate-limiting step for cap dependent translation initiation in euk aryotes 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 Caenor habditis 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. el egans 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 o f the three proteins. All three eIF4E isoforms were detected in extrac ts of C. elegans and were retained on m(7)GTP-Sepharose. One eIF4E iso form, IFE-1, was also retained on m,2 2 7GTP-Sepharose. Furthermore, b inding 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 dif ferent affinities. In conjunction with IFE-3, these eIF4E isoforms wou ld permit cap-dependent recruitment of all C, elegans mRNAs to the rib osome.