Rate-limiting Step Of Translation Research Articles

Interaction of three Caenorhabditis elegans isoforms of translation initiation factor eIF4E with mono- and trimethylated mRNA 5' cap analogues.

Translation initiation factor eIF4E binds the m(7)G cap of eukaryotic mRNAs and mediates recruitment of mRNA to the ribosome during cap-dependent translation initiation. This event is the rate-limiting step of translation and a major target for translational control. In the nematode Caenorhabditis elegans, about 70% of genes express mRNAs with an unusual cap structure containing m(3)(2,2,7)G, which is poorly recognized by mammalian eIF4E. C. elegans expresses five isoforms of eIF4E (IFE-1, IFE-2, etc.). Three of these (IFE-3, IFE-4 and IFE-5) were investigated by means of spectroscopy and structural modelling based on mouse eIF4E bound to m(7)GDP. Intrinsic fluorescence quenching of Trp residues in the IFEs by iodide ions indicated structural differences between the apo and m(7)G cap bound proteins. Fluorescence quenching by selected cap analogues showed that only IFE-5 forms specific complexes with both m(7)G- and m(3)(2,2,7)G-containing caps (K(as) 2 x 10(6) M(-1) to 7 x 10(6) M(-1)) whereas IFE-3 and IFE-4 discriminated strongly in favor of m(7)G-containing caps. These spectroscopic results quantitatively confirm earlier qualitative data derived from affinity chromatography. The dependence of K(as) on pH indicated optimal cap binding of IFE-3, IFE-4 and IFE-5 at pH 7.2, lower by 0.4 pH units than that of eIF4E from human erythrocytes. These results provide insight into the molecular mechanism of recognition of structurally different caps by the highly homologous IFEs.

Protein-protein interactions required during translation.

Protein synthesis requires the involvement of numerous accessory factors that assist the ribosome in translation initiation, elongation, and termination. Extensive protein-protein and protein-RNA interactions are required to bring together the accessory factors, tRNAs, ribosomes, and mRNA into a productive complex and these interactions undergo dynamic alterations during each step of the translation initiation process. Initiation represents the most complex aspect of translation, requiring more accessory proteins, called initiation factors, than either elongation or termination. Not surprisingly, initiation is most often the rate-limiting step of translation and, as such, most (but not all) examples of translational regulation involve the regulation of protein-protein or protein-RNA interactions of the initiation complex. In this review, we focus on those interactions required for efficient translation initiation and how such interactions are regulated by developmental or environmental signals.

Assignment of the human gene encoding eukaryotic initiation factor 4E (EIF4E) to the region q21-25 on chromosome 4.

We recently cloned genomic sequences containing the promoter region for the messenger RNA cap binding protein (eIF4E). As the rate-limiting step in translation, eukaryotic initiation factor 4E is important in cellular growth control. Using oligonucleotide primers specific for the promoter region in polymerase chain reactions (PCR), we amplified the human gene in a chromosome 4-specific human/rodent somatic cell panel. This panel mapped single copy genomic sequences for eIF4E in the region 4q21 to 4q25.