Deaggregation of eIF4E induced by mRNA 5' cap binding

miR-483-5p orchestrates the initiation of protein synthesis by facilitating the decrease in phosphorylated Ser209eIF4E and 4E-BP1 levels

Eukaryotic initiation factor 4E (eIF4E) is a pivotal protein involved in the regulatory mechanism for global protein synthesis in both physiological and pathological conditions. MicroRNAs (miRNAs) play a significant role in regulating gene expression by targeting mRNA. However, the ability of miRNAs to regulate eIF4E and its phosphorylation remains relatively unknown. In this study, we predicted and experimentally verified targets for miR-483-5p, including eukaryotic translation initiation factor eIF4E and its binding proteins, 4E-BPs, that regulate protein synthesis. Using the Web of Science database, we identified 28 experimentally verified miR-483-5p targets, and by the TargetScan database, we found 1818 predicted mRNA targets, including EIF4E, EIF4EBP1, and EIF4EBP2. We verified that miR-483-5p significantly reduced ERK1 and MKNK1 mRNA levels in HEK293 cells. Furthermore, we discovered that miR-483-5p suppressed EIF4EBP1 and EIF4EBP2, but not EIF4E. Finally, we found that miR-483-5p reduced the level of phosphorylated eIF4E (pSer209eIF4E) but not total eIF4E. In conclusion, our study suggests that miR-483-5p's multi-targeting effect on the ERK1/ MKNK1 axis modulates the phosphorylation state of eIF4E. Unlike siRNA, miRNA can have multiple targets in the pathway, and thereby exploring the role of miR-483-5p in various cancer models may uncover therapeutic options.

Kinetic mechanism for assembly of the m7GpppG.eIF4E.eIF4G complex

Interaction of the mRNA cap with the translational machinery is a critical and early step in the initiation of protein synthesis. To better understand this process, we determined kinetic constants for the interaction of m(7)GpppG with human eIF4E by stopped-flow fluorescence quenching in the presence of a 90-amino acid fragment of human eIF4G that contains the eIF4E-binding domain (eIF4G(557-646)). The values obtained, k(on) = 179 x 10(6) m(-1) s(-1) and k(off) = 79 s(-1), were the same as reported previously in the absence of an eIF4G-derived peptide. We also used surface plasmon resonance to determine kinetic constants for the binding of eIF4E to eIF4G(557-646), both in the presence and absence of m(7)GpppG. The results indicated that eIF4G(557-646) binds eIF4E and eIF4E.m(7)GpppG at the same rate, with k(on) = 3 x 10(6) m(-1) s(-1) and k(off) = 0.01 s(-1). Our data represent the first full kinetic description of the interaction of eIF4E with its two specific ligands. The results demonstrate that the formation of the m(7)GpppG.eIF4E.eIF4G(557-646) complex obeys a sequential, random kinetic mechanism and that there is no preferential pathway for its formation. Thus, even though eIF4G(557-646) binds eIF4E tightly, it does not increase the affinity of eIF4E for m(7)GpppG, as has been claimed in several previous publications. We did, in fact, observe increased binding to m(7)GTP-Sepharose in the presence of eIF4G(557-646), but only with recombinant eIF4E that was prepared from inclusion bodies.

Cap-free structure of eIF4E suggests a basis for conformational regulation by its ligands

The activity of the eukaryotic translation initiation factor eIF4E is modulated through conformational response to its ligands. For example, eIF4G and eIF4E-binding proteins (4E-BPs) modulate cap affinity, and thus physiological activity of eIF4E, by binding a site distal to the 7-methylguanosine cap-binding site. Further, cap binding substantially modulates eIF4E's affinity for eIF4G and the 4E-BPs. To date, only cap-bound eIF4E structures were reported. In the absence of structural information on the apo form, the molecular underpinnings of this conformational response mechanism cannot be established. We report here the first cap-free eIF4E structure. Apo-eIF4E exhibits structural differences in the cap-binding site and dorsal surface relative to cap-eIF4E. Analysis of structure and dynamics of apo-eIF4E, and changes observed upon ligand binding, reveal a molecular basis for eIF4E's conformational response to these ligands. In particular, alterations in the S4-H4 loop, distal to either the cap or eIF4G binding sites, appear key to modulating these effects. Mutation in this loop mimics these effects. Overall, our studies have important implications for the regulation of eIF4E.