Characterization of the Two eIF4A-binding Sites on Human eIF4G-1

Abstract

Eukaryotic translation initiation factor 4G-1 (eIF4G) plays a critical role in the recruitment of mRNA to the 43 S preinitiation complex. eIF4G has two binding sites for the RNA helicase eIF4A, one in the central domain and one in the COOH-terminal domain. Recombinant eIF4G fragments that contained each of these sites separately bound eIF4A with a 1:1 stoichiometry, but fragments containing both sites bound eIF4A with a 1:2 stoichiometry. eIF3 did not interfere with eIF4A binding to the central site. Interestingly, at the same concentration of free eIF4A, more eIF4A was bound to an eIF4G fragment containing both eIF4A sites than the sum of binding to fragments containing the single sites, indicating cooperative binding. Binding of eIF4A to an immobilized fragment of eIF4G containing the COOH-terminal site was competed by a soluble eIF4G fragment containing the central site, indicating that a single eIF4A molecule cannot bind simultaneously to both sites. The association rate constant, dissociation rate constant, and dissociation equilibrium constant for each site were determined by surface plasmon resonance and found to be, respectively, 1. The initiation of translation of most eukaryotic mRNAs involves the sequential recruitment of Met-tRNA i , mRNA, and the 60 S ribosomal subunit to the 40 S ribosomal subunit, catalyzed by the various groups of initiation factors (1). One of the most highly regulated steps is the recruitment of mRNA, which requires recognition of the 5Ј-terminal m 7 GTP-containing cap and 3Ј-terminal poly(A) tract, unwinding of 5Ј-terminal secondary structure, and binding to the 43 S initiation complex. These steps are mediated by members of the eIF4 1 group of initiation factors (eIF4A, eIF4B, eIF4E, and eIF4G) as well as poly(A)-binding protein.eIF4A is the prototypical member of the DEXD/H-box protein family of nucleic acid helicases (2, 3). It functions as an ATPdependent, bi-directional RNA helicase and RNA-dependent ATPase (4 -7). The ␥-phosphate on the bound nucleotide has been shown to mediate changes in eIF4A conformation and RNA affinity. ATP binding and hydrolysis produce conformational changes in eIF4A that alter the RNA-protein interactions (8, 9, 3). A current model for protein synthesis initiation envisions eIF4A in the role of unwinding mRNA secondary structure in the 5Ј-untranslated region to allow the 40 S ribosomal subunit to bind the mRNA and/or scan it for the first AUG. The observation that dominant negative variants of eIF4A inhibit translation is consistent with such a role (10). Interestingly, the RNA helicase activity of eIF4A is ϳ20 times greater when bound to eIF4G than as a free protein (6, 11).There are at least two genes for eIF4G in humans (12-14), wheat germ (15), and yeast (16). In mammals, these are termed eIF4G-1 and eIF4G-2.2 eIF4G serves to colocalize initiation factors involved in mRNA recruitment to the 43 S initiation complex. It directly binds RNA (17-19), poly(A)-binding protein (13), eIF4E (20, 21), the 40 S-binding protein complex eIF3 (20), eIF4A (...

Conducting the initiation of protein synthesis: the role of eIF4G

“…Kinetic binding by surface plasmon resonance indicated that eIF4A associates and dissociates faster with the central than with the carboxyterminal binding site. These results suggest that two molecules of eIF4A can bind in a co-operative manner to each individual binding site on eIF4GI (Korneeva et al, 2001). …”

“…The presence of two binding sites raises the question of whether one or two molecules of eIF4A can simultaneously be bound on eIF4GI. This was directly addressed by Korneeva and colleagues who have used a combination of purified eIF4A and recombinant expressed eIF4GI fragments to show that eIF4A can interact with each of the individual binding site in a 1:1 ratio, whereas fragments of eIF4GI containing the two binding sites exhibited a 1:2 stoichiometry (Korneeva et al, 2001). Kinetic binding by surface plasmon resonance indicated that eIF4A associates and dissociates faster with the central than with the carboxyterminal binding site.…”

“…Finally, the exact position of the carboxy-terminus eIF4A binding site has been mapped between aa 1201-1411 by Morino et al (2000) while the upstream binding site is defined as amino acids 672-970 by Morino et al (2000), 672-876 by Korneeva et al (2000Korneeva et al ( , 2001) and 722-949 by Lomakin et al (2000) (see Fig. 2).…”

Starting the protein synthesis machine: eukaryotic translation initiation

“…(43) There is no consensus as to whether eIF4G binds simultaneously to just one or two eIF4A molecules. (44,45) At the extreme C terminus of metazoan eIF4G is another conserved region termed W2 domain (Fig. 3), with homology to eIF5 and eIF2Be.…”

DAP5 and IRES-mediated translation during programmed cell death

“…Phosphorylation of eIF4E increases its binding affinity to the cap structure and stabilizes the entire eIF4F complex, this way providing positive feedback on the initiation of protein synthesis. 27,29,[32][33][34][35][36][37] Another mechanism of translation initiation, which does not require the mRNA cap structure, exists as well. In this case, instead of scanning of the preinitiation complex from the 5 0 cap toward the first initiation codon, the ribosome lands directly on the internal AUG.…”