Binding of ATP and messenger RNA by the β-subunit of eukaryotic initiation factor 2

In addition to forming a ternary complex with Met-tRNA_f and GTP, eukaryotic initiation factor 2 (eIF-2) recognizes a specific site in mRNA molecules. Both binding activities are regulated by ATP, which itself binds tightly and specifically to eIF-2. Denaturation of eIF-2 with urea leads to complete loss of Met-tRNA_f binding activity, while mRNA binding activity is stable. Hence, distinct conformational features in eIF-2 are required for ternary complex formation and for binding of mRNA. Chromatography of eIF-2 over ATP-agarose, in denaturing conditions that induce polypeptide subunit dissociation, results in selective retention of the β-subunit of eIF-2. Isolated β-subunit is capable of binding mRNA as well as ATP. Cibacron blue 3G-A binds tightly to eIF-2 and inhibits the binding of mRNA. This inhibition is relieved upon addition of ATP, showing that Cibacron blue 3G-A competes with ATP for eIF-2. eIF-2β subunit, active in binding of mRNA, is recovered upon chromatography of eIF-2 in denaturing conditions over matrix-bound Cibacron blue 3G-A. These results show that the ability of eIF-2 to bind mRNA and its ability to bind ATP are both lodged within remarkably stable domains of its β-subunit. During initiation of protein synthesis, the eIF-2β subunit may thus interact with three ligands important for translational control: Met-tRNA_f, mRNA and ATP.