Isolation of a heme-controlled inhibitor of translation that blocks the interaction between messenger rna and eukaryotic initiation factor 2

Modification of eukaryotic initiation factor 4F during infection by influenza virus

Influenza virus infection of cells is accompanied by a striking shutoff of cellular protein synthesis, resulting in the exclusive translation of viral mRNAs. The mechanism for control of cellular protein synthesis by influenza virus is poorly understood, but several translation properties of influenza virus mRNAs which are potentially involved have been described. Influenza virus mRNAs possess the surprising ability to translate in the presence of inhibitory levels of inactive (phosphorylated) eukaryotic initiation factor 2 (eIF-2). In addition, influenza virus mRNAs were shown to be capable of translating in cells during the late phase of adenovirus infection but not in cells infected by poliovirus. Since both adenovirus and poliovirus facilitate virus-specific translation by impairing the activity of initiation factor eIF-4F (cap-binding protein complex) but through different mechanisms, we investigated the translation properties of influenza virus mRNAs in more detail. We show that influenza virus infection is associated with the significant dephosphorylation and inactivation of eIF-4E (cap-binding protein), a component of eIF-4F, and accordingly that influenza virus mRNAs possess a moderate ability to translate by using low levels of eIF-4F. We also confirm the ability of influenza virus mRNAs to translate in the presence of high levels of inactive (phosphorylated) eIF-2 but to a more limited extent than reported previously. We suggest a potential mechanism for the regulation of protein synthesis by influenza virus involving a decreased requirement for large pools of active eIF-4F and eIF-2.

Binding of ATP and messenger RNA by the beta-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 beta-subunit of eIF-2. Isolated beta-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-2 beta 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 beta-subunit. During initiation of protein synthesis, the eIF-2 beta subunit may thus interact with three ligands important for translational control: Met-tRNA(f), mRNA and ATP.

Distinct epitopes in eukaryotic initiation factor 2 for binding of mRNA and for ternary complex formation with methionyl-tRNA(f) and GTP

Eukaryotic initiation factor 2 (eIF-2) forms a ternary complex with methionyl-tRNA(fMet) and GTP on one hand, and it binds to a specific site in mRNA molecules on the other. Antibodies directed against eIF-2 were used to analyze these dual binding activities. A monoclonal antibody directed against the beta-subunit of eIF-2, 5A4, is able to inhibit ternary complex formation as well as binding of mRNA, showing that this subunit is essential for both binding activities of eIF-2. However, a polyclonal antibody, PR1, is able to distinguish between these activities in the eIF-2 molecule. In the presence of PR1, binding of mRNA by eIF-2 is inhibited completely, yet ternary complex formation with methionyl-tRNA(fMet) and GTP is stimulated more than 5-fold. Apparently, specific antibodies to eIF-2 can induce a conformational change in inactive factor molecules that permits them to form ternary complexes. These results show that distinct epitopes in eIF-2 are involved in binding of mRNA and in ternary complex formation with methionyl-tRNA(fMet) and GTP.

Elevated adenosine deaminase activity and hereditary hemolytic anemia. Evidence for abnormal translational control of protein synthesis

We have investigated the molecular basis of the marked elevation in erythrocyte adenosine deaminase (ADA) activity in a kindred with hereditary hemolytic anemia. Red cell ADA-specific activity was verified to be 70- to 100-fold normal levels. Western blots demonstrated a corresponding increase in erythrocyte ADA-specific immunoreactive protein. Analysis of genomic DNA revealed no evidence for amplification or major structural changes in the ADA gene. ADA-specific messenger RNA (mRNA) from proband reticulocytes was comparable in size and amount to mRNA from control reticulocytes. Translation of proband poly A+ reticulocyte mRNA in a rabbit reticulocyte lysate system and immunoprecipitation of 35S-labeled protein products with anti-ADA antibody yielded a band of approximately 42,000 apparent mol wt that was absent in translation products from control reticulocyte mRNAs. These data suggest that the increased ADA activity in red cells in this disorder results from the increased translation of an aberrant ADA mRNA.