Regulation of eukaryotic initiation factor-2B activity by polyamines and amino acid starvation in rabbit reticulocyte lysate

We recently reported that the translational control of protein synthesis by glucose 6-phosphate in gel-filtered, rabbit reticulocyte lysate is exerted on the activity of eukaryotic initiation factor (eIF)-2B, the factor that catalyzes the exchange of GTP for GDP bound to eIF-2, by a mechanism that is independent of the phosphorylation of eIF-2 (alpha subunit). We now demonstrate that two other conditions regulate the activity of eIF-2B in rabbit reticulocyte lysate: polyamines (spermidine and spermine) and amino acid deficiency. In the absence of added polyamines, protein synthesis in gel-filtered lysate is reduced to about 70% and eIF-2B activity to about 35% of optimal. The former is likely a result of the latter, since we find that reticulocyte lysate has about twice the eIF-2B necessary to recycle the eIF-2.GDP generated under conditions of optimal protein synthesis. In contrast, the reduction in eIF-2B activity (to about 50% of optimal) occurring in the absence of added amino acids in unfractionated or gel-filtered lysate is insufficient, by itself, to slow the rate of protein synthesis, and the inhibition of protein synthesis that does occur with amino acid deficiency is exerted on polypeptide chain elongation, not initiation. The reduction in eIF-2B activity occurring with amino acid deficiency cannot be reversed by adding more glucose 6-phosphate or polyamines nor can the reduced eIF-2B activity seen with polyamine deficiency be overcome by increasing the glucose 6-phosphate, suggesting that these three components regulate eIF-2B activity by different mechanisms.