THE CONVERSION OF EIF-2.GDP TO EIF-2.GTP BY EIF-2B REQUIRES MET-TRANSFER RNAFMET

We have investigated why the recycling of eIF-2·GDP to eIF-2·GTP, mediated by the guanine nucleotide exchange factor eIF-2B, is rapid in rabbit reticulocyte lysate, reconstituted for optimal protein synthesis, but slow in an isolated reaction with purified eIF-2B. We have found that purified eIF-2B dissociates eIF-2·[3H]GDP as efficiently in the presence of GTP as it does in the presence of GDP provided Met-tRNAfMet is added. tRNAfMed is ineffective, and there is no Met-tRNAfMet requirement for exchange with GDP. Exchange of eIF-2 bound GDP for GTP is completely dependent upon Met-tRNAfMet in the presence of ATP, suggesting that under physiological conditions efficient recycling of eIF-2·GDP to eIF-2·GTP requires conversion of the latter, a relatively unstable complex, to a more stable Met-tRNAfMet·eIF-2·GTP complex. © 1991.