The structural specificity of the purified protein synthesis initiation factor 2 (eIF-2) from X. laevis ovary towards analogs of GTP and GDP was studied. The relative affinity of the structural analogs was measured by their capacity to inhibit the formation of the [3H]GDP·eIF-2 binary complex. The results obtained demonstrate that modifications in the ribose moiety are well tolerated by eIF-2 which binds dGTP, 2′,3′-dialdehyde GTP (oGTP) and 2′,3′-dialdehyde GDP (oGDP) and even the dinucleotide cytidylyl(5′-3′)guanosine 5′-triphosphate (pppGpC). Substitution in the polyphosphate chain by phosphorothioate groups in the β and γ positions (GDPβS or GTPγS) does not abolish the affinity for the nucleotides and the presence of an imido group between the β and γ phosphates in guanyl-5′-yl imidodiphosphate (GppNHp) still permits a weaker but significant binding. Guanine 5′-O-(2-fluorodiphosphate) (GDPβF) has an affinity considerably lower than GDPβS. Methylation of position 7 of the guanine (7-m GDP), however, completely eliminates the interaction of GDP with eIF-2. The analogs tested can be listed in the following order of descending affinities: GDP > GDPβS > oGDP⩾ GTPγS > GDPβF > pppGpC > GTP > GppNHp > oGTP ⪢ 7-m GDP. Assays of the capacity of GTP analogs to form a ternary complex of the type met-tRNAi·GTP·eIF-2 or of GDP analogs to inhibit the formation of this complex reflect, in general, the same order of relative affinities except for pppGpC, which is weaker in its capacity to form a ternary complex than GppNHp or oGTP, although it has a higher affinity than these compounds in the formation of a binary complex.
