Interaction of protein synthesis initiation factor 2 from Xenopus laevis oocytes with GDP and GTP analogs

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 X 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 beta and gamma positions (GDP beta S or GTP gamma S) does not abolish the affinity for the nucleotides and the presence of an imido group between the beta and gamma phosphates in guanyl-5'-yl imidodiphosphate (GppNHp) still permits a weaker but significant binding. Guanine 5'-O-(2-fluorodiphosphate) (GDP beta F) has an affinity considerably lower than GDP beta S. 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 greater than GDP beta S greater than oGDP greater than or equal to GTP gamma S greater than GDP beta F greater than pppGpC greater than GTP greater than GppNHp greater than oGTP much greater than 7-m GDP. Assays of the capacity of GTP analogs to form a ternary complex of the type met-tRNAi X GTP X 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.