Role of GTP in eukaryotic polypeptide-chain initiation. Purification and properties of a factor from Artemia salina embryos which interacts with initiator transfer RNA and guanine nucleotides.

1. A factor, which makes a ternary complex with GTP and eukaryotic initiator tRNA (Met-tRNAi), has been purified 100-fold from developed cysts of Artemia salina. Some of the properties of the purified factor have been studied. 2. Mg2+ appears to inhibit ternary complex formation. 3. Little or no ternary complex is formed when 5 muM GTP is replaced by an identical concentration of UTP, CTP or ATP. The analog, guanosine 5'-(beta, gamma-imino)triphosphate [GMP-P(NHP)] seems to be a much better substitute for GTP than guanosine 5'-(beta, gamma-methylene)triphosphate [GMP-P(CH2)P]. Since GMP-P(NH)P is as effective as GTP in ternary complex formation, it would appear that GTP plays the role of an allosteric effector in this step of eukaryotic polypeptide chain initiation. 4. GDP inhibits both the rate and extent of ternary complex formation. The inhibition is largely reversed by adding a 5-fold molar excess of GTP over GDP. DGDP is slightly less inhibitory than GDP. UDP and CDP are much less inhibitory than GDP and very little inhibition is obtained with ADP. 5. The preformed ternary complex is rapidly and completely destroyed in the presence of N-ethylmaleimide. The results suggest that free--SH groups of the factor may be essential for maintaining the integrity of the ternary complex.