The first step in elongation requires two different activities; elongation factor (EF)-1␣ transfers aminoacyltRNA to the ribosome and is released upon hydrolysis of GTP, EF-1␥␦ catalyzes exchange of GDP on EF-1␣ with GTP. To analyze the role of the individual subunits of EF-1 in elongation, the cDNAs for the , ␥, and ␦ subunits of EF-1 from rabbit were cloned, and proteins of 225, 437, and 280 amino acids, respectively, were expressed in Escherichia coli. The purified recombinant  subunit migrates as a dimer and the ␥ subunit as a trimer upon gel filtration, whereas the ␦ subunit forms a large aggregate. Complexes of ␥, ␥␦ and ␥␦ were formed by selfassociation and eluted with a molecular mass of approximately 160, 530, and 670 kDa, respectively; no interaction was observed between  and ␦. The activity of the recombinant subunits was determined with native EF-1␣ by measuring stimulation of the rate of elongation by poly(U)-directed polyphenylalanine synthesis. Recombinant  and ␦ alone stimulated the rate of elongation by 10-fold, with a ratio of 5␣:2 or ␦. The ␥␦ complex stimulated EF-1␣ activity up to 10-fold with a ratio of 20␣ to 1␥␦. Phosphorylation of the  and ␦ subunits alone or in ␥␦ by protein kinase CKII had no effect on the rate of elongation.
Eukaryotic elongation factor (EF)1 1 consists of four subunits, EF-1␣, , ␥, and ␦. EF-1␣ (50 kDa) forms a ternary complex with GTP and aminoacyl-tRNA and transfers the aminoacyl-tRNA to 80 S ribosomes with the hydrolysis of GTP. EF-1␥␦ facilitates the exchange of the GDP bound to EF-1␣ for GTP, initiating another round of elongation (1, 2). EF-1 and ␦ contain the GTP exchange activity (3); ␥ is tightly associated to  and is removed only under denaturing conditions (4). The cDNAs for  and ␥ have been cloned and sequenced from a number of different organisms and tissues (5-13). Recently, EF-1␦ cDNA has been sequenced from human (14) and Xenopus (15), and partial amino acid sequences have been obtained from Artemia (14). A leucine zipper motif in the amino terminus of ␦ is found in all three species (14). EF-1 and ␦ have a highly homologous carboxyl terminus that contains the GDP/GTP exchange activity, whereas the amino-terminal domains differ and appear to be important for regulation of EF-1 activity. The function of the ␥ subunit is unknown, although there is evidence that ␥ can stimulate the nucleotide exchange activity of  (16). EF-1␥ may anchor the complex to the membrane (16) and has been shown to contain a sequence homologous to glutathione S-transferase in the amino-terminal domain, which has been postulated to be involved in regulation of the assembly of multisubunit complexes (17).Eukaryotic EF-1 has been isolated from a variety of organisms and tissues with molecular weights ranging from 50,000 to about 1 ϫ 10 6 . The low molecular weight form is EF-1␣, the intermediate form is EF-1␣␥␦, and the high molecular weight form is a complex of five polypeptides, valyl-tRNA synthetase (ValRS) and .Using purified subunits of EF-1 from Artemia, t...