Role of Yeast Peptide Elongation Factor 3 (EF-3) at the AA-tRNA Binding Step1

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Abstract
The stimulatory effect of peptide elongation factor 3 (EF-3) which is uniquely required for the yeast elongation cycle, on the step of binding of aminoacyl-tRNA (AA-tRNA) to ribosomes has been investigated in detail. Yeast EF-lα apparently functions in a stoichiometric manner in the binding reaction of AA-tRNA to the ribosomes.The addition of EF-3 and ATP to this binding system strikingly stimulated the binding reaction, and the stimulated reaction proceeded catalytically with respect to both EF-lα and EF-3, ac companied by ATP hydrolysis, indicating that EF-3 stimulated the AA-tRNA binding reaction by releasing EF-lα from the ribosomal complex, thus recycling it. This binding stimulation by EF-3 was in many respects distinct from that by EF-1βγ The idea that EF-3 may participate in the regeneration of GTP from ATP and the formed GDP, as indicated by the findings that the addition of EF-3 along with ATP allowed the AA-tRNA binding and Phepolymerization reactions to proceed even in the presence of GDP in place of GTP, was not verified by the results of direct measurement of [32P] formation from [α-32P] and GDP under various conditions. Examination of the stability of thebound AA-tRNA disclosed the different binding states of AA-tRNA on ribosomes between in the cases of the complexes formed with EF-lα alone, or factor-independently, and with EF-lα and EF-3. Furthermore, we found thatwhereas the EF-lα-promoted reaction allowed the binding of noncognate AA-tRNA to a certain extent, the EF-3-stimulated reaction strictly selected to bind only cognate AA-tRNA correctly pairing between codon and anticodon. Thus, we concluded that, for AA-tRNA binding to ribosomes, at first EF-lα carries AA-tRNA in the ternary complex with GTP to the ribosomal introducing site (I-site) with a little binding of noncognate AA-tRNA, and thenEF-3 plays a key role in the strict selection of cognate AA-tRNA and in its transfer from the I- to the A-site, by changing its binding state, accompanied by ATP hydrolysis.

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