RESUMEN
A cyclic hexapeptide, RA-VII isolated from the Rubiaceae family of plants, has high cytotoxic activity. Although RA-VII has been shown to inhibit protein synthesis in eukaryotic cells, the molecular mode of its action is not clear. Here we investigate the mechanism of the RAVII action on the translation apparatus. Biochemical functional assays showed that RA-VII inhibits poly(U)-dependent polyphenylalanine synthesis in the presence of animal elongation factors eEF1A and eEF2. Furthermore, RAVII prevented eEF2/ribosome-dependent GTPase activity, but not eEF-1A/ribosome-dependent activity. A filter binding assay demonstrated that RA-VII markedly enhances the binding affinity of eEF2 for GTP, but not for GDP, and prevents exchange of GTP in the eEF2-GTP complex, even after addition of a large excess of GTP/GDP. Limited proteolysis experiments indicated that RA-VII prevents the digestion of eEF2 in the presence of either GTP or GMPPCP, but not with GDP. Further footprint analysis and a translocation assay showed that the eEF2â¢GMPPNPâ¢RA-VII complex binds to the conserved rRNA regions at the factor-binding center of the ribosome and retains the ability to translocate the A site-bound tRNA to the P-site. These results suggest that RA-VII tightly stabilizes the GTPâ¢eEF2 complex structure, which is able to bind to the ribosomal functional site, but seems to suppress normal turnover of eEF2 after translocation. The properties of RA-VII make it a novel ligand for probing the action of eEF2 in the process of translocation on the ribosome.