Conformational transition of polypeptide chain elongation factor G as determined by electron spin resonance

The conformational transition of the polypeptide chain elongation factor G (EF-G) induced by interaction with guanine nucleotide has been investigated by means of the spin-labeling technique. Various spin-label probes were attached specifically to the sulfhydryl group of the protein that is essential for binding to ribosomes, and the effects of these ligands on the electron spin resonance (ESR) spectra were examined. It was found that the ESR spectra of EF-G labeled with nitroxide maleimide reagents were modified by the addition of various guanine nucleotides such as GDP, GTP and, to a lesser extent, by Gpp(NH)p and Gpp(CH2)p, indicating that conformational changes accompany the binding of nucleotide ligand. However, the ESR spectra of labeled EF-G-GDP and EF-G-GTP were almost identical. On the other hand, when EF-G was labeled with nitroxide iodoacetamide reagents, a clear difference in the ESR spectra of EF-G-GDP and EF-G-GTP derivatives was observed. In this case, the spectral shape of the spin-labeled EF-G in the presence of GTP or its analogs, Gpp(NH)p or Gpp(CH2)p, was quite similar to that of free, unliganded EF-G derivative. These results, together with those previously obtained using hydrophobic probes (Arai, Arai, & Kaziro (1975) J. Biochem. 78, 243-246) demonstrate the existence of an EF-G-guanine nucleotide binary complex. They also indicate that there is a substantial difference in conformation between free EF-G, EF-G-GDP, and EF-G-GTP near the active site essential for interaction with ribosomes.