The Conformational Properties of Elongation Factor G and the Mechanism of Translocation

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Abstract
The elongation phase of protein synthesis is promoted by two G proteins, elongation factor Tu (EF-Tu), which delivers aminoacyl tRNAs to the ribosome, and elongation factor G (EF-G), which catalyzes translocation. Crystallographic investigations have revealed that EF-G·GDP resembles the EF-Tu·GTP·(aminoacyl tRNA) complex, and it has been proposed that the translocase function of EF-G is derived from this similarity [Nissen, P., et al. (1995) Science 270, 1464]. However, its significance is uncertain because the affinity of EF-G·GDP for the ribosome is much lower than that of the ternary complex it resembles and because EF-Tu·GDP, the form of EF-Tu that has low ribosome affinity, has a conformation radically different from that of EF-Tu·GTP or EF-Tu in the ternary complex. The small-angle X-ray scattering study described here was undertaken to ascertain if the form of EF-G that has high ribosome affinity, EF-G·GTP, the structure of which is unknown, could be a mimic of EF-Tu·GDP. The data show that nucleotide-free EF-G, EF-G·GDP, EF-G·GTP, and EF-G·GMPPCP cannot be distinguished by solution scattering and that it is likely they all resemble crystalline EF-G·GDP. Since an EF-Tu-like change would easily have been detected, it follows that it does not occur in EF-G. These observations have significant implications for the mechanism of translocation.