Role and timing of GTP binding and hydrolysis during EF-G-dependent tRNA translocation on the ribosome

The translocation of tRNA and mRNA through the ribosome is promoted by elongation factor G (EF-G), a GTPase that hydrolyzes GTP during the reaction. Recently, it was reported that, in contrast to previous observations, the affinity of EF-G was much weaker for GTP than for GDP and that ribosome-catalyzed GDP-GTP exchange would be required for translocation [Zavialov AV, Hauryliuk VV, Ehrenberg M (2005) J Biol 4:9]. We have reinvestigated GTP/GDP binding and show that EF-G binds GTP and GDP with affinities in the 20 to 40 mu M range (37 degrees C), in accordance with earlier reports. Furthermore, GDP exchange, which is extremely rapid on unbound EF-G, is retarded, rather than accelerated, on the ribosome, which, therefore, is not a nucleotide-exchange factor for EF-G. The EFG-GDPNP complex, which is very labile, is stabilized 30,000-fold by binding to the ribosome. These findings, together with earlier kinetic results, reveal that EF-G enters the pretranslocation ribosome in the GTP-bound form and indicate that, upon ribosomecomplex formation, the nucleotide-binding pocket of EF-G is closed, presumably in conjunction with GTPase activation. GTP hydrolysis is required for rapid tRNA-mRNA movement, and Pi release induces further rearrangements of both EF-G and the ribosome that are required for EF-G turnover.