Structural basis for interaction of the ribosome with the switch regions of GTP-bound elongation factors

Elongation factor G (EF-G) catalyzes tRNA translocation on the ribosome. Here a cryo-EM reconstruction of the 70S(.)EF-G ribosomal complex at 7.3 angstrom resolution and the crystal structure of EF-G-2(.)GTP, an EF-G homolog, at 2.2 angstrom resolution are presented. EF-G-2(.)GTP is structurally distinct from previous EF-G structures, and in the context of the cryo-EM structure, the conformational changes are associated with ribosome binding and activation of the GTP binding pocket. The P loop and switch 11 approach A2660-A2662 in helix 95 of the 23S rRNA, indicating an important role for these conserved bases. Furthermore, the ordering of the functionally important switch I and 11 regions, which interact with the bound GTP, is dependent on interactions with the ribosome in the ratcheted conformation. Therefore, a network of interaction with the ribosome establishes the active GTP conformation of EF-G and thus facilitates GTP hydrolysis and tRNA translocation.