title: Structure of the ribosome with elongation factor G trapped in the pretranslocation state.
abstract: During protein synthesis, tRNAs and their associated mRNA codons move sequentially on the ribosome from the A (aminoacyl) site to the P (peptidyl) site to the E (exit) site in a process catalyzed by a universally conserved ribosome-dependent GTPase [elongation factor G (EF-G) in prokaryotes and elongation factor 2 (EF-2) in eukaryotes]. Although the high-resolution structure of EF-G bound to the posttranslocation ribosome has been determined, the pretranslocation conformation of the ribosome bound with EF-G and A-site tRNA has evaded visualization owing to the transient nature of this state. Here we use electron cryomicroscopy to determine the structure of the 70S ribosome with EF-G, which is trapped in the pretranslocation state using antibiotic viomycin. Comparison with the posttranslocation ribosome shows that the small subunit of the pretranslocation ribosome is rotated by ∼12° relative to the large subunit. Domain IV of EF-G is positioned in the cleft between the body and head of the small subunit outwardly of the A site and contacts the A-site tRNA. Our findings suggest a model in which domain IV of EF-G promotes the translocation of tRNA from the A to the P site as the small ribosome subunit spontaneously rotates back from the hybrid, rotated state into the nonrotated posttranslocation state.
authors: Department of Biochemistry, Rosenstiel Basic Medical Sciences Research Center, Brandeis University, Waltham, MA 02454.
authors: Axel F
authors: AF
authors: Brilot
authors: Andrei A
authors: AA
authors: Korostelev
authors: Dmitri N
authors: DN
authors: Ermolenko
authors: Nikolaus
authors: N
authors: Grigorieff
mesh_terms: D020285:Cryoelectron Microscopy; D008958:Models, Molecular; D009690:Nucleic Acid Conformation; D020653:Peptide Elongation Factor G / Q000737:chemistry*; D014176:Protein Biosynthesis / Q000502:physiology*; D012270:Ribosomes / Q000737:chemistry*
chemical_list: D020653:Peptide Elongation Factor G
keywords: A/P* hybrid state; FREALIGN; real-space refinement; single particle analysis; translation