Control of ribosomal subunit rotation by elongation factor G.
Protein synthesis by the ribosome requires the translocation of transfer RNAs
and messenger RNA by one codon after each peptide bond is formed, a reaction
that requires ribosomal subunit rotation and is catalyzed by the guanosine
triphosphatase (GTPase) elongation factor G (EF-G). We determined 3 angstrom
resolution x-ray crystal structures of EF-G complexed with a nonhydrolyzable
guanosine 5'-triphosphate (GTP) analog and bound to the Escherichia coli
ribosome in different states of ribosomal subunit rotation. The structures
reveal that EF-G binding to the ribosome stabilizes switch regions in the GTPase
active site, resulting in a compact EF-G conformation that favors an
intermediate state of ribosomal subunit rotation. These structures suggest that
EF-G controls the translocation reaction by cycles of conformational rigidity
and relaxation before and after GTP hydrolysis.