Guanine-nucleotide exchange on ribosome-bound elongation factor G initiates the translocation of tRNAs.
BACKGROUND
During the translation of mRNA into polypeptide, elongation factor G (EF-G) catalyzes the translocation of peptidyl-tRNA from the A site to the P site of the ribosome. According to the 'classical' model, EF-G in the GTP-bound form promotes translocation, while hydrolysis of the bound GTP promotes dissociation of the factor from the post-translocation ribosome. According to a more recent model, EF-G operates like a 'motor protein' and drives translocation of the peptidyl-tRNA after GTP hydrolysis. In both the classical and motor protein models, GDP-to-GTP exchange is assumed to occur spontaneously on 'free' EF-G even in the absence of a guanine-nucleotide exchange factor (GEF).


RESULTS
We have made a number of findings that challenge both models. First, free EF-G in the cell is likely to be in the GDP-bound form. Second, the ribosome acts as the GEF for EF-G. Third, after guanine-nucleotide exchange, EF-G in the GTP-bound form moves the tRNA2-mRNA complex to an intermediate translocation state in which the mRNA is partially translocated. Fourth, subsequent accommodation of the tRNA2-mRNA complex in the post-translocation state requires GTP hydrolysis.


CONCLUSIONS
These results, in conjunction with previously published cryo-electron microscopy reconstructions of the ribosome in various functional states, suggest a novel mechanism for translocation of tRNAs on the ribosome by EF-G. Our observations suggest that the ribosome is a universal guanosine-nucleotide exchange factor for EF-G as previously shown for the class-II peptide-release factor 3.
Department of Cell and Molecular Biology, Biomedical Center, Uppsala University, SE-75124 Uppsala, Sweden.
Andrey V
AV
Zavialov
Vasili V
VV
Hauryliuk
Måns
M
Ehrenberg
D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism; D006150:Guanine Nucleotides / Q000235:genetics / Q000378:metabolism*; D008954:Models, Biological; D020653:Peptide Elongation Factor G / Q000235:genetics / Q000378:metabolism*; D014176:Protein Biosynthesis; D012346:RNA, Transfer, Amino Acyl / Q000378:metabolism*; D012270:Ribosomes / Q000235:genetics / Q000378:metabolism*
D006150:Guanine Nucleotides; D020653:Peptide Elongation Factor G; D012346:RNA, Transfer, Amino Acyl