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Nat Struct Mol Biol. 2005 Dec;12(12):1145-9. Epub 2005 Nov 13.

Conformational transition of initiation factor 2 from the GTP- to GDP-bound state visualized on the ribosome.

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Department of Structural Biology and Genomics, Institute of Genetics and Molecular and Cellular Biology, Centre National de la Recherche Scientifique/Institut National de la Santé et de la Recherche Médicale, Université Louis Pasteur, Illkirch, France.


Initiation of protein synthesis is a universally conserved event that requires initiation factors IF1, IF2 and IF3 in prokaryotes. IF2 is a GTPase essential for binding initiator transfer RNA to the 30S ribosomal subunit and recruiting the 50S subunit into the 70S initiation complex. We present two cryo-EM structures of the assembled 70S initiation complex comprising mRNA, fMet-tRNA(fMet) and IF2 with either a non-hydrolyzable GTP analog or GDP. Transition from the GTP-bound to the GDP-bound state involves substantial conformational changes of IF2 and of the entire ribosome. In the GTP analog-bound state, IF2 interacts mostly with the 30S subunit and extends to the initiator tRNA in the peptidyl (P) site, whereas in the GDP-bound state IF2 steps back and adopts a 'ready-to-leave' conformation. Our data also provide insights into the molecular mechanism guiding release of IF1 and IF3.

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