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-tRNAfMet 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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We thank J. Thompson (Brown University, Providence, Rhode Island, USA) for providing IF2 strains and D. Moras, P. Schultz, J.-C. Thierry and V. Mallouh for sustained support. We are grateful to L. Jenner for comments on the manuscript and to M. Schatz and R. Schmidt for support with the IMAGIC-5 software. A.G.M. and S.M. are recipients of a postdoctoral fellowship from the CNRS, and A.S. benefited from a fellowship from the Camerino University exchange program. The electron microscope was financed by the Alsace Region, the INSERM, the CNRS and the Association pour la Recherche sur le Cancer.
The authors declare no competing financial interests.
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Myasnikov, A., Marzi, S., Simonetti, A. et al. Conformational transition of initiation factor 2 from the GTP- to GDP-bound state visualized on the ribosome. Nat Struct Mol Biol 12, 1145–1149 (2005). https://doi.org/10.1038/nsmb1012
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