Specific Interaction between EF-G and RRF and Its Implication for GTP-Dependent Ribosome Splitting into Subunits

After termination of protein synthesis, the bacterial ribosome is split into its 30S and 50S subunits by the action of ribosome recycling factor (RRF) and elongation factor G (EF-G) in a guanosine 5′-triphosphate (GTP)-hydrolysis-dependent manner. Based on a previous cryo-electron microscopy study of ribosomal complexes, we have proposed that the binding of EF-G to an RRF-containing posttermination ribosome triggers an interdomain rotation of RRF, which destabilizes two strong intersubunit bridges (B2a and B3) and, ultimately, separates the two subunits. Here, we present a 9-Å (Fourier shell correlation cutoff of 0.5) cryo-electron microscopy map of a 50S·EF-G·guanosine 5′-[(βγ)-imido]triphosphate·RRF complex and a quasi-atomic model derived from it, showing the interaction between EF-G and RRF on the 50S subunit in the presence of the noncleavable GTP analogue guanosine 5′-[(βγ)-imido]triphosphate. The detailed information in this model and a comparative analysis of EF-G structures in various nucleotide- and ribosome-bound states show how rotation of the RRF head domain may be triggered by various domains of EF-G. For validation of our structural model, all known mutations in EF-G and RRF that relate to ribosome recycling have been taken into account. More importantly, our results indicate a substantial conformational change in the Switch I region of EF-G, suggesting that a conformational signal transduction mechanism, similar to that employed in transfer RNA translocation on the ribosome by EF-G, translates a large-scale movement of EF-G's domain IV, induced by GTP hydrolysis, into the domain rotation of RRF that eventually splits the ribosome into subunits.