Ribosome dynamics and tRNA movement by time-resolved electron cryomicroscopy

The translocation step of protein synthesis entails large-scale rearrangements of the ribosome–transfer RNA (tRNA) complex. Here we have followed tRNA movement through the ribosome during translocation by time-resolved single-particle electron cryomicroscopy (cryo-EM). Unbiased computational sorting of cryo-EM images yielded 50 distinct three-dimensional reconstructions, showing the tRNAs in classical, hybrid and various novel intermediate states that provide trajectories and kinetic information about tRNA movement through the ribosome. The structures indicate how tRNA movement is coupled with global and local conformational changes of the ribosome, in particular of the head and body of the small ribosomal subunit, and show that dynamic interactions between tRNAs and ribosomal residues confine the path of the tRNAs through the ribosome. The temperature dependence of ribosome dynamics reveals a surprisingly flat energy landscape of conformational variations at physiological temperature. The ribosome functions as a Brownian machine that couples spontaneous conformational changes driven by thermal energy to directed movement. During protein synthesis, transfer RNAs move sequentially through the A, P and E sites of the ribosome as their attached amino acids are transferred to the growing peptide chain. Large conformational movements accompany their translocation. Holger Stark and colleagues have processed a staggering 1.9 million single-particle electron cryomicroscopy images of the ribosome to visualize these dynamic changes. They conclude that the conformational changes are thermally driven (or 'Brownian') and that they cause directed movement of transfer RNAs on a narrow path through the ribosome. During protein synthesis within the ribosome, transfer RNAs (tRNAs) move sequentially through different sites as their attached amino acids are transferred onto the growing protein chain. Large conformational movements accompany this process. Here, a staggering 1.9 million electron cryomicroscopy images of the ribosome have been processed to visualize these changes. The results reveal that the ribosome functions as a Brownian machine that couples spontaneous changes driven by thermal energy to directed movement.