Almost lost in translation. Cryo-EM of a dynamic macromolecular complex: the ribosome.

Ribosomes are dynamic biological machines that perform numerous tasks during translation, the biosynthesis of proteins. Translocation, the movement of transfer RNAs (tRNAs) and messenger RNA (mRNA) to progress in the reading frame of codons in the mRNA, takes place after the addition of each amino acid. This process involves large ribosome conformational changes, where tRNAs proceed through intermediate states. The structural characterization of these translocation intermediates has remained elusive. Cryo-electron microscopy (cryo-EM) produces three-dimensional averages, and translocating ribosomes poise distinct conformational states, and hence, structurally heterogeneous populations. During the last decade, the quest for visualization of translocation intermediates has progressed together with the development of classification tools in cryo-EM. Some of these new tools have recently been tested in ribosomal translocation, uncovering a clearer picture of the process. This success goes along with the latest advances in cryo-EM and illustrates how the technique offers multiple possibilities for studying macromolecular complexes engaged in dynamic reactions.