Electrons and X-rays gang up on the ribosome

Electron microscopy of functional ribosome complexes

Cryoelectron microscopy has made a number of significant contributions to our understanding of the translation process. The method of single-particle reconstruction is particularly well suited for the study of the dynamics of ribosome-ligand interactions. This review follows the events of the functional cycle and discusses the findings in the context provided by the recently published x-ray structures.

Determining the structure of biological macromolecules by transmission electron microscopy, single particle analysis and 3D reconstruction

Single particle analysis and 3D reconstruction of molecules imaged by transmission electron microscopy have provided a wealth of medium to low resolution structures of biological molecules and macromolecular complexes, such as the ribosome, viruses, molecular chaperones and photosystem II. In this review, the principles of these techniques are introduced in a non-mathematical way, and single particle analysis is compared to other methods used for structural studies. In particular, the recent X-ray structures of the ribosome and of ribosomal subunits allow a critical comparison of single particle analysis and X-ray crystallography. This has emphasised the rapidity with which single particle analysis can produce medium resolution structures of complexes that are difficult to crystallise. Once crystals are available, X-ray crystallography can produce structures at a much higher resolution. The great similarities now seen between the structures obtained by the two techniques reinforce confidence in the use of single particle analysis and 3D reconstruction, and show that for electron cryo-microscopy structure distortion during sample preparation and imaging has not been a significant problem. The ability to analyse conformational flexibility and the ease with which time-resolved studies can be performed are significant advantages for single particle analysis. Future improvements in single particle analysis and electron microscopy should increase the attainable resolution. Combining single particle analysis of macromolecular complexes and electron tomography of subcellular structures with high-resolution X-ray structures may enable us to realise the ultimate dream of structural biology-a complete description of the macromolecular complexes of the cell in their different functional states.

The ribosome--a macromolecular machine par excellence

The ribosome is the site in the cell where proteins are synthesized. Cryo-electron microscopy and X-ray crystallography have revealed the ribosome as a particle made of two subunits, each formed as an intricate mesh of RNAs and many proteins. Ligand-binding experiments followed by cryo-electron microscopy have helped to determine some of the key stages of interaction between the ribosome and the main ligand molecules.