Cryo-electron microscopy

Electron microscopy uses beams of electrons to form images of macromolecules and complexes. Electron microscopy can reveal smaller details than light microscopes owing to the wavelength differences between electrons and photons. Images of macromolecules are obtained in artificially-stained states or frozen in glassy ‘vitreous’ ice layers (Figure 23). Images from a large number of orientations of a macromolecule can be combined computationally to give a 3D reconstruction of the electron density. For many years the resolution of EM-deduced data was lower than that which could be determined from X-ray crystallography or NMR spectroscopy. However, over the last decade, a so called “resolution revolution” has taken place. Improvements in sample preparation, microscope design, and data processing (coupled with high performance computing), have together enabled the determination of high-resolution ab initio structures. Even when this is not possible, high resolution X-ray or NMR structure can still be positioned in electron density volumes to allow for meaningful structural interpretation. Both ‘fitted’ and directly calculated structures can be found in the PDB. Reconstructed 3D density maps are available from the Electron Microscopy Data Bank (EMDB).

CryoEM image of GroEL suspended in vitreous ice
Figure 23 CryoEM image of GroEL suspended in vitreous ice at 50,000X magnification (image from Wikipedia).