Electron Microscopy

Electron microscopy (EM) in combination with image analysis is a powerful technique to study protein structure at low- and high resolution. Since electron micrographs of biological objects are very noisy, substantial improvement of image quality can be obtained by averaging of individual projections. Averaging procedures can be divided into crystallographic and non-crystallographic methods and both will be described. Crystallographic averaging, based on two-dimensional crystals of rather small proteins, has the potential of solving a structure to atomic resolution just as the more common techniques of X-ray diffraction and NMR. Single particle analysis is an alternative method for large proteins, viruses and all non-crystallizable proteins. It is a fast method to reveal the low-resolution structure with details in the range of maximally 10–15 Å. Results of EM on Light-harvesting complex II (LHC-II) and Photosystem I will be presented as examples for the crystallographic averaging. Trimeric Photosystem I complexes and dimeric Photosystem II complexes will be discussed as examples for the potential of single-particle averaging