Electron Crystallography in Photosynthesis Research

Abstract

Electron crystallography covers the analysis of crystals by electron microscopy based methods in order to infer into the structure of the crystallized molecules. Electron crystallography of two-dimensional crystals has become a powerful alternative to X-ray crystallography and NMR spectroscopy for the determination of the structure of proteins or protein complexes. The electron crystallography approach is particularly suitable for the study of membrane proteins, which due to their amphiphilic properties are stable in lipid bilayers, their natural environment, where they can form crystalline arrays. This method of structural analysis has also proved suitable for the study of soluble proteins, which can be induced to form two-dimensional lattices, as well as the study of very thin three-dimensional crystals. The preparation of two-dimensional crystals normally requires less protein and at a lower concentration than required for three-dimensional crystallization. Overall, the structural analysis based on electron microscopy is more amenable to the study of conformational variations associated with different functional states of membrane proteins than other techniques used in structural biology. Furthermore, the analysis of two-dimensional crystals enables the calculation of preliminary structural maps at relatively low resolutions, which at early stages in the optimization crystallization and image analysis may render valuable information on the overall protein architecture. Here we present a summary of the methods involved in protein electron crystallography, regarding both the crystallization process itself and the fundaments of electron microscopy and image analysis required. We also show how electron crystallography has greatly contributed for the development of the present knowledge on the structural organization of photosynthetic protein complexes, focusing on the studies involving transmembrane pigmented proteins and protein complexes.