Recent Advances in Electron Crystallography

The ultimate goal of structural biology is to understand the protein function and its physiological mechanisms by determining the three-dimensional (3D) structure. Several techniques have been used for the purpose, including X-ray crystallography, nuclear magnetic resonance (NMR), and electron microscopy (EM). X-ray crystallography is the most successful technique for the structural study of many proteins (Berman et al., 2000). In a traditional X-ray crystallography, X-ray beam is directed towards protein crystal, resulting in the scattering portions of X-ray photons to form a diffraction pattern. During the process, the crystal is rotated at small angles and other diffraction patterns are recorded. Eventually, after repeated iterations, the structure of the protein can be reconstructed from the accumulated diffraction patterns (Shi et al., 2013). Although, there is a great development in X-ray crystallography, it still remains a bottleneck for protein structure determination. The crystal used in X-ray crystallography must be large well-ordered. However, some proteins such as membrane proteins and protein complexes, may never yield the large well-ordered crystals and it takes significant time and resources to optimize the initial small crystals found during the screening process (Bill et al., 2011). Since early 1940s, electron diffraction has been used to solve the crystallographic problems (Bendersky & Gayle, 2001). The basic principle of electron crystallography is similar to X-ray crystallography in a concept that protein crystals scatters electron beam to produce a diffraction pattern. The crystals that are used in the electron crystallography are needed to be thinner than X-ray crystal as the interacting power of electron is much stronger than that of X-ray photons (Henderson & Unwin, 1975; Kimura et al., 1997; Kuhlbrandt et al., 1994). Over the past decades, the electron crystallography has been successfully used to determine the structure of several difficult proteins with two-dimensional crystals (2D electron crystallography) (Wisedchaisri et al., 2011). However, this technique is normally only possible to produce one diffraction pattern from each crystal as electrons have very high energy which causes a large amount of radiation damage to the sample crystals, resulting in the loss of structural information (Glaeser, 1971). To overcome this, the electron diffraction patterns that are produced from hundreds of individual crystals are merged to generate a single data set. Not surprisingly, several studies have conducted the electron crystallography using 3D protein crystals over