The Nobel Prize in Chemistry 2017: High-Resolution Cryo-Electron Microscopy

Abstract

Since the structure of myoglobin was first determined in the 1950s by X-ray crystallography, more than 130,000 biomolecular structures, including protein domain, protein/ protein complex, protein/DNA complex, and protein/ RNA complex, have been deposited in the Protein Data Bank, which is a central repository for three-dimensional (3D) biomolecular structural data. The knowledge of these biomolecular structures has significantly advanced our understanding of biological phenomena at the molecular level and has facilitated pharmaceutical development. Despite the considerable success of X-ray crystallography in determining the structures of biomolecules, this method still has fundamental limitations. Large amounts of high purity proteins are required for crystallization and procuring adequate well-organized crystal, which is sufficient to determine the atomic structure, is sometimes very difficult. Therefore, many of macromolecular complex (>100 kDa), polymerizing proteins, integral membrane proteins, and proteins with multiple conformations or with flexible domain have been awaiting the birth of new technology, high-resolution CryoEM, for their structure analysis. The invention of transmission electron microscopy (TEM) by Ernst Ruska in the 1930s was immediately welcomed by physicists and materials scientists owing to its wide range of applications. However, this was not the case for biologists mainly because of the intrinsic problems associated with biomolecules. In particular, biomolecules are sensitive to radiation damage, can be easily dehydrated under the high vacuum conditions required for TEM, and have poor electron scattering capability due to their low weight elemental composition. To overcome these issues, negative staining, which involves embedding a biomolecule in a layer of dried heavy metal solution, such as uranyl formate or uranyl acetate, was first employed in the 1940s and was further refined during the next 20 years (Brenner & Horne, 1959; Hall et al., 1945; Huxley & Zubay, 1961). This specimen preparation for TEM is quick and easy, and allows the collection of detailed information about the morphology of bacteria, viruses, organelles, and protein complexes. Nevertheless, staining with the heavy metal solution and drying often caused the structural collapse of biomolecules, and the resolution is limited by the granularity of heavy metal used for negative stain. Therefore, the development of an alternative sample preparation method that enables the study of unstained CC This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0) which permits unrestricted noncommercial use, distribution, and reproduction in any medium, provided the original work is properly cited. Copyrights They all contributed to the development of a Cryo-electron microscopy (EM) technique for determining the high-resolution structures of biomolecules in solution, particularly without crystal and with much less amount of biomolecules than X-ray crystallography. In this brief commentary, we address the major advances made by these three Nobel laureates as well as the current status and future prospects of this Cryo-EM technique.