Three-dimensional electron cryo-microscopy as a powerful structural tool in molecular medicine

Electron cryo-microscopy has established itself as a valuable method for the structure determination of protein molecules, protein complexes, and cell organelles. This contribution presents an introduction to the various aspects of three-dimensional electron cryomicroscopy. This includes the need for sample preservation in the microscope vacuum, strategies for minimizing radiation damage, methods of improving the poor signal-to-noise ratio in electron micrographs of unstained specimens, and the various methods of three-dimensional image reconstruction from projections. The various specimen types (e.g., flat and tubular two-dimensional crystals, protein filaments, individual protein molecules, and large complexes) require different means of three-dimensional reconstruction, and we review the five major reconstruction techniques (electron crystallography, helical reconstruction, icosahedral reconstruction, single-particle reconstruction, and electron tomography), with an emphasis on electron crystallography. Several medically relevant three-dimensional protein structures are chosen to illustrate the potential of electron cryo-microscopy and image reconstruction techniques. Among the structural methods, electron cryo-microscopy is the only tool for studying objects that range in size from small proteins over macromolecular complexes to cell organelles or even cells.