Molecular electron microscopy is used to determine the three-dimensional structure of individual proteins or protein complexes (as compared to traditional electron microscopy, which uses thin sections to visualize the structure of cells or tissues). To achieve the much higher resolution required for the visualization of protein structures, molecular electron microscopy takes advantage of low-dose imaging techniques to reduce beam damage of the specimen and computational averaging techniques to improve the signal-to-noise ratio.
The two main techniques used in molecular electron microscopy are electron crystallography and single particle averaging.
In electron crystallography, the protein is crystallized in two dimensions and the structure determined using diffraction techniques. This approach is particularly powerful for membrane proteins and can yield an atomic structure for the protein.
In single particle averaging approaches, individual molecules are imaged and the structure determined by back projection algorithms in real space. Although the resolution of the resulting density maps is typically only around 25 Å, single particle methods can yield structural information for macromolecular complexes that are often very difficult to solve by X-ray crystallography. Particularly powerful is the combination of molecular electron microscopy with X-ray crystallography for macromolecular complexes, where the atomic structures of individual components, as determined by X-ray crystallography, are fit into the molecular envelope of the entire complex determined by electron microscopy.