3D reconstruction from electron micrographs of tilted 2D crystal: Structure of a human water channel

In order to understand at the atomic level how a biological macromolecule functions, a detailed knowledge of its 3-dimensional structure is essential. Unlike soluble proteins, integral membrane proteins are usually recalcitrant to the growth of large, well-ordered 3-D crystals, which is necessary for high-resolution x-ray crystallographic analyses. An alternative approach is to grow thin, one-molecule thick 2-D crystals in lipid bilayers and apply electron crystallography to solve the structures. Lipids surround the membrane protein in such a 2-D crystal, which allows for a direct assay of function. Another notable advantage of electron crystallography is that phases can be directly obtained from the images unlike in the case of x-ray where phases must be determined indirectly by methods such as isomorphous replacement etc. The availability of the phase information partially compensates for the lack of data at the highest resolution (typically approximately 3.5 angstroms and beyond) because of low-contrast in the images. We briefly review the method of recording high-resolution data from many tilted views of a 2-D crystal, merging of phase and amplitudes from images and diffraction patterns respectively and the calculation of a 3-D density map. The results from such an analysis applied to the human water channel is discussed in the context of its structure/function relationship.