Fidelity of structure representation in electron micrographs of negatively stained protein molecules.

We have investigated the fidelity of structure representation in electron micrographs of negatively stained proteins by conducting a systematic evaluation of such micrographs in terms of a known molecular structure, solved by x-ray crystallography. Microcrystals of immunoglobulin G Dob were used as specimens in this comparison between micrograph images, optimized by computer image processing, and reference images derived computationally from the crystal structure. To an effective resolution of 2 nm, we observed a remarkably good correlation between the experimental images and their idealized counterparts, which are unaffected by those factors--electron irradiation and dehydration--that are thought to be primarily responsible for perturbation of protein structure during electron microscopy. Separate structural features resolved in these micrographs do not, in general, correspond to specific components of individual molecules but arise instead from complex superpositions involving several overlapping molecules.