How Cryo-EM Became so Hot

Abstract

The Royal Swedish Academy of Sciences awarded the 2017 Nobel Prize for Chemistry to Jacques Dubochet, Joachim Frank, and Richard Henderson for "developing cryoelectron microscopy for the high-resolution structure determination of biomolecules in solution." Achieving this goal, which required innovation, persistence, and uncommon physical insight, has broadened horizons for structural studies in molecular and cell biology.

Figure 1.. Rapid Freezing Results in Vitreous Ice

Biological macromolecules must be surrounded by water if they are to retain their native structure and function. In order to prevent that water from evaporating when a fully hydrated sample is inserted into the vacuum of an electron microscope, Dubochet et al. (1988) used the elegant but simple apparatus shown here to freeze samples. In such frozen samples, water molecules remain in a nearly random arrangement, as they are in the liquid, and thus, the biological macromolecules remain in a near-native state.

Figure 2.. Crystals Consist of Identical Objects with a Known Spatial and Angular Relationship

(A and B) A 2D protein crystal is made up o f a layer of molecules that, in this cartoon, all have the same orientation and w hose respective locations are specified by points on a regular lattice (A). Henderson and Unwin (1975) took advantage of 2D crystals to facilitate merging data from many thousands of bacteriorhodopsin molecules, thus building up sufficient statistical definition of the signal without exposing any one molecule to more radiation than it could tolerate. Such images had to then be recorded for different crystals, tilted by different amounts, in order to obtain the data needed for a 3D density map. Rather than relying on crystalline forms, however, single-particle cryo-EM uses an ensemble of randomly dispersed macromolecules (B). Saxton and Frank (1977) demonstrated that, in principle, information about the locations and orientations of these molecules could be obtained computationally from their images. In doing so, “virtual crystals” would effectively be grown in silico, in the sense that it is then possible to merge data from many thousands of identical molecules.