Macromolecular structures without crystals

Electron microscopy has many attractive capabilities as a tool for experimentally visualizing the structures of biological cells and macromolecules. Particles can be imaged while freely suspended in biochemically “native” buffers, and electrons can be focused to resolutions exceeding anything that a biochemist's heart might desire. To be sure, biological electron microscopy also is subject to certain physical limitations, perhaps the most problematic of which arise from the fact that short-wavelength electrons also are a form of ionizing radiation. However, among the goals for which technology rather than physical principles currently appears to be the limitation, many regard the most desirable to be the production of 3D density maps that show the positions and rotamer conformations of every amino acid by using specimens in which the proteins are in the form of well dispersed, single particles. The structures of such proteins would not only be free of crystal-packing constraints, they would also be free of the influences of the associated crystallization buffers, which often have unphysiological pH, ionic strength, ionic composition, or water activity. With the publication of the structure of the icosahedral “inner capsid particle” of a human rotavirus in this issue of PNAS, Zhang et al. (1) have come remarkably close to that goal. The technological frontier now moves to accomplishing the same results with macromolecular complexes that are significantly smaller than viruses and with structures for which a high degree of symmetry cannot be exploited to ease the task.

*E-mail: rmglaeser{at}lbl.gov