Electron Cryomicroscopy of Viruses at Near-Atomic Resolutions.

Recently, dozens of virus structures have been solved to resolutions between 2.5 and 5.0 Å by means of electron cryomicroscopy. With these structures we are now firmly within the "atomic age" of electron cryomicroscopy, as these studies can reveal atomic details of protein and nucleic acid topology and interactions between specific residues. This improvement in resolution has been the result of direct electron detectors and image processing advances. Although enforcing symmetry facilitates reaching near-atomic resolution with fewer particle images, it unfortunately obscures some biologically interesting components of a virus. New approaches on relaxing symmetry and exploring structure dynamics and heterogeneity of viral assemblies have revealed important insights into genome packaging, virion assembly, cell entry, and other stages of the viral life cycle. In the future, novel methods will be required to reveal yet-unknown structural conformations of viruses, relevant to their biological activities. Ultimately, these results hold the promise of answering many unresolved questions linking structural diversity of viruses to their biological functions.