Most cellular activities are outcomes of interactions among many components, including proteins, nucleic acids and lipids. Electron cryo-microscopy of isolated macromolecular complexes ("single particle cryo-EM") can now visualize icosahedral viruses at near-atomic resolution (, [1
]), and it should soon achieve similar resolution with less symmetrical particles. In a seminal review 15 years ago, Henderson predicted the success of single-particle techniques in visualizing detail at 3 – 4 Å resolution [10
], and Glaeser extended the analysis a few years later [11
]. A major step towards this goal was visualization of the hepatitis B icosahedral capsid at sub-nanometer resolution [12
], allowing for the first time the identification of a protein fold using single-particle cryo-EM. Reconstructions at sub-nanometer resolution of particles with lower symmetry followed, and it is now possible to obtain reconstructions of ribosomes at about 5 Å resolution [14
Table 1 Chronological list of icosahedral virus structures imaged at near-atomic resolution. "Averaged subunits" is the number of molecules (or molecular units) averaged to give the stated resolution. References to image processing software: Signature, CTFTILT/CTFFIND3[ (more ...)
The leading role of icosahedral viruses in achieving near-atomic resolution is due primarily to: (i) their high symmetry, which effectively increases the size of the data set 60-fold, or more if quasi-equivalent subunits can be averaged; (ii) their large molecular mass, producing strong image contrast and hence more accurate alignments and reconstructions; and (iii) their rigidity and uniformity, ensuring near-perfect superposition of structural features in three-dimensional (3D) reconstructions. In this review, we consider technological improvements made over the last decade that have enabled the recent successes, explain some of the remaining challenges and discuss some of the resulting functional conclusions about the organization of non-enveloped virions.