Single-particle applications at intermediate resolution.

Electron microscopy together with single-particle image processing is an excellent method for structure determination of biological assemblies that exist in multiple identical copies. Typical assemblies contain several proteins and/or nucleic acids in a defined and reproducible arrangement. Coherent averaging of electron microscopic images of 5000-100,000 copies of these assemblies allows the determination of three-dimensional structures at ca. 1-3-nm resolution. At this intermediate resolution, it is possible to map individual subunits and thus to understand the architecture and quaternary structure of the assemblies. The intermediate resolution structural information gives a solid basis on which pseudo-atomic models of the assemblies can be modeled provided that high-resolution structures of smaller entities are known. The architecture of the assemblies, their pseudo-atomic models, and knowledge on their plasticity during function give a comprehensive understanding of large-scale structural dynamics of multicopy biological complexes. In this review, we will introduce the experimental pipeline and discuss selected examples.