Dear colleagues,

The characterization of structure and associated conformational changes of biological macromolecular complexes is important for understanding how macromolecular assemblies fulfil their complex roles in the living cell. Several techniques may be used to determine their 3D macromolecular structure. X-ray crystallography may yield atomic resolution of the structure, but it requires a crystallization process, which is difficult for complexes showing conformational heterogeneity, for example. Nuclear magnetic resonance may provide unique information about dynamics and interactions, but atomic structure determination is restricted to small complexes. Cryo-electron microscopy (cryo-EM) allows studying frozen hydrated samples within a thin layer of amorphous ice, at cryogenic temperatures (generally liquid nitrogen temperatures). Cryo-EM allows the observation of biological specimens in close-to-physiological conditions as well as studying flexible complexes. Single particle analysis and single particle tomography are two cryo-EM approaches that are extensively used for obtaining 3D structural information of biological macromolecular complexes.

The cryo-EM field has been experiencing a very fast evolution in recent years. New technological developments (e.g., direct electron detector, phase plates, and new image processing methods) have opened the way to obtain quasi-atomic resolution for a large range of macromolecular complexes, including relatively small ones. Moreover, new technological developments have increased the richness of the data, requiring novel image processing methods to extract all the available structural information. Additionally, these recent developments now allow extracting information about the dynamics of complexes by exploring the conformational heterogeneity of the sample. Finally, old questions such as how to validate the reconstructed EM density map or how to measure its resolution are just as topical as ever.

This Special Issue aims at providing a platform for discussion of cutting-edge cryo-EM image analysis and 3D reconstruction methods and their structural biology and biomedical applications.

Dr. Khanh Huy Bui
Dr. Javier Vargas
Guest Editors

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• Cryo-electron microscopy
• single particle analysis, cryo-electron tomography
• image analysis
• methods development