Current approaches for the fitting and refinement of atomic models into cryo-EM maps using CCP-EM

By R. Nicholls, Michal Tykač, O. Kovalevskiy, G. Murshudov. 2018

79 citations recorded by the source corpus.

Recent advances in instrumentation and software have resulted in cryo-EM rapidly becoming the method of choice for structural biologists, especially for those studying the three-dimensional structures of very large macromolecular complexes. In this contribution, the tools available for macromolecular structure refinement into cryo-EM reconstructions that are available via CCP-EM are reviewed, specifically focusing on REFMAC5 and related tools. Whilst originally designed with a view to refinement against X-ray diffraction data, some of these tools have been able to be repurposed for cryo-EM owing to the same principles being applicable to refinement against cryo-EM maps. Since both techniques are used to elucidate macromolecular structures, tools encapsulating prior knowledge about macromolecules can easily be transferred. However, there are some significant qualitative differences that must be acknowledged and accounted for; relevant differences between these techniques are highlighted. The importance of phases is considered and the potential utility of replacing inaccurate amplitudes with their expectations is justified. More pragmatically, an upper bound on the correlation between observed and calculated Fourier coefficients, expressed in terms of the Fourier shell correlation between half-maps, is demonstrated. The importance of selecting appropriate levels of map blurring/sharpening is emphasized, which may be facilitated by considering the behaviour of the average map amplitude at different resolutions, as well as the utility of simultaneously viewing multiple blurred/sharpened maps. Features that are important for the purposes of computational efficiency are discussed, notably the Divide and Conquer pipeline for the parallel refinement of large macromolecular complexes. Techniques that have recently been developed or improved in Coot to facilitate and expedite the building, fitting and refinement of atomic models into cryo-EM maps are summarized. Finally, a tool for symmetry identification from a given map or coordinate set, ProSHADE, which can identify the point group of a map and thus may be used during deposition as well as during molecular visualization, is introduced.

Subfields: Materials Chemistry, Molecular Biology, Structural Biology

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Citations in the corpus

Current approaches for automated model building into cryo-EM maps using Buccaneer with CCP-EM (2020, methodology): “Brown et al. (2015) have implemented an EM mode in REFMAC5 to facilitate the refinement of structures solved by cryo-EM.”
Advances in methods for atomic resolution macromolecular structure determination (2020, methodology): “…to reconstruct density maps with near-atomic resolution required the development of methods and software 76 that could be used to build and refine 77–79 molecular models from cryo-EM data, in some cases utilizing complex automated modeling 80,81 or molecular dynamics calculations 82–85 .”