Refinement of Atomic Structures Against cryo-EM Maps.

Area of Science:

• Structural Biology
• Biophysics
• Biochemistry

Background:

• Single-particle cryo-electron microscopy (cryo-EM) generates 3D density maps of macromolecules.
• Atomic structure refinement (fitting) is crucial for interpreting these maps into detailed molecular models.
• Macromolecular X-ray crystallography offers established methods for structure analysis.

Purpose of the Study:

• To review methods for atomic structure refinement into medium/high-resolution cryo-EM maps.
• To discuss the applicability of X-ray crystallography tools in cryo-EM.
• To identify and discourage problematic practices in cryo-EM model building.

Main Methods:

• Adaptation of crystallographic software for cryo-EM data.
• Development of cryo-EM specific likelihood functions.
• Evaluation of model refinement strategies for cryo-EM maps.

Main Results:

• Many crystallographic tools can be adapted for cryo-EM, but require modified likelihood functions.
• Certain crystallographic techniques (e.g., 2Fo-Fc maps, solvent flattening) are unsuitable for cryo-EM.
• Overclaiming resolution and map oversharpening lead to inaccurate and misleading atomic models.

Conclusions:

• Careful selection and adaptation of refinement tools are necessary for accurate cryo-EM structures.
• Avoiding common pitfalls like oversharpening is critical for reliable model building.
• Accurate atomic models derived from cryo-EM are essential for advancing structural biology.