Visualizing flexibility at molecular resolution: analysis of heterogeneity in single-particle electron microscopy reconstructions.

It is becoming increasingly clear that many macromolecules are intrinsically flexible and exist in multiple conformations in solution. Single-particle reconstruction of vitrified samples (cryo-electron microscopy, or cryo-EM) is uniquely positioned to visualize this conformational flexibility in its native state. Although heterogeneity remains a significant challenge in cryo-EM single-particle analysis, recent efforts in the field point to a future where it will be possible to tap into this rich source of biological information on a routine basis. In this article, we review the basic principles behind a few relatively new and generally applicable methods that show particular promise as tools to analyze macromolecular flexibility. We also discuss some of their recent applications to problems of biological interest.