A major impediment in the determination of high resolution protein structures by single particle cryo-electron microscopy has been the presence of sample heterogeneity. Oftentimes, heterogeneity is due the dynamic nature of protein complexes, which can exist in multiple different conformational states in solution. Recent advancements in cryo-electron image processing have provided tools to accurately sort vitrified protein complexes into distinct populations that can be subsequently used to determine structures corresponding to each of the subpopulations. Such sorting algorithms have allowed high resolution structures of multiple conformations to be elucidated from a single cryo-electron microscopic grid. Besides sorting different conformations, classification can also be used to assess the influence of specific perturbations upon conformational state at level of individual protein complexes. By modulating the concentrations of different components of the system prior to vitrification, it is possible to measure the fraction of complexes belonging to each conformational state as a function of the concentration of each component and thus characterize how specific components influence the equilibrium between conformational states.


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