Proteins fold into intricate shapes, known as conformations. The activation of many signal transduction proteins, kinases, and transcription factors requires a change in their conformations. Thus the conformation of a protein can indicate its biological activity. This importance of conformational changes has stimulated the development of numerous methods for analyzing protein conformations and interactions, such as native mass spectrometry and cryoelectron microscopy. These methods may achieve detailed characterizations of protein conformations, but they require highly purified proteins; they are challenged by the complexity of in vivo proteomes.