Quantifying the heterogeneity of macromolecular machines by mass photometry

Sample purity is central to in vitro studies of protein function and regulation, and to the efficiency and success of structural studies using techniques such as x-ray crystallography and cryo-electron microscopy (cryo-EM). Here, we show that mass photometry (MP) can accurately characterize the heterogeneity of a sample using minimal material with high resolution within a matter of minutes. To benchmark our approach, we use negative stain electron microscopy (nsEM), a popular method for EM sample screening. We include typical workflows developed for structure determination that involve multi-step purification of a multi-subunit ubiquitin ligase and chemical cross-linking steps. When assessing the integrity and stability of large molecular complexes such as the proteasome, we detect and quantify assemblies invisible to nsEM. Our results illustrate the unique advantages of MP over current methods for rapid sample characterization, prioritization and workflow optimization. Mass photometry is a label-free optical approach capable of detecting, imaging and accurately measuring the mass of single biomolecules in solution. Here, the authors demonstrate the potential of mass photometry for quantitatively characterizing sample heterogeneity of purified protein complexes with implications for structural studies specifically and in vitro studies more generally.