Matrix-Landing Mass Spectrometry for Electron Microscopy Imaging of Native Protein Complexes

Recently, we described the use of a chemical matrix for landing and preserving the cations of protein–protein complexes within a mass spectrometer (MS) instrument. By use of a glycerol-landing matrix, we used negative stain transmission electron microscopy (TEM) to obtain a three-dimensional (3D) reconstruction of landed GroEL complexes. Here, we investigate the utilities of other chemical matrices for their abilities to land, preserve, and allow for direct imaging of these cationic particles using TEM. We report here that poly­(propylene) glycol (PPG) offers superior performance over glycerol for matrix landing. We demonstrated the utility of the PPG matrix landing using three protein–protein complexesGroEL, the 20S proteasome core particle, and β-galactosidaseand obtained a 3D reconstruction of each complex from matrix-landed particles. These structures have no detectable differences from the structures obtained using conventional preparation methods, suggesting the structures are well preserved at least to the resolution limit of the reconstructions (∼20 Å). We conclude that matrix landing offers a direct approach to couple native MS with TEM for protein structure determination.