2.2 Å Resolution Cryo-EM Structure of β-galactosidase in Complex with a Cell-permeant Inhibitor
Recent advances in cryo–electron microscopy allow structures of large macromolecules to be determined at near-atomic resolution. So far, though, resolutions approaching 2 Å, where features key to drug design are revealed, remain the territory of x-ray crystallography. Bartesaghi et al. achieved a resolution of 2.2 Å for a 465-kD ligand-bound protein complex using cryo-EM. The density map is detailed enough to show close to 800 water molecules, magnesium and sodium ions, and precise side-chain conformations. These results bring routine use of cryo-EM in rational drug design a step closer.
Cryo-electron microscopy (cryo-EM) is rapidly emerging as a powerful tool for protein structure determination at high resolution. Here we report the structure of a complex between Escherichia coli β-galactosidase and the cell-permeant inhibitor phenylethyl β-D-thiogalactopyranoside (PETG), determined by cryo-EM at an average resolution of ~2.2 angstroms (Å). Besides the PETG ligand, we identified densities in the map for ~800 water molecules and for magnesium and sodium ions. Although it is likely that continued advances in detector technology may further enhance resolution, our findings demonstrate that preparation of specimens of adequate quality and intrinsic protein flexibility, rather than imaging or image-processing technologies, now represent the major bottlenecks to routinely achieving resolutions close to 2 Å using single-particle cryo-EM.