A high-resolution electron cryo-microscope equipped with a top-entry specimen stage has been refined by modifying a previously described superfluid helium stage. Instruments equipped with such a cryo-stage achieve a resolution of better than 2.0 Å and have proved extremely powerful in the high-resolution structure analysis of membrane proteins. Improvement of the electron microscopic system in combination with improved specimen preparation techniques allowed the structure of bR to be analyzed to a resolution of 3.0 Å. The 3D structure of bR, especially the surface features, revealed the structural basis for the efficient guidance of protons to the entrance of the transmembrane channel. Based on the characteristic difference of the atomic scattering factors for electrons of ionized atoms versus neutral atoms as well as the data analysis, charged and uncharged amino acid residues could be discriminated. Thus, electron crystallography is providing us with new and exciting insights into the structure of membrane proteins because it not only enables us to determine the structure of a membrane protein, but allows us to study its interaction with the surrounding lipid molecules and to determine its ionization state.

The structural study of membrane proteins by electron crystallography

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