Comparison of photosystem II 3D structure as determined by electron crystallography of frozen-hydrated and negatively stained specimens

The efficiency of electron crystallographic analysis for 3D structure determination has been demonstrated for an increasing number of proteins and protein complexes. Here, we report cryo-electron crystallography data leading to the first 3D structure of unstained, frozen-hydrated photosystem II (PSII), a multi-protein, membrane-integral complex comprised of more than 20 polypeptides. These data are directly comparable with the 3D structure of negatively stained PSII and reveal a high correlation between the two types of specimens at 3 nm resolution, confirming the earlier assignments of subunits to well-defined domains as well as the functionally important intramolecular cavity. The cavity is at least partially capped on the lumenal surface of the complex and opens out to a 3-nm diameter internal water-filled space that dominates the lumenal side of PSII. The data are discussed in the light of recent studies on PSII subcomplexes. It is concluded that the fidelity of data on transmembrane proteins in negative stain is mainly affected by a flattening of the specimen, partial-depth staining and limited stain penetration into the lipid bilayer.