Revealing the structure of the oxygen-evolving core dimer of photosystem II by cryoelectron crystallography

Here we present cryoelectron crystallographic analysis of an isolated dimer ic oxygen-evolving complex of photosystem II (at a resolution of similar to 0.9 nm), revealing that the D1-D2 reaction center (RC) proteins are centra lly located between the chlorophyll-binding proteins, CP43 and CP47. This c onclusion supports the hypothesis that photosystems I and II have similar s tructural features and share a common evolutionary origin. Additional densi ty connecting the two halves of the dimer, which was not observed in a rece ntly described CP47-RC complex that did not include CP43, may be attributed to the small subunits that are involved in regulating secondary electron t ransfer, such as PsbH. These subunits are possibly also required for stabil ization of the dimeric photosystem II complex. This complex, containing at least 29 transmembrane helices in its asymmetric unit, represents one of th e largest membrane protein complexes studied at this resolution.