Projection structure of photosystem II in vivo determined by Cryo-electron crystallography

Photosystem II (PSII) is a protein-pigment complex situated in the thylakoid membranes of plants and cyanobacteria where it catalyses the conversion of light into chemical energy. This energy is used to extract electrons from water, during which process oxygen is evolved. Owing to its extreme fragility and the large number of polypeptides (> 20) it is composed of, the complex has so far proven recalcitrant to high-resolution structural studies. Cryo-electron crystallography of 2-D crystals (a=15.4 nm, b=23.1 nm, γ=97.2°, pl) comprising in situ PSII revealed the first projection structure of the native complex. The unit cell contains one monomeric complex in which three central domains straddle an elongated intramolecular cavity. In conjunction with earlier data, these central domains were assigned to the reaction center core subunits of PSII consisting of CP43, CP47, the reaction center heterodimer D1/D2 and cytochrome b-559. The data are discussed in view of the evolution of reaction centers from anoxygenic to oxygenic photosynthesis.