Photosystem II (PSII) catalyzes a key step in photosynthesis, the oxidation of water to oxygen. Excellent structural models exist for the dimeric PSII core complex of cyanobacteria, but higher order physiological assemblies readily dissociate when solubilized from the native thylakoid membrane with detergent. Here, we describe the crystallization of PSII from Thermosynechococcus elongatus with a postcrystallization treatment involving extraction of the detergent C12E8. This resulted in a transition from Type II to Type I-like membrane protein crystals and improved diffraction to 2.44 Å resolution. The obtained PSII packing in precise rows, interconnected by specific pairs of galactolipids and a loop in the PsbO subunit specific to cyanobacteria, is superimposable with previous electron microscopy images of the thylakoid membrane. The study provides a detailed model of such a superstructure and its organization of light-harvesting pigments with possible implications for the understanding of their efficient use of solar energy.
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