Three-dimensional structure of photosystem II

THE photosynthetic oxidation of water occurs within a multi-protein complex called photosystem II (PSII), located in the thylakoid membranes of plants and cyanobacteria. Little information on PSII architecture is available; even the oligomeric nature of the complex is contentious, with various predictions of monomeric1, dimeric and tetrameric2–4 forms. Biochemistry suggests that it consists of an outer shell of easily removable light-harvesting proteins and a more resistant core5. The core itself can be split into further light-harvesting proteins and a reaction centre5,6. The reaction centre polypeptides bind the electron transfer components, and probably contain amino-acid residues liganded to the cluster of four Mn atoms that is believed to constitute the water 'splitting' site6–8. We report here the first, to our knowledge, three-dimensional functionally intact structure of PSII of higher plants, complete with antennae and core light-harvesting proteins. Our model has been obtained by digital image processing of ordered two-dimensional arrays of the complex, such as have often been observed