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 4,9-12.
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