3-DIMENSIONAL STRUCTURE OF PHOTOSYSTEM-II

THE photosynthetic oxidation of water occurs within a multiprotein com plex called photosystem II (PSII), located in the thylakoid membranes of plants and cyanobacteria. Little information on PSII architecture i s available; even the oligomeric nature of the complex is contentious, with various predictions of monomeric1, dimeric and tetrameric2-4 for ms. Biochemistry suggests that it consists of an outer shell of easily removable light-harvesting proteins and a more resistant core5. The c ore itself can be split into further light-harvesting proteins and a r eaction 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 wa ter 'splitting' site6-8. We report here the first, to our knowledge, t hree-dimensional functionally intact structure of PSII higher plants, complete with antennae and core light-harvesting proteins. Our model h as been obtained by digital image processing of ordered two-dimensiona l arrays of the complex, such as have often been observed4,9-12.