Characterization by electron microscopy of dimeric Photosystem II core complexes from spinach with and without CP43

Dimeric associations of the D1-D2-CP47 and D1-D2-CP47-CP43 complexes of Photosystem II from spinach were isolated and purified with sucrose density gradient centrifugation and gel filtration chromatography and analyzed by electron microscopy and image analysis. Images of both preparations show characteristic details in protein density. The location of the CP43 subunit and the way the dimers are associated could be determined from a comparison between diamond-like monomeric projections of the D1-D2-CP47-CP43 complex (maximal dimensions along the diagonals 10-12 and 7-8 nm) and triangle-like monomeric projections of the D1-D2-CP47 complex (dimensions 8-9 and 7-8 nm). Both isolated complexes have different dimeric configurations than observed before in several other dimeric complexes, and based on biochemical considerations we conclude that both newly observed configurations are artificial. The observation of the artificial aggregates, however, allows conclusions on the organization of Photosystem II in two-dimensional crystals and on the size of the monomeric unit. We propose a model for the location of D1, D2, CP43 and CP47 in the Photosystem II core complex in which CP43 and CP47 are positioned at the tips of the monomeric unit, closely connected to D2 and D1, respectively. (C) 1997 Elsevier Science B.V.