3D map of the plant photosystem II supercomplex obtained by cryoelectron microscopy and single particle analysis

Here we describe the first 3D structure of the photosystem II (PSII) superc omplex of higher plants, constructed by single particle analysis of images obtained by cryoelectron microscopy. This large multisubunit membrane prote in complex functions to absorb light energy and catalyze the oxidation of w ater and reduction of plastoquinone. The resolution of the 3D structure is 24 Angstrom and emphasizes the dimeric nature of the supercomplex. The extr insic proteins of the oxygen-evolving complex (OEC) are readily observed as a tetrameric cluster bound to the lumenal surface. By considering higher r esolution data, obtained from electron crystallography, it has been possibl e to relate the binding sites of the OEC proteins with the underlying intri nsic membrane subunits of the photochemical reaction center core. The model suggests that the 33 kDa OEC protein is located towards the CP47/D2 side o f the reaction center but is also positioned over the C-terminal helices of the D1 protein including its CD lumenal loop. In contrast, the model predi cts that the 23/17 kDa OEC proteins are positioned at the N-terminus of the D1 protein incorporating the AB lumenal [GRAPH] loop of this protein and t wo other unidentified transmembrane helices. Overall the 3D model represent s a significant step forward in revealing the structure of the photosynthet ic OEC whose activity is required to sustain the aerobic atmosphere on our planet.