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

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Abstract
Here we describe the first 3D structure of the photosystem II (PSII) supercomplex of higher plants, constructed by single particle analysis of images obtained by cryoelectron microscopy. This large multisubunit membrane protein complex functions to absorb light energy and catalyze the oxidation of water and reduction of plastoquinone. The resolution of the 3D structure is 24 A and emphasizes the dimeric nature of the supercomplex. The extrinsic proteins of the oxygen-evolving complex (OEC) are readily observed as a tetrameric cluster bound to the lumenal surface. By considering higher resolution data, obtained from electron crystallography, it has been possible to relate the binding sites of the OEC proteins with the underlying intrinsic membrane subunits of the photochemical reaction center core. The model suggests that the 33 kDa OEC protein is located towards the CP47/D2 side of 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 predicts that the 23/17 kDa OEC proteins are positioned at the N-terminus of the D1 protein incorporating the AB lumenal loop of this protein and two other unidentified transmembrane helices. Overall the 3D model represents a significant step forward in revealing the structure of the photosynthetic OEC whose activity is required to sustain the aerobic atmosphere on our planet.

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