Crystal structure of the PsbQ polypeptide of photosystem II from higher plants

The smallest extrinsic polypeptide of the water oxidizing complex (PsbQ) was extracted and purified from spinach photosystem II membranes. It was then crystallized in the presence of Zn2+ and its structure determined by X-ray diffraction at 1.95 Å resolution, using the MAD method with the zinc as anomalous scatterer. The crystal structure reveals that the core of the protein is a four-helix bundle, whereas the N-terminal portion, which probably interacts with the rest of the photosystem, is disordered in the crystal. The distribution of positive and negative charges on the protein surface is discussed.