Identification of a calcium-binding site in the PsbO protein of photosystem II.

Analysis of the anomalous X-diffraction data reported by Ferreira et al. (PDB entry 1S5L) for crystals of photosystem II isolated from Thermosynechococcus elongatus indicates that a calcium ion is bound to the PsbO protein. The Ca2+-binding site is located close to the lumenal exit of a putative proton channel leading from the water splitting site.