Binding of ⁴⁵Ca by Photosystem II Polypeptides

Abstract

It is now well established that photosystem II reactions have an absolute requirement for calcium ion(s). The calcium ions are required for both the normal functioning of the water oxidising complex^1,2 and for electron flow from the secondary electron donor, Z, to the reaction center³. The number of calcium binding sites is controversial and is reported to range between 1 and 3 per reaction center^4,5. The role of calcium in photosystem II and the location of the calcium binding sites is unknown. Exogenously added calcium can partially reverse the inhibition of oxygen evolution due to the removal of the hydrophilic polypeptides (33, 23, 16 and 10kDa) by various salt treatments^2,6,7,8 such observations have led to the suggestion that these hydrophilic polypeptides provide a binding site(s) for calcium ion required for water oxidation. However, conflicting results suggest that light is required for the release of calcium ion from photosystem II membranes depleted of the hydrophilic polypeptides⁹ and indicate that the calcium binding site is associated with an intrinsic photosystem II component. In order to understand the role of calcium in photosystem II it is clearly important to identify the calcium binding sites. Here we report the identification of individual polypeptides in photosystem II which are able to bind calcium ions when immobilised to nitrocellulose membranes.