Chloride relations of photosystem II membrane preparations depleted of, and resupplied with, their 17 and 23 kDa extrinsic polypeptides

Abstract

Photosystem II membranes prepared from thylakoids of Phytolacca americana chloroplasts were depleted of their intrinsic 17 and 23 kDa polypeptides, and the effects of a reconstitution of these polypeptides on the Cl⁻ requirements of O₂ evolution activity were analyzed. It was found that the activating effectiveness of limiting amounts of added Cl⁻ was increased several fold by an addition of the 23 kDa polypeptide. When it was supplemented by the 17 kDa species, only a small additional increase occurred, but Cl⁻ retention in Cl⁻ free media was enhanced greatly. Addition of the 17 kDa polypeptide alone was without effect because it is known that it cannot bind to its native site unless the 23 kDa polypeptide is in place.

Optimal enhancements of the effectiveness of activating added Cl⁻ were observed when the assays were done in the presence of the reconstituting polypeptides. When the reconstituting treatment with the polypeptides, and the assay of the Cl⁻ relations, were separated, it was advantageous to have Cl⁻ present in the reconstituting medium, and not to add Ca²⁺, another cofactor of photosynthetic water oxidation. Those requirements are attributed to the labilizing effects Cl⁻ free conditions and divalent cations have on the association of especially the 23 kDa polypeptides with the water oxidizing complex, and to a possible aggregation of the membranes under the influence of Ca²⁺ which might have impeded proper polypeptide binding.