IDENTIFICATION OF CP26 OF PHOTOSYSTEM-II AS A COPPER-BINDING PROTEIN

Photosystem II particles from spinach contained 0.1-1.1 copper/300 chl orophyll, which was resistant to EDTA. This value was increased to 1.2 when the isolation was made in the presence of 10 muM CuSO4. No corre lation was round between the copper content and the oxygen evolving ca pacity of the photosystem II particles. In order to identify the coppe r binding protein, we developed a fractionation procedure including so lubilisation of photosystem II particles followed by precipitation wit h poly-ethylene glycol. A 22 fold purification of copper with respect to protein, was achieved for a 28 kDa protein. Partial amino acid sequ ence determination, after V8 (endo Glu-C) protease cleavage, of a 13 k Da fragment showed 14 amino acid identity with CP26 (Jansson et al. 19 92, Plant Mol. Biol. Rep. 10 : 238-250; CP29 type-1 from tomato, Piche rsky et al. 1991, Mol. Gen. Gen. 227 : 277-284; LHCII(c) from barley, Morishige and Thornberg 1992, Plant Physiol. 98 : 238-245). EPR measur ement on the purified protein suggest oxygen and/or nitrogen as ligand s for copper but tends to exclude sulfur. We conclude that the 28 kDa apoprotein of CP26 from spinach possess properties to bind one copper per molecule of CP26. A possible function for this copper protein in t he xanthophyll cycle is discussed.