THE 28-KDA APOPROTEIN OF CP-26 IN PS-II BINDS COPPER

Photosystem II (PS II) particles isolated from spinach in the presence of 10 muM CuSO4 contained 1.2 copper/300 Chl that was resistant to ED TA. When CuSO4 was not added during the isolation, PS II particles con tained variable amounts of copper resistant to EDTA (0.1-1.1 copper/30 0 Chl). No correlation was found between copper content and oxygen evo lving capacity of the PS II particles. To identify the copper binding protein, we developed a fractionation procedure which included solubil isation of PS II particles followed by precipitation with polyethylene glycol. A 22-fold purification of copper with respect to protein was achieved for a 28 kDa protein. Partial amino acid sequence of a 13 kDa fragment, obtained after V8 (endo Glu-C) protease treatment, showed i dentity with CP 26 over a 14 amino acid stretch. EPR measurements on t he purified protein suggest oxygen and/or nitrogen as ligands for copp er but tend to exclude sulfur. We conclude that the 28 kDa apoprotein of CP 26 from spinach binds one copper per molecule of CP 26. A possib le function for this copper protein in the xanthophyll cycle is discus sed.