ISOLATION, PURIFICATION AND PARTIAL CHARACTERIZATION OF A 30-KDA CHLOROPHYLL-ALPHA-BETA-BINDING PROTEIN FROM SPINACH

A 30-kDa chlorophyll-a/b-binding protein was purified from photosystem II membrane fragments using Ca2+-chelating Sepharose 6B chromatography. The protein binds approximately four chlorophyll a molecules, one chlorophyll b molecule and carotenoids. Its 77-K fluorescence-emission spectrum exhibits a maximum at 680 +/- 1 nm. The protein has a high tendency to form a dimer in the presence of Ca2+ . Ca2+ binding affects the low-temperature fluorescence-emission maximum, leading to a decrease in its intensity and a blue shift of 1 nm. Similar spectral changes were obtained in the presence of Mg2+, possibly indicating a common binding domain for both cations. We interpret these observations as cation-induced conformational changes of the protein, which were reversible upon subsequent incubation in EDTA. Evidence is presented for the involvement of carboxyl groups in the coordination sphere of the bivalent cations. The possible structural and functional role of the protein is discussed.