A 30-kDa chlorophyll-a/b-binding protein was purified from photosystem I1 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 k 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.The primary physicochemical events of photosynthesis comprise light absorption, excitation, energy transfer and exciton transformation into electrochemical free energy by the formation of a sufficiently stable radical pair within a functional unit, referred to as the reaction-centre complex. In higher plants, different types of chlorophyll (Ch1)-containing protein complexes are incorporated into the thylakoid membrane: Chl-a/b-containing peripheral antenna complexes exhibiting lateral mobility; immobile core antenna complexes containing either Chl a/b or Chl a and reaction-centre complexes of photosystem I and 11, respectively (for recent reviews see [I -31). The mobile peripheral light-harvesting antenna complex of PS I1 (LHC 11) carries about 120 Chl a/ h molecules bound to at least two distinct polypeptides with relative molecular masses of 25 kDa and 27 kDa [I]. The immobile antenna is composed of about 80 k 20 Chl a/b molecules bound to polypeptides of 24 (CP24), 26 (CP26) and 29 (CP29) kDa [I -51. The pigment distribution among these polypeptides is a matter of debate. In addition, two Chl-acontaining proteins of 43 kDa and 47 kDa are closely associated with the PS I1 reaction-centre complex. Each of these polypeptides is assumed to bind about 25 Chl a molecules [6, 71. Both proteins presumably also function as core antennae, but for CP47 in particular an additional role in maintaining the structural integrity of the reaction-centre complex is not excluded [5,8,[9][10][11][12].The isolation and characterization of Chl-binding proteins is difficult to achieve with commonly used protein-chemistry Abbreviations. Chl, chlorophyll; CP, chlorophyll-protein complex; LHC, light-harvesting complex; P680, reaction-centre chlorophyll u molccule of photosystem 11; PS I and PS 11, photosystems I and 11.techniques because of the tendency of such proteins to selfassociate into oligomers, their similarities in relative molecular masses, isoelectric points and hydrophobicities and structural homologies. Many of these polypeptides have therefore been p...