Protein-protein interactions of light-harvesting pigment protein from spinach chloroplasts. I.Ca-2+ binding and its relation to protein association.

The role of divalent cations in the regulation of the distribution of excitation energy between the two photosystems involved in green plants photosynthesis has led us to search for a better understanding of how such phenomena might occur at the molecular level. Since small changes in orientation of and distance between pigment molecules could greatly affect the distribution of excitation energy, we have decided to study the effects of ions on the light-harvesting pigment protein from spinach chloroplasts. The light-harvesting pigment protein is shown to have two types of binding sites for Ca-2+. Binding studies and analytical ultracentrifugation indicate that site I (K-d equals 2.5 mu-M, n equals 1.5-4.0 mu-mol Ca-2+ bound/mg chlorophyll) is lost as the protein associates. Site II (K-d equals 32 mu-M, n equals 9,5 mu-mol Ca-2+/mg chlorophyll) is not affected by the association of the protein. This site is responsible, however, for a further divalent cation-dependent association of the protein. The possible role of this protein in grana stacking and control of spillover is discussed.