IN-VITRO RECONSTITUTION OF THE PHOTOSYSTEM-I LIGHT-HARVESTING COMPLEXLHCI-730 - HETERODIMERIZATION IS REQUIRED FOR ANTENNA PIGMENT ORGANIZATION

Here we describe the in vitro reconstitution of photosystem I light-ha rvesting complexes with pigments and proteins (Lhca1 and Lhca4) obtain ed by overexpression of tomato Lhca genes in Escherichia coil. Using L hca1 and Lhca4 individually for reconstitution results in monomeric pi gment-proteins, whereas a combination thereof yields a dimeric complex . Interactions of the apoproteins is highly specific, as reconstitutio n of either of the two constituent proteins in combination with a ligh t-harvesting protein of photosystem II does not result in dimerization . The reconstituted Lhca1/4, but not complexes obtained with either Lh ca1 or Lhca4 alone, closely resembles the native LHCI-730 dimer from t omato leaves with regard to spectroscopic properties, pigment composit ion, and stoichiometry. Monomeric complexes of Lhcal or Lhca4 possess lower pigment/protein ratios, indicating that interactions of the two subunits not only facilitates pigment reorganization but also recruitm ent of additional pigments. In addition to higher averages of chloroph yll a/b ratios in monomeric complexes than in LHCI-730, comparative fl uorescence and CD spectra demonstrate that heterodimerization involves preferential ligation of more chlorophyll b.