Functional reconstitution of photosystem I reaction center from cyanobacteriumSynechocystis sp PCC6803 into liposomes using a new reconstitution procedure

Abstract

Photosystem I reaction center from the cyanobacteriumSynechocystis sp PCC6803 was reconstituted into phosphatidylcholine/phosphatidic acid liposomes. Liposomes prepared by reversephase evaporation were treated with various amounts of different detergents and protein incorporation was analyzed at each step of the solubilization process. After detergent removal the activities of the resulting proteoliposomes were measured. The most efficient reconstitution was obtained by insertion of the protein complex into preformed liposomes destabilized by saturating amounts of octylglucoside. In the presence of N-methylphenazonium methosulfate and ascorbic acid, liposomes containing the reaction center catalyzed a light-dependent net H⁺ uptake as measured by the 9-aminoacridine fluorescence quenching and the pH meter. An important benefit of the new reconstitution procedure is that it produces a homogeneous population of large-size proteoliposomes with a low ionic permeability and with a majority inwardly directed H⁺ transport activity. In optimal conditions, a light-induced δpH of about 1.8 units could be sustained at 20‡C in the presence of valinomycin. In the absence of valinomycin, a “back-pressure” effect of an electrical transmembrane potential decreased both the rate and the extent of the H⁺ transport. The reaction center was also co-reconstituted with F₀F₁ H⁺-ATPases from chloroplasts and from the thermophilic bacterium, PS3. The coreconstituted system was shown to catalyze a light-dependent phosphorylation which could only be measured in the presence of a high concentration of PSI (low lipid/PSI ratios) while no δpH could be detected.