Characterization of Photosystem I Chlorophyll-Protein Complexes Reconstituted into Phosphatidylcholine Liposomes

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Chlorophyll-protein complexes associated with photosystem I were isolated from native photosystem I particles (PS I-200) of spinach thylakoids by centrifugation in SDS-sucrose density gradients. These complexes were designated CPIa (Chl/P700 ratio of 〓 160), CPI' (Chl/P700 ratio of 〓 70), and LHCI (light-harvesting Chl a/b-protein complex associated with photosystem I). CPI' was reconstituted with and without LHCI into phosphatidylcholine liposomes by a freeze-thaw technique. The first-order rate constant for P700-photooxidation in proteoliposomes reconstituted with CPI' plus LHCI increased with an increase in the concentration of phosphatidylcholine. When the concentration of phosphatidylcholine was more than 20 times (by weight) that of chlorophyll in the complexes, the rate constant under light-limiting conditions was approximately double that of a mixture of two complexes not reconstituted into liposomes. The fluorescence emission spectrum (77 K) of the proteoliposomes reconstituted with CPI' plus LHCI displayed a longer wavelength band at 730-733 nm which was very similar to the spectrum of CPIa and which was not displayed in the spectrum of a mixture of CPI' and LHCI without liposomes. The circular dichroism spectrum of a mixture of CPI' and LHCI indicated that the intensity of both a positive peak at 665 nm and a negative peak at 686 nm increased when a mixture of the two complexes was reconstituted into liposomes. These results suggest that some alteration of chlorophyll organization occurs in proteoliposomes reconstituted with both CPI' and LHCI, facilitating energy transfer from LHCI to the reaction center of photosystem I.

Characterization of Photosystem I Chlorophyll-Protein Complexes Reconstituted into Phosphatidylcholine Liposomes

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