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

Chlorophyll-protein complexes associated with photosystem Iwere isolated from native photosystem I particles (PS I-200)of spinach thylakoids by centrifugation in SDS-sucrose densitygradients. These complexes were designated CPIa (Chl/P700 ratioof 160), CPI' (CW/P700 ratio of 70), and LHCI (light-harvestingChl a/bprotein complex associated with photosystem I). CPI'was reconstituted with and without LHCI into phosphatidylcholineliposomes by a freeze-thaw technique. The first-order rate constantfor P700-photooxidation in proteoliposomes reconstituted withCPI' plus LHCI increased with an increase in the concentrationof phosphatidylcholine. When the concentration of phosphatidylcholinewas more than 20 times (by weight) that of chlorophyll in thecomplexes, the rate constant under lightlimiting conditionswas approximately double that of a mixture of two complexesnot reconstituted into liposomes. The fluorescence emissionspectrum (77 K) of the proteoliposomes reconstituted with CPI'plus LHCI displayed a longer wavelength band at 730–733nm which was very similar to the spectrum of CPIa and whichwas not displayed in the spectrum of a mixture of CPI' and LHCIwithout liposomes. The circular dichroism spectrum of a mixtureof CPI' and LHCI indicated that the intensity of both a positivepeak at 665 nm and a negative peak at 686 nm increased whena mixture of the two complexes was reconstituted into liposomes.These results suggest that some alteration of chlorophyll organizationoccurs in proteoliposomes reconstituted with both CPI' and LHCI,facilitating energy transfer from LHCI to the reaction centerof photosystem I. (Received July 18, 1986; Accepted March 12, 1987)