Structural role of lipids in molecular architecture of photosystem I in thylakoid membranes.

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Chlorophyll-protein complexes were isolated from spinach thylakoids. These complexes were designated CPI' (reaction-center enriched photosystem I), and LHCI (light-harvesting Chl a/b-protein complex associated with photosystem I). When CPI' was reconstituted with LHCI into liposomes of soybean phosphatidylcholine (PC), the first-order rate constant for P700-photooxidation (kP700) in proteoliposomes 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 of the two complexes reconstituted into soybean PC displayed a longer wavelength band at 730-733 nm which was not dispalyed in the spectrum of a mixture of CPI' and LHCI without liposomes. The circular dichroism spectrum of the two complexes indicated that the intensities of both a positive peak at 665 nm and a negative peak at 686 nm increased when the two complexes were reconstituted into soybean PC. I … More n order to investigate whether reconstitution with thylakoid lipids would affect kP700, CPI' was reconstituted with LHCI into liposomes of lipids isolated from spinach thylakoids. Reconstitution with acidic lipids (consisted of sulfoquinovosyldiacylglycerol and phosphatidylglycerol) resulted in no effect on kP700. On the other hand, kP700 of a mixture of CPI' and LHCI increased significantly when the two complexes were reconstituted into neutral lipids. In fact reconstitution with monogalactosyldiacylglycerol (containing 20%(W/W) soybean PC for making liposomes) gave a 75% stimulation above kP700 in a mixture of the two complexes without liposomes, and digalactosyldiacylglycerol (containing 20% soybean PC) gave about 30% stimulation. These results 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, and monogalactosyldiacylglyceride plays an important role in the incorporation of chlorophyll-protein complexes associated with photosystem I into the bilayer of the thylakoid membranes. Less