Function of phosphatidylglycerol in assembly and stabilization of photosynthetic machinery

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Phosphatidylglycerol (PG) is one of the ubiquitous lipid components in thylakoid membranes from chloroplasts and cyanobacteria. The major lipids found in thylakoid membranes are glycolipids and the phospholipid PG. Because PG is the only phospholipid in thylakoid membranes and is a negatively charged molecule at neutral pH, it is likely that PG mediates indispensable interactions with the components of photosynthetic complexes in thylakoid membranes and that PG plays a specific role in photosynthesis for which glycolipids cannot substitute.In this study, we have investigated the function of PG in assembly and stabilization of photosynthetic machinery with the pgsA mutant of the cyanobacterium Synechocystis sp. PCC6803, which is defective for the biosynthesis of PG. The present study shows that PG also plays an important role in the electron donor side of PSII, namely the oxygen-evolving system. Detailed analyses of purified PSII complexes indicated that PSII from PG-depleted pgsA mutant cells sustained only 〜50% of the oxygen-evolving activity compared to wild-type cells. Dissociation of the extrinsic proteins PsbO, PsbV, and PsbU, which are required for stabilization of the Mn-cluster, followed by the release of a manganese atom, was observed in PSII of the PG-depleted mutant cells. The released PsbO re-bound to PSII when PG was added back to the PG-depleted mutant cells, even when de novo protein synthesis was inhibited. Changes in photosynthetic activity of the PG-depleted pgsA mutant cells induced by heat treatment or dark incubation resembled those of ΔpsbO, ΔpsbV and ΔpsbU mutant cells. These results suggest that PG plays an important role in the binding of extrinsic proteins required for sustaining a functional Mn-cluster on the donor side of PSII.