Function of phosphatidylglycerol in assembly and stabilization of photosynthetic machinery

磷脂酰甘油(PG)是叶绿体和蓝藻类囊体膜中普遍存在的一种脂质成分。类囊体膜中的主要脂类是糖脂和磷脂PG。由于PG是类囊体膜中唯一的磷脂，并且在中性pH条件下是一个带负电荷的分子，因此PG很可能与类囊体膜上的光合作用复合体成分之间的相互作用是必不可少的，PG在光合作用中发挥着糖脂不可替代的特殊作用。PCC6803，不利于PG的生物合成。本研究表明，PG在PSII的电子供给侧，即放氧系统中也起着重要的作用。对纯化的PSII复合体的详细分析表明，PG缺失的pgsA突变细胞的PSII放氧活性仅为野生型细胞的50%左右。在PG缺失突变细胞的PSII中观察到稳定Mn簇所需的外源蛋白PsbO、PsbV和PsbU的解离，随后一个锰原子被释放。当PG被重新加入到PG耗尽的突变细胞中时，释放的PsbO重新结合到PSII上，即使从头合成蛋白质被抑制。热处理或暗培养诱导的PG缺失突变细胞的光合活性变化与Δ、Δ和Δ突变细胞相似。这些结果表明，PG在PSII供体侧维持功能的Mn-簇所需的外源蛋白的结合中发挥着重要作用。

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.