Chapter 12 The Anionic Chloroplast Membrane Lipids: Phosphatidylglycerol and Sulfoquinovosyldiacylglycerol

Abstract

Four polar lipids provide the basic building blocks for the membranes of chloroplasts of all plants and cyanobacteria: the two galactolipids mono-and digalactosyldiacylglycerol, the sulfolipid sulfoquinovosyldiacylglycerol, and the phospholipid phosphatidylglycerol. All four lipids are glycerolipids with a diacylglycerol backbone, but only one is a phosphoglycerolipid. While the two galactolipids are not charged at biological pH, the sulfolipid and phospholipid carry a negative charge. The focus here is on the two chloroplast anionic lipids. Enzymes involved in their assembly are well described and are localized in the chloroplasts. Mutants affecting the biosynthesis of one or both anionic lipids of the model plant Arabidopsis thaliana , the unicellular algae Chlamydomonas reinhardtii or different cyanobacteria are available providing tools for the functional analysis of these lipids. This genetic analysis has shown that sulfolipid can substitute to some extent for phosphatidylglycerol under phosphate limiting growth conditions, when plants replace phospholipids with non-phosphorous glycolipids. However, phosphatidylglycerol mutants have a much more severe phenotype than sulfolipid mutants suggesting specific roles for phosphatidylglycerol in the photosynthetic membrane that cannot be filled by the sulfolipid. Double mutant analysis suggests that the net negative charge of the photosynthetic membrane is critical for proper function.