The Role of Phosphatidylglycerol in Photosynthesis

Abstract

Since the first identification of phosphatidylglycerol (PG) in the green alga Scenedesmus by Benson and Maruo in 1958, this lipid has been recognized as a ubiquitous phospholipid that is present in almost all organisms. PG is an anionic phospholipid, with a negatively charged phosphate group at neutral pH. In photosynthetic organisms, such as cyanobacteria and higher plants that perform oxygenic photosynthesis, the majority of PG is found in thylakoid membranes, which are the site of photosynthetic light reactions and electron transport. Thylakoid membranes are composed predominantly of glycoli-pids, such as monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG), and sulfo-quinovosyldiacylglycerol (SQDG), and PG is the only major phospholipid that is present in thylakoid membranes. Thus, it has been suggested that PG might play an important role in the primary processes of photosynthesis in thylakoid membranes. Recent x-ray crystallographic analyses of the cyanobacterial photosystem I (PS I) and photosystem II (PS II) complexes that are involved in the photosynthetic transport of electrons in thylakoid membranes have identified four and 25 lipid molecules per monomer in the respective complexes. Three and two of these lipid molecules are PG in the PS I and PS II complexes, respectively. These findings suggest that lipids, including PG, might play important roles not only in the formation of the lipid bilayers of thylakoid membranes but also in the folding and assembly of the protein subunits in each complex. Genetic and biochemical studies of the role of PG, using mutants and genetically manipulated strains of cyanobacteria and higher plants with levels of PG different from those in the corresponding wild-type strains, have confirmed that PG is essential for the growth of cyanobacteria and higher plants, moreover, that it is crucial to the photosynthetic transport of electrons, the development of chloroplasts, and tolerance to chilling. In this review, we summarize our present understanding of the biochemical and physiological roles of PG in photosynthesis.