Abstract
The thylakoid membrane is the site of photochemical and electron transport reactions of oxygenic photosynthesis. The lipid composition of the thylakoid membrane, with two galactolipids, one sulfolipid, and one phospholipid, is highly conserved among oxygenic photosynthetic organisms. Besides providing a lipid bilayer matrix, thylakoid lipids are integrated in photosynthetic complexes particularly in photosystems I and II and play important roles in electron transport processes. Thylakoid lipids are differentially allocated to photosynthetic complexes and the lipid bilayer fraction, but distribution of each lipid in the thylakoid membrane is unclear. In this study, based on published crystallographic and biochemical data, we estimated the proportions of photosynthetic complex-associated and bilayer-associated lipids in thylakoid membranes of cyanobacteria and plants. The data suggest that ∼30 mol% of phosphatidylglycerol (PG), the only major phospholipid in thylakoid membranes, is allocated to photosystem complexes, whereas glycolipids are mostly distributed to the lipid bilayer fraction and constitute the membrane lipid matrix. Because PG is essential for the structure and function of both photosystems, PG buried in these complexes might have been selectively conserved among oxygenic phototrophs. The specific and substantial allocation of PG to the deep sites of photosystems may need a unique mechanism to incorporate PG into the complexes possibly in coordination with the synthesis of photosynthetic proteins and pigments.
Highlights
The thylakoid membrane is the site of photochemical and electron transport reactions in oxygenic phototrophs
Besides providing a lipid bilayer matrix, thylakoid lipids are integrated in photosystem II (PSII) (Loll et al, 2005, 2007; Sakurai et al, 2006; Umena et al, 2011; Wei et al, 2016), Cyt b6/f (Kurisu et al, 2003; Stroebel et al, 2003; Baniulis et al, 2009; Hasan et al, 2011), photosystem I (PSI) (Jordan et al, 2001; Kubota et al, 2010; Qin et al, 2015; Mazor et al, 2017), and light-harvesting complexes (LHCs) (Liu et al, 2004; Standfuss et al, 2005; Wei et al, 2016) and play important roles in electron transport processes (Kobayashi et al, 2016b)
The relative abundance of PSI, PSII, and LHCII in plants greatly differs according to growth conditions, in this study, we estimated lipid content in photosystem complexes based on the data in spinach thylakoids (Table 1) (Kirchhoff et al, 2002)
Summary
The thylakoid membrane is the site of photochemical and electron transport reactions in oxygenic phototrophs. Glycerolipids form a bilayer matrix, in which photosynthetic protein–cofactor complexes such as photosystem I (PSI), photosystem II (PSII), cytochrome (Cyt) b6/f complex and ATP synthase are embedded. The hydrophobic lipid bilayer prevents free diffusion of ions and allows for creating a proton gradient while enabling lateral diffusion of the photosynthetic complexes and the mobile electron carriers for efficient electron transport reactions. The lipid composition of the thylakoid membrane is highly conserved among oxygenic phototrophs (Murata et al, 1981; Mendiola-Morgenthaler et al, 1985; Dorne et al, 1990)
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