Digalactosyldiacylglycerol Is Required for Stabilization of the Oxygen-Evolving Complex in Photosystem II

Isamu Sakurai,Hajime Wada,Naoki Sato,Naoki Mizusawa

doi:10.1104/pp.107.106781

Isamu Sakurai, Hajime Wada + Show 2 more

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https://doi.org/10.1104/pp.107.106781

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Abstract

The galactolipid digalactosyldiacylglycerol (DGDG) is present in the thylakoid membranes of oxygenic photosynthetic organisms such as higher plants and cyanobacteria. Recent x-ray crystallographic analysis of protein-cofactor supercomplexes in thylakoid membranes revealed that DGDG molecules are present in the photosystem II (PSII) complex (four molecules per monomer), suggesting that DGDG molecules play important roles in folding and assembly of subunits in the PSII complex. However, the specific role of DGDG in PSII has not been fully clarified. In this study, we identified the dgdA gene (slr1508, a ycf82 homolog) of Synechocystis sp. PCC6803 that presumably encodes a DGDG synthase involved in the biosynthesis of DGDG by comparison of genomic sequence data. Disruption of the dgdA gene resulted in a mutant defective in DGDG synthesis. Despite the lack of DGDG, the mutant cells grew as rapidly as the wild-type cells, indicating that DGDG is not essential for growth in Synechocystis. However, we found that oxygen-evolving activity of PSII was significantly decreased in the mutant. Analyses of the PSII complex purified from the mutant cells indicated that the extrinsic proteins PsbU, PsbV, and PsbO, which stabilize the oxygen-evolving complex, were substantially dissociated from the PSII complex. In addition, we found that heat susceptibility but not dark-induced inactivation of oxygen-evolving activity was notably increased in the mutant cells in comparison to the wild-type cells, suggesting that the PsbU subunit is dissociated from the PSII complex even in vivo. These results demonstrate that DGDG plays important roles in PSII through the binding of extrinsic proteins required for stabilization of the oxygen-evolving complex.

Highlights

The galactolipid digalactosyldiacylglycerol (DGDG) is present in the thylakoid membranes of oxygenic photosynthetic organisms such as higher plants and cyanobacteria
We exploited the comparative genomics tools developed in the laboratory of one of the authors (Naoki Sato) to find putative glycosyltransferases that are shared by cyanobacteria and C. merolae but not by green plants
Low-temperature fluorescence spectra of intact cells excited at 590 nm indicated that fluorescence from photosystem II (PSII) increased significantly in the dgdA mutant cells compared to the wild-type cells (Fig. 4B)

Summary

Introduction

The galactolipid digalactosyldiacylglycerol (DGDG) is present in the thylakoid membranes of oxygenic photosynthetic organisms such as higher plants and cyanobacteria. We found that heat susceptibility but not dark-induced inactivation of oxygen-evolving activity was notably increased in the mutant cells in comparison to the wild-type cells, suggesting that the PsbU subunit is dissociated from the PSII complex even in vivo. These results demonstrate that DGDG plays important roles in PSII through the binding of extrinsic proteins required for stabilization of the oxygen-evolving complex. A dgd mutant that has a point mutation in the DGD1 gene contained a strongly decreased amount of DGDG (1% of total lipids compared to 15% in the wild type), and its growth and photosynthetic activity were severely affected (Dormann et al, 1995). The double mutant containing only a trace amount of DGDG showed a dwarf phenotype and altered parameters of chlorophyll (Chl) fluorescence, suggesting that DGDG has a crucial function in normal plant development and photosynthesis

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