Phosphatidylglycerol Is Essential for Oligomerization of Photosystem I Reaction Center

Abstract

Our earlier studies with the pgsA mutant of Synechocystis PCC6803 demonstrated the important role of phosphatidylglycerol (PG) in PSII dimer formation and in electron transport between the primary and secondary electron-accepting plastoquinones of PSII. Using a long-term depletion of PG from pgsA mutant cells, we could induce a decrease not only in PSII but also in PSI activity. Simultaneously with the decrease in PSI activity, dramatic structural changes of the PSI complex were detected. A 21-d PG depletion resulted in the degradation of PSI trimers and concomitant accumulation of monomer PSI. The analyses of PSI particles isolated by MonoQ chromatography showed that, following the 21-d depletion, PSI trimers were no longer detectable in the thylakoid membranes. Immunoblot analyses revealed that the PSI monomers accumulating in the PG-depleted mutant cells do not contain PsaL, the protein subunit thought to be responsible for the trimer formation. Nevertheless, the trimeric structure of PSI reaction center could be restored by readdition of PG, even in the presence of the protein synthesis inhibitor lincomycin, indicating that free PsaL was present in thylakoid membranes following the 21-d PG depletion. Our data suggest an indispensable role for PG in the PsaL-mediated assembly of the PSI reaction center.Phosphatidylglycerol (PG) is an integral component of photosynthetic membranes. PG molecules are important for both the formation and functioning of photosynthetic apparatus. In cyanobacterial cells, PG is the only representative of the phospholipid family (Wada and Murata, 1998), and the majority of PG molecules are localized in the thylakoid membranes that are the site of oxygenic electron transport. Photosynthesis is the basic energy source of cyanobacteria and generally for photosynthetic organisms. The availability of the complete genomic sequence of Synechocystis PCC6803 (Kaneko et al., 1996) opened the way for studying the structural and functional roles of PG via molecular genetic approaches. The pgsA gene encoding PG phosphate synthase was inactivated in Synechocystis PCC6803 cells by inserting a kanamycin resistance gene cassette (Hagio et al., 2000). Maintenance of the recently generated pgsA mutant strain requires exogenously supplied PG. A 40% decrease in photosynthetic oxygen-evolving activity could be detected following a 3-d depletion of PG, which resulted in an approximately 50% decrease in the amount of PG molecules in the cellular membranes. The photosynthetic processes, as measured by fluorescence induction in the PG-depleted cells, slowed down, and a perturbation of the surroundings of the secondary quinone acceptor (Q B ) was observed (Gombos et al., 2002). This perturbation was similar to the effect of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), which can block electron transport between primary quinone acceptor (Q A ) and Q B . Additionally, Fragata et al. (1991) demonstrated that in isolated barley (Hordeum vulgare) PSII particles, oxygen-evolving activity strongly depends on the pr...

Evidence for a Role of VIPP1 in the Structural Organization of the Photosynthetic Apparatus in Chlamydomonas

“…PG-depleted Synechocystis mutants were highly susceptible to photobleaching, which was explained by an increased formation of singlet oxygen as a consequence of reduced electron transfer from Q A to Q B that enhances the occurrence of the dangerous triplet state of chlorophyll (Sakurai et al, 2003;Domonkos et al, 2008). Moreover, loss of PSI activity upon PG depletion was found to come along with an accumulation of PSI monomers at the expense of PSI trimers (Domonkos et al, 2004), similar to the observation of Fuhrmann et al (2009a) in a vipp1 merodiploid strain.…”

Changes in membrane lipids and carotenoids during light acclimation in a marine cyanobacterium Synechococcus sp.

“…C18 chains are preferred at the sn-1 position of the glycerol backbone, while C16 chains are preferred at the sn-2 position (Zepke et al 1978;Hölzl and Dörmann 2007;Sato and Wada 2009). An essential role for PG has been found in photosynthesis (Sato et al 2000;Domonkos et al 2004;Sato 2004;Wada and Murata 2007) and regulation of carotenoid biosynthesis (Domonkos et al 2009); SQDG and/or DGDG seem to substitute PG under phosphate limitation (Güler et al 1996;Dörmann and Benning 2002;Sato 2004;Frentzen 2004;Awai et al 2007;Kobayashi et al 2009); and MGDG or the MGDG/DGDG ratio has been bound to the upkeep of the PSII complex functional configuration and efficient energy transfer between the photosystem II antenna complexes and core complex (Grzyb et al 2008;Leng et al 2008;Zhou et al 2009). …”

Responses of Unsaturated Fatty Acid in Membrane Lipid and Antioxidant Enzymes to Chilling Stress in Sweet Sorghum (Sorghum bicolor (L.) Moench) Seedling

“…Among them, PG is a phospholipid containing trans sixteen carbon triple acid, and is widely present in the biological membranes. The majority of PG molecules are localized in the thylakoid membranes and functions as the site of oxygenic electron transport in PSII (Domonkos et al, 2004;Murata, 1983;Wada & Murata, 1998;Yu & Benning, 2003). An increase in the proportion of unsaturated fatty acids results in the membrane remaining functional at lower temperatures (O'Kane et al, 1996).…”