A model for chloroplast thylakoid membranes involving orderly arrangements of negatively charged lipidic particles containing sulphoquinovosyldiacylglycerol

Regulation of LHCII aggregation by different thylakoid membrane lipids

In the present study the influence of the lipid environment on the organization of the main light-harvesting complex of photosystem II (LHCII) was investigated by 77K fluorescence spectroscopy. Measurements were carried out with a lipid-depleted and highly aggregated LHCII which was supplemented with the different thylakoid membrane lipids. The results show that the thylakoid lipids are able to modulate the spectroscopic properties of the LHCII aggregates and that the extent of the lipid effect depends on both the lipid species and the lipid concentration. Addition of the neutral galactolipids monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) seems to induce a modification of the disorganized structures of the lipid-depleted LHCII and to support the aggregated state of the complex. In contrast, we found that the anionic lipids sulfoquinovosyldiacylglycerol (SQDG) and phosphatidylglycerol (PG) exert a strong disaggregating effect on the isolated LHCII. LHCII disaggregation was partly suppressed under a high proton concentration and in the presence of cations. The strongest suppression was visible at the lowest pH value (pH 5) and the highest Mg(2+) concentration (40 mM) used in the present study. This suggests that the negative charge of the anionic lipids in conjunction with negatively charged domains of the LHCII proteins is responsible for the disaggregation. Additional measurements by photon correlation spectroscopy and sucrose gradient centrifugation, which were used to gain information about the size and molecular mass of the LHCII aggregates, confirmed the results of the fluorescence spectroscopy. LHCII treated with MGDG and DGDG formed an increased number of aggregates with large particle sizes in the micromm-range, whereas the incubation with anionic lipids led to much smaller LHCII particles (around 40 nm in the case of PG) with a homogeneous distribution.

Structural and functional consequences of galactolipids on thylakoid membrane organization

Photosynthetic membranes of higher plant chloroplasts are composed primarily of polar, but uncharged, galactolipids unlike most mammalian membranes which contain large amounts of phosphatidylcholine. It is unclear what role(s) the galactolipids play in maintaining the differentiated thylakoid membranes, or in stabilizing the photosynthetically active enzyme complexes. Some of the membrane complexes show no lipid selectivity for maintaining structural or functional integrity. Others are poisoned or dissociated in the presence of high concentrations of a trace lipid class. The efficiency of energy transfer and the reconstitution of protein complexes into liposomes are dependent on the lipid class employed. The lipids are asymmetrically arranged along and across the thylakoid membranes but not as distinctly as the proteins.

Micellar enzymology

Experimental approaches to modelling the enzymatic function of biological membranes are discussed. Emphasis is given to pseudohomogeneous systems such as proteolipid complexes and enzymes in organic solvents; the latter are solubilized with phospholipids or synthetic surfactants. Methods for producing and studying such micellar systems are considered. The key research problems of micellar enzymology are formulated and its relation to enzyme membranology is discussed. Finally, the new potentialities are noted of applied enzymology (biotechnology) offered by application of a colloidal solution of water in organic solvents as a microheterogeneous medium for enzymatic reactions.

Reconstitution of energy transfer and electron transfer between solubilised pigment-protein complexes from thylakoid membranes. The role of acyl lipids

Solubilisation of thylakoid membranes from young leaves of Pisum sativum in the presence of Triton X-100 resulted in an almost complete loss of quenching of light-harvesting chlorophyll-protein (LHCP) fluorescence, as measured at 77°K. There were concomitant changes in the kinetics of light-saturation curves of electron transport from 2,6-dichlorophenolindophenol/ascorbate to methyl viologen. These effects were accompenied by a physical dissociation of LHCP polypeptides from photosystem I (PSI) and photosystem II (PSII) polypeptides, as determined by polyacrylamide gel-electrophoresis. Detergent-dialysis in the presence of exogenous purified galactolipids, about 80% of which were linoleoyl molecular species, only partially reversed these effects. However, detergent-dialysis using the phospholipids, phosphatidylglycerol and phosphatidylcholine, resulted in the substantial restoration of 77°K fluorescence quenching and the restoration of both emission spectra and electron transport kinetics of both Photosystems I and II that were typical of native membranes.

Lipid specificity for the reconstitution of well-coupled ATPase proteoliposomes and a new method for lipid isolation from photosynthetic membranes

The lipid specificity for the enzymatic and proton-translocating functions of a reconstituted thermophilic ATPase complex has been investigated. The proteoliposomes were prepared from the ATPase complex of the thermophilic cyanobacterium Synechococcus 6716 and various lipids and lipid mixtures extracted from this organism and from a related mesophilic strain. Some commercial lipids were used as well. An improved method of lipid extraction from chlorophyll-containing membranes is presented. This method is based on acetone extraction and additional chlorophyll separation and results in higher yields, less chlorophyll contamination and a simpler procedure than the conventional methods based on chloroform/methanol extraction. The lipids of Synechococcus 6716 thus extracted were fractionated by thin-layer chromatography. The fatty acyl chain composition of the separated lipids was analyzed by gas chromatography. The coupling quality of the reconstituted ATPase proteoliposomes made of different lipids was tested by a membrane-bound fluorescent probe and uncoupler stimulation of ATP hydrolysis. None of the separated lipids alone was able to produce a well-coupled system. The best results were obtained with the native lipid mixture. The minimum requirement was the combination of a typical bilayer-forming lipid and the non-bilayer (hexagonal II structure)-forming monogalactosyldiacylglycerol. Lipids from the mesophilic Synechococcus 6301 and commercial lipids (also mesophilic) produced poorly coupled vesicles but significant improvement was obtained when thermophilic monogalactosyldiacylglycerol was included. Both the reconstituted and solubilized ATPase complex have a sharp temperature optimum at 50 degrees C. The effect of reconstitution and measurement temperatures on the yield of well-coupled vesicles from different lipid sources was also studied.