A chloroplast membrane lacking photosystem I. Changes in unstacked membrane regions

Studying the Supramolecular Organization of Photosynthetic Membranes within Freeze-fractured Leaf Tissues by Cryo-scanning Electron Microscopy

Cryo-scanning electron microscopy (SEM) of freeze-fractured samples allows investigation of biological structures at near native conditions. Here, we describe a technique for studying the supramolecular organization of photosynthetic (thylakoid) membranes within leaf samples. This is achieved by high-pressure freezing of leaf tissues, freeze-fracturing, double-layer coating and finally cryo-SEM imaging. Use of the double-layer coating method allows acquiring high magnification (>100,000X) images with minimal beam damage to the frozen-hydrated samples as well as minimal charging effects. Using the described procedures we investigated the alterations in supramolecular distribution of photosystem and light-harvesting antenna protein complexes that take place during dehydration of the resurrection plant Craterostigma pumilum, in situ.

Composition, architecture and dynamics of the photosynthetic apparatus in higher plants

The process of oxygenic photosynthesis enabled and still sustains aerobic life on Earth. The most elaborate form of the apparatus that carries out the primary steps of this vital process is the one present in higher plants. Here, we review the overall composition and supramolecular organization of this apparatus, as well as the complex architecture of the lamellar system within which it is harbored. Along the way, we refer to the genetic, biochemical, spectroscopic and, in particular, microscopic studies that have been employed to elucidate the structure and working of this remarkable molecular energy conversion device. As an example of the highly dynamic nature of the apparatus, we discuss the molecular and structural events that enable it to maintain high photosynthetic yields under fluctuating light conditions. We conclude the review with a summary of the hypotheses made over the years about the driving forces that underlie the partition of the lamellar system of higher plants and certain green algae into appressed and non-appressed membrane domains and the segregation of the photosynthetic protein complexes within these domains.

Plastoquinol diffusion in linear photosynthetic electron transport

The diffusion of plastoquinol and its binding to the cytochrome bf complex, which occurs during linear photosynthetic electron transport and is analogous to reaction sequences found in most energy-converting membranes, has been studied in intact thylakoid membranes. The flash-induced electron transfer between the laterally separated photosystems II and photosystems I was measured by following the sigmoidal reduction kinetics of P-700(+) after previous oxidation of the intersystem electron carriers. The amount of flash-induced plastoquinol produced at photosystem II was (a) reduced by inhibition with dichlorophenyl-dimethylurea and (b) increased by giving a second saturating flash. These signals were simulated by a new model which combines a deterministic simulation of reaction kinetics with a Monte Carlo approach to the diffusion of plastoquinol, taking into account the known structural features of the thylakoid membrane. The plastoquinol molecules were assumed to be oxidized by either a diffusion-limited or a nondiffusion-limited step in a collisional mechanism or after binding to the cytochrome bf complex. The model was able to account for the experimental observations with a nondiffusion-limited collisional mechanism or with a binding mechanism, giving minimum values for the diffusion coefficient of plastoquinol of 2 x 10(-8) cm(2)s(-1) and 3 x 10(-7) cm(2)s(-1), respectively.

Granal stacking of thylakoid membranes in higher plant chloroplasts: the physicochemical forces at work and the functional consequences that ensue

The formation of grana in chloroplasts of higher plants is examined in terms of the subtle interplay of physicochemical forces of attraction and repulsion. The attractive forces between two adjacent membranes comprise (1) van der Waals attraction that depends on the abundance and type of atoms in each membrane, on the distance between the membranes and on the dielectric constant, (2) depletion attraction that generates local order by granal stacking at the expense of greater disorder (i.e. entropy) in the stroma, and (3) an electrostatic attraction of opposite charges located on adjacent membranes. The repulsive forces comprise (1) electrostatic repulsion due to the net negative charge on the outer surface of thylakoid membranes, (2) hydration repulsion that operates at small separations between thylakoid membranes due to layers of bound water molecules, and (3) steric hindrance due to bulky protrusions of Photosystem I (PSI) and ATP synthase into the stroma. In addition, specific interactions may occur, but they await experimental demonstration. Although grana are not essential for photosynthesis, they are ubiquitous in higher plants. Grana may have been selected during evolution for the functional advantages that they confer on higher plants. The functional consequences of grana stacking include (1) enhancement of light capture through a vastly increased area-to-volume ratio and connectivity of several PSIIs with large functional antenna size, (2) the ability to control the lateral separation of PSI from PSII and, therefore, the balanced distribution of excitation energy between two photosystems working in series, (3) the reversible fine-tuning of energy distribution between the photosystems by State 1-State 2 transitions, (4) the ability to regulate light-harvesting via controlled thermal dissipation of excess excitation energy, detected as non-photochemical quenching, (5) dynamic flexibility in the light reactions mediated by a granal structure in response to regulation by a trans-thylakoid pH gradient, (6) delaying the premature degradation of D1 and D2 reaction-centre protein(s) in PSII by harbouring photoinactived PSIIs in appressed granal domains, (7) enhancement of the rate of non-cyclic synthesis of adenosine triphosphate (ATP) as well as the regulation of non-cyclic vs. cyclic ATP synthesis, and (8) the potential increase of photosynthetic capacity for a given composition of chloroplast constituents in full sunlight, concomitantly with enhancement of photochemical efficiency in canopy shade. Hence chloroplast ultrastructure and function are intimately intertwined.

Entropy-assisted stacking of thylakoid membranes

Chloroplasts in plants and some green algae contain a continuous thylakoid membrane system that is structurally differentiated into stacked granal membranes interconnected by unstacked thylakoids, the stromal lamellae. Experiments were conducted to test the hypothesis that the thermodynamic tendency to increase entropy in chloroplasts contributes to thylakoid stacking to form grana. We show that the addition of bovine serum albumin or dextran, two very different water-soluble macromolecules, to a suspension of envelope-free chloroplasts with initially unstacked thylakoids induced thylakoid stacking. This novel restacking of thylakoids occurred spontaneously, accompanied by lateral segregation of PSII from PSI, thereby mimicking the natural situation. We suggest that such granal formation, induced by the macromolecules, is partly explained as a means of generating more volume for the diffusion of macromolecules in a crowded stromal environment, i.e., greater entropy overall. This mechanism may be relevant in vivo where the stroma has a very high concentration of enzymes of carbon metabolism, and where high metabolic fluxes are required.

Structural characteristics of thylakoid membranes of Arabidopsis mutants deficient in lipid fatty acid desaturation

The ultrastructure of thylakoid membranes from Arabidopsis thaliana wild-type, JB67 and LK3 fatty acid desaturation deficient mutants was studied by thin-section and freeze-fracture electron microscopy. There was a decrease in the amount of the appressed and non-appressed membranes in JB67 and LK3 Arbidopsis mutants when compared to the wild type, resulting in a reduction in the length of photosynthetic membrane per plastid. The results from freeze-fracture showed a decrease in size and a marked increase in packing density of membrane-associated particles on the exo- and endoplasmic fracture faces of the mutants. In addition, areas of the appressed membranes of the mutants contained particles in regular arrays under conditions where no such arrays were observed in wild-type thylakoid membranes. These observations suggest, that the decreased level of lipid fatty acid unsaturation affects the ability of the lipid matrix to mediate the assembly of chloroplast membrane components. The role of polyunsaturated membrane lipids is considered in terms of their ability to promote functional oligomeric assemblies of components of the photosynthetic apparatus.

Isolation of protein(s) containing chloride ion transport activity from thylakoid membranes

Extracts of detergent-treated thylakoids, when reconstituted into azolectin/cholesterol/dicetyl phosphate vesicles, stimulate chloride ion efflux as measured with a Cl--sensitive electrode. This stimulation is inhibited by piretanide, an active chloride ion transport inhibitor in fish intestinal epithelia. This stimulation is also abolished by pretreatment of extracted proteins with trypsin. Antiserum raised to efflux-active proteins inhibits a cation-driven Cl- influx in nonenergized thylakoids, as measured by a flow dialysis technique.

Correlation of structure and function of chloroplast membranes at the supramolecular level

Freeze-fracture electron microscopy has revealed that different size classes of intramembrane particles of chloroplast membranes are nonrandomly distributed between appressed grana and nonappressed stroma membrane regions. It is now generally assumed that thylakoid membranes contain five major functional complexes, each of which can give rise to an intramembrane particle of a defined size. These are the photosystem II complex, the photosystem I complex, the cytochrome f/b6 complex, the chlorophyll a/b light-harvesting complex, and the CF0 -CF1 ATP synthetase complex. By mapping the distribution of the different categories of intramembrane particles, information on the lateral organization of functional membrane units of thylakoid membranes can be determined. In this review, we present a brief summary of the evidence supporting the correlation of specific categories of intramembrane particles with known biochemical entities. In addition, we discuss studies showing that ions and phosphorylation of the membrane adhesion factor, the chlorophyll a/b light-harvesting complex, can affect the lateral organization of chloroplast membrane components and thereby regulate membrane function.

Regulation of chloroplast membrane function: protein phosphorylation changes the spatial organization of membrane components

A chlorophyll-protein complex of chloroplast membranes, which simultaneously serves as light-harvesting antenna and membrane adhesion factor, undergoes reversible, lateral diffusion between appressed and nonappressed membrane regions under the control of a protein kinase. The phosphorylation-dependent migration process regulates the amount of light energy that is delivered to the reaction centers of photosystems I and II (PS I and PS II), and thereby regulates their rate of turnover. This regulatory mechanism provides a rationale for the finding that the two photosystems are physically separated in chloroplast membranes (PS II in appressed, grana membranes, and PS I in nonappressed, stroma membranes). The feedback system involves the following steps: a membrane-bound kinase senses the rate of PS II vs. PS I turnover via the oxidation-reduction state of the plastoquinone pool, which shuttles electrons from PS II via cytochrome f to PS I. If activated, the kinase adds negative charge (phosphate) to a grana-localized pigment-protein complex. The change in its surface charge at a site critical for promoting membrane adhesion results in increased electrostatic repulsion between the membranes, unstacking, the lateral movement of the complex to adjacent stroma membranes, which differ in their functional composition. The general significance of this type of membrane regulatory mechanism is discussed.

Reconstitution of cytochrome f/b6 and CF0-CF1 ATP synthetase complexes into phospholipid and galactolipid liposomes

Cytochrome f/b6 and ATP synthetase (CF0-CF1) complexes from spinach chloroplasts have been reconstituted into liposomes prepared from soybean phospholipids and purified spinach galactolipids. Freeze-fracture analysis revealed homogeneous populations of particles spanning the lipid bilayers with their elongated axes perpendicular to the membrane plane. The lipid composition of the liposomes had no effect on the size of the reconstituted complexes, the average diameter of cytochrome f/b6 complex measuring 8.5 nm, and of the CF0 base piece of the ATP synthetase, 9.5 nm. When reconstituted cytochrome f/b6 complexes were cross-linked by means of antibodies prepared against the whole complex, the thus aggregated particles formed either hexagonal or square arrays. In both instances the center-to-center spacing of the particles was 8.3 nm, thereby suggesting that this value could be closer to the real diameter of the complexes than the one obtained from measuring individual particles. Assuming an ellipsoidal shape for these particles, and using a measured height of 11 nm, a molecular weight of approximately 280,000 could be calculated for the reconstituted cytochrome f/b6 complex, consistent with a dimeric configuration. In many instances the crystalline sheets of antibody-aggregated cytochrome f/b6 complexes were found to be free in the buffer solution; apparently the antibody-induced strains caused the sheet-like aggregates to pop out of the liposomal membranes. Agglutination studies of inside-out and right-side-out thylakoid vesicles revealed the antigenic determinants of the cytochrome f and cytochrome b6 polypeptides to be exposed on the inner thylakoid surface and to be present in stacked and unstacked membrane regions. The molecular weight calculated from the size of freeze-fractured CF0 base pieces was over twice the value determined by x-ray scattering data. This discrepancy may be caused by significant lipid domains within the base piece, or by an unusual fracturing behavior of the base piece in reconstituted liposomes.

Lateral mobility of the light-harvesting complex in chloroplast membranes controls excitation energy distribution in higher plants

Chloroplast thylakoid protein phosphorylation produces changes in light-harvesting properties and in membrane structure as revealed by freeze-fracture electron microscopy. Protein phosphorylation resulted in an increase in the 77 degrees K fluorescence signal at 735 nm relative to that at 685 nm. In addition, a decrease in connectivity between Photosystem II centers (PS II) and a dynamic quenching of the room temperature variable fluorescence was observed upon phosphorylation. Accompanying these fluorescence changes was a 23% decrease in the amount of stacked membranes. Microscopic analyses indicated that 8.0-nm particles fracturing on the P-face moved from the stacked into the unstacked regions upon phosphorylation. The movement of the 8.0-nm particles was accompanied by the appearance of chlorophyll b and 25 to 29 kD polypeptides in isolated stroma lamellae fractions. We conclude that phosphorylation of a population of the light-harvesting chlorophyll a/b protein complexes (LHC) in grana partitions causes the migration of these pigment proteins from the PS II-rich appressed membranes into the Photosystem I (PS I) enriched unstacked regions. This increases the absorptive cross section of PS I. In addition, we suggest that the mobile population of LHC functions to interconnect PS II centers in grana partitions; removal of this population of LHC upon phosphorylation limits PS II leads to PS II energy transfer and thereby favors spillover of energy from PS II to PS I.

The role of chlorophyll-protein complexes in the function and structure of chloroplast thylakoids

The photosynthetic pigments of chloroplast thylakoid membranes are complexed with specific intrinsic polypeptides which are included in three supramolecular complexes, photosystem I complex, photosystem II complex and the light-harvesting complex. There is marked lateral heterogeneity in the distribution of these complexes along the membrane with photosystem II complex and its associated light-harvesting complex being located mainly in the stacked membranes of the grana partitions, while photosystem I complex is found mainly in unstacked thylakoids together with ATP synthetase. In contrast, the intermediate electron transport complex, the cylochrome b-f complex, is rather uniformly distributed in these two membrane regions. The consequences of this lateral heterogeneity in the location of the thylakoid complexes are considered in relation to the function and structure of chloroplasts of higher plants.

Lateral heterogeneity in the distribution of chlorophyll-protein complexes of the thylakoid membranes of spinach chloroplasts

The lateral distribution of the main chlorophyll-protein complexes between appressed and non-appressed thylakoid membranes has been studied. The reaction centre complexes of Photosystems I and II and the light-harvesting complex have been resolved by an SDS-polyacrylamide gel electrophoretic method which permits most of the chlorophyll to remain protein-bound. The analyses were applied to subchloroplast fractions shown to be derived from different thylakoid regions. Stroma thylakoids were separated from grana stacks by centrifugation following chloroplast disruption by press treatment or digitonin. Vesicles derived from the grana partitions were isolated by aqueous polymer two-phase partition. A substantial depletion in the amount of Photosystem I chlorophyll-protein complex and an enrichment in the Photosystem II reaction centre complex and the light-harvesting complex occurrred in the appressed grana partition region. The high enrichment in this fraction compared to grana stack fractions derived from press or digitonin-treatments, suggests that the grana Photosystem I is restricted mainly to the non-appressed grana end membranes and margins, and that the grana partitions possess mainly Photosystem II reaction centre complex and the light-harvesting complex. In contrast, stroma thylakoids are highly enriched in the Photosystem I reaction centre complex. They possess also some 10--20% of the total Photosystem II reaction centre complex and the light-harvesting complex. The ratio of light-harvesting complex to Photosystem II reaction centre complex is rather constant in all subchloroplast fractions suggesting a close association between these complexes. This was not so for the ratio of light-harvesting complex and the Photosystem I reaction centre complex. The lateral heterogeneity in the distribution of the photosystems between appressed and non-appressed membranes must have a profound impact on current understanding of both the distribution of excitation energy and photosynthetic electron transport between the photosystems.