Light-harvesting chlorophyll a/b-protein: Three-dimensional structure of a reconstituted membrane lattice in negative stain

Characterization of fluorescent chlorophyll charge-transfer states as intermediates in the excited state quenching of light-harvesting complex II

Light-harvesting complex II (LHCII) is the major antenna complex in higher plants and green algae. It has been suggested that a major part of the excited state energy dissipation in the so-called "non-photochemical quenching" (NPQ) is located in this antenna complex. We have performed an ultrafast kinetics study of the low-energy fluorescent states related to quenching in LHCII in both aggregated and the crystalline form. In both sample types the chlorophyll (Chl) excited states of LHCII are strongly quenched in a similar fashion. Quenching is accompanied by the appearance of new far-red (FR) fluorescence bands from energetically low-lying Chl excited states. The kinetics of quenching, its temperature dependence down to 4 K, and the properties of the FR-emitting states are very similar both in LHCII aggregates and in the crystal. No such FR-emitting states are found in unquenched trimeric LHCII. We conclude that these states represent weakly emitting Chl-Chl charge-transfer (CT) states, whose formation is part of the quenching process. Quantum chemical calculations of the lowest energy exciton and CT states, explicitly including the coupling to the specific protein environment, provide detailed insight into the chemical nature of the CT states and the mechanism of CT quenching. The experimental data combined with the results of the calculations strongly suggest that the quenching mechanism consists of a sequence of two proton-coupled electron transfer steps involving the three quenching center Chls 610/611/612. The FR-emitting CT states are reaction intermediates in this sequence. The polarity-controlled internal reprotonation of the E175/K179 aa pair is suggested as the switch controlling quenching. A unified model is proposed that is able to explain all known conditions of quenching or non-quenching of LHCII, depending on the environment without invoking any major conformational changes of the protein.

Two-dimensional crystallization and preliminary structure analysis of LHC-II from cucumber and spinach

Large and well-ordered two-dimensional (2D) crystals of the light-harvesting chlorophyll a/b protein complexes (LHC-II) from cucumber and spinach chloroplasts were produced by the so-called batch method. The twodimensional structures of these crystals were examined at about 1.5 nm resolution by electron microscopy and image processing. The projection maps showed that there were similar, but not identical, structure features between two different LHC-II complexes. A cmparison between 2D crystal formations of the two different LHC-II complexes was done and some factors affecting 2D crystallization of the membrane proteins were analyzed. The relations of the structures of the LHC-II complexes to their polypeptide components and Chl a/b ratio were also discussed.

Determination of the aggregate size in detergent solution of the light-harvesting chlorophyll a/b-protein complex from chloroplast membranes

The molecular mass of an oligomeric integral membrane protein, the light-harvesting chlorophyll a/b-protein complex from the photosynthetic membranes of chloroplasts, has been determined in detergent solution by analytical ultracentrifugation and measurement of the density increment at constant chemical potential of all diffusible solutes. The technique used eliminates any problems resulting from detergent binding to the protein, is independent of the particular detergent used (in this case the nonionic n-octyl beta-D-glucopyranoside), and gives the apparent weight-average molecular mass at different protein concentrations, allowing extrapolation to zero concentration. It means that the solutions of the complex must be brought to dialysis equilibrium with the solvent detergent solution and also requires a reliable method for measuring the protein concentration, for which amino acid analysis was used. The detergent-solubilized complex was a trimer that dissociated into monomers and dimers at low protein concentration. The accurate concentration determinations also allowed the molar chlorophyll-to-protein ratio to be measured as 15, corresponding to 8 chlorophyll a and 7 chlorophyll b molecules.

Two-dimensional crystallogenesis of transmembrane proteins

Two-dimensional crystallogenesis is a crucial step in the long road that leads to the determination of macromolecules structure via electron crystallography. The necessity of having large and highly ordered samples can hold back the resolution of structural works for a long time, and this, despite improvements made in electron microscopes or image processing. Today, finding good conditions for growing two-dimensional crystals still rely on either "biocrystallo-cooks" or on lucky ones. The present review presents the field by first describing the different crystals that one can encounter and the different crystallisation methods used. Then, the effects of different components (such as protein, lipids, detergent, buffer, and temperature) and the different methods (dialysis, hydrophobic adsorption) are discussed. This discussion is punctuated by correspondences made to the world of three-dimensional crystallogenesis. Finally, a guide for setting up 2D crystallogenesis experiments, built on the discussion mentioned before, is proposed to the reader. More than giving recipes, this review is meant to open up the discussions in this field.

A current assessment of photosystem II structure

This review covers the recent progress in the elucidation of the structure of photosystem II (PSII). Because much of the structural information for this membrane protein complex has been revealed by electron microscopy (EM), the review will also consider the specific technical and interpretation problems that arise with EM where they are of particular relevance to the structural data. Most recent reviews of photosystem II structure have concentrated on molecular studies of the PSII genes and on the likely roles of the subunits that they encode or they were mainly concerned with the biophysical data and fast absorption spectroscopy largely relating to electron transfer in various purified PSII preparations. In this review, we will focus on the approaches to the three-dimensional architecture of the complex and the lipid bilayer in which it is located (the thylakoid membrane) with special emphasis placed upon electron microscopical studies of PSII-containing thylakoid membranes. There are a few reports of 3D crystals of PSII and of associated X-ray diffraction measurements and although little structural information has so far been obtained from such studies (because of the lack of 3D crystals of sufficient quality), the prospects for such studies are also assessed.

2D crystallization: from art to science

The techniques as well as the principles of the 2D crystallization of membrane and water-soluble proteins for electron crystallography are reviewed. First, the biophysics of the interactions between proteins, lipids and detergents is surveyed. Second, crystallization of membrane proteins in situ and by reconstitution methods is discussed, and the various factors involved are addressed. Third, we elaborate on the 2D crystallization of water-soluble proteins, both in solution and at interfaces, such as lipid monolayers, mica, carbon film or mercury surfaces. Finally, techniques and instrumentations that are required for 2D crystallization are described.

Two-dimensional crystallization of membrane proteins

In spite of several great breakthroughs, the overall rate of progress in determining high-resolution structures of membrane proteins has been slow. This is entirely due to the scarcity of suitable, well-ordered crystals. Most membrane proteins are multimeric complexes with a composite molecular mass in excess of 50000 Da which puts them outside the range of current solution NMR techniques. For the foreseeable future, detailed information about the structure of large membrane proteins will therefore depend on crystallographic methods.

Characterization by electron microscopy of isolated particles and two-dimensional crystals of the CP47-D1-D2-cytochrome b-559 complex of photosystem II

A photosystem II complex containing the reaction center proteins D1 and D2, a 47-kDa chlorophyll-binding protein (CP47), and cytochrome b-559 was isolated with high yield, purity, and homogeneity; small but well-ordered two-dimensional crystals were prepared from the particles. The crystals and the isolated particles were analyzed by electron microscopy using negatively stained specimens. The information of 20 different digitized crystals was combined by alignment programs based on correlation methods to obtain a final average. The calculated diffraction pattern, with spots up to a resolution of 2.5 nm, and the optical diffraction pattern of a single crystal indicate that the plane group is p22121 (also called p2gg) and that the unit cell is rectangular with parameters of 23.5 x 16.0 nm, containing four stain-excluding monomers (two face-up and two face-down). In projection, the monomers have an asymmetrical shape with a length of 10 nm, a maximal width of 7.5 nm, and a height of 6 nm; their molecular mass is 175 +/- 40 kDa.

Expression of the Major Light-Harvesting Chlorophyll a/b-Protein and Its Import into Thylakoids of Mesophyll and Bundle Sheath Chloroplasts of Maize

Distribution of the major light-harvesting chlorophyll a/b-protein (LHCII) and its mRNA within bundle sheath and mesophyll cells of maize (Zea mays L.) was studied using in situ immunolocalization and hybridization, respectively. In situ hybridization with specific LHCII RNA probes from maize and Lemna gibba definitively shows the presence of high levels of mRNA for LHCII in both bundle sheath cells and mesophyll cells. In situ immuno-localization studies, using an LHCII monoclonal antibody, demonstrate the presence of LHCII polypeptides in chloroplasts of both cell types. The polypeptide composition of LHCII and the amount of LHCII in bundle sheath cells are different from those in mesophyll cells. Both mesophyll and bundle sheath chloroplasts can take up, import and process the in vitro transcribed and translated LHCII precursor protein from L. gibba. Although bundle sheath chloroplasts incorporate LHCII into the pigmented light-harvesting complex, the efficiency is lower than that in mesophyll chloroplasts.

A hydrophobic, carboxy-proximal region of a light-harvesting chlorophyll a/b protein is necessary for stable integration into thylakoid membranes

Proteins synthesized as soluble precursors in the cytoplasm of eukaryotic cells often cross organellar membrane barriers and then insert into lipid bilayers. One such polypeptide, the light-harvesting chlorophyll a/b-binding protein (LHCP), must also associate with pigment molecules and be assembled into the photosystem II light-harvesting complex in the chloroplast thylakoid membrane. A study of the import of mutant LHCPs into isolated chloroplasts has shown that a putative alpha-helical membrane-spanning domain near the carboxy terminus (helix 3) is essential for the stable insertion of LHCP in the thylakoid. Protease digestion experiments are consistent with the carboxy terminus of the protein being in the lumen. This report also shows that helix 3, when fused to a soluble protein, can target it to the thylakoids of isolated, intact chloroplasts. Although helix 3 is required for the insertion of LHCP and mutant derivatives into the thylakoid, the full insertion of helix 3 itself requires additionally the presence of other regions of LHCP. Thus, LHCP targeting and integration into thylakoid membranes requires a complex interaction involving a number of different domains of the LHCP polypeptide.

Structural determination of the photosystem II core complex from spinach

A photosystem II core complex was purified with high yield from spinach by solubilization with beta-dodecylmaltoside. The complex consisted of polypeptides with molecular mass 47, 43, 34, 31, 9 and 4 kDa and some minor components, as detected by silver-staining of polyacrylamide gels. There was no indication for the chlorophyll-a/b-binding, light-harvesting complex polypeptides. The core complex revealed electron-transfer activity (1,5-diphenylcarbazide----2,6-dichloroindophenol) of about 30 mumol reduced 2,6-dichloroindophenol/mg chlorophyll/h. The structural integrity was analyzed by electron microscopy. The detergent-solubilized protein complex has the shape of a triangular disk with a maximum diameter of 13 nm and a maximum height of 6.8 nm. The shape of this core complex differs considerably from that of cyanobacterial photosystem II membrane fragments, which are elongated particles. The structural differences between both the complexes of higher plants and cyanobacteria are discussed with special emphasis on their association with the antenna apparatus in the photosynthetic membranes.

Regulation and expression of the multigene family coding light-harvesting chlorophyll a/b-binding proteins of photosystem II

The current state of knowledge concerning the expression of the nuclear genes that code the light-harvesting chlorophyll a/b-binding polypeptides of photosystem II is presented. This review covers the structure of these genes, the complex multistep pathway involved in their expression, and the environmental and other factors which regulate their expression. Some of the effects of these factors are mediated, at least in part, at the level of transcription, but other effects can be explained only by the existence of multiple posttranscriptional regulatory steps.

Two-dimensional structure of the light-harvesting chlorophyll a/b complex by cryoelectron microscopy

The light-harvesting chlorophyll a/b complex (LHC-II) found in green plants has at least three functions: it absorbs light energy for transfer to the reaction centers, it is involved in keeping the photosynthetic membranes stacked, and it regulates energy distribution between the two photosystems. We have developed a procedure to produce large vesicles consisting almost exclusively of two-dimensional crystalline domains of LHC-II in which LHC-II is biochemically and structurally intact, as shown by SDS-PAGE, response to cations, and 77K fluorescence excitation spectra. The vesicles were examined by cryoelectron microscopy and analyzed, in projection, to a resolution of 17 A. Their surface lattice consists of trimers arranged in interlocking circles; the two-sided plane group is p321 (unit cell dimension, a = 124 A) with two, oppositely facing trimers/unit cell. Individual trimers consist of matter arranged in a ring, around a central cavity, an appearance similar to that obtained in some conditions using negative stain (Li, J., 1985. Proc. Natl. Acad. Sci. USA. 82:386-390). The monomer (approximately 45 x 20 A) is seen as two domains of slightly different size at this resolution. The thickness of single layers is approximately 48 A, measured from edge-on views of the frozen vesicles. Based on these dimensions, the molecular mass of the monomer is approximately 30 kD. Therefore, each monomer appears to be composed of a single polypeptide and its associated pigments.

The major light-harvesting complex of Photosystem II: aspects of its molecular and cell biology

The light-harvesting complex of photosystem II (LHC II) contains one major (LHC IIb) and at least three minor chlorophyll-protein components. The apoproteins of LHC IIb (LHCP) are encoded by nuclear genes and synthesized in the cytoplasm as a higher molecular weight precursor(s) (pLHCP). Several genes coding for pLHCP have been cloned from various higher plant species. The expression of these genes is dependent upon a variety of factors such as light, the developmental stage of the plastids and the plant. After its synthesis in the cytoplasm, pLHCP is imported into plastids, inserted into thylakoids, processed to its mature form, and assembled into LHC IIb. The pathway of assembly of LHC IIb in the thylakoid membranes is currently being investigated in several laboratories. We present a model that gives some details of the steps in the assembly process. Many of the steps involved in the synthesis and assembly are dependent on light and the stage of plastid development.

The structure of NADH:ubiquinone oxidoreductase from beef-heart mitochondria. Crystals containing an octameric arrangement of iron-sulphur protein fragments

We have investigated the structure of two-dimensional crystals from preparations of NADH:ubiquinone oxidoreductase from beef-heart mitochondria. The crystal structure of these crystals was previously determined to be equivalent with two native enzyme molecules per unit cell, i.e. a p2 symmetry [Boekema, E. J., Van Heel, M. G. & Van Bruggen, E. F. J. (1984) Biochim. Biophys. Acta 787, 19-26]. However, the optical diffraction patterns of the crystals displayed a clear fourfold symmetry. A Fourier analysis carried out on the calculated diffraction pattern proved unambiguously that the crystal symmetry was p42(1)2. Following crystallographic rules the unit cell therefore contained eight identical molecules. As a consequence, only a subcomplex of the enzyme rather than the intact enzyme formed the crystal. Electron microscopy of isolated, single molecules of the iron-sulphur protein, a dissociation product of complex I, revealed the presence of square complexes with sides of approximately 15 nm. Since these complexes were indistinguishable from the building blocks (unit cells) of the two-dimensional crystals, the crystals could be composed of Fe-S protein fragments only. The nature of the fragments in the unit cell was probed by immuno-labelling with monovalent antibodies (Fab's), raised against the 75-kDa subunit from the Fe-S protein, followed by image analysis. We found at least four binding sites for the anti-(75-kDa subunit) Fab per unit cell, indicating the presence of at least four copies of the antigen. In order to account for these observations we postulate the hypothesis that the two-dimensional crystals obtained from complex I are composed of iron-sulphur protein molecules in an octameric arrangement.

Monoclonal antibodies to the light-harvesting chlorophyll a/b protein complex of photosystem II

A collection of 17 monoclonal antibodies elicited against the light-harvesting chlorophyll a/b protein complex which serves photosystem II (LHC-II) of Pisum sativum shows six classes of binding specificity. Antibodies of two of the classes recognize a single polypeptide (the 28- or the 26- kD polypeptides), thereby suggesting that the two proteins are not derived from a common precursor. Other classes of antibodies cross-react with several polypeptides of LHC-II or with polypeptides of both LHC-II and the light-harvesting chlorophyll a/b polypeptides of photosystem I (LHC-I), indicating that there are structural similarities among the polypeptides of LHC-II and LHC-I. The evidence for protein processing by which the 26-, 25.5-, and 24.5-kD polypeptides are derived from a common precursor polypeptide is discussed. Binding studies using antibodies specific for individual LHC-II polypeptides were used to quantify the number of antigenic polypeptides in the thylakoid membrane. 27 copies of the 26-kD polypeptide and two copies of the 28-kD polypeptide were found per 400 chlorophylls. In the chlorina f2 mutant of barley, and in intermittent light-treated barley seedlings, the amount of the 26-kD polypeptide in the thylakoid membranes was greatly reduced, while the amount of 28-kD polypeptide was apparently not affected. We propose that stable insertion and assembly of the 28-kD polypeptide, unlike the 26-kD polypeptide, is not regulated by the presence of chlorophyll b.

Functional and mutational analysis of the light-harvesting chlorophyll a/b protein of thylakoid membranes

The precursor for a Lemna light-harvesting chlorophyll a/b protein (pLHCP) has been synthesized in vitro from a single member of the nuclear LHCP multigene family. We report the sequence of this gene. When incubated with Lemna chloroplasts, the pLHCP is imported and processed into several polypeptides, and the mature form is assembled into the light-harvesting complex of photosystem II (LHC II). The accumulation of the processed LHCP is enhanced by the addition to the chloroplasts of a precursor and a co-factor for chlorophyll biosynthesis. Using a model for the arrangement of the mature polypeptide in the thylakoid membrane as a guide, we have created mutations that lie within the mature coding region. We have studied the processing, the integration into thylakoid membranes, and the assembly into light-harvesting complexes of six of these deletions. Four different mutant LHCPs are found as processed proteins in the thylakoid membrane, but only one appears to have an orientation in the membrane that is similar to that of the wild type. No mutant LHCP appears in LHC II. The other two mutant LHCPs cannot be detected within the chloroplasts. We conclude that stable complex formation is not required for the processing and insertion of altered LHCPs into the thylakoid membrane. We discuss the results in light of our model.

The light-harvesting chlorophyll a/b protein acts as a torque aligning chloroplasts in a magnetic field

Displacement of particles from the purified light-harvesting chlorophyll a/b protein aggregate (LHC) was studied in magnetic fields of various strengths (0 to 1.6 T) by polarized fluorescence measurements. Macromolecular aggregates of LHC have a considerable magnetic susceptibility which enables the particles to rotate and align with their nematic axes parallel with H. As LHC is embedded in a transmembrane direction thylakoids should align perpendicular to H, the mode of alignment experimentally observed in thylakoids. The value of the magnetic susceptibility could be estimated by relating it to the integral susceptibility of the chlorophyll molecules in LHC. The fitting of this value with the field strength dependency of the fluorescence polarization ratio (FP) revealed a relationship between the LHC content of various photosynthetic membranes and their capacity for alignment, which suggested that LHC might be the torque ordering chloroplasts in a magnetic field.