Flexibility and size heterogeneity of the LH1 light harvesting complex revealed by atomic force microscopy: functional significance for bacterial photosynthesis

The organization of LH2 complexes in membranes from Rhodobacter sphaeroides

The mapping of the photosynthetic membrane of Rhodobacter sphaeroides by atomic force microscopy (AFM) revealed a unique organization of arrays of dimeric reaction center-light harvesting I-PufX (RC-LH1-PufX) core complexes surrounded and interconnected by light-harvesting LH2 complexes (Bahatyrova, S., Frese, R. N., Siebert, C. A., Olsen, J. D., van der Werf, K. O., van Grondelle, R., Niederman, R. A., Bullough, P. A., Otto, C., and Hunter, C. N. (2004) Nature 430, 1058-1062). However, membrane regions consisting solely of LH2 complexes were under-represented in these images because these small, highly curved areas of membrane rendered them difficult to image even using gentle tapping mode AFM and impossible with contact mode AFM. We report AFM imaging of membranes prepared from a mutant of R. sphaeroides, DPF2G, that synthesizes only the LH2 complexes, which assembles spherical intracytoplasmic membrane vesicles of approximately 53 nm diameter in vivo. By opening these vesicles and adsorbing them onto mica to form small, < or =120 nm, largely flat sheets we have been able to visualize the organization of these LH2-only membranes for the first time. The transition from highly curved vesicle to the planar sheet is accompanied by a change in the packing of the LH2 complexes such that approximately half of the complexes are raised off the mica surface by approximately 1 nm relative to the rest. This vertical displacement produces a very regular corrugated appearance of the planar membrane sheets. Analysis of the topographs was used to measure the distances and angles between the complexes. These data are used to model the organization of LH2 complexes in the original, curved membrane. The implications of this architecture for the light harvesting function and diffusion of quinones in native membranes of R. sphaeroides are discussed.

The peripheral light-harvesting complexes from purple sulfur bacteria have different 'ring' sizes

The integral membrane light-harvesting (LH) proteins from purple photosynthetic bacteria form circular oligomers of an elementary unit that is composed of two very hydrophobic polypeptides, termed alpha and beta. These apoprotein dimers are known to associate into closed circular arrays of 8, 9 and 16 alpha/beta-mers. We report the existence of peripheral LH proteins purified from Allochromatium vinosum with two intermediate ring sizes and postulate that one is a 13 alpha/beta-mer. This shows that LH proteins are able to form membrane rings of continuously increasing diameter from 68 to 115A. The presence of these new ring sizes warrants further study, as it will help to further validate the structure-function models of LH proteins currently found in the literature.

Intrinsic curvature properties of photosynthetic proteins in chromatophores

In purple bacteria, photosynthesis is carried out on large indentations of the bacterial plasma membrane termed chromatophores. Acting as primitive organelles, chromatophores are densely packed with the membrane proteins necessary for photosynthesis, including light harvesting complexes LH1 and LH2, reaction center (RC), and cytochrome bc(1). The shape of chromatophores is primarily dependent on species, and is typically spherical or flat. How these shapes arise from the protein-protein and protein-membrane interactions is still unknown. Now, using molecular dynamics simulations, we have observed the dynamic curvature of membranes caused by proteins in the chromatophore. A membrane-embedded array of LH2s was found to relax to a curved state, both for LH2 from Rps. acidophila and a homology-modeled LH2 from Rb. sphaeroides. A modeled LH1-RC-PufX dimer was found to develop a bend at the dimerizing interface resulting in a curved shape as well. In contrast, the bc(1) complex, which has not been imaged yet in native chromatophores, did not induce a preferred membrane curvature in simulation. Based on these results, a model for how the different photosynthetic proteins influence chromatophore shape is presented.

Atomic force microscopy imaging of Xenopus laevis oocyte plasma membrane purified by ultracentrifugation

Atomic force microscopy (AFM) was used to investigate the native plasma membrane of Xenopus laevis (X. laevis) oocyte purified by means of ultracentrifugation on sucrose gradient and subsequently adsorbed on mica leaves through a physisorption process. Reproducible AFM topography images were collected, analyzed, and compared. AFM images showed the presence of large single or double bilayer membrane sheets covered with protein complexes. The lateral dimension and height of protein complexes imaged in air showed a normal distribution centred on 15.4 +/- 0.4 nm (mean +/- SE; n = 59) and 3.9 +/- 0.2 nm (mean +/- SE; n = 57), respectively. A density of about 270 protein complexes per square micron was calculated. Less frequently, ordered nanometer domains with densely packed protein complexes arranged in hexagonal patterns were also visualized in AFM images, confirming previously published data. Their lateral dimension and height showed a normal distribution centred on 23.0 +/- 0.4 nm (mean +/- SE; n = 42) and 1.5 +/- 0.6 nm (mean +/- SE; n = 90), respectively. A density of about 870 protein complexes per square micrometer was calculated. Advantages and drawbacks of this new sample preparation for AFM imaging are discussed.

Three-dimensional reconstruction of a membrane-bending complex: the RC-LH1-PufX core dimer of Rhodobacter sphaeroides

A three-dimensional model of the dimeric reaction center-light harvesting I-PufX (RC-LH1-PufX) complex from Rhodobacter sphaeroides, calculated from electron microscope single particle analysis of negatively stained complexes, shows that the two halves of the dimer molecule incline toward each other on the periplasmic side, creating a remarkable V-shaped structure. The distribution of negative stain is consistent with loose packing of the LH1 ring near the 14th LH1 alpha/beta pair, which could facilitate the migration of quinone and quinol molecules across the LH1 boundary. The three-dimensional model encloses a space near the reaction center Q(B) site and the 14th LH1 alpha/beta pair, which is approximately 20 angstroms in diameter, sufficient to sequester a quinone pool. Helical arrays of dimers were used to construct a three-dimensional membrane model, which matches the packing lattice deduced from electron microscope analysis of the tubular dimer-only membranes found in mutants of Rba. sphaeroides lacking the LH2 complex. The intrinsic curvature of the dimer explains the shape and approximately 70-nm diameter of these membrane tubules, and at least partially accounts for the spherical membrane invaginations found in wild-type Rba. sphaeroides. A model of dimer aggregation and membrane curvature in these spherical membrane invaginations is presented.

Electrostatic effect of surfactant molecules on bacteriochlorophyll a and carotenoid binding sites in the LH1 complex isolated from Rhodospirillum rubrum S1 probed by Stark spectroscopy

The LH1 complexes were isolated from the purple photosynthetic bacterium Rhodospirillum rubrum strain S1. They were initially solubilized using LDAO and then purified in the presence of Triton X-100. The purified complexes were then either used directly or following an exchange into LDAO. Stark spectroscopy was applied to probe the electrostatic field around the bacteriochlorophyll a (BChl a) and carotenoid binding sites in the LH1 complexes surrounded by these two different surfactant molecules. Polarizabilty change (deltaalpha)) and dipole moment change (deltamicrom) upon photoexcitation were determined for the BChl a Q(y) band. Both of these parameters show smaller values in the presence of LDAO than in Triton X-100. This indicates that polar detergent molecules, like LDAO, affect the electrostatic environment around BChl a, and modify the nonlinear optical parameters (deltaalpha and deltamicrom values). The electrostatic field around the BChl a binding site, which is generated by the presence of LDAO, was determined to be |E ( L )| = approximately 3.9 x 10(5) [V/cm]. Interestingly, this kind of electrostatic effect was not observed for the carotenoid-binding site. The present study demonstrates a unique electrostatic interaction between the polar detergent molecules surrounding the LH1 complex and the Q(y) absorption band of BChl a that is bound to the LH1 complex.

Construction of hybrid photosynthetic units using peripheral and core antennae from two different species of photosynthetic bacteria: detection of the energy transfer from bacteriochlorophyll a in LH2 to bacteriochlorophyll b in LH1

Typical purple bacterial photosynthetic units consist of supra-molecular arrays of peripheral (LH2) and core (LH1-RC) antenna complexes. Recent atomic force microscopy pictures of photosynthetic units in intact membranes have revealed that the architecture of these units is variable (Scheuring et al. (2005) Biochim Bhiophys Acta 1712:109-127). In this study, we describe methods for the construction of heterologous photosynthetic units in lipid-bilayers from mixtures of purified LH2 (from Rhodopseudomonas acidophila) and LH1-RC (from Rhodopseudomonas viridis) core complexes. The architecture of these reconstituted photosynthetic units can be varied by controlling ratio of added LH2 to core complexes. The arrangement of the complexes was visualized by electron-microscopy in combination with Fourier analysis. The regular trigonal array of the core complexes seen in the native photosynthetic membrane could be regenerated in the reconstituted membranes by temperature cycling. In the presence of added LH2 complexes, this trigonal symmetry was replaced with orthorhombic symmetry. The small lattice lengths for the latter suggest that the constituent unit of the orthorhombic lattice is the LH2. Fluorescence and fluorescence-excitation spectroscopy was applied to the set of the reconstituted membranes prepared with various proportions of LH2 to core complexes. Remarkably, even though the LH2 complexes contain bacteriochlorophyll a, and the core complexes contain bacteriochlorophyll b, it was possible to demonstrate energy transfer from LH2 to the core complexes. These experiments provide a first step along the path toward investigating how changing the architecture of purple bacterial photosynthetic units affects the overall efficiency of light-harvesting.

Directed assembly of functional light harvesting antenna complexes onto chemically patterned surfaces

We report the directed assembly of the photosynthetic membrane proteins LH1 and LH2 isolated from the purple bacterium Rhodobacter sphaeroides onto chemically patterned substrates. Nanoimprint lithography was used to pattern discrete regions of amino- and fluoro-terminated or poly(ethylene glycol) self-assembled monolayers onto a glass substrate. Densely packed layers of assembled protein complexes were observed with atomic force microscopy. The protein complexes attached selectively to the amino-terminated regions by electrostatic interactions. Spectral images generated with a hybrid scanning probe and fluorescence microscope confirmed that the patterned proteins retained their native optical signatures.

Spectral dynamics of individual bacterial light-harvesting complexes: alternative disorder model

The bacterial (Rhodopseudomonas acidophila) photosynthetic peripheral light-harvesting complex of type 2 (LH2) exhibits rich fluorescence spectral dynamics at room temperature. The fluorescence spectrum of individual LH2 shifts either to the blue or to the red during the experimental observation time of a few minutes. These spectral changes are often reversible and occur between levels of a distinctly different peak wavelength. Furthermore, they are accompanied by a change of the spectral line shape. To interpret the dynamics of spectral changes, an energetic disorder model associated with easily explainable structural changes of the protein is proposed. This model assumes that each pigment in the tightly coupled ring of bacteriochlorophylls can be in two states of electronic transition energy due to the protein-pigment interaction. The transition between these structural, and hence spectroscopic, states occurs through the thermally induced conformational potential energy barrier crossing. Although simplified, the model allows us to reproduce the bulk fluorescence spectrum, the distribution of the single-molecule spectral peak wavelength and its changes, and the statistics of the duration of the spectral states. It also provides an intuitively clear picture of possible protein dynamics in LH2. At the same time, it requires additional sophistication since it essentially does not reproduce the red occurrences of single LH2 spectra.

Protein shape and crowding drive domain formation and curvature in biological membranes

Folding, curvature, and domain formation are characteristics of many biological membranes. Yet the mechanisms that drive both curvature and the formation of specialized domains enriched in particular protein complexes are unknown. For this reason, studies in membranes whose shape and organization are known under physiological conditions are of great value. We therefore conducted atomic force microscopy and polarized spectroscopy experiments on membranes of the photosynthetic bacterium Rhodobacter sphaeroides. These membranes are densely populated with peripheral light harvesting (LH2) complexes, physically and functionally connected to dimeric reaction center-light harvesting (RC-LH1-PufX) complexes. Here, we show that even when converting the dimeric RC-LH1-PufX complex into RC-LH1 monomers by deleting the gene encoding PufX, both the appearance of protein domains and the associated membrane curvature are retained. This suggests that a general mechanism may govern membrane organization and shape. Monte Carlo simulations of a membrane model accounting for crowding and protein geometry alone confirm that these features are sufficient to induce domain formation and membrane curvature. Our results suggest that coexisting ordered and fluid domains of like proteins can arise solely from asymmetries in protein size and shape, without the need to invoke specific interactions. Functionally, coexisting domains of different fluidity are of enormous importance to allow for diffusive processes to occur in crowded conditions.

On the effects of PufX on the absorption properties of the light-harvesting complexes of Rhodobacter sphaeroides

Some species of purple bacteria as, e.g., Rhodobacter sphaeroides contain the protein PufX. Concurrently, the light harvesting complexes 1 (LH1) form dimers of open rings. In mutants without PufX, the LH1s are closed rings and photosynthesis breaks down, because the ubiquinone exchange at the reaction center is blocked. However, the main purpose of the LH1 is light harvesting. We therefore investigate the effects that the PufX-induced dimerization has on the absorption properties of the core complexes. Calculations with a dipole model, which compare the photosynthetic efficiency of various configurations of monomeric and dimeric core complexes, show that the dimer can absorb photons directly into the reaction centers more efficiently, but that the performance of the more sophisticated dimeric LH1 antenna degrades faster with structural perturbations. The calculations predict an optimal orientation of the reaction centers relative to the LH1 dimer, which agrees well with the experimentally found configuration. Based on experimental observations indicating that the dimeric core complexes are indeed rather rigid, we hypothesize that in PufX(+) species the association between the LH1 and the reaction centers is enhanced. This mechanical stabilization of the core complexes would lead to the observed quinone blockage, when PufX is missing.

Discrepancy between experimental and theoretical excitation transfer rates in LH2 bacteriochlorophyll-protein complexes of purple bacteria

Discrepancy is revealed between the values of excitation transfer times measured experimentally, and those calculated, for the atomic structures of B800 --> B850 bacteriochlorophylls within the LH2 light-harvesting pigment-protein complex of the purple bacterium Rhodopseudomonas acidophila. The value 2.9-3.2 ps for the B800 --> B850 excitation transfer, calculated on the basis of atomic structure of LH2, is about 4-times longer than that measured for this bacterium (0.7 ps). This discrepancy appears common in at least two purple bacteria. Possible sources responsible for this discrepancy are discussed. It may either signify some drawback/s/ in our notions about the precise in vivo structure of LH2 complexes, for example, possible changes of LH2 structure during crystallization, or it may reflect our ignorance of some mechanisms involved in excitation migration.

Self-Assembled Monolayer of Light-Harvesting Core Complexes from Photosynthetic Bacteria on a Gold Electrode Modified with Alkanethiols

Light-harvesting antenna core (LH1-RC) complexes isolated from Rhodoseudomonas palustris were self-assembled on a gold electrode modified with self-assembled monolayers (SAMs) of the alkanethiols NH2(CH2)nSH, n = 2, 6, 8, 11; HOOC(CH2)7SH; and CH3(CH2)7SH, respectively. Adsorption of the LH1-RC complexes on the SAMs depended on the terminating group of the alkanethiols, where the adsoption increased in the following order for the terminating groups: amino groups > carboxylic acid groups > methyl groups. Further, the adsorption on a gold electrode modified with SAMs of NH2(CH2)nSH, n = 2, 6, 8, 11, depended on the methylene chain length, where the adsorption increased with increasing the methylene chain length. The presence of the well-known light-harvesting and reaction center peaks of the near infrared (NIR) absorption spectra of the LH1-RC complexes indicated that these complexes were only fully stable on the SAM gold electrodes modified with the amino group. In the case of modification with the carboxyl group, the complexes were partially stable, while in the presence of the terminal methyl group the complexes were extensively denatured. An efficient photocurrent response of these complexes on the SAMs of NH2(CH2)nSH, n = 2, 6, 8, 11, was observed upon illumination at 880 nm. The photocurrent depended on the methylene chain length (n), where the maximum photocurrent response was observed at n = 6, which corresponds to a distance between the amino terminal group in NH2(CH2)6SH and the gold surface of 1.0 nm.

Stabilization of charge separation and cardiolipin confinement in antenna-reaction center complexes purified from Rhodobacter sphaeroides

The reaction center-light harvesting complex 1 (RC-LH1) purified from the photosynthetic bacterium Rhodobacter sphaeroides has been studied with respect to the kinetics of charge recombination and to the phospholipid and ubiquinone (UQ) complements tightly associated with it. In the antenna-RC complexes, at 6.5<pH<9.0, P(+)Q(B)(-) recombines with a pH independent average rate constant <k> more than three times smaller than that measured in LH1-deprived RCs. At increasing pH values, for which <k> increases, the deceleration observed in RC-LH1 complexes is reduced, vanishing at pH >11.0. In both systems kinetics are described by a continuous rate distribution, which broadens at pH >9.5, revealing a strong kinetic heterogeneity, more pronounced in the RC-LH1 complex. In the presence of the antenna the Q(A)Q(B)(-) state is stabilized by about 40 meV at 6.5<pH<9.0, while it is destabilized at pH >11. The phospholipid/RC and UQ/RC ratios have been compared in chromatophore membranes, in RC-LH1 complexes and in the isolated peripheral antenna (LH2). The UQ concentration in the lipid phase of the RC-LH1 complexes is about one order of magnitude larger than the average concentration in chromatophores and in LH2 complexes. Following detergent washing RC-LH1 complexes retain 80-90 phospholipid and 10-15 ubiquinone molecules per monomer. The fractional composition of the lipid domain tightly bound to the RC-LH1 (determined by TLC and (31)P-NMR) differs markedly from that of chromatophores and of the peripheral antenna. The content of cardiolipin, close to 10% weight in chromatophores and LH2 complexes, becomes dominant in the RC-LH1 complexes. We propose that the quinone and cardiolipin confinement observed in core complexes reflects the in vivo heterogeneous distributions of these components. Stabilization of the charge separated state in the RC-LH1 complexes is tentatively ascribed to local electrostatic perturbations due to cardiolipin.

Combining optical tweezers and scanning probe microscopy to study DNA-protein interactions

We present the first results obtained with a new instrument designed and built to study DNA-protein interactions at the single molecule level. This microscope combines optical tweezers with scanning probe microscopy and allows us to locate DNA-binding proteins on a single suspended DNA molecule. A single DNA molecule is stretched taut using the optical tweezers, while a probe is scanned along the molecule. Interaction forces between the probe and the sample are measured with the optical tweezers. The instrument thus enables us to correlate mechanical and functional properties of bound proteins with the tension within the DNA molecule. The typical friction force between a micropipette used as probe and a naked DNA molecule was found to be <1 pN. A 16 micro m DNA molecule with approximately 10-15 digoxygenin (DIG) molecules located over a 90 nm range in the middle of the DNA was used as a model system. By scanning with an antidigoxygenin (alpha-DIG) antibody-coated pipette we were able to localize these sites by exploiting the high binding affinity between this antibody-antigen pair. The estimated experimental resolution assuming an infinitesimally thin and rigid probe and a single alpha-DIG/DIG bond was 15 nm.

Self-assembled monolayer of light-harvesting core complexes of photosynthetic bacteria on an amino-terminated ITO electrode

Light-harvesting antenna core (LH1-RC) complexes isolated from Rhodospirillum rubrum and Rhodopseudomonas palustris were successfully self-assembled on an ITO electrode modified with 3-aminopropyltriethoxysilane. Near infra-red (NIR) absorption, fluorescence, and IR spectra of these LH1-RC complexes indicated that these LH1-RC complexes on the electrode were stable on the electrode. An efficient energy transfer and photocurrent responses of these LH1-RC complexes on the electrode were observed upon illumination of the LH1 complex at 880 nm.

Conformational transformations induced by the charge-curvature interaction: Mean-field approach

A simple phenomenological model for describing the conformational dynamics of biological macromolecules via the nonlinearity-induced instabilities is proposed. It is shown that the interaction between charges and bending degrees of freedom of closed molecular aggregates may act as drivers giving impetus to conformational dynamics of biopolymers. It is demonstrated that initially circular aggregates may undergo transformation to polygonal shapes and possible application to aggregates of bacteriochlorophyl a molecules is considered.

Hexacoordination of bacteriochlorophyll in photosynthetic antenna LH1

The ability of chlorophylls to coordinate ligands is of fundamental structural importance for photosynthetic pigment-protein complexes, where in virtually all cases the pigment is thought to be in a pentacoordinated state. In this study, the correlation of the Q(X) transition energy with the coordination state of the central metal in bacteriochlorophyll is applied in investigating the pigment coordination state in bacterial photosynthetic antenna LH1. To facilitate a detailed spectral analysis in the Q(X) region, carotenoid-depleted forms of LH1 are prepared and model LH1 are constructed with non-native carotenoids having blue-shifted absorption. The deconvolution of the Q(X) envelope in LH1 reveals that the band is the sum of two transitions, which peak near 590 and 607 nm, showing that a significant fraction (up to 25%) of hexacoordinated bacteriochlorophyll is present in the complex. The hexacoordination can be seen also in LH1 antennae from other species of purple photosynthetic bacteria. It seems correlated with the LH1 aggregation state and probably is a consequence of the structural flexibility of the assembled complex. The sixth ligand probably originates from the apoprotein and seems not to affect the chromophore core size. These findings show that in light-harvesting complexes a hexacoordinated state of bacteriochlorophyll is not uncommon. Its presence may be relevant to a correct assembly of the antenna and have functional consequences, as it results in a splitting of the pigment S2 excited state (Q(X)), i.e., the carotenoid excitation acceptor state, what might affect intracomplex carotenoid-to-bacteriochlorophyll energy transfer.

Spectral trends in the fluorescence of single bacterial light-harvesting complexes: experiments and modified redfield simulations

In this work we present and discuss the single-molecule fluorescence spectra of a variety of species of light-harvesting complexes: LH2 of Rhodopseudomonas acidophila, Rhodobacter sphaeroides, and Rhodospirillum molischianum and LH1 of Rhodobacter sphaeroides. The emission spectrum of these complexes varies as a function of time as was described in earlier work. For each type of complex, we observe a pronounced and well-reproducible characteristic relationship between the fluorescence spectral parameters of the peak wavelength, width, and asymmetry. This dependence for the LH2 complexes can be quantitatively explained on the basis of a disordered exciton model by varying the static disorder and phonon coupling parameters. In addition, a correlation of the pigment site energies has to be assumed to interpret the behavior of the LH1 complex.

Comparative study of spectral flexibilities of bacterial light-harvesting complexes: structural implications

This work presents a comparative study of the frequencies of spectral jumping of individual light-harvesting complexes of six different types: LH2 of Rhodopseudomonas acidophila, Rhodobacter sphaeroides, and Rhodospirillum molischianum; LH1 of Rhodobacter sphaeroides; and two "domain swap mutants" of LH2 of Rhodobacter sphaeroides: PACLH1 and PACLH2mol, in which the alpha-polypeptide C-terminus is exchanged with the corresponding sequence from LH1 of Rhodobacter sphaeroides or LH2 of Rhodospirillum molischianum, respectively. The quasistable states of fluorescence peak wavelength that were previously observed for the LH2 of Rps. acidophila were confirmed for other species. We also observed occurrences of extremely blue-shifted spectra, which were associated with reversible bleaching of one of the chromophore rings. Different jumping behavior is observed for single complexes of different types investigated with the same equivalent excitation intensity. The differences in spectral diffusion are associated with subtle differences of the binding pocket of B850 pigments and the structural flexibility of the different types of complexes.

On the role of basic residues in adapting the reaction centre-LH1 complex for growth at elevated temperatures in purple bacteria

The purple photosynthetic bacterium Thermochromatium tepidum is a moderate thermophile, with a growth optimum of 48-50 degrees C. The X-ray crystal structure of the reaction centre from this organism has been determined, and compared with that from mesophilic bacteria such as Blastochloris viridis and Rhodobacter sphaeroides (Nogi T et al. (2000) Proc Natl Acad Sci USA 97: 13561-13566). Structural features that could contribute to the enhanced thermal stability of the Thermochromatium tepidum reaction centre were discussed, including three arginine residues exposed at the periplasmic side of the membrane that are not present in reaction centres from mesophilic organisms, and potentially could increase the affinity of the complex for the surrounding membrane. In the present report these arginine residues, plus a histidine identified from an extensive sequence alignment, were engineered into structurally homologous positions in the Rhodobacter sphaeroides reaction centre, and the effect on the thermal stability of the Rhodobacter sphaeroides complex was examined. We find that these residues do not enhance the thermal stability of the reaction centre, as assessed by absorbance spectroscopy of the bacteriochlorin cofactors in membrane-bound reaction centres. Possible roles of these residues in the Thermochromatium tepidum reaction centre are discussed, and it is proposed that they facilitate stronger binding of the reaction centre to the encircling LH1 antenna complex, through ionic interactions with acidic residues at the C-terminal end of the LH1 alpha-polypeptide. Such an interaction could enhance the stability of the so-called 'RC-LH1 core' complex that is formed between the reaction centre and the LH1 antenna, and which represents the minimal functional photosynthetic unit in all known purple photosynthetic bacteria. Stronger bonding interactions between the two complexes could also contribute to an increase in the rigidity of the photosynthetic membrane in Thermochromatium tepidum, in accord with the general finding that the cytoplasmic membrane from thermophilic eubacteria is less fluid than its counterpart in mesophilic bacteria.

Characterization of a highly purified, fully active, crystallizable RC-LH1-PufX core complex from Rhodobacter sphaeroides

Photosynthetic complexes in bacteria absorb light and undergo photochemistry with high quantum efficiency. We describe the isolation of a highly purified, active, reaction center-light-harvesting 1-PufX complex (RC-LH1-PufX core complex) from a strain of the photosynthetic bacterium, Rhodobacter sphaeroides, which lacks the light-harvesting 2 (LH2) and contains a 6 histidine tag on the H subunit of the RC. The complex was solubilized with diheptanoyl-sn-glycero-3-phosphocholine (DHPC), and purified by Ni-affinity, size-exclusion and ion-exchange chromatography in dodecyl maltoside. SDS-PAGE analysis shows the complex to be highly purified. The quantum efficiency was determined by measuring the charge separation (DQA --> D+QA -) in the RC as a function of light intensity. The RC-LH1-PufX complex had a quantum efficiency of 0.95 +/- 0.05, indicating full activity. The stoichiometry of LH1 subunits per RC was determined by two independent methods: (i) solvent extraction and absorbance spectroscopy of bacteriochlorophyll, and (ii) density scanning of the SDS-PAGE bands. The average stoichiometry from the two measurements was 13.3 +/- 0.9 LH1/RC. The presence of PufX was observed in SDS-PAGE gels at a stoichiometry of 1.1 +/- 0.1/RC. Crystals of the core complex have been obtained which diffract X-rays to 12 A. A preliminary analysis of the space group and unit cell analysis indicated a P1 space group with unit cell dimensions of a = 76.3 A, b = 137.2 A, c = 137.5 A; alpha = 60.0 degrees , beta = 89.95 degrees , gamma =90.02 degrees .

Microscopy and single molecule detection in photosynthesis

Progress in various fields of microscopy techniques brought up enormous possibilities to study the photosynthesis down to the level of individual pigment-protein complexes. The aim of this review is to present recent developments in the photosynthesis research obtained using such highly advanced techniques. Three areas of microscopy techniques covering optical microscopy, electron microscopy and scanning probe microscopy are reviewed. Whereas the electron microscopy and scanning probe microscopy are used in photosynthesis mainly for structural studies of photosynthetic pigment-protein complexes, the optical microscopy is used also for functional studies.

The 8.5A projection structure of the core RC-LH1-PufX dimer of Rhodobacter sphaeroides

Two-dimensional crystals of dimeric photosynthetic reaction centre-LH1-PufX complexes have been analysed by cryoelectron microscopy. The 8.5A resolution projection map extends previous analyses of complexes within native membranes to reveal the alpha-helical structure of two reaction centres and 28 LH1 alphabeta subunits within the dimer. For the first time, we have achieved sufficient resolution to suggest a possible location for the PufX transmembrane helix, the orientation of the RC and the arrangement of helices within the surrounding LH1 complex. Whereas low-resolution projections have shown an apparent break in the LH1, our current map reveals a diffuse density within this region, possibly reflecting high mobility. Within this region the separation between beta14 of one monomer and beta2 of the other monomer is approximately 6A larger than the average beta-beta spacing within LH1; we propose that this is sufficient for exchange of quinol at the RC Q(B) site. We have determined the position and orientation of the RC within the dimer, which places its Q(B) site adjacent to the putative PufX, with access to the point in LH1 that appears most easily breached. PufX appears to occupy a strategic position between the mobile alphabeta14 subunit and the Q(B) site, suggesting how the structure, possibly coupled with a flexible ring, plays a role in optimizing quinone exchange during photosynthesis.

Watching the components of photosynthetic bacterial membranes and their in situ organisation by atomic force microscopy

The atomic force microscope has developed into a powerful tool in structural biology allowing information to be acquired at submolecular resolution on the protruding structures of membrane proteins. It is now a complementary technique to X-ray crystallography and electron microscopy for structure determination of individual membrane proteins after extraction, purification and reconstitution into lipid bilayers. Moving on from the structures of individual components of biological membranes, atomic force microscopy has recently been demonstrated to be a unique tool to identify in situ the individual components of multi-protein assemblies and to study the supramolecular architecture of these components allowing the efficient performance of a complex biological function. Here, recent atomic force microscopy studies of native membranes of different photosynthetic bacteria with different polypeptide contents are reviewed. Technology, advantages, feasibilities, restrictions and limits of atomic force microscopy for the acquisition of highly resolved images of up to 10 A lateral resolution under native conditions are discussed. From a biological point of view, the new insights contributed by the images are analysed and discussed in the context of the strongly debated organisation of the interconnected network of membrane-associated chlorophyll-protein complexes composing the photosynthetic apparatus in different species of purple bacteria.

Structure and functional role of supercomplexes of IsiA and Photosystem I in cyanobacterial photosynthesis

Cyanobacteria express large quantities of the iron stress-inducible protein IsiA under iron deficiency. IsiA can assemble into numerous types of single or double rings surrounding Photosystem I. These supercomplexes are functional in light-harvesting, empty IsiA rings are effective energy dissipaters. Electron microscopy studies of these supercomplexes show that Photosystem I trimers bind 18 IsiA copies in a single ring, whereas monomers may bind up to 35 copies in two rings. Work on mutants indicates that the PsaF/J and PsaL subunits facilitate the formation of closed rings around Photosystem I monomers but are not obligatory components in the formation of Photosystem I-IsiA supercomplexes.

Near-IR absorption and fluorescence spectra and AFM observation of the light-harvesting 1 complex on a mica substrate refolded from the subunit light-harvesting 1 complexes of photosynthetic bacteria Rhodospirillum rubrum

The subunit light-harvesting 1 (LH 1) complexes isolated from photosynthetic bacteria Rhodospirillum rubrum using n-octyl-beta-glucoside were reassociated and adsorbed on a mica substrate using spin-coat methods with the aim of using this LH complex in a nanodevice. The near-IR absorption and fluorescence spectra of the LH 1 complexes indicated that the LH 1 complex on the mica was stable, and efficient energy transfer from a carotenoid to a bacteriochlorophyll a was observed. Atomic force microscopy of the reassociated LH 1 complexes, under air, showed the expected ringlike structure. The outer and inner diameters of the ringlike structure of the LH 1 complex were approximately 30 and 8 nm, respectively, and the ringlike structure protruded by 0.2-0.6 nm.

Design and Expression of Cysteine-Bearing Hydrophobic Polypeptides and Their Self-Assembling Properties with Bacteriochlorophyll a Derivatives as a Mimic of Bacterial Photosynthetic Antenna Complexes. Effect of Steric Confinement and Orientation of the Polypeptides on the Pigment/Polypeptide Assembly Process

A series of cysteine-bearing hydrophobic polypeptides analogous to a light-harvesting one betapolypeptide (LH1beta) from the LH1 complex from the purple photosynthetic bacterium, Rhodobacter sphaeroides, was synthesized using an Escherichia coli expression system. The cysteine was placed in the C- or N-terminal regions of the polypeptide to investigate the influence of steric confinement and orientation of the polypeptides via disulfide linkages as they were self-assembled with zinc-substituted bacteriochlorophyll a ([Zn]-BChl a). The polypeptides were expressed as water-soluble fusion proteins with maltose-binding protein (MBP). The fusion proteins formed a subunit-type complex with the [Zn]-BChl a in an n-octyl-beta-d-glucopyranoside (OG) micellar solution regardless of the cross-links or the cleavage of the cysteines, judging from absorption, CD, and fluorescence spectra. Following treatment with trypsin, the polypeptides were detached from the MBP portion. Such trypsin-digested polypeptides formed a subunit-type LH complex at 25 degrees C, which also showed that the disulfide linkage was not crucial for the subunit formation. When a polypeptide having cysteine on the C-terminus was assembled at 4 degrees C, the Qy absorption band was remarkably red-shifted to approximately 836 nm, suggesting that the cleavage of the large MBP portion liberates the polypeptides to form the progressive type of complex similar to LH1-type complex. The trypsin-treated polypeptides bearing cysteines in both terminal regions, which are randomly cross-linked, did not form the LH1-type complex under oxidative conditions but did form the complex under reductive conditions. This observation suggests that the polypeptide orientation strongly influences the LH1-type complex formation. The progressive assembly from the subunit to the holo-LH1-type complex following cleavage of MBP portion in a lipid bilayer is also briefly discussed.

Combined AFM and confocal fluorescence microscope for applications in bio-nanotechnology

We present a custom-designed atomic force fluorescence microscope (AFFM), which can perform simultaneous optical and topographic measurements with single molecule sensitivity throughout the whole visible to near-infrared spectral region. Integration of atomic force microscopy (AFM) and confocal fluorescence microscopy combines the high-resolution topographical imaging of AFM with the reliable (bio)-chemical identification capability of optical methods. The AFFM is equipped with a spectrograph enabling combined topographic and fluorescence spectral imaging, which significantly enhances discrimination of spectroscopically distinct objects. The modular design allows easy switching between different modes of operation such as tip-scanning, sample-scanning or mechanical manipulation, all of which are combined with synchronous optical detection. We demonstrate that coupling the AFM with the fluorescence microscope does not compromise its ability to image with a high spatial resolution. Examples of several modes of operation of the AFFM are shown using two-dimensional crystals and membranes containing light-harvesting complexes from the photosynthetic bacterium Rhodobacter sphaeroides.

Variable LH2 stoichiometry and core clustering in native membranes of Rhodospirillum photometricum

The individual components of the photosynthetic unit (PSU), the light-harvesting complexes (LH2 and LH1) and the reaction center (RC), are structurally and functionally known in great detail. An important current challenge is the study of their assembly within native membranes. Here, we present AFM topographs at 12 A resolution of native membranes containing all constituents of the PSU from Rhodospirillum photometricum. Besides the major technical advance represented by the acquisition of such highly resolved data of a complex membrane, the images give new insights into the organization of this energy generating apparatus in Rsp. photometricum: (i) there is a variable stoichiometry of LH2, (ii) the RC is completely encircled by a closed LH1 assembly, (iii) the LH1 assembly around the RC forms an ellipse, (iv) the PSU proteins cluster together segregating out of protein free lipid bilayers, (v) core complexes cluster although enough LH2 are present to prevent core-core contacts, and (vi) there is no cytochrome bc1 complex visible in close proximity to the RCs. The functional significance of all these findings is discussed.

The native architecture of a photosynthetic membrane

In photosynthesis, the harvesting of solar energy and its subsequent conversion into a stable charge separation are dependent upon an interconnected macromolecular network of membrane-associated chlorophyll-protein complexes. Although the detailed structure of each complex has been determined, the size and organization of this network are unknown. Here we show the use of atomic force microscopy to directly reveal a native bacterial photosynthetic membrane. This first view of any multi-component membrane shows the relative positions and associations of the photosynthetic complexes and reveals crucial new features of the organization of the network: we found that the membrane is divided into specialized domains each with a different network organization and in which one type of complex predominates. Two types of organization were found for the peripheral light-harvesting LH2 complex. In the first, groups of 10-20 molecules of LH2 form light-capture domains that interconnect linear arrays of dimers of core reaction centre (RC)-light-harvesting 1 (RC-LH1-PufX) complexes; in the second they were found outside these arrays in larger clusters. The LH1 complex is ideally positioned to function as an energy collection hub, temporarily storing it before transfer to the RC where photochemistry occurs: the elegant economy of the photosynthetic membrane is demonstrated by the close packing of these linear arrays, which are often only separated by narrow 'energy conduits' of LH2 just two or three complexes wide.

Circular dichroism of carotenoids in bacterial light-harvesting complexes: experiments and modeling

In this work we investigate the origin and characteristics of the circular dichroism (CD) spectrum of rhodopin glucoside and lycopene in the light-harvesting 2 complex of Rhodopseudomonas acidophila and Rhodospirillum molischianum, respectively. We successfully model their absorption and CD spectra based on the high-resolution structures. We assume that these spectra originate from seven interacting transition dipole moments: the first corresponds to the 0-0 transition of the carotenoid, whereas the remaining six represent higher vibronic components of the S2 state. From the absorption spectra we get an estimate of the Franck-Condon factors of these transitions. Furthermore, we investigate the broadening mechanisms that lead to the final shape of the spectra and get an insight into the interaction energy between carotenoids. Finally, we examine the consequences of rotations of the carotenoid transition dipole moment and of deformations in the light-harvesting 2 complex rings. Comparison of the modeled carotenoid spectra with modeled spectra of the bacteriochlorophyll QY region leads to a refinement of the modeling procedure and an improvement of all calculated results. We therefore propose that the combined carotenoid and bacteriochlorophyll CD can be used as an accurate reflection of the overall structure of the light-harvesting complexes.