Preliminary characterization by X-ray diffraction of crystals of photochemical reaction centres from wild-type Rhodopseudomonas spheroides

Stability and solubility of integral membrane proteins from photosynthetic bacteria solubilized in different detergents

As a first step toward the establishment of practical guidelines for the search for crystallization conditions, stability and solubility were examined for integral membrane proteins from photosynthetic bacteria in the presence of different detergents. The results obtained from their stability provided practical information on the proper choice of detergent type in the preparation process and the subsequent crystallization experiment. In addition, the determination of a solubility diagram provided a practical method for quantifying the correct choice of detergent concentration and for setting up the suitable precipitant concentration in the crystallization experiment.

A motif for quinone binding sites in respiratory and photosynthetic systems

Many of the membrane-bound protein complexes of respiratory and photosynthetic systems are reactive with quinones. To date, no clear structural relationship between sites that bind quinone has been defined, apart from that in the homologous family of "type II" photosynthetic reaction centres. We show here that a structural element containing a weak sequence motif is common to the Q(A) and Q(B) sites of bacterial reaction centres and the Q(i) site of the mitochondrial bc(1) complex. Analyses of sequence databases indicate that this element may also be present in the PsaA/B subunits of photosystem I, in the ND4 and ND5 subunits of complex I and, possibly, in the mitochondrial alternative quinol oxidase. This represents a first step in the structural classification of quinone binding sites.

Photosystem I at 4 A resolution represents the first structural model of a joint photosynthetic reaction centre and core antenna system

The 4 A X-ray structure model of trimeric photosystem I of the cyanobacterium Synechococcus elongatus reveals 31 transmembrane, nine surface and three stromal alpha-helices per monomer, assigned to the 11 protein subunits: PsaA and PsaB are related by a pseudo two-fold axis normal to the membrane plane, along which the electron transfer pigments are arranged. 65 antenna chlorophyll a (Chl a) molecules separated by < or = 16 A form an oval, clustered net continuous with the electron transfer chain through the second and third Chl a pairs of the electron transfer system. This suggests a dual role for these Chl a both in excitation energy and electron transfer. The architecture of the protein core indicates quinone and iron-sulphur type reaction centres to have a common ancestor.

Crystallization of the reaction center from Rhodobacter sphaeroides in a new tetragonal form

The reaction center from the nonsulfur purple bacterium Rhodobacter sphaeroides has been crystallized in a new form. The crystals grew in the presence of polyethylene glycol 4000, the detergent beta-octyl glucoside, and the amphiphiles heptane triol and benzamidine hydrochloride, using the sitting drop method. The space group of these crystals is tetragonal, P4(1)(4(3))2(1)2, and the cell constants area a = b = 141.5 A and c = 276.7 A with probably 2 proteins per asymmetric unit. A native data set has been set collected to a resolution of 2.8 A consisting of 56,332 unique reflections (50,731 with F > 2 sigma) with an Rsym of 9.5%. Analysis of the diffraction data is underway using molecular and isomorphous replacement.

Structure of the photosynthetic reaction centre from Rhodobacter sphaeroides at 2.65 A resolution: cofactors and protein-cofactor interactions

BACKGROUND

Photosynthetic reaction centres (RCs) catalyze light-driven electron, transport across photosynthetic membranes. The photosynthetic bacterium Rhodobacter, sphaeroides is often used for studies of RCs, and three groups have determined the structure of its reaction centre. There are discrepancies between these structures, however, and to resolve these we have determined the structure to higher resolution than before, using a new crystal form.

RESULTS

The new structure provides a more detailed description of the Rb. sphaeroides RC, and allows us to compare it with the structure of the RC from Rhodopseudomonas viridis. We find no evidence to support most of the published differences in cofactor binding between the RCs from Rps. viridis and Rb. sphaeroides. Generally, the mode of cofactor binding is conserved, particularly along the electron transfer pathway. Substantial differences are only found at ring V of one bacteriochlorophyll of the 'special pair' and for the secondary quinone, QB. A water chain with a length of about 23 A including 14 water molecules extends from the QB to the cytoplasmic side of the RC.

CONCLUSIONS

The cofactor arrangement and the mode of binding to the protein seem to be very similar among the non-sulphur bacterial photosynthetic RCs. The functional role of the displaced QB molecule, which might be present as quinol, rather than quinone, is not yet clear. The newly discovered water chain to the QB binding site suggests a pathway for the protonation of the secondary quinone QB.

Determination of the number of detergent molecules associated with the reaction center protein isolated from the photosynthetic bacterium Rhodopseudomonas viridis. Effects of the amphiphilic molecule 1,2,3-heptanetriol

Detergent-free reaction center (RC) proteins from the photosynthetic bacterium Rhodopseudomonas viridis were obtained using Bio-Beads SM-2. With these RCs, the amount of detergent molecules associated with the protein was measured by determining the detergent concentration at which re-solubilization occurred as a function of the RC concentration. For N,N-dimethyl dodecylamine-N-oxide (LDAO), Triton X-100 and beta-octylglucoside 260 +/- 30,105 +/- 10 and 360 +/- 100 detergent molecules were necessary to dissolve the protein, respectively. With this technique we have studied the effect of the amphiphilic molecule 1,2,3-heptanetriol, which is essential in the crystallization process of these RCs. Addition of 5% 1,2,3-heptanetriol reduces the value for LDAO to 120 +/- 20 LDAO/RC, supporting the notion that crystallization of the RCs is promoted by increasing the number of protein-protein contacts.

Involvement of the protein-protein interactions in the thermodynamics of the electron-transfer process in the reaction centers from Rhodopseudomonas viridis

Reaction centers from Rhodopseudomonas viridis were reconstituted into dimyristoylphosphatidylcholine (DMPC) and dielaidoylphosphatidylcholine (DEPC) liposomes. Freeze-fracture electron micrographs were performed on the samples frozen from temperatures above and below the phase transition temperatures of those lipids (Tc = 23 and 9.5 degrees C, in DMPC and DEPC, respectively). Above Tc, in the fluid conformation of the lipids, the reaction centers are randomly distributed in the vesicle membranes. Below Tc, aggregation of the proteins occurs. The Arrhenius plots of the rate constants of the charge recombination between P+ and QA- display a break at about 24 degrees C in DMPC vesicles and about 10 degrees C in DEPC vesicles (P represents the primary electron donor, a dimer of bacteriochlorophyll, and QA the primary quinone electron acceptor). This is in contrast to what was previously observed for the proteoliposomes of egg yolk phosphatidylcholine and for chromatophores [Baciou, L., Rivas, E., & Sebban, P. (1990) Biochemistry 29, 2966-2976], for which Arrhenius plots were linear. In DMPC and DEPC proteoliposomes, the activation parameters were very different on the two sides of Tc (delta H degrees for T less than Tc = 2.5 times delta H degrees for T greater than Tc), leading however, to the same delta G degrees values. Taking into account the structural and thermodynamic data, we suggest that, in vivo, protein-protein interactions play a role in the thermodynamic parameters associated with the energy stabilization process within the reaction centers.

P+QA- and P+QB- charge recombinations in Rhodopseudomonas viridis chromatophores and in reaction centers reconstituted in phosphatidylcholine liposomes. Existence of two conformational states of the reaction centers and effects of pH and o-phenanthroline

The P+QA- and P+QB- charge recombination decay kinetics were studied in reaction centers from Rhodopseudomonas viridis reconstituted in phosphatidylcholine bilayer vesicles (proteoliposomes) and in chromatophores. P represents the primary electron donor, a dimer of bacteriochlorophyll; QA and QB are the primary and secondary stable quinone electron acceptors, respectively. In agreement with recent findings for reaction centers isolated in detergent [Sebban, P., & Wraight, C.A. (1989) Biochim. Biophys. Acta 974, 54-65] the P+QA- decay kinetics were biphasic (kfast and kslow). Arrhenius plots of the kinetics were linear, in agreement with the hypothesis of a thermally activated process (probably via P+I-; I is the first electron acceptor, a bacteriopheophytin) for the P+QA- charge recombination. Similar activation free energies (delta G) for this process were found in chromatophores and in proteoliposomes. Significant pH dependences of kfast and kslow were observed in chromtophores and in proteoliposomes. In the pH range 5.5-11, the pH titration curves of kfast and kslow were interpreted in terms of the existence of three protonable groups, situated between I- and QA-, which modulate the free energy difference between P+I- and P+QA-. In proteoliposomes, a marked effect of o-phenanthroline was observed on two of the three pKs, shifting one of them by more than 2 pH units. On the basis of recent structural data, we suggest a possible interpretation for this effect, which is much smaller in Rhodobacter sphaeroides. The decay kinetics of P+QB- were also biphasic. Marked pH dependences of the rate constants and of the relative proportions of both phases were also detected for these decays. The major conclusion of this work comes from the biphasicity of the P+QB- decay kinetics. We had suggested previously that biphasicity of the P+QA- charge recombination in Rps. viridis comes from nonequilibrium between protonation states of the reaction centers due to comparable rates of the protonation events and charge recombination. This hypothesis does not hold since the P+QB- decays occur on a time scale (tau approximately 300 ms at pH 8) much longer than protonation events. This leads to the conclusion that kfast and kslow (for both P+QA- and P+QB-) are related to conformational states of the reaction centers, existing before the flash. In addition, the fast and slow decays of P+QB- are related to those measured for P+QA-, via the calculations of the QA-QB in equilibrium QAQB- apparent equilibrium constants, K2.(ABSTRACT TRUNCATED AT 400 WORDS)

Structure of spheroidene in the photosynthetic reaction center from Y Rhodobacter sphaeroides

The structure of the reaction center of Y Rhodobacter sphaeroides has been solved at 3 A resolution, using the atomic coordinates of the reaction center from the carotenoidless mutant R26 Rhodobacter sphaeroides. The structure has been refined by a stimulated annealing with the computer program X-PLOR, leading to a crystallographic R factor of 0.22 using reflections between 8 and 3 A. The spheroidene molecule which is bound to the Y reaction center has been fitted in the electron density map as a 15-cis isomer with a highly asymmetric structure. The cis-bond is located at proximity from ring 1 of the accessory bacteriochlorophyll on the inactive M side. The nature of the cis-bond was confirmed by resonance Raman spectra obtained from Y reaction center crystals. The structure of spheroidene in Y reaction center is compared to that proposed for 1,2-dihydroneurosporene in Rhodopseudomonas viridis reaction center crystals.

Three-dimensional crystallization of membrane proteins

As recently as 10 years ago, the prospect of solving the structure of any membrane protein by X-ray crystallography seemed remote. Since then, the threedimensional (3-D) structures of two membrane protein complexes, the bacterial photosynthetic reaction centres ofRhodopseudomonas viridis(Deisenhoferet al.1984, 1985) and ofRhodobacter sphaeroides(Allenet al.1986, 1987a, 6; Changet al.1986) have been determined at high resolution. This astonishing progress would not have been possible without the pioneering work of Michel and Garavito who first succeeded in growing 3-D crystals of the membrane proteins bacteriorhodopsin (Michel & Oesterhelt, 1980) and matrix porin (Garavito & Rosenbusch, 1980). X-ray crystallography is still the only routine method for determining the 3-D structures of biological macromolecules at high resolution and well-ordered 3-D crystals of sufficient size are the essential prerequisite.

How carotenoids function in photosynthetic bacteria

Carotenoids are essential for the survival of photosynthetic organisms. They function as light-harvesting molecules and provide photoprotection. In this review, the molecular features which determine the efficiencies of the various photophysical and photochemical processes of carotenoids are discussed. The behavior of carotenoids in photosynthetic bacterial reaction centers and light-harvesting complexes is correlated with data from experiments carried out on carotenoids and model systems in vitro. The status of the carotenoid structural determinations in vivo is reviewed.