Orientation of chromatophores and spheroplast-derived membrane vesicles of Rhodopseudomonas sphaeroides: analysis by localization of enzyme activities

Chemical Tagging of Membrane Proteins Enables Oriented Binding on Solid Surfaces

In biological systems, membrane proteins play major roles in energy conversion, transport, sensing, and signal transduction. Of special interest are the photosynthetic reaction centers involved in the initial process of light energy conversion to electrical and chemical energies. The oriented binding of membrane proteins to solid surfaces is important for biotechnological applications. In some cases, novel properties are generated as a result of the interaction between proteins and solid surfaces. We developed a novel approach for the oriented tagging of membrane proteins. In this unique process, bifunctional molecules are used to chemically tag the exposed surfaces of membrane proteins at selected sides of membrane vesicles. The isolated tagged membrane proteins were self-assembled on solid surfaces, leading to the fabrication of dens-oriented layers on metal and glass surfaces, as seen from the atomic force microscopy (AFM) images. In this work, we used chromatophores and membrane vesicles containing protein chlorophyll complexes for the isolation of the bacterial reaction center and photosystem I, from photosynthetic bacteria and cyanobacteria, respectively. The oriented layers, which were fabricated on metal surfaces, were functional and generated light-induced photovoltage that was measured by the Kalvin probe apparatus. The polarity of the photovoltage depended on the orientation of proteins in the layers. Other membrane proteins can be tagged by the same method. However, we preferred the use of reaction centers because their orientation can be easily detected by the polarity of their photovoltages.

Examination of the putative Ca2+-binding site in the light-harvesting complex 1 of thermophilic purple sulfur bacterium Thermochromatium tepidum

The core light-harvesting complex (LH1) of purple sulfur photosynthetic bacterium Thermochromatium tepidum exhibits an unusual absorption maximum at 915 nm for the Q (y) transition, and is highly stable when copurified with reaction center (RC) in a LH1-RC complex form. In previous studies, we demonstrated that the calcium ions are involved in both the large red shift and the enhanced thermal stability, and possible Ca(2+)-binding sites were proposed. In this study, we further examine the putative binding sites in the LH1 polypeptides using purified chromatophores. Incubation of the chromatophores in the presence of EDTA revealed no substantial change in the absorption maximum of LH1 Q (y) transition, whereas further addition of detergents to the chromatophores-EDTA solution resulted in a blue-shift for the LH1 Q (y) peak with the final position at 892 nm. The change of the LH1 Q (y) peak to shorter wavelengths was relatively slow compared to that of the purified LH1-RC complex. The blue-shifted LH1 Q (y) transition in chromatophores can be restored to its original position by addition of Ca(2+) ions. The results suggest that the Ca(2+)-binding site is exposed on the inner surface of chromatophores, corresponding to the C-terminal region of LH1. An Asp-rich fragment in the LH1 α-polypeptide is considered to form a crucial part of the binding network. The slow response of LH1 Q (y) transition upon exposure to EDTA is discussed in terms of the membrane environment in the chromatophores.

The LH1-RC core complex of Rhodobacter sphaeroides: interaction between components, time-dependent assembly, and topology of the PufX protein

Mutant strains of the photosynthetic bacterium Rhodobacter sphaeroides, lacking either LH1, the RC or PufX, were analysed by mild detergent fractionation of the cores. This reveals a hierarchy of binding of PufX in the order RC:LH1 > LH1 > RC. The assembly of photosynthetic membranes was studied by switching highly aerated cells to conditions of low aeration in the dark. The RC-H subunit appears before other components, followed by the pufBALMX then pufBA transcripts. Synthesis of the PufX polypeptide precedes that of LH1alpha and beta, which suggests that PufX associates with a limited amount of LH1alpha, beta and the RC, and prior to the encirclement of the RC by the rest of the LH1 complex. The topology of PufX within the intracytoplasmic membrane was determined by proteolytic treatment of membrane vesicles followed by protein sequencing; PufX is N-terminally exposed on the cytoplasmic surface of the photosynthetic membrane.

Topological organization of the Rieske iron-sulphur protein and subunit IV in the cytochrome bc1 complex of Rhodobacter sphaeroides

The ubiquinol-cytochrome c2 oxidoreductases (cytochrome bc1 complex) of Rhodobacter sphaeroides contains highly conserved cytochrome b, cytochrome c1 and Rieske FeS subunits, as well as a unique 14 kDa polypeptide, designated as subunit IV, thought to function as a ubiquinol-binding protein [Yu and Yu (1991) Biochemistry 30, 4934-4939]. As the topology of subunit IV is unknown and that of the FeS subunit remains a matter of debate, both the inner (cytoplasmic) and outer (periplasmic) surfaces of the intracytoplasmic membrane (ICM) were digested with proteinase K, and cleavage products were identified by immunoblotting. In uniformly oriented chromatophore vesicles (inner ICM surface exposed), fragments of approx. 4 and 1 kDa were removed from subunit IV and the FeS protein respectively. Neither subunit IV nor the FeS protein was cleaved from the outer ICM surface as exposed in osmotically protected spheroplasts or as presented to proteinase K after microencapsulation of the protease in unilamellar liposomes and fusion of these structures to chromatophore vesicles. Studies with the isolated bc1 complex, however, suggested that the C-terminal domain of the Rieske FeS, thought to reside on the periplasmic side of the ICM, was resistant to proteinase K. Overall, these results suggest a single N-terminal transmembrane helix for the FeS protein, with exposure of the N-terminus to the cytoplasm and an orientation in which a major, N-terminal portion of subunit IV is located in the cytoplasm with the predicted C-terminal transmembrane domain anchoring this polypeptide to the membrane.

Spheroplast-derived membrane vesicles from Rhodobacter capsulatus cells catalyzing nucleotide transport

Rodobacter capsulatus cells, which were cultured anaerobically in high light intensity, had fewer foldings in the cytoplasmic membrane than those which were grown in lower light intensities. Spheroplast-derived membrane fractions obtained from cells cultured under high light intensity contained a high yield of large right-side-out membrane vesicles. The right-side-out vesicles catalyzed reversible light-induced proton efflux as did intact cells. Nucleotide transport activity was also catalyzed by these membrane vesicles. This activity was indirectly monitored by measurement of photophosphorylation or hydrolysis of externally added diphospho- and triphosphonucleosides. These enzymatic activities occur inside the cytoplasmic membrane of spheroplasts and membrane vesicles and therefore require the transport of the externally added reagents. The indirect measurements of transport were complemented by the demonstration of direct uptake of radiolabeled nucleotides into the membrane vesicles. These data support the suggestion that a nucleotide transporter located in the cytoplasmic membrane of R. capsulatus bacteria mediates these activities.

Use of the membrane-impermeable guanidinating reagent 2-S-[14C]thiuroniumethanesulfonate to demonstrate the orientation of light-harvesting proteins in Rhodobacter sphaeroides

The radiolabeled guanidinating reagent 2-S-[14C]thiuroniumethanesulfonate reacts with the epsilon-amino groups of accessible lysyl residues of membrane proteins under relatively mild labeling conditions, yielding labeled homoarginyl residues. Model studies have shown that the resulting homoarginyl residues do act as new cleavage sites for trypsin, but only at a very slow rate of hydrolysis. The reagent has been shown to be impermeable to the intracytoplasmic membranes of Rhodobacter sphaeroides: when cytoplasmic-side-out chromatophores were treated with the reagent, it reacted with all four of the light-harvesting proteins, all of which have one or more lysyl residues on the N-terminal sides of their hydrophobic regions. However, when periplasmic-side-out vesicles, prepared by cytochrome c affinity chromatography, were treated with the guanidinating reagent, three of the light-harvesting proteins (B850 alpha, B850 beta, and B870 beta) were not labeled. The only light-harvesting protein to be labeled (B870 alpha) was the only one of the four to have a lysyl residue on the C-terminal side of its hydrophobic region. Guanidinated B870 alpha polypeptides from both the cytoplasmic-side-out chromatophores and the periplasmic-side-out membrane vesicles were purified and digested with trypsin. The resulting peptide fragments were then separated by high-performance liquid chromatography and analyzed for radioactivity. The results have confirmed the asymmetric orientation of the light-harvesting proteins of R. sphaeroides, with their N-termini on the cytoplasmic side of the intracytoplasmic membrane. In the case of the B870 alpha subunit, the protein has been shown to be transmembrane with its C-terminus on the periplasmic side of the membrane.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of reaction center and B800-850 antenna pigment proteins in membranes of Rhodobacter sphaeroides

The localization of the N- and C-terminal regions of pigment-binding polypeptides of the bacterial photosynthetic apparatus of Rhodobacter sphaeroides was investigated by proteinase K treatment of chromatophore and spheroplast-derived vesicles and amino acid sequence determination. Under conditions of proteinase K treatment of chromatophores, which left the in vivo absorption spectrum and the membrane intact, 15 and 46 amino acyl residues from the N-terminal regions of the L and M subunits, respectively, of the reaction center polypeptides were removed. The N termini are therefore exposed on the cytoplasmic surface of the membrane. The C-terminal domain of the light-harvesting B800-850 alpha and B870 alpha polypeptides was found to be exposed on the periplasmic surface of the membrane. A total of 9 and 13 amino acyl residues were cleaved from the B800-850 alpha and B870 alpha polypeptides, respectively, when spheroplasts were treated with proteinase K. The N-terminal regions of the alpha polypeptides were not digested in either membrane preparation and were apparently protected from proteolytic attack. Seven N-terminal amino acyl residues of the B800-850 beta polypeptide were removed after the digestion of chromatophores. C-terminal residues were not removed after the digestion of chromatophores or spheroplasts. The C termini seem to be protected from protease attack by interaction with the membrane. Therefore, the N-terminal regions of the beta polypeptides are exposed on the cytoplasmic membrane surface. The C termini of the beta polypeptides are believed to point to the periplasmic space.

Transverse membrane topography of the B875 light-harvesting polypeptides of wild-type Rhodobacter sphaeroides

Purified B875 light-harvesting complex, chromatophores, and spheroplast-derived vesicles from wild-type Rhodobacter sphaeroides were treated with proteinase K or trypsin, and the alpha and beta polypeptides were analyzed by electrophoretic, immunochemical, and protein-sequencing methods. With the purified complex, proteinase K digested both polypeptides and completely eliminated the A875 peak. Trypsin digested the alpha polypeptide and reduced the A875 by 50%. Proteinase K cleaved the beta polypeptide of chromatophores and the alpha polypeptide of spheroplast-derived vesicles. Sequence analyses of polypeptides extracted from proteinase K-treated chromatophores revealed that the beta polypeptide was cleaved between amino acids 4 and 5 from the N terminus. The N terminus of the alpha polypeptide was intact. We concluded that the N terminus of the beta polypeptide is exposed on the cytoplasmic membrane surface, and the difference in the digestion patterns between the spheroplast-derived vesicles and chromatophores suggested that the C terminus of the alpha polypeptide is exposed on the periplasmic surface.

Kinetics of the c-cytochromes in chromatophores from Rhodopseudomonas sphaeroides as a function of the concentration of cytochrome c2. Influence of this concentration on the oscillation of the secondary acceptor of the reaction centers QB

The oxidation kinetics of Cyt c1 and c2 have been measured in normal chromatophores and in chromatophores fused with liposomes in order to increase the internal volume. The kinetics of Cyt c1 oxidation were found to be dependent on Cyt c2 concentration. The initial rate of Cyt c1 oxidation decreased after fusion by a factor of about two, indicating a process dependent on diffusion. The results do not allow a clear distinction between a diffusion of Cyt c2 along the inner membrane surface or through the inner volume of the vesicle; two- and three-dimensional models are discussed. In contrast to Cyt c1, the kinetics of oxidation of Cyt c2 were not influenced by changes in concentration. It is concluded that reduced Cyt c2 is preferentially bound to the reaction centers. A binary pattern as a function of flash number from the dark-adapted state was measured in the turn-over of the two-electron gate of the reaction center. In chromatophores with more than 0.5 cytochrome c2 molecules per reaction center, this binary pattern titrated out with a midpoint around 340 mV on reduction of the suspension. In experiments with chromatophores with a low Cyt c2 content, or with spheroplast-derived vesicles which had lost Cyt c2, the binary oscillation in the two-electron gate could be observed at much lower potentials. The results suggest that the binding of reduced cytochrome c2 modifies the behavior of the two-electron gate. A model in which reaction center dimers are stabilized by Cyt c2 is proposed to explain the effect.

Reduction of cytochrome b-561 through the antimycin-sensitive site of the ubiquinol-cytochrome c2 oxidoreductase complex of Rhodopseudomonas sphaeroides

Cytochrome b-561 of the ubiquinol-cytochrome c2 oxidoreductase complex of Rhodopseudomonas sphaeroides is reduced after flash illumination in the presence of myxothiazol in an antimycin-sensitive reaction. Flash-induced reduction was observed over the redox range in which cytochrome b-561 and the Q-pool are both oxidized before the flash. The extent of reduction increased with increasing pH, and was maximal at pH greater than 10.0 where the extent approached that observed in the presence of antimycin following a group of flashes. Reduction of cytochrome b-561 in the presence of myxothiazol showed a lag of approximately 1 ms after the flash, followed by reduction with t 1/2 approximately 6 ms; by analogy with the similar kinetics of the quinol oxidase site, we suggest that the rate is determined by collision with the QH2 produced in the pool on flash excitation.

Cell-cycle-specific biosynthesis of the photosynthetic membrane of Rhodopseudomonas sphaeroides. Structural implications

Structural changes association with the intracytoplasmic membrane during the cell cycle of the photosynthetic bacterium Rhodopseudomonas sphaeroides have been studied by freeze-fracture electron microscopy. The isolated intracytoplasmic membrane vesicles, chromatophores, were fused in order to obtain large fracture faces, allowing more precise measurements and statistical analysis of both intramembrane particle density and size determinations. The intramembrane particle density of the protoplasmic face (PF) of the intracytoplasmic membrane, (from 4970 to 8290/micrometers 2), was shown to be a linear function of the protein/phospholipid ratio (from 2.5 to 5.1, w/w) of the intracytoplasmic membrane. Under constant light intensity, both the average particle size and particle size distribution remained unchanged during the cell cycle. These results provide the structural basis for the earlier reported cell-cycle-specific variations in both protein/phospholipid ratio and alternation in phospholipid structure of the intracytoplasmic membrane of R. sphaeroides during photosynthetic growth. The average particle diameter in the PF face of the intracytoplasmic membrane was 8.25, 9.08 and 9.75 nm at incident light intensities of 4000, 500 and 30 ft X cd, respectively. When chromatophores were fused with small, unilamellar liposomes, the intramembrane particle density decreased as input liposome phospholipid increased, whereas the particle size remained constant and particle distribution became random.

Topography of reaction center subunits in the membrane of the photosynthetic bacterium, rhodopseudomonas sphaeroides

The localization of the reaction center polypeptides (L, M, and H) in the membranes of both the wild-type, strain 2.4.1, and the carotenoidless mutant, R-26, of Rhodopseudomonas sphaeroides was determined by using affinity-purified antibodies specific for these proteins. Binding of the antibodies to reaction center subunits in spheroplasts was visualized in the electron microscope by immunoferritin labeling. The H and M subunits were labeled at both the cytoplasmic and the periplasmic surfaces of the membrane, whereas the L subunit was labeled only at the periplasmic surface of the membrane. Thus, the reaction center is asymmetrically oriented in the membrane with at least two subunits (H and M) spanning the membrane.

Phospholipid topography of the photosynthetic membrane of Rhodopseudomonas sphaeroides

The topography of phospholipids in the photosynthetic membranes of Rhodopseudomonas sphaeroides was investigated by using purified chromatophores and spheroplast-derived vesicles (SDVs). Chromatophores are closed vesicles oriented inside out with respect to the cytoplasmic membrane (cytoplasmic side out) and obtained from French-pressed cell lysates. SDVs are oriented right side out (periplasmic side out) and are obtained after osmotic lysis of lysozyme-treated cells. Phosphatidylethanolamine (PE) comprised approximately 62% and phosphatidylglycerol (PG) comprised approximately 33% of the total phospholipid of both vesicle preparations. The relatively membrane impermeable reagent trinitrobenzenesulfonate (TNBS) at 3 mM concentration and 5 degrees C modified chromatophore and SDV PE with kinetics indicating the occurrence of fast- and slow-reacting pools of PE. The fast-reacting pools comprised 33% and 55% of the total PE of chromatophores and SDVs, respectively. The slow-reacting pools comprised 61% and 32% of the total PE of chromatophores and SDVs, respectively. Phospholipase A2 treatment of chromatophores (1 unit/mg of vesicle protein) for 1 h at 37 degrees C resulted in hydrolysis of 73% and 77% of the total PG and PE, respectively. Similar enzyme treatment of SDVs resulted in 14% and 60% hydrolysis of the total PG and PE, respectively. Phospholipase A2 treatment inhibited 60% of the succinate dehydrogenase activity of chromatophores but only 8% of the activity of SDVs, indicating the membrane impermeability of phospholipase A2. Incubation of chromatophores for 10 min with 3 mM TNBS at 5 degrees C and then treatment with phospholipase A2 for 10 min and 1 h resulted in the hydrolysis of 10% and 61%, respectively, of unmodified PE. The results indicate asymmetric distributions of PE polar head groups (32-33% cytoplasmic side, 55-61% periplasmic side) and PG (73% cytoplasmic side, 14% periplasmic side) across the membrane. Also, a rapid and unidirectional transbilayer movement of PE polar head groups from the periplasmic to cytoplasmic surfaces of the membrane appears to occur during phospholipase A2 hydrolysis on the chromatophore surfaces.

Penicillin-binding proteins of Rhodopseudomonas sphaeroides and their membrane localization

Cytoplasmic membranes (CM) prepared from both chemotrophic and phototrophic cells of Rhodopseudomonas sphaeroides possess penicillin-binding proteins (PBPs), as demonstrated by binding of [125]furazlocillin to isolated membranes, the subsequent separation of the constituent PBPs by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and their detection by autoradiography. The major PBP present in CM from R. sphaeroides corresponds in molecular weight to PBP-5, the predominant PBP present in CM of Escherichia coli. In contrast, the outer membrane of R. sphaeroides shows only low-level furazlocillin-binding activity on a per milligram of protein basis compared with chemotrophic CM. The intracytoplasmic membrane (ICM) derived from phototrophic cells contains less than 5% of the furazlocillin-binding activity of the CM. Based on the specific localization of PBPs in the CM, it is possible to provide quantitative estimates of the extent of CM present in preparations of ICM. This method demonstrates that highly purified preparations of ICM contain less than 5% CM. Additionally, the assay for PBPs demonstrates that during ICM remodeling, which occurs upon a shift from phototrophic to chemotrophic growth, there is no significant insertion of PBPs into the ICM over the first two generations after a shift to chemotrophic growth.

In vivo intermembrane transfer of phospholipids in the photosynthetic bacterium Rhodopseudomonas sphaeroides

The kinetics of accumulation of phospholipids into the intracytoplasmic membrane of Rhodopseudomonas sphaeroides have been examined. We have previously demonstrated that accumulation of phospholipids in the intracytoplasmic membrane is discontinuous with respect to the cell cycle. In this study we demonstrated a sevenfold increase in the rate of phospholipid incorporation into the intracytoplasmic membrane concurrent with the onset of cell division. Pulse-chase labeling studies revealed that the increase in the rate of phospholipid accumulation into the intracytoplasmic membrane results from the transfer of phospholipid from a site other than the intracytoplasmic membrane, and that the transfer of phospholipid, rather than synthesis of phospholipid, is most likely subject to cell cycle-specific regulation. The rates of synthesis of the individual phospholipid species (phosphatidylethanolamine, phosphatidyglycerol, and an unknown phospholipid) remained constant with respect to one another throughout the cell cycle. Similarly, each of these phospholipid species appeared to be transferred simultaneously to the intracytoplasmic membrane. We also present preliminary kinetic evidence which suggested that phosphatidylethanolamine may be converted to phosphatidycholine within the intracytoplasmic membrane.

Assessment of Rhodopseudomonas sphaeroides chromatophore membrane asymmetry through bilateral antiserum adsorption studies

The asymmetric structure of the Rhodopseudomonas sphaeroides chromatophore membrane was examined in detail by crossed immunoelectrophoresis techniques. Because these methods are quantitative and allow increased resolution and sensitivity, it was possible to analyze simultaneously the relative transmembrane distribution of a number of previously identified antigenic components. This was demonstrated by analysis of immunoglobulin samples that were adsorbed by preincubation with either isolated chromatophores or osmotically protected spheroplasts. The photochemical reaction center, the light-harvesting bacteriochlorophyll a-protein complex, the L-lactate dehydrogenase, and reduced nicotinamide adenine dinucleotide dehydrogenase (EC 1.6.99.3) were found to be exposed on the chromatophore surface (cytoplasmic aspect of the membrane within the cell). Other antigenic components were found to be exposed on the surface of spheroplasts (periplasmic aspect of the in vivo chromatophore membrane). Antigens with determinants expressed on both sides of the chromatophore membrane were also identified. Charge shift crossed immunoelectrophoresis confirmed the suggested amphiphilic character of the pigment-protein complexes and identified several additional amphiphilic membrane components.