Structure of a bacterial photosynthetic membrane. Isolation, polypeptide composition, and selective proteolysis

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.

Nanodissection and high-resolution imaging of the Rhodopseudomonas viridis photosynthetic core complex in native membranes by AFM. Atomic force microscopy

In photosynthesis, highly organized multiprotein assemblies convert sunlight into biochemical energy with high efficiency. A challenge in structural biology is to analyze such supramolecular complexes in native membranes. Atomic force microscopy (AFM) with high lateral resolution, high signal-to-noise ratio, and the possibility to nanodissect biological samples is a unique tool to investigate multiprotein complexes at molecular resolution in situ. Here we present high-resolution AFM of the photosynthetic core complex in native Rhodopseudomonas viridis membranes. Topographs at 10-A lateral and approximately 1-A vertical resolution reveal a single reaction center (RC) surrounded by a closed ellipsoid of 16 light-harvesting (LH1) subunits. Nanodissection of the tetraheme cytochrome (4Hcyt) subunit from the RC allows demonstration that the L and M subunits exhibit an asymmetric topography intimately associated to the LH1 subunits located at the short ellipsis axis. This architecture implies a distance distribution between the antenna and the RC compared with a centered location of the RC within a circular LH1, which may influence the energy transfer within the core complex. The LH1 subunits rearrange into a circle after removal of the RC from the core complex.

Redox properties of an H-subunit-depleted photosynthetic reaction center from Rhodopseudomonas viridis

Recently, we reported that a H-subunit-depleted photosynthetic reaction center (RC-H) was purified from purple nonsulfer photosynthetic bacterium Rhodopseudomonas viridis (Rps. viridis) using a strong detergent sodium alkyl ether sulfate. We compared the redox properties of a native photosynthetic reaction center (RC) and RC-H of Rps. viridis. In RC-H prepared by our method, secondary quinone (QB) was removed while primary quinone (QA) was retained. Absorption spectrum of RC-H was similar to that of RC. After reconstitution of ubiquinone 10 into QB sites, RC-H showed electron transfer activity that was the same as that for native RC. This is the first report about the redox properties of RC-H of Rps. viridis.

Molecular handling of photosynthetic proteins for molecular assembly construction

Methods of constructing proteins were examined with special reference to the molecular assembly using photosynthetic RCs as membrane proteins. Molecular assemblies at the interfaces were studied by LB films, adsorption to the surface and reconstitution into liposomes and bilayer lipid membranes. The applications of biological specific ligands (recognition and binding), combinatorial chemical method, 2-D and 3-D order array assemblies and modification of protein molecules to make fusion proteins, as well as physical methods of manipulation of molecules by AFM tips and electric fields were reviewed.

Langmuir-Blodgett monolayer films of bacterial photosynthetic membranes and isolated reaction centers: preparation, spectrophotometric and electrochemical characterization

The Langmuir-Blodgett (LB) film technique has been successfully applied to the construction of stable and photo-active films of chromatophore membranes and isolated reaction centers from two species of photosynthetic bacteria, Rhodobacter sphaeroides and Rhodopseudomonas viridis. LB films of these preparations were characterized at the air/water interface through compression isotherms and film stabilities. Films deposited on glass slides were analyzed by spectrophotometric and redox potentiometric techniques. The results obtained indicate that the in vivo properties of the photosynthetic apparatus in the deposited films are essentially unchanged. Furthermore, the pigments and redox cofactors in the films are highly oriented and offer a unique opportunity for structural and functional studies of the kind described in the accompanying paper (Biochim. Biophys. Acta 1057 (1991) 258-272).

The three-dimensional structure of the Na,K-ATPase from electron microscopy

The structure of Na,K-ATPase has been studied by electron microscopy and image reconstruction. A three-dimensional structure of this enzyme has been obtained to an overall resolution of 2.5 nm using data from specimens of negatively stained dimer sheets tilted through a range of angles +/- 60 degrees. The reconstruction shows a complex mass distribution consisting of ribbons of paired molecules extending approximately 6.0 nm from the cytoplasmic side of the membrane. The molecular envelope consists of a massive "body" with "lobe" and "arm" structures projecting from it. The body has a columnar shape and is tilted with respect to the plane of the membrane. The region of interaction responsible for dimer formation is located between two bodies and is clearly visible in the reconstruction. It has been identified as a segment in the amino-terminal portion of the alpha subunit. The arms that interconnect the ribbons are located close to the membrane and are most probably formed by the beta subunits.

The Rhodopseudomonas viridis photosynthetic membrane: arrangement in situ

The organization of photosynthetic membranes in the cytoplasm of the photosynthetic bacterium Rh. viridis has been examined by several techniques for electron microscopy. Thin sections of membrane stacks show that the regular lattice of membrane subunits reported in other studies can be observed in thin section. Tilting of sections in the electron microscope shows that the regular lattices of several membranes overlap in a way that suggests they are in register with each other. This observation can be confirmed by freeze-fracture images in which a regular arrangement of membrane lattices can be observed, each perfectly aligned. Analysis of the spacings of membrane pairs shows that the photosynthetic membranes of Rh. viridis are very closely apposed. The mean diameter of two membranes is 160A, and the average space between two such membranes is only 42A. When a recently developed atomic level model of Rh. viridis reaction center is superimposed against these spacings, each reaction center extends from the surface of its respective membrane far enough to make contact with an apposing membrane. The limited free space between membranes and regular alignment of lattices has a number of implications for how this membrane is organized to carry out the process of energy transfer.

Pigment-protein complexes from Rhodopseudomonas palustris: isolation, characterization, and reconstitution into liposomes

We have employed detergent solubilization and sucrose density gradient centrifugation to obtain pigment-protein complexes from Rhodopseudomonas palustris. Two types of detergent buffers were used, containing either octyl-beta-glucopyranoside (OG) plus sodium dodecyl sulfate (SDS) or OG alone. The fractions thus obtained were analyzed spectrophotometrically and by polyacrylamide gel electrophoresis to determine their pigment and protein composition. OG-SDS solubilization yields four fractions. The least dense of these fractions (OG-SDS a and b) are nonspecific mixtures of peptides and pigments. The next fraction, OG-SDS c, is an accessory light-harvesting complex, LHII, called B800-850. The largest particle, OG-SDS d, is a combination of reaction center (RC) and primary light-harvesting complex (LHI), B880. Solubilization using OG alone yields one fraction, a single large complex consisting of RC, LHI, and LHII. We have inserted the two large OG-SDS complexes and the OG complex into phospholipid liposomes to determine the size of such complexes in freeze-fractured membranes. On the basis of morphological, biochemical, and available biophysical data, we propose the following models for pigment-protein complexes in R. palustris membranes: 5-nm particles as free RC or LHI tetramers, 7.5-nm particles as LHI or LHII octamers (or both); 10-nm particles as RC-LHI core complexes (1 RC plus 12 LHI) or large LHII oligomers (or both), and large particles of 12.5 and 15 nm and LHII associated with the RC-LHI core complex.

Oxido-reduction of B800-850 and B880 holochromes isolated from three species of photosynthetic bacteria as studied by electron-paramagnetic resonance and optical spectroscopy

Certain redox properties of bacteriochlorophyll alpha were used to probe the structure of several light-harvesting pigment-protein complexes or holochromes. To attribute redox properties unequivocally to a given holochrome, we worked with purified holochromes. We developed purification procedures for the B880 holochromes from Rhodospirillum rubrum, Rhodopseudomonas sphaeroides and Ectothiorhodospira sp. and for the B800-850 holochromes from the latter two species. In all these holochromes, bacteriochlorophyll alpha could be oxidized by ferricyanide as witnessed by the bleaching of their near-infrared absorption bands. However, only in B880 holochromes was this oxidation reversible. Another important difference between the B800-850 and the B880 holochromes is that oxidation of the latter gives rise to a g = 2.0025 electron paramagnetic resonance (EPR) signal with linewidth varying, according to species, from 0.37 mT to 0.48 mT. Both the reversible EPR signal and absorption changes titrate with a midpoint redox potential (pH 8.0) of approximately 570 mV. Linewidth narrowing can be interpreted by delocalization of the free electron spin over approximately 12 bacteriochlorophyll molecules. While the B880 holochromes from the three species considered had indistinguishable redox properties, the B800-850 holochromes differed from one another by their circular dichroic spectra and by the relative ease of oxidation of their 800-nm and 850-nm bands. This indicates that, contrary to the B880 holochromes, the B800-850 holochromes may not form a homogeneous class.

Structure of a bacterial photosynthetic membrane: integrity of reaction centers following proteolysis and detergent solubilization

The photosynthetic membranes of the purple bacterium Rhodopseudomonas viridis are composed of a semi-crystalline lattice of subunits. Proteolysis of isolated membranes with trypsin or pronase results in the degradation of polypeptides associated with the photosynthetic reaction center. However, two low molecular weight peptides which may form the light-harvesting complex survive the enzymatic treatment. The proteolysis does not affect the major absorbance peak (830 nm) associated with the reaction center. However, treatment of proteolyzed membranes with detergents such as LDAO abolishes the 830 nm absorbance peak. The 830 nm peak is stable following LDAO solubilization of non-proteolyzed membranes. These results suggest that a combination of covalent and non-covalent interactions are important in maintaining the configuration of the reaction center, and are consistent with a model of membrane organization in which the light-harvesting components are buried in a lipid phase of the membrane and reaction center components form the large structures which electron microscope studies have shown to extend from either membrane surface.

Two-dimensional crystals formed from photosynthetic reaction centers

Photosynthetic reaction centers from the bacterium Rhodopseudomonas viridis were prepared after detergent solubilization of photosynthetic membranes. The purified reaction centers, in agreement with reports from other laboratories, contain four distinct polypeptides ranging in molecular weight from 28,000 to 41,000. When the detergent was gradually removed by dialysis under appropriate conditions, large two-dimensional sheets of reaction centers were formed, suitable for analysis by electron microscopy. The crystals were rectangular, and the dimensions of a single unit cell were 121 X 129 A. Each unit cell contained four distinct subunits, each with approximate dimensions of 45 X 60 A. The thickness of the sheet was 60 A. Preliminary studies of the sheets with negative staining indicated that the sheets show a high degree of order: as many as six orders are visible in transforms of the images. Because of the fact that in R. viridis the native membrane from which these reaction centers were purified also displays a crystal-like structure, comparative studies between a membrane and one of its components, each analyzed by Fourier techniques, are now possible.