Photosystem II 3-D structure and the role of the extrinsic subunits in photosynthetic oxygen evolution

Assembly of monomeric human cytomegalovirus pUL104 into portal structures

In order for human cytomegalovirus (HCMV) to replicate, concatemeric DNA has to be cleaved into unit-length genomes and packaged into preformed capsids. For packaging to take place and DNA to be translocated, a channel is required in the capsid. Viral capsid channels are generally formed by portal proteins. Here, we show by cross-linking, native gel electrophoresis of infected cells and gel permeation chromatography that the HCMV portal candidate protein pUL104 can form dimers and higher order multimers. Electron microscopy of purified monomeric pUL104 after 5 min incubation revealed that the protein had assembled into a multimeric form and that this form closely resembles complete portal assembly. This is the first study to show that pUL104 monomers have the ability to form portal complexes without additional viral proteins.

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.

Segregation of the photosystems in thylakoids depends on their size

Lateral segregation of two types of photosystems in thylakoid membranes of green plants is one of the key factors that provide the stability and fine-tuning of the light quanta supply by pigment proteins and non-cyclic electron transport. Due to this specific feature of the membrane structural organization, the photosynthetic units function in the green plants with optimal performance. In this report a mesoscopic theory is outlined to address the physical aspects of segregation phenomenon. Results of theoretical studies and computer simulations suggest that charge mismatch and the size difference between two photosystems in grana are most responsible for their lateral segregation, which is driven by the screened electrostatic and lipid-induced interactions. Comparative simulations of photosystems of different sizes show the crucial dependence of their ordering on a geometrical parameter. It seems that the size effect alone may prevent photosystems from segregated arrangement in cyanobacterial thylakoids.

Dynamic Interaction between the D1 protein, CP43 and OEC33 at the lumenal side of photosystem II in spinach chloroplasts: evidence from light-induced cross-Linking of the proteins in the donor-side photoinhibition

During the donor-side photoinhibition of spinach photosystem II, the reaction center D1 protein cross-linked with the antenna chlorophyll binding protein CP43 of photosystem II lacking the oxygen-evolving complex (OEC) subunit proteins. The cross-linking did not occur upon illumination of photosystem II samples that retained the OEC33, nor when OEC33-depleted photosystem II samples were reconstituted with the OEC33 prior to illumination. These results suggest that the D1 protein, CP43 and the OEC33 are located in close proximity at the lumenal side of photosystem II, and that the OEC33 suppresses the unnecessary contact between the D1 protein and CP43. Previously we presented data showing the D1 protein located adjacent to CP43 on the stromal side of photosystem II [Ishikawa et al. (1999) BIOCHIM: Biophys. Acta 1413: 147]. The present data suggest that the spatial arrangement of the D1 protein and CP43 at the lumenal side of photosystem II in spinach chloroplasts is similar to that at the stromal side of photosystem II and is consistent with the assignment of these proteins recently proposed on the crystal structures of the photosystem II complexes from cyanobacteria [Zouni et al. (2001) Nature 409: 739, Kamiya and Shen 2003 PROC: Natl. Acad. Sci. USA, 100: 98]. Moreover, the data suggest that the binding condition and positioning of the OEC33 in the photosystem II complex from higher plants may be different from those in cyanobacteria.

An alternative model for photosystem II/light harvesting complex II in grana membranes based on cryo-electron microscopy studies

The photosynthetic protein complexes in plants are located in the chloroplast thylakoid membranes. These membranes have an ultrastructure that consists of tightly stacked 'grana' regions interconnected by unstacked membrane regions. The structure of isolated grana membranes has been studied here by cryo-electron microscopy. The data reveals an unusual arrangement of the photosynthetic protein complexes, staggered over two tightly stacked planes. Chaotrope treatment of the paired grana membranes has allowed the separation and isolation of two biochemically distinct membrane fractions. These data have led us to an alternative model of the ultrastructure of the grana where segregation exists within the grana itself. This arrangement would change the existing view of plant photosynthesis, and suggests potential links between cyanobacterial and plant photosystem II light harvesting systems.

The location of the mobile electron carrier ferredoxin in vascular plant photosystem I

In this study, we present the location of the ferredoxin-binding site in photosystem I from spinach. Image analysis of negatively stained two-dimensional crystals indicates that the addition of ferredoxin and chemical cross-linkers do not significantly alter the unit cell parameters (for untreated photosystem I, a = 26.4 nm, b = 27.6 nm, and gamma = 90 degrees, space group p22(1)2(1) and for ferredoxin cross-linked photosystem I, a = 26.2 nm, b = 27.2 nm, and gamma = 90 degrees, space group p22(1)2(1)). Fourier difference analysis reveals that ferredoxin is bound on top of the stromal ridge principally interacting with the extrinsic subunits PsaC and PsaE. This location would be accessible to the stroma, thereby promoting efficient electron transfer away from photosystem I. This observation is significantly different from that of the ferredoxin binding site proposed for cyanobacteria. A model for the binding of ferredoxin in vascular plants is proposed and is discussed relative to observations in cyanobacteria.

Structural analysis of photosystem II in far-red-light-adapted thylakoid membranes. New crystal forms provide evidence for a dynamic reorganization of light-harvesting antennae subunits

We studied two-dimensional crystals of the major pigment-protein complex, photosystem II, in far-red-light-adapted thylakoid membranes of the viridis-zb63 mutant of barley. Significantly larger grana membranes were produced with an increased synthesis of the entire photosystem II complex. These red-light-adapted membranes also contained two-dimensional crystals with a high frequency. Three different crystal forms of photosystem II were observed, providing the following data which further our understanding of the architecture of the native complex. (a) The oligomeric form of photosystem II in the membrane was monomeric in all crystal forms, but with a clear non-crystallographic pseudo-twofold symmetry. This was more apparent on the lumenal face of the complex. (b) The variability of unit cell contacts in different crystal forms implied that the peripheral light-harvesting antenna complex and the core of the complex were loosely connected. These peripheral subunits were predicted to rearrange so that they can either encircle the core complex or associate in parallel channels separated by lines of core complexes. (c) Grana membranes were found to retain a double-layered inside-out character, with a stromal face-to-stromal face packing. However, the presence of a crystal in one membrane did not necessarily impose crystallinity on its pair.

Investigation of the structure of spinach photosystem II reaction center complex

The photosystem II (PSII) reaction center (RC) complex was isolated from spinach and characterized by gel electrophoresis, gel filtration and analytical ultracentrifugation. The purified complex contained the PsbA, PsbD, PsbE, PsbF and PsbI subunits. Gel filtration and analytical ultracentrifugation indicated the presence of a homogeneous complex. The mass of the RC complexes was found to be 107 kDa by analytical ultracentrifugation and 132 kDa by scanning transmission electron microscopy (STEM). The mass obtained showed the isolated complex to exist as a monomer and only one cytochrome b559 (cyt b559) to be associated with the RC complex. Digital images of negatively stained RC complexes were recorded by STEM and analyzed by single-particle averaging. The complex was 9 nm long and 5 nm wide, and exhibited a pronounced quasi-twofold symmetry. This supports the symmetric organization of the PSII complex, with the PsbA and the PsbD proteins in the center and symmetrically arranged PsbB and PsbC proteins at the periphery of the monomeric complex.

Secondary structure and thermal stability of the extrinsic 23 kDa protein of photosystem II studied by Fourier transform infrared spectroscopy

The secondary structure and thermal stability of the extrinsic 23 kDa protein (OEC23) of spinach photosystem II have been characterized in solution between 25 and 75 degrees C using Fourier transform infrared spectroscopy. Quantitative analysis of the amide I band (1700-1600 cm(-1)) shows that OEC23 contains 5% alpha-helix, 37% beta-sheet, 24% turn, and 34% disorder structures at 25 degrees C. No appreciable conformational changes occur below 45 degrees C. At elevated temperatures, the beta-sheet structure is unfolded into the disorder structure with a major conformational transition occurring at 55 degrees C. Implications of these results for the functions of OEC23 in photosystem II are discussed.

Structural Analysis of Photosystem II: Comparative Study of Cyanobacterial and Higher Plant Photosystem II Complexes

Oxygen evolving photosystem II (PSII-OEC) complexes and PSII core complexes were isolated from spinach and the thermophilic cyanobacterium Synechococcus sp. OD24 and characterized by gel electrophoresis, immunoblotting, and absorbance spectroscopy. The mass of the core complexes was determined by scanning transmission electron microscopy (STEM) and found to be 281 ± 65 kDa for spinach and 313 ± 52 kDa for Synechococcus sp. OD24. The mass of the spinach PSII-OEC complex was 327 ± 64 kDa. Digital images of negatively stained PSII-OEC and PSII core complexes were recorded by STEM and analyzed by single particle averaging. All monomeric complexes showed similar morphologies and were of comparable length (14 nm) and width (10 nm). The averages revealed a pseudo-twofold symmetry axis, which is a prominent structural element of the monomeric form. Difference maps between the averaged projections of the oxygen evolving complexes and the core complexes from both species indicated where the 33-kDa extrinsic manganese stabilizing protein is bound. A symmetric organization of the PSII complex, with the PsbA and the PsbD proteins in the center and symmetrically arranged PsbB and PsbC proteins at the periphery of the monomeric complex, is proposed.

Electron and atomic force microscopy of membrane proteins

Electron crystallography is becoming a powerful tool for the resolution of membrane protein structures. The past year has seen the production of a bacteriorhodopsin model at 3.5 A and the structure of aquaporin 1 approaching atomic resolution. Determination of surface topographies of 2D crystals using the atomic force microscope is similarly advancing to a level that reveals submolecular details. As the latter is operated in solution, membrane proteins can be observed at work.

The three-dimensional structure of a photosystem II core complex determined by electron crystallography

BACKGROUND

Photosystem II (PSII) is a multisubunit protein complex which is embedded in the photosynthetic membranes of plants. It uses light energy to split water into molecular oxygen and reducing equivalents. PSII can be isolated with varying degrees of complexity in terms of its subunit composition and activity. To date, no three-dimensional (3-D) structure of the PSII complex has been determined which allows location of the proteins within the PSII complex and their orientation in relation to the thylakoid membrane.

RESULTS

Two-dimensional (2-D) PSII core complex crystals composed of the two reaction centre proteins, D1 and D2, two chlorophyll-binding proteins, CP47 and CP43, cytb559 and associated low molecular weight proteins were formed after reconstituting the isolated complex into purified thylakoid lipids. Electron micrographs of negatively stained crystals were used for 2-D and 3-D image analyses. In the resulting maps, the PSII complex is composed of two halves related by twofold rotational symmetry, thus, confirming the dimeric nature of the complex; each monomer appears to contain five domains. Comparison of the 3-D images with platinum shadowed images of the crystals allowed the likely lumenal and stromal surfaces of the complex to be identified and regions contained within the membrane to be inferred. The projection structure of 2-D crystals of a smaller CP47-D1-D2-cytb559 complex was used to identify the domains apparently associated with CP43.

CONCLUSION

The results indicate that PSII probably exists as a dimer in vivo. The extensive proteinaceous protrusions from the lumenal surface have been tentatively assigned to hydrophilic loops of CP47 and CP43; the positioning of these loops possibly implies their involvement in the water-splitting process.

STRUCTURE AND MEMBRANE ORGANIZATION OF PHOTOSYSTEM II IN GREEN PLANTS

Photosystem II (PSII) is the pigment protein complex embedded in the thylakoid membrane of higher plants, algae, and cyanobacteria that uses solar energy to drive the photosynthetic water-splitting reaction. This chapter reviews the primary, secondary, tertiary, and quaternary structures of PSII as well as the function of its constituent subunits. The understanding of in vivo organization of PSII is based in part on freeze-etched and freeze-fracture images of thylakoid membranes. These images show a resolution of about 40-50 A and so provide information mainly on the localization, heterogeneity, dimensions, and shapes of membrane-embedded PSII complexes. Higher resolution of about 15-40 A has been obtained from single particle images of isolated PSII complexes of defined and differing subunit composition and from electron crystallography of 2-D crystals. Observations are discussed in terms of the oligomeric state and subunit organization of PSII and its antenna components.

Structure of the multidrug resistance P-glycoprotein to 2.5 nm resolution determined by electron microscopy and image analysis

P-glycoprotein (P-gp) is a member of the ATP binding cassette superfamily of active transporters and can confer multidrug resistance on cells and tumors by pumping chemotherapeutic drugs from the cytoplasm. P-gp was purified from CHrB30 cells and retained the ability to bind substrates and hydrolyze ATP. Labeling of P-gp with lectin-gold particles suggested it is monomeric. An initial structure of purified P-gp was determined to 2.5 nm resolution by electron microscopy and single particle image analysis of both detergent-solubilized and lipid-reconstituted protein. The structure was further refined by three dimensional reconstructions from single particle images and by Fourier projection maps of small two-dimensional crystalline arrays (unit cell parameters: a, 14.2 nm; b, 18.5 nm; and gamma, 91.6 degrees ). When viewed from above the membrane plane the protein is toroidal, with 6-fold symmetry and a diameter of about 10 nm. There is a large central pore of about 5 nm in diameter, which is closed at the inner (cytoplasmic) face of the membrane, forming an aqueous chamber within the membrane. An opening from this chamber to the lipid phase is present. The projection of the protein perpendicular to the membrane is roughly rectangular with a maximum depth of 8 nm and two 3-nm lobes exposed at the cytoplasmic face of the membrane, likely to correspond to the nucleotide binding domains. This study provides the first experimental insight into the three-dimensional architecture of any ATP binding cassette transporter.

Isolation and biochemical characterisation of monomeric and dimeric photosystem II complexes from spinach and their relevance to the organisation of photosystem II in vivo

Membranes enriched in photosystem II were isolated from spinach and further solubilised using n-octyl beta-D-glucopyranoside (OctGlc) and n-dodecyl beta-D-maltoside (DodGlc2). The OctGlc preparation had high rates of oxygen evolution and when subjected to size-exclusion HPLC and sucrose density gradient centrifugation, in the presence of DodGlc2, separated into dimeric (430 kDa), monomeric (236 kDa) photosystem II cores and a fraction containing photosystem II light-harvesting complex (Lhcb) proteins. The dimeric core fraction was more stable, contained higher levels of chlorophyll, beta-carotene and plastoquinone per photosystem II reaction centre and had a higher oxygen-evolving activity than the monomeric cores. Their subunit composition was similar (CP43, CP47, D1, D2, cytochrome b 559 and several lower-molecular-mass components) except that the level of 33-kDa extrinsic protein was lower in the monomeric fraction. Direct solubilisation of photosystem-II-enriched membranes with DodGlc2, followed by sucrose density gradient centrifugation, yielded a super complex (700 kDa) containing the dimeric form of the photosystem II core and Lhcb proteins: Lhcb1, Lhcb2, Lhcb4 (CP29), and Lhcb5 (CP26). Like the dimeric and monomeric photosystem II core complexes, the photosystem II-LHCII complex had lost the 23-kDa and 17-kDa extrinsic proteins, but maintained the 33-kDa protein and the ability to evolve oxygen. It is suggested, with a proposed model, that the isolated photosystem II-LHCII super complex represents an in vivo organisation that can sometimes form a lattice in granal membranes of the type detected by freeze-etch electron microscopy [Seibert, M., DeWit, M. & Staehelin, L. A. (1987) J. Cell Biol. 105, 2257-2265].

Progress towards structural elucidation of Photosystem II

In recent years Photosystem II, and in particular the oxygen evolving component of the enzyme, have been the subject of intense biochemical and biophysical analysis. To date no high resolution structural model of the complex has been produced. As a consequence unambiguous interpretation of much experimental data has proven difficult, leading to a lack of consensus over many basic questions regarding the mechanisms involved, the oligomerization state of the enzyme in vivo and even the exact biochemical composition.This review is a summary of the progress towards the production of a structural model of PS II-derived from either X-ray crystallography or electron microscopy based techniques-and the current opinions, which have arisen from these structural analyses, on the structural topology and assemblage of the various subunits that constitute the complex.

A current assessment of photosystem II structure

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