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

Electronic Structure of Tyrosyl D Radical of Photosystem II, as Revealed by 2D-Hyperfine Sublevel Correlation Spectroscopy

The biological water oxidation takes place in Photosystem II (PSII), a multi-subunit protein located in thylakoid membranes of higher plant chloroplasts and cyanobacteria. The catalytic site of PSII is a Mn4Ca cluster and is known as the oxygen evolving complex (OEC) of PSII. Two tyrosine residues D1-Tyr161 (YZ) and D2-Tyr160 (YD) are symmetrically placed in the two core subunits D1 and D2 and participate in proton coupled electron transfer reactions. YZ of PSII is near the OEC and mediates electron coupled proton transfer from Mn4Ca to the photooxidizable chlorophyll species P680+. YD does not directly interact with OEC, but is crucial for modulating the various S oxidation states of the OEC. In PSII from higher plants the environment of YD• radical has been extensively characterized only in spinach (Spinacia oleracea) Mn-depleted non functional PSII membranes. Here, we present a 2D-HYSCORE investigation in functional PSII of spinach to determine the electronic structure of YD• radical. The hyperfine couplings of the protons that interact with the YD• radical are determined and the relevant assignment is provided. A discussion on the similarities and differences between the present results and the results from studies performed in non functional PSII membranes from higher plants and PSII preparations from other organisms is given.

Molecular functions of PsbP and PsbQ proteins in the photosystem II supercomplex

The PsbP and PsbQ proteins are extrinsic subunits of the photosystem II (PSII) supercomplex, which are found in green plants including higher plants and green algae. These proteins are thought to have evolved from their cyanobacterial homologs; cyanoP and cyanoQ respectively. It has been suggested that the functions of PsbP and PsbQ have largely changed from those of cyanoP and cyanoQ. In addition, multiple isoforms and homologs of PsbP and PsbQ were found in green plants, indicating that the acquisition of PsbP and PsbQ in PSII is not a direct path but a result of intensive functional divergence during evolution from cyanobacterial endosymbiont to chloroplast. In this review, we highlight newly introduced topics related to the functions and structures of both PsbP and PsbQ proteins. The present data suggest that PsbP together with PsbQ have specific and important roles in coordinating the activity of the donor and acceptor sides of PSII and stabilizing the active form of the PSII-light-harvesting complex II (LHCII) supercomplex.

Probing tyrosine Z oxidation in Photosystem II core complex isolated from spinach by EPR at liquid helium temperatures

Tyrosine Z (Tyr(Z)) oxidation observed at liquid helium temperatures provides new insights into the structure and function of Tyr(Z) in active Photosystem II (PSII). However, it has not been reported in PSII core complex from higher plants. Here, we report Tyr(Z) oxidation in the S(1) and S(2) states in PSII core complex from spinach for the first time. Moreover, we identified a 500 G-wide symmetric EPR signal (peak position g = 2.18, trough position g = 1.85) together with the g = 2.03 signal induced by visible light at 10 K in the S(1) state in the PSII core complex. These two signals decay with a similar rate in the dark and both disappear in the presence of 6% methanol. We tentatively assign this new feature to the hyperfine structure of the S(1)Tyr(Z)(*) EPR signal. Furthermore, EPR signals of the S(2) state of the Mn-cluster, the oxidation of the non-heme iron, and the S(1)Tyr(Z)(*) in PSII core complexes and PSII-enriched membranes from spinach are compared, which clearly indicate that both the donor and acceptor sides of the reaction center are undisturbed after the removal of LHCII. These results suggest that the new spinach PSII core complex is suitable for the electron transfer study of PSII at cryogenic temperatures.

A comparative approach towards thylakoid membrane proteome analysis of unicellular green alga Scenedesmus obliquus

The chlorophyll (Chl)-containing membrane protein complexes from the green alga Scenedesmus obliquus have been isolated from the thylakoid membranes by solubilization with dodecyl-beta-maltoside and fractionation using a sucrose density gradient. The Chl-containing protein fractions were characterized by absorption spectroscopy, tricine SDS PAGE, BN-PAGE, and dynamic light scattering (DLS). BN-PAGE showed the presence of seven protein complexes with molecular weights in the range of 68, 118, 157, 320, 494, 828 and 955 kDa, respectively. Furthermore, light scattering reveals the simultaneous presence of particles of different sizes in the 3-4 nm and 6.0-7.5 nm range, respectively. The smaller size is related to the hydrodynamic radius of the trimer Light Harvesting Complex (LHCII), whereas the larger size is associated with the presence of photosystem I and photosystem II reaction centers. Additionally, functional information regarding protein-protein interactions was deconvoluted using coupling 2-D BN-PAGE, MALDI-TOF MS and a detailed mapping of S. obliquus photosynthetic proteome of the solubilized thylakoid membranes is therefore presented.

The 33 kDa Protein Can Be Removed without Affecting the Association of the 23 and 17 kDa Proteins with the Luminal Side of PS II of Spinach

An earlier study shows that a 30 min incubation of spinach PS II submembrane fragments at pH 6.3 in the presence of 10 microM HgCl(2) induces a 40% depletion of the 33 kDa protein without the apparent release of the 17 and 23 kDa proteins [Bernier, M., and Carpentier, R. (1995) FEBS Lett. 360, 251-254]. Here we report that the photosystem II 33 kDa extrinsic protein is fully removed by HgCl(2) added at micromolar and higher concentrations (0.25, 20, and 50 microM), with the 17 and 23 kDa extrinsic proteins and other intrinsic proteins remaining bound to the reaction center. The data presented here put in doubt the "regulatory cap" model of PS II, which follows the OEC-33 kDa-23 kDa-17 kDa binding order, as these results directly demonstrate that the 33 kDa protein can be removed without affecting the binding of the 23 and 17 kDa proteins to the intrinsic subunits of PS II. This suggests that each extrinsic protein may possess its own binding site on PS II. A possible mechanism for HgCl(2) upon the release of the 33 kDa protein is discussed.

Structure and electrophysiological properties of the YscC secretin from the type III secretion system of Yersinia enterocolitica

YscC is the integral outer membrane component of the type III protein secretion machinery of Yersinia enterocolitica and belongs to the family of secretins. This group of proteins forms stable ring-like oligomers in the outer membrane, which are thought to function as transport channels for macromolecules. The YscC oligomer was purified after solubilization from the membrane with a nonionic detergent. Sodium dodecyl sulfate did not dissociate the oligomer, but it caused a change in electrophoretic mobility and an increase in protease susceptibility, indicating partial denaturation of the subunits within the oligomer. The mass of the homo-oligomer, as determined by scanning transmission electron microscopy, was approximately 1 MDa. Analysis of the angular power spectrum from averaged top views of negatively stained YscC oligomers revealed a 13-fold angular order, suggesting that the oligomer consists of 13 subunits. Reconstituted in planar lipid bilayers, the YscC oligomer displayed a constant voltage-independent conductance of approximately 3 nS, thus forming a stable pore. However, in vivo, the expression of YscC did not lead to an increased permeability of the outer membrane. Electron microscopy revealed that the YscC oligomer is composed of three domains, two stacked rings attached to a conical domain. This structure is consistent with the notion that the secretin forms the upper part of the basal body of the needle structure of the type III secreton.

Structural studies of the manganese stabilizing subunit in photosystem II

Photosystem II (PSII) is the plant photosynthetic reaction center that carries out the light driven oxidation of water. The water splitting reactions are catalyzed at a tetranuclear manganese cluster. The manganese stabilizing protein (MSP) of PSII stabilizes the manganese cluster and accelerates the rate of oxygen evolution. MSP can be removed from PSII, with an accompanying decrease in activity. Either an Escherichia coli expressed version of MSP or native, plant MSP can be rebound to the PSII reaction center; MSP reconstitution reverses the deleterious effects associated with MSP removal. We have employed Fourier transform infrared (FTIR) spectroscopy and solution small angle x-ray scattering (SAXS) techniques to investigate the structure of MSP in solution and to define the structural changes that occur before and after reconstitution to PSII. FTIR and SAXS are complementary, because FTIR spectroscopy detects changes in MSP secondary structure and SAXS detects changes in MSP size/shape. From the SAXS data, we conclude that the size/shape and domain structure of MSP do not change when MSP binds to PSII. From FTIR data acquired before and after reconstitution, we conclude that the reconstitution-induced increase in beta-sheet content, which was previously reported, persists after MSP is removed from the PSII reaction center. However, the secondary structural change in MSP is metastable after removal from PSII, which indicates that this form of MSP is not the lowest energy conformation in solution.

Photosystem II: the solid structural era

Understanding the precise role of photosystem II as an element of oxygenic photosynthesis requires knowledge of the molecular structure of this membrane protein complex. The past few years have been particularly exciting because the structural era of the plant photosystem II has begun. Although the atomic structure has yet to be determined, the map obtained at 6 A resolution by electron crystallography allows assignment of the key reaction center subunits with their associated pigment molecules. In the following, we first review the structural details that have recently emerged and then discuss the primary and secondary photochemical reaction pathways. Finally, in an attempt to establish the evolutionary link between the oxygenic and the anoxygenic photosynthesis, a framework structure common to all photosynthetic reaction centers has been defined, and the implications have been described.

Design of functionalized lipids and evidence for their binding to photosystem II core complex by oxygen evolution measurements, atomic force microscopy, and scanning near-field optical microscopy

Photosystem II core complex (PSII CC) absorbs light energy and triggers a series of electron transfer reactions by oxidizing water while producing molecular oxygen. Synthetic lipids with different alkyl chains and spacer lengths bearing functionalized headgroups were specifically designed to bind the Q(B) site and to anchor this large photosynthetic complex (240 kDa) in order to attempt two-dimensional crystallization. Among the series of different compounds that have been tested, oxygen evolution measurements have shown that dichlorophenyl urea (DCPU) binds very efficiently to the Q(B) site of PSII CC, and therefore, that moiety has been linked covalently to the headgroup of synthetic lipids. The analysis of the monolayer behavior of these DCPU-lipids has allowed us to select ones bearing long spacers for the anchoring of PSII CC. Oxygen evolution measurements demonstrated that these long-spacer DCPU-lipids specifically bind to PSII CC and inhibit electron transfer. With the use of atomic force microscopy (AFM) and scanning near-field optical microscopy (SNOM), it was possible to visualize domains of PSII CC bound to DCPU-lipid monolayers. SNOM imaging has enabled us to confirm that domains observed by AFM were composed of PSII CC. Indeed, the SNOM topography images presented similar domains as those observed by AFM, but in addition, it allowed us to determine that these domains are fluorescent. Electron microscopy of these domains, however, has shown that the bound PSII CC was not crystalline.

Interaction of membrane proteins and lipids with solubilizing detergents

Detergents are indispensable in the isolation of integral membrane proteins from biological membranes to study their intrinsic structural and functional properties. Solubilization involves a number of intermediary states that can be studied by a variety of physicochemical and kinetic methods; it usually starts by destabilization of the lipid component of the membranes, a process that is accompanied by a transition of detergent binding by the membrane from a noncooperative to a cooperative interaction already below the critical micellar concentration (CMC). This leads to the formation of membrane fragments of proteins and lipids with detergent-shielded edges. In the final stage of solubilization membrane proteins are present as protomers, with the membrane inserted sectors covered by detergent. We consider in detail the nature of this interaction and conclude that in general binding as a monolayer ring, rather than as a micelle, is the most probable mechanism. This mode of interaction is supported by neutron diffraction investigations on the disposition of detergent in 3-D crystals of membrane proteins. Finally, we briefly discuss the use of techniques such as analytical ultracentrifugation, size exclusion chromatography, and mass spectrometry relevant for the structural investigation of detergent solubilized membrane proteins.

Molecular weight determination of membrane proteins by sedimentation equilibrium at the sucrose or nycodenz-adjusted density of the hydrated detergent micelle

The determination of the molecular weight of a membrane protein by sedimentation equilibrium is complicated by the fact that these proteins interact with detergents and form complexes of unknown density. These effects become marginal when running sedimentation equilibrium at gravitational transparency, i.e., at the density corresponding to that of the hydrated detergent micelles. Dodecyl-maltoside and octyl-glucoside are commonly used for dissolving membrane proteins. The density of micelles thereof was measured in sucrose or Nycodenz. Both proved to be about 50% lower than those of the corresponding non-hydrated micelles. Several membrane proteins were centrifuged at sedimentation equilibrium in sucrose- and in Nycodenz-enriched solutions of various densities. Their molecular weights were then calculated by using the resulting slope value at the density of the hydrated detergent micelles, i.e. at gravitational transparency, and the partial specific volume corrected for a 50% hydration of the membrane protein. The molecular weights of all measured membrane proteins, i.e. of photosystem II complex, reaction center of Rhodobacter sphaeroides R26, spinach photosystem II reaction center (core complex), bacteriorhodopsin, OmpF-porin and rhodopsin from Bovine retina corresponded within +/-15% to those reported previously, indicating a general applicability of this approach.

Spectroscopic properties of the CP43 core antenna protein of photosystem II

CP43 is a chlorophyll-protein complex that funnels excitation energy from the main light-harvesting system of photosystem II to the photochemical reaction center. We purified CP43 from spinach photosystem II membranes in the presence of the nonionic detergent n-dodecyl-beta,D-maltoside and recorded its spectroscopic properties at various temperatures between 4 and 293 K by a number of polarized absorption and fluorescence techniques, fluorescence line narrowing, and Stark spectroscopy. The results indicate two "red" states in the Q(y) absorption region of the chlorophylls. The first peaks at 682.5 nm at 4 K, has an extremely narrow bandwidth with a full width at half-maximum of approximately 2.7 nm (58 cm(-1)) at 4 K, and has the oscillator strength of a single chlorophyll. The second peaks at approximately 679 nm, has a much broader bandshape, is caused by several excitonically interacting chlorophylls, and is responsible for all 4 K absorption at wavelengths longer than 685 nm. The Stark spectrum of CP43 resembles the first derivative of the absorption spectrum and has an exceptionally small overall size, which we attribute to opposing orientations of the monomer dipole moments of the excitonically coupled pigments.

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.

Hydrodynamic studies on the manganese-stabilizing protein of photosystem II

The solution conformation of the manganese-stabilizing protein of photosystem II was examined by analytical ultracentrifugation. Sedimentation velocity and sedimentation equilibrium studies were performed. These experiments yielded values for of 2.26 S with a diffusion constant, D, of 7.7 x 10(-)7 cm2 s-1. This s value is significantly lower than the apparent s value of 2.6 S previously reported [Miyao, M., and Murata, N. (1989) Biochim. Biophys. Acta 977, 315-321]. The molecular mass of the protein, 26.531 kDa, was verified by MALDI mass spectrometry. The diffusion coefficient was also determined by dynamic light scattering. The z-weighted average of D was 6.8 x 10(-)7 cm2 s-1. This result was somewhat lower than that observed by analytical ultracentrifugation due to the presence of slowly diffusing components in the sample. A two-component exponential fit of the dynamic light scattering data, however, gave D = 7.52 x 10(-)7 cm2 s-1 for the major component of the sample, which is in excellent agreement with the value determined by analytical ultracentrifugation. The value of s, the apparent sedimentation coefficient, was found to depend on the concentration of the protein and varied about 4% per milligram of protein. This is a feature of proteins which are asymmetric in solution. This asymmetry was examined using both the v-bar and Teller methods. Both methods indicated a significant degree of asymmetry for the manganese-stabilizing protein. Our findings indicate that the prolate ellipsoid model for the manganese-stabilizing protein is elongated in solution, with approximate dimensions of about 12.6 nm x 3.0 nm, yielding an axial ratio of 4.2.

Mass Measurement in the Scanning Transmission Electron Microscope: A Powerful Tool for Studying Membrane Proteins

The technique of mass measurement in the scanning transmission electron microscope is briefly presented. Results obtained for membrane proteins, with particular emphasis on the channel forming proteins, are discussed. The data illustrate the versatility of the technique which is applicable to particulate, filamentous, and sheet-like structures. When combined with composition analysis, the absolute mass values measured with the STEM allow protein stoichiometries to be unambiguously defined. Copyright 1998 Academic Press.

Multiple crystal types reveal photosystem II to be a dimer

Three types of photosystem II (PS II) crystals have been produced using a variety of detergents. Intermediate stages of crystal formation were examined and it was determined that each crystal probably originates from a single grana membrane. Each crystal type was examined by electron microscopy and image processing, providing three different projection maps. The highest resolution results came from type 1 and type 2 crystals. Projection maps from these crystals were examined for two-fold symmetry via difference maps between the unsymmetrized averages and their 180 degrees rotation. A comparison of the final maps shows a high degree of two-fold symmetry, with only slight differences noted in the low density regions of the two halves of the structure. The interpretation is that PS II is a dimer, with the further suggestion that the two reaction center cores may have slightly different complements of antennae polypeptides.

Scanning transmission electron microscopy study of the molecular mass of amphipol/cytochrome b6f complexes

The composition and mass of complexes between Chlamydomonas reinhardtii cytochrome b6f and low molecular mass amphipathic polymers ('amphipols') have been studied using biochemical analysis and scanning transmission electron microscopy at liquid helium temperature (cryo-STEM). Cytochrome b6f was trapped by amphipols either under its native 14-meric state or as a delipidated, lighter form. A good consistency was observed between the masses of either form calculated from their biochemical composition and those determined by cryo-STEM. These data show that association with amphipols preserved the original original state of the protein in detergent solution. Complexation with amphipols appears to facilitate preparation of the samples and mass determination by cryo-STEM as compared to conventional solubilization with detergents.

The 23 and 17 kDa extrinsic proteins of photosystem II modulate the magnetic properties of the S1-state manganese cluster

An S1-state parallel polarization "multiline" EPR signal arising from the oxygen-evolving complex has been detected in spinach (PSII) membrane and core preparations depleted of the 23 and 17 kDa extrinsic polypeptides, but retaining the 33 kDa extrinsic protein. This S1-state multiline signal, with an effective g value of 12 and at least 18 hyperfine lines, has previously been detected only in PSII preparations from the cyanobacterium sp. Synechocystis sp. PCC6803 [Campbell, K. A., Peloquin, J. M., Pham, D. P., Debus, R. J., and Britt, R. D. (1998) J. Am. Chem. Soc. 120, 447-448]. It is absent in PSII spinach membrane and core preparations that either fully retain or completely lack the 33, 23, and 17 kDa extrinsic proteins. The S1-state multiline signal detected in spinach PSII cores and membranes has the same effective g value and hyperfine spacing as the signal detected in Synechocystis PSII particles. This signal provides direct evidence for the influence of the extrinsic PSII proteins on the magnetic properties of the Mn cluster.

Adsorption of biological molecules to a solid support for scanning probe microscopy

Scanning probe microscopes are now established tools to study the surface structure of biological macromolecules under physiological conditions. Sample preparation methods for this microscopy all have the objective to attach the specimen firmly to a support. Here we analyse the commonly used method of adsorbing biological specimens to freshly cleaved mica. This is facilitated by adjusting the electrolyte concentration and the pH of the buffer solution. Native macromolecular systems absorbed to mica in this way can be reproducibly imaged at submolecular resolution.