Refined purification and further characterization of oxygen-evolving and Tris-treated Photosystem II particles from the thermophilic Cyanobacterium synechococcus sp

Coupled rows of PBS cores and PSII dimers in cyanobacteria: symmetry and structure

Phycobilisome (PBS) is a giant water-soluble photosynthetic antenna transferring the energy of absorbed light mainly to the photosystem II (PSII) in cyanobacteria. Under the low light conditions, PBSs and PSII dimers form coupled rows where each PBS is attached to the cytoplasmic surface of PSII dimer, and PBSs come into contact with their face surfaces (state 1). The model structure of the PBS core that we have developed earlier by comparison and combination of different fine allophycocyanin crystals, as reported in Zlenko et al. (Photosynth Res 130(1):347-356, 2016b), provides a natural way of the PBS core face-to-face stacking. According to our model, the structure of the protein-protein contact between the neighboring PBS cores in the rows is the same as the contact between the APC hexamers inside the PBS core. As a result, the rates of energy transfer between the cores can occur, and the row of PBS cores acts as an integral PBS "supercore" providing energy transfer between the individual PBS cores. The PBS cores row pitch in our elaborated model (12.4 nm) is very close to the PSII dimers row pitch obtained by the electron microscopy (12.2 nm) that allowed to unite a model of the PBS cores row with a model of the PSII dimers row. Analyzing the resulting model, we have determined the most probable locations of ApcD and ApcE terminal emitter subunits inside the bottom PBS core cylinders and also revealed the chlorophyll molecules of PSII gathering energy from the PBS.

Molecular Dynamics of Photosystem II Embedded in the Thylakoid Membrane

Photosystem II (PSII) is one of the key protein complexes in photosynthesis. We introduce a coarse grained model of PSII and present the analysis of 60 μs molecular dynamics simulations of PSII in both monomeric and dimeric form, embedded in a thylakoid membrane model that reflects its native lipid composition. We describe in detail the setup of the protein complex and the many natural cofactors and characterize their mobility. Overall we find that the protein subunits and cofactors are more flexible toward the periphery of the complex as well as near the PLQ exchange cavity and at the dimer interface. Of all cofactors, β-carotenes show the highest mobility. Some of the β-carotenes diffuse in and out of the protein complex via the thylakoid membrane. In contrast with the PSII dimer, the monomeric form adopts a tilted conformation in the membrane, with strong interactions between the soluble PsbO subunit and the glycolipid headgroups. Interestingly, the tilted conformation causes buckling of the membrane. Together, our results provide an unprecedented view of PSII dynamics on a microsecond time scale. Our data may be used as basis for the interpretation of experimental data as well as for theoretical models describing exciton energy transfer.

Dynamic properties of photosystem II membranes at physiological temperatures characterized by elastic incoherent neutron scattering. Increased flexibility associated with the inactivation of the oxygen evolving complex

Elastic incoherent neutron scattering (EINS), a non-invasive technique which is capable of measuring the mean square displacement of atoms in the sample, has been widely used in biology for exploring the dynamics of proteins and lipid membranes but studies on photosynthetic systems are scarce. In this study we investigated the dynamic characteristics of Photosystem II (PSII) membrane fragments between 280 and 340 K, i.e., in the physiological temperature range and in the range of thermal denaturation of some of the protein complexes. The mean square displacement values revealed the presence of a hydration-sensitive transition in the sample between 310 and 320 K, suggesting that the oxygen evolving complex (OEC) plays an important role in the transition. Indeed, in samples in which the OEC had been removed by TRIS- or heat-treatments (323 and 333 K) no such transition was found. Further support on the main role of OEC in these reorganizations is provided by data obtained from differential scanning calorimetry experiments, showing marked differences between the untreated and TRIS-treated samples. In contrast, circular dichroism spectra exhibited only minor changes in the excitonic interactions below 323 K, showing that the molecular organization of the pigment-protein complexes remains essentially unaffected. Our data, along with earlier incoherent neutron scattering data on PSII membranes at cryogenic temperatures (Pieper et al., Biochemistry 46:11398-11409, 2007), demonstrate that this technique can be applied to characterize the dynamic features of PSII membranes, and can be used to investigate photosynthetic membranes under physiologically relevant experimental conditions.

Localization of 13 one-helix integral membrane proteins in photosystem II subcomplexes

Photosystem II is a multimeric protein complex of the thylakoid membrane in chloroplasts. Approximately half of the at least 26 different integral membrane protein subunits have molecular masses lower than 10 kDa. After one-dimensional (1D) or two-dimensional (2D) polyacrylamide gel electrophoresis (PAGE) separation, followed by enzymatic digestion of detected proteins, hardly any of these low-molecular-weight (LMW) subunits are detectable. Therefore, we developed a method for the analysis of highly hydrophobic LMW proteins. Intact proteins are extracted from acrylamide gels using a mixture of formic acid and organic solvent, precipitated with acetone, and analyzed by "top-down" mass spectrometry (MS). After offline nanoESI (electrospray ionization) MS, all LMW one-helix proteins from photosystem II were detected. In the four detected photosystem II supercomplexes of Nicotiana tabacum wild-type plants, 11 different one-helix proteins were identified as PsbE, -F, -H, -I, -K, -L, -M, -Tc, -W, and two isoforms of PsbX. The proteins PsbJ, -Y1, and -Y2 were localized in the buffer front after blue native (BN) PAGE, indicating their release during solubilization. Assembled PsbW is detected exclusively in supercomplexes, whereas it is absent in photosystem II core complexes, corroborating the protein's function for assembly of the light-harvesting complexes. This approach will substantiate gel-blot immunoanalysis for localization and identification of LMW protein subunits in any membrane protein complex.

Functional and Structural Characterization of the First Prokaryotic Member of the L-Amino Acid Transporter (LAT) Family

We have identified YkbA from Bacillus subtilis as a novel member of the L-amino acid transporter (LAT) family of amino acid transporters. The protein is approximately 30% identical in amino acid sequence to the light subunits of human heteromeric amino acid transporters. Purified His-tagged YkbA from Escherichia coli membranes reconstituted in proteoliposomes exhibited sodium-independent, obligatory exchange activity for L-serine and L-threonine and also for aromatic amino acids, albeit with less activity. Thus, we propose that YkbA be renamed SteT (Ser/Thr exchanger transporter). Kinetic analysis supports a sequential mechanism of exchange for SteT. Freeze-fracture analysis of purified, functionally active SteT in proteoliposomes, together with blue native polyacrylamide gel electrophoresis and transmission electron microscopy of detergent-solubilized purified SteT, suggest that the transporter exists in a monomeric form. Freeze-fracture analysis showed spherical particles with a diameter of 7.4 nm. Transmission electron microscopy revealed elliptical particles (diameters 6 x 7 nm) with a distinct central depression. To our knowledge, this is the first functional characterization of a prokaryotic member of the LAT family and the first structural data on an APC (amino acids, polyamines, and choline for organocations) transporter. SteT represents an excellent model to study the molecular architecture of the light subunits of heteromeric amino acid transporters and other APC transporters.

Functional and structural characterization of a prokaryotic peptide transporter with features similar to mammalian PEPT1

The ydgR gene of Escherichia coli encodes a protein of the proton-dependent oligopeptide transporter (POT) family. We cloned YdgR and overexpressed the His-tagged fusion protein in E. coli BL21 cells. Bacterial growth inhibition in the presence of the toxic phosphonopeptide alafosfalin established YgdR functionality. Transport was abolished in the presence of the proton ionophore carbonyl cyanide p-chlorophenylhydrazone, suggesting a proton-coupled transport mechanism. YdgR transports selectively only di- and tripeptides and structurally related peptidomimetics (such as aminocephalosporins) with a substrate recognition pattern almost identical to the mammalian peptide transporter PEPT1. The YdgR protein was purified to homogeneity from E. coli membranes. Blue native-polyacrylamide gel electrophoresis and transmission electron microscopy of detergent-solubilized YdgR suggest that it exists in monomeric form. Transmission electron microscopy revealed a crown-like structure with a diameter of approximately 8 nm and a central density. These are the first structural data obtained from a proton-dependent peptide transporter, and the YgdR protein seems an excellent model for studies on substrate and inhibitor interactions as well as on the molecular architecture of cell membrane peptide transporters.

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.

Supramolecular organization of thylakoid membrane proteins in green plants

The light reactions of photosynthesis in green plants are mediated by four large protein complexes, embedded in the thylakoid membrane of the chloroplast. Photosystem I (PSI) and Photosystem II (PSII) are both organized into large supercomplexes with variable amounts of membrane-bound peripheral antenna complexes. PSI consists of a monomeric core complex with single copies of four different LHCI proteins and has binding sites for additional LHCI and/or LHCII complexes. PSII supercomplexes are dimeric and contain usually two to four copies of trimeric LHCII complexes. These supercomplexes have a further tendency to associate into megacomplexes or into crystalline domains, of which several types have been characterized. Together with the specific lipid composition, the structural features of the main protein complexes of the thylakoid membranes form the main trigger for the segregation of PSII and LHCII from PSI and ATPase into stacked grana membranes. We suggest that the margins, the strongly folded regions of the membranes that connect the grana, are essentially protein-free, and that protein-protein interactions in the lumen also determine the shape of the grana. We also discuss which mechanisms determine the stacking of the thylakoid membranes and how the supramolecular organization of the pigment-protein complexes in the thylakoid membrane and their flexibility may play roles in various regulatory mechanisms of green plant photosynthesis.

Purification, characterisation and crystallisation of photosystem II from Thermosynechococcus elongatus cultivated in a new type of photobioreactor

The thermophilic cyanobacterium Thermosynechococcus elongatus was cultivated under controlled growth conditions using a new type of photobioreactor, allowing us to optimise growth conditions and the biomass yield. A fast large-scale purification method for monomeric and dimeric photosystem II (PSII) solubilized from thylakoid membranes of this cyanobacterium was developed using fast protein liquid chromatography (FPLC). The obtained PSII core complexes (PSIIcc) were analysed for their pigment stoichiometry, photochemical and oxygen evolution activities, as well as lipid and detergent composition. Thirty-six chlorophyll a (Chla), 2 pheophytin a (Pheoa), 9+/- 1 beta-carotene (Car), 2.9+/-0.8 plastoquinone 9 (PQ9) and 3.8+/-0.5 Mn were found per active centre. For the monomeric and dimeric PSIIcc, 18 and 20 lipid as well as 145 and 220 detergent molecules were found in the detergent shell, respectively. The monomeric and dimeric complexes showed high oxygen evolution activity with 1/4 O(2) released per 37-38 Chla and flash in the best cases. Crystals were obtained from dimeric PSIIcc by a micro-batch method. They diffract synchrotron X-rays to a maximum resolution of 2.9-A, resulting in complete data sets of 3.2 A resolution.

Separation methods in the analysis of protein membrane complexes

The separation of membrane protein complexes can be divided into two categories. One category, which is operated on a relatively large scale, aims to purify the membrane protein complex from membrane fractions while retaining its native form, mainly to characterize its nature. The other category aims to analyze the constituents of the membrane protein complex, usually on a small scale. Both of these face the difficulty of isolating the membrane protein complex without interference originating from the hydrophobic nature of membrane proteins or from the close association with membrane lipids. To overcome this difficulty, many methods have been employed. Crystallized membrane protein complexes are the most successful example of the former category. In these purification methods, special efforts are made in the steps prior to the column chromatography to enrich the target membrane protein complexes. Although there are specific aspects for each complex, the most popular method for isolating these membrane protein complexes is anion-exchange column chromatography, especially using weak anion-exchange columns. Another remarkable trend is metal affinity column chromatography, which purifies the membrane protein complex as an intact complex in one step. Such protein complexes contain subunit proteins which are genetically engineered so as to include multiple-histidine tags at carboxyl- or amino-termini. The key to these successes for multi-subunit complex isolation is the idea of keeping the expression at its physiological level, rather than overexpression. On the other hand, affinity purification using the Fv fragment, in which a Strep tag is genetically introduced, is ideal because this method does not introduce any change to the target protein. These purification methods supported by affinity interaction can be applied to minor membrane protein complexes in the membrane system. Isoelectric focusing (IEF) and blue native (BN) electrophoresis have also been employed to prepare membrane protein complexes. Generally, a combination of two or more chromatographic and/or electrophoretic methods is conducted to separate membrane protein complexes. IEF or BN electrophoresis followed by 2nd dimension electrophoresis serve as useful tools for analytical demand. However, some problems still exist in the 2D electrophoresis using IEF. To resolve such problems, many attempts have been made, e.g. introduction of new chaotropes, surfactants, reductants or supporting matrices. This review will focus in particular on two topics: the preparative methods that achieved purification of membrane protein complexes in the native (intact) form, and the analytical methods oriented to resolve the membrane proteins. The characteristics of these purification and analytical methods will be discussed along with plausible future developments taking into account the nature of membrane protein complexes.

Photosystem II single crystals studied by transient EPR: the light-induced triplet state

Transient electron paramagnetic resonance (TR EPR) at 9.8 GHz has been used to study the light-induced triplet state in single crystals of Photosystem II (PS II). The crystals were grown from a solution of PS II core complexes from the thermophilic cyanobacterium Synechococcus elongatus. The core complexes contain at least 17 subunits, including the water-oxidizing complex, and 32 chlorophyll a molecules per PS II complex. The PS II complexes are active in light-induced electron transfer and water oxidation. The crystals belong to the orthorhombic space group P2(1)2(1)2(1), with four dimers of PS II complexes per unit cell. Laser excitation was used to generate the recombination triplet state in PS II which was then studied by EPR at low temperatures (10 K). The crystal spectra show the same magnitude of the zero-field splitting (ZFS) values D, E as spectra obtained earlier for the triplet state of PS II in frozen solution. The orientation of the ZFS tensor D of the triplet state with respect to the crystallographic axes has been deduced from the analysis of angular-dependent EPR spectra. Knowledge of the orientation of the D tensor component perpendicular to the plane of the chlorophyll (D(Z)) allows an assignment on which chlorophyll of the reaction centre the triplet state is localized at low temperatures. Furthermore, the orientation of the D(X) and D(Y) components of the D tensor yielded the in-plane orientation of the respective chlorophyll in the reaction centre providing first experimental evidence for the orientation of this molecule in the PS II.

Pathways for energy transfer in the core light-harvesting complexes CP43 and CP47 of photosystem II

The pigment-protein complexes CP43 and CP47 transfer excitation energy from the peripheral antenna of photosystem II toward the photochemical reaction center. We measured the excitation dynamics of the chlorophylls in isolated CP43 and CP47 complexes at 77 K by time-resolved absorbance-difference and fluorescence spectroscopy. The spectral relaxation appeared to occur with rates of 0.2-0.4 ps and 2-3 ps in both complexes, whereas an additional relaxation of 17 ps was observed only in CP47. Using the 3.8-A crystal structure of the photosystem II core complex from Synechococcus elongatus (A. Zouni, H.-T. Witt, J. Kern, P. Fromme, N. Krauss, W. Saenger, and P. Orth, 2001, Nature, 409:739-743), excitation energy transfer kinetics were calculated and a Monte Carlo simulation of the absorption spectra was performed. In both complexes, the rate of 0.2-0.4 ps can be ascribed to excitation energy transfer within a layer of chlorophylls near the stromal side of the membrane, and the slower 2-3-ps process to excitation energy transfer to the calculated lowest excitonic state. We conclude that excitation energy transfer within CP43 and CP47 is fast and does not contribute significantly to the well-known slow trapping of excitation energy in photosystem II.

A giant chlorophyll-protein complex induced by iron deficiency in cyanobacteria

Cyanobacteria are abundant throughout most of the world's water bodies and contribute significantly to global primary productivity through oxygenic photosynthesis. This reaction is catalysed by two membrane-bound protein complexes, photosystem I (PSI) and photosystem II (PSII), which both contain chlorophyll-binding subunits functioning as an internal antenna. In addition, phycobilisomes act as peripheral antenna systems, but no additional light-harvesting systems have been found under normal growth conditions. Iron deficiency, which is often the limiting factor for cyanobacterial growth in aquatic ecosystems, leads to the induction of additional proteins such as IsiA (ref. 3). Although IsiA has been implicated in chlorophyll storage, energy absorption and protection against excessive light, its precise molecular function and association to other proteins is unknown. Here we report the purification of a specific PSI-IsiA supercomplex, which is abundant under conditions of iron limitation. Electron microscopy shows that this supercomplex consists of trimeric PSI surrounded by a closed ring of 18 IsiA proteins binding around 180 chlorophyll molecules. We provide a structural characterization of an additional chlorophyll-containing, membrane-integral antenna in a cyanobacterial photosystem.

Photosystem II single crystals studied by EPR spectroscopy at 94 GHz: the tyrosine radical Y(D)(*)

Electron paramagnetic resonance (EPR) spectroscopy at 94 GHz is used to study the dark-stable tyrosine radical Y(D)(*) in single crystals of photosystem II core complexes (cc) isolated from the thermophilic cyanobacterium Synechococcus elongatus. These complexes contain at least 17 subunits, including the water-oxidizing complex (WOC), and 32 chlorophyll a molecules/PS II; they are active in light-induced electron transfer and water oxidation. The crystals belong to the orthorhombic space group P2(1)2(1)2(1), with four PS II dimers per unit cell. High-frequency EPR is used for enhancing the sensitivity of experiments performed on small single crystals as well as for increasing the spectral resolution of the g tensor components and of the different crystal sites. Magnitude and orientation of the g tensor of Y(D)(*) and related information on several proton hyperfine tensors are deduced from analysis of angular-dependent EPR spectra. The precise orientation of tyrosine Y(D)(*) in PS II is obtained as a first step in the EPR characterization of paramagnetic species in these single crystals.

Effects of tail-like substituents on the binding of competitive inhibitors to the Q(B) site of photosystem II

The QB quinone-binding site of photosystem II is an important target for herbicides. Two major classes of herbicides are based on s-triazine and phenylurea moieties. A small library of triazine and phenylurea compounds has been synthesized which have tail-like substituents in order to test the effects of charge, hydrophobicity and size of the tail on binding properties. It is found that a tail can be attached to one of the alkylamino groups of triazine-type herbicides or to the para position of phenylurea-type herbicides without loss of binding, provided that the tail is hydrophobic. This indicates that the herbicides must be oriented in the QB site such that these positions point toward the natural isoprenyl tail-binding pocket that extends out of the Q(B) site. In turn, the requirement that the tail must extend out of the QB site constrains the size of the other herbicide substituents in the pocket. This is in agreement with the presumed orientation and fit of ligands in the QB site. When longer hydrophobic tails are used, the binding penalty that occurs upon adding a charged substituent at the distal end is reduced. This allows the use of a series of tail substituents possessing a distal charge as an approximate molecular ruler to measure the distance from the QB site to the aqueous phase. Even a 10-carbon alkyl chain still shows a 4-fold effect from the presence or absence of a distal charge. Such a chain does not appear to be long enough to extend from the bulk aqueous phase to the QB site because binding is completely lost when a large hydrophilic domain (PEG(4000)) is attached to the distal end. Longer tails are effective only if they are sufficiently hydrophobic. An effort was made to use tailed herbicides for affinity binding of photosystem II. It was found that hydrophobic linkers promote nonspecific binding, but careful choice of solvent conditions, such as the use of excess nonionic detergent well above its critical micelle concentration, might obviate this problem during affinity-binding applications.

The position of cytochrome b(559) relative to Q(A) in photosystem II studied by electron-electron double resonance (ELDOR)

The electron-electron double resonance (ELDOR) method was applied to measure the dipole interaction between cytochrome (Cyt) b(+)(559) and the primary acceptor quinone (Q(-)(A)), observed at g=2.0045 with the peak to peak width of about 9 G, in Photosystem II (PS II) in which the non-heme Fe(2+) was substituted by Zn(2+). The paramagnetic centers of Cyt b(+)(559)Y(D)Q(-)(A) were trapped by illumination at 273 K for 8 min, followed by dark adaptation for 3 min and freezing into 77 K. The distance between the pair Cyt b(+)(559)-Q(-)(A) was estimated from the dipole interaction constant fitted to the observed ELDOR time profile to be 40+/-1 A. In the membrane oriented PS II particles the angle between the vector from Q(A) to Cyt b(559) and the membrane normal was determined to be 80+/-5 degrees. The position of Cyt b(559) relative to Q(A) suggests that the heme plane is located on the stromal side of the thylakoid membrane. ELDOR was not observed for Cyt b(+)(559) Y(D) spin pair, suggesting the distance between them is more than 50 A.

Towards structural determination of the water-splitting enzyme. Purification, crystallization, and preliminary crystallographic studies of photosystem II from a thermophilic cyanobacterium

A photosystem II preparation from the thermophilic cyanobacterium Synechococcus elongatus, which is especially suitable for three-dimensional crystallization in a fully active form was developed. The efficient purification method applied here yielded 10 mg of protein of a homogenous dimeric complex of about 500 kDa within 2 days. Detailed characterization of the preparation demonstrated a fully active electron transport chain from the manganese cluster to plastoquinone in the Q(B) binding site. The oxygen-evolving activity, 5000-6000 micromol of O(2)/(h.mg of chlorophyll), was the highest so far reported and is maintained even at temperatures as high as 50 degrees C. The crystals obtained by the vapor diffusion method diffracted to a resolution of 4.3 A. The space group was determined to be P2(1)2(1)2(1) with four photosystem II dimers per unit cell. Analysis of the redissolved crystals revealed that activity, supramolecular organization, and subunit composition were maintained during crystallization.

First photosystem II crystals capable of water oxidation

Oxygen evolution and proton release of crystallised photosystem II core complexes isolated from Synechococcus elongatus were measured. The yields show that the crystals themselves are capable of highly active water oxidation. This opens the possibility for the structural analysis of the outstanding water-oxidising apparatus.

Cooperative binding of oxygen to the water-splitting enzyme in the filamentous cyanobacterium Oscillatoria chalybea

In the filamentous cyanobacterium Oscillatoria chalybea photolysis of water does not take place in the complete absence of oxygen. A catalytic oxygen partial pressure of 15x10(-6) Torr has to be present for effective water splitting to occur. By means of mass spectrometry we measured the photosynthetic oxygen evolution in the presence of H(2)(18)O in dependence on the oxygen partial pressure of the atmosphere and analysed the liberations of (16)O(2), (16)O(18)O and (18)O(2) simultaneously. The observed dependences of the light-induced oxygen evolution on bound oxygen yield sigmoidal curves. Hill coefficient values of 3.0, 3.1 and 3.2, respectively, suggest that the binding is cooperative and that four molecules of oxygen have to be bound per chain to the oxygen evolving complex. Oxygen seems to prime the water-splitting reaction by redox steering of the S-state system, putting it in the dark into the condition from which water splitting can start. It appears that in O. chalybea an interaction of oxygen with S(0) and S(1) leads to S(2) and S(3), thus yielding the typical oxygen evolution pattern in which even after extensive dark adaptation substantial amounts of Y(1) and Y(2) are found. The interacting oxygen is apparently reduced to hydrogen peroxide. Mass spectrometry permits to distinguish this highly specific oxygen requirement from the interaction of bulk atmospheric oxygen with the oxygen evolving complex of the cyanobacterium. This interaction leads to the formation H(2)O(2) which is decomposed under O(2) evolution in the light. The dependence on oxygen-partial pressure and temperature is analysed. Structural peculiarities of the cyanobacterial reaction centre of photosystem II referring to the extrinsic peptides might play a role.

Supramolecular organization of photosystem II and its light-harvesting antenna in partially solubilized photosystem II membranes

We present an extended analysis of the organization of green plant photosystem II and its associated light-harvesting antenna using electron microscopy and image analysis. The analysis is based on a large dataset of 16 600 projections of negatively stained PSII-LHCII supercomplexes and megacomplexes prepared by means of three different pretreatments. In addition to our previous work on this system [Boekema, E.J., van Roon, H., Calkoen, F., Bassi, R. and Dekker, J.P. (1999) Biochemistry 38, 2233-2239], the following results were obtained. The rotational orientation of trimeric LHCII at the S, M and L binding positions was determined. It was found that compared to the S trimer, the M and L trimers are rotationally shifted by about -20 degrees and -50 degrees, respectively. The number of projections with empty CP29, CP26 and CP24 binding sites was found to be about 0, 18 and 4%, respectively. We suggest that CP26 and CP24 are not required for the binding of trimeric LHCII at any of the three binding positions. A new type of megacomplex was observed with a characteristic windmill-like shape. This type III megacomplex consists of two C2S2 supercomplexes connected at their CP26 tips. Structural variation in the region of the central dimeric photosystem II complex was found to occur at one specific position near the periphery of the complex. We attribute this variation to the partial absence of an extrinsic polypeptide or one or more small intrinsic membrane proteins.

Localization of cyanobacterial photosystem II donor-side subunits by electron microscopy and the supramolecular organization ofphotosystem II in the thylakoid membrane

A large set of electron microscopy projections of photosystem II (PSII) dimers isolated from the cyanobacterium Synechococcus elongatus was characterized by single particle image analysis. In addition to previously published maps at lower resolution [Boekema, E.J., Hankamer, B., Bald, D., Kruip, J., Nield, J., Boonstra, A.F., Barber, J. & Rögner, M. (1995) Proc. Natl Acad. Sci. USA 92, 175-179], the new side-view projections show densities of all three lumenal extrinsic proteins, i.e. the 33-kDa, 12-kDa and the cytochrome c-550 subunit encoded by psbO, psbU and psbV, respectively. Analysis of the size and shape of the top-view projections revealed a small number of photosystem II particles of about double the size of the usual dimers. Size and quantity of these 'double dimers' correlates with a small fraction of 1000-kDa particles found with HPLC-size-exclusion chromatographic analysis. Because many cyanobacteria contain dimeric photosystem II complexes arranged in rows within the membrane, the double dimers can be considered as the breakdown fragments of these rows. Their analysis enabled the detection of the arrangement of photosystem II within the rows, in which the dimers interact with other dimers mostly with their tips, leaving a rather open center at the interfaces of two dimers. The dimers have a repeating distance of only 11.7 nm. As a consequence, the phycobilisomes, located on top of PSII and functioning in light-harvesting, must be closely packed or almost touch each other, in a manner similar to a recently suggested model [Bald, D., Kruip, J. & Rögner, M. (1996) Photosynthesis Res. 49, 103-118].

Stoichiometry of proton release from the catalytic center in photosynthetic water oxidation. Reexamination by a glass electrode study at ph 5.5-7.2

The catalytic center (CC) of water oxidation in photosystem II passes through four stepwise increased oxidized states (S(0)-S(4)) before O(2) evolution takes place from 2H(2)O in the S(4) --> S(0) transition. The pattern of the release of the four protons from the CC cannot be followed directly in the medium, because proton release from unknown amino acid residues also takes place. However, pH-independent net charge oscillations of 0:0:1:1 in S(0):S(1):S(2):S(3) have been considered as an intrinsic indicator for the H(+) release from the CC. The net charges have been proposed to be created as the charge difference between electron abstraction and H(+) release from the CC. Then the H(+) release from the CC is 1:0:1:2 for the S(0) --> S(1) --> S(2) --> S(3) --> S(0) transition. Strong support for this conclusion is given in this work with the analysis of the pH-dependent pattern of H(+) release in the medium measured directly by a glass electrode between pH 5.5 and 7.2. Improved and crystallizable photosystem II core complexes from the cyanobacterium Synechococcus elongatus were used as material. The pattern can be explained by protons released from the CC with a stoichiometry of 1:0:1:2 and protons from an amino acid group (pK approximately 5.7) that is deprotonated and reprotonated through electrostatic interaction with the oscillating net charges 0:0:1:1 in S(0):S(1):S(2):S(3). Possible water derivatives that circulate through the S states have been named.

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.

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.

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].

A comparative fluorescence kinetics study of Photosystem I monomers and trimers from Synechocystis PCC 6803

Picosecond time-resolved fluorescence measurements have been performed as a function of emission wavelengths in order to investigate the possible functional differences between monomeric and trimeric Photosystem I (PS I) particles from a cyanobacterium Synechocystis. Applying global analysis, four kinetic components were found necessary to describe the fluorescecne decay for both monomers and trimers of PS I. The lifetimes and spectra of the respective components are quite similar, indicating that they can be attributed to identical processes in both the monomers and trimers. It is concluded that both forms of PS I are capable of efficient energy transfer and charge separation, in agreement with a physiological role of both forms. Small differences in the fluorescence decays are discussed in terms of a slightly higher ratio of red emitting pigments per reaction centre in trimers of PS I. A comparison to Synechococcus PS I particles reveals the higher red chlorophyll content of the latter.

Supramolecular architecture of cyanobacterial thylakoid membranes: How is the phycobilisome connected with the photosystems?

Cyanobacteria, as the most simple organisms to perform oxygenic photosynthesis differ from higher plants especially with respect to the thylakoid membrane structure and the antenna system used to capture light energy. Cyanobacterial antenna systems, the phycobilisomes (PBS), have been shown to be associated with Photosystem 2 (PS 2) at the cytoplasmic side, forming a PS 2-PBS-supercomplex, the structure of which is not well understood. Based on structural data of PBS and PS 2, a model for such a supercomplex is presented. Its key features are the PS 2 dimer as prerequisite for formation of the supercomplex and the antiparallel orientation of PBS-cores and the two PS 2 monomers which form the 'contact area' within the supercomplex. Possible consequences for the formation of 'superstructures' (PS 2-PBS rows) within the thylakoid membrane under so-called 'state 1' conditions are discussed. As there are also indications for specific functional connections of PBS with Photosystem 1 (PS 1) under so-called 'state 2' conditions, we show a model which reconciles the need for a structural interaction between PBS and PS 1 with the difference in structural symmetry (2-fold rotational symmetry of PBS-cores, 3-fold rotational symmetry of trimeric PS 1). Finally, the process of dynamic coupling and uncoupling of PBS to PS 1 and PS 2, based on the presented models, shows analogies to mechanisms for the regulation of photosynthetic electron flow in higher plants-despite the very different organization of their thylakoid membranes in comparison to cyanobacteria.

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.

The PSII-S protein of higher plants: a new type of pigment-binding protein

An intrinsic 22 kDa protein of photosystem II has been shown to possess high sequence homology with the CAB gene products, but differs from these proteins by an additional putative fourth transmembrane helix. This protein, designated PSII-S in accordance with the assignment of the name psbS to its gene, has been isolated by nonionic detergents and preparative isoelectric focusing in this study. The isolated PSII-S protein was shown to bind 5 chlorophyll molecules (a and b) per protein unit and also several different kinds of carotenoids. The room temperature absorption spectrum of the Qy transition of the chlorophylls bound to the isolated protein is characterized by a broad band with a maximum at 671 nm. The 77 K fluorescence spectrum exhibits a peak at 672 nm. A single photon counting technique was applied to resolve the room temperature decay kinetics of the first excited singlet states in the chlorophyll ensemble of the PSII-S protein. The data can be satisfactorily described by triexponential kinetics with lifetimes of tau 1 = 1.8 ns, tau 2 = 4.4 ns, and tau 3 = 6.1 ns and normalized amplitudes of 0.09, 0.60, and 0.31, respectively. Circular dichroism spectra suggest that, in contrast to LHCII, virtually no pigment coupling exists in the PSII-S protein. Two copies of the PSII-S protein were found per PSII in spinach thylakoids. It displays an unusually extreme lateral heterogeneity, since the PSII beta centers located in the stroma exposed thylakoid regions contained only residual amounts of the PSII-S protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification and characterization of oxygen-evolving photosystem II core complexes from the green alga Chlamydomonas reinhardtii

Oxygen-evolving photosystem II complexes were isolated from the green alga Chlamydomonas reinhardtii by selective solubilization of thylakoid membranes with dodecyl maltoside followed by density gradient centrifugation and anion-exchange chromatography. In the presence of CaCl2 and K3[Fe(CN)6] the complexes evolved oxygen at rates exceeding 1000 mumol (mg of chl)-1 h-1. The particles contained 40 chlorophylls a and had properties very similar to those of PSII isolated from higher plants. Chlamydomonas reinhardtii is now the first organism which can be used for both site-directed mutagenesis and detailed biochemical and biophysical characterization of oxygen-evolving photosystem II. It seems therefore to be an ideal model organism for investigation of structure-function relationships in photosynthetic oxygen evolution.

Evidence for the existence of trimeric and monomeric Photosystem I complexes in thylakoid membranes from cyanobacteria

In cyanobacteria, solubilization of thylakoid membranes by detergents yields both monomeric and trimeric Photosystem I (PS I) complexes in variable amounts. We present evidence for the existence of both monomeric and trimeric PS I in cyanobacterial thylakoid membranes with the oligomeric state depending 'in vitro' on the ion concentration. At low salt concentrations (i.e.≤10 mM MgSO4) PS I is mainly extracted as a trimer from these membranes and at high salt concentrations (i.e.≥150 mM MgSO4) nearly exclusively as a monomer, irrespective of the type of salt used (i.e. mono- or bivalent ions) and the temperature (i.e. 4°C or 20°C). Once solubilized, the PS I trimer is stable over a wide range of ion concentrations (i.e. beyond 0.5 M). A model is presented which suggests a monomer-oligomer equilibrium of PS I, but also of PS II and the cyt. b6/f-complex in the cyanobacterial thylakoid membrane. The possible physiological role of this equilibrium in the regulation of state transitions is discussed.

Biochemical and spectroscopic characterization of the reaction center-LH1 complex and the carotenoid-containing B820 subunit of Chromatium purpuratum

Two complexes, the reaction center light-harvesting complex 1 (RC-LH1) and the B820 subunit of the LH1, have been isolated and characterized from the purple-sulfur photosynthetic bacterium Chromatium purpuratum. The RC-LH1 consists of the B870 antenna and a P-870 RC with an associated tetraheme cytochrome. This complex can be further fractionated to yield the B820 subunit of the LH1. The C. purpuratum B820 subunit is the first isolated from a purple-sulfur bacterium. It is also the first that retains its carotenoid absorption properties. CD spectra in the Qy region of bacteriochlorophyll a in both the RC-LH1 and the B820 subunit are bathochromically shifted as compared to other such complexes. Comparison of the sequence of the LH1 beta polypeptide to other LH1 beta s reveals the presence of additional aromatic amino acids in the vicinity of both of the conserved histidines in the C. purpuratum beta polypeptide. The CD spectra of these C. purpuratum pigment-protein complexes can be interpreted in terms of exciton interaction between bacteriochlorophylls in the B820 subunit of the LH1 and in the B870, with additional spectral characteristics arising from interactions of the pigments with their protein environment.

Characterization of the chloroplast cytochrome b6f complex as a structural and functional dimer

Size analysis of the cytochrome b6f complex by FPLC Superose-12 chromatography and Blue Native PAGE indicated a predominantly dimeric component with M(r) = (1.9-2.5) x 10(5). The true dimer molecular weight including bound lipid, but not detergent, was estimated to be 2.3 x 10(5). Size and shape analysis by negative-stain single-particle electron microscopy indicated that the preparation of dimeric complexes contains a major population that has a protein cross section 40% larger than the monomer, binds more negative stain, and has a geometry with a distinct 2-fold axis of symmetry compared to the monomeric complex. The dimeric species is more stable at higher ionic strength with respect to conversion to the monomeric species. SDS-PAGE of monomer and dimer preparations indicated that both contain the four major polypeptides in approximately equal stoichiometry and also contain the petG M(r) 4000 subunit. One bound chlorophyll a per monomer, part of the bound lipid, is present in monomer and dimer. The in vitro electron-transport activity (decyl-PQH2-->PC-ferricyanide) of the separated dimer was comparable to that of the isolated b6f complex and was 4-5-fold greater than that of the monomer preparation, whose activity could be attributed to residual dimer. No difference in the properties of the dimer and monomer was detected by SDS-PAGE or redox difference spectrophotometry that could account for the difference in activities. However, the concentration of the Rieske [2Fe-2S] center was found by EPR analysis of the gy = 1.90 signal to be lower in the monomer fraction by a factor of 3.5 relative to the dimer.(ABSTRACT TRUNCATED AT 250 WORDS)

Electron microscopic structural analysis of Photosystem I, Photosystem II, and the cytochrome b6/f complex from green plants and cyanobacteria

Electron microscopy (EM) in combination with image analysis is a powerful technique to study protein structure at low- and high resolution. Since electron micrographs of biological objects are very noisy, substantial improvement of image quality can be obtained by averaging individual projections. Crystallographic and noncrystallographic averaging methods are available and have been applied to study projections of the large protein complexes embedded in photosynthetic membranes from cyanobacteria and higher plants. Results of EM on monomeric and trimeric Photosystem I complexes, on monomeric and dimeric Photosystem II complexes, and on the monomeric cytochrome b6/f complex are discussed.

Functional characterisation of a purified homogeneous Photosystem II core complex with high oxygen evolution capacity from spinach

The functional properties of a purified homogeneous spinach PS II-core complex with high oxygen evolution capacity (Haag et al. 1990a) were investigated in detail by measuring thermoluminescence and oscillation patterns of flash induced oxygen evolution and fluorescence quantum yield changes. The following results were obtained: a) Depending on the illumination conditions the PS II-core complexes exhibit several thermoluminescence bands corresponding to the A band, Q band and Zv band in PS II membrane fragments. The lifetime of the Q band (Tmax=10°C) was determined to be 8s at T=10°C. No B band corresponding to S2QB (-) or S3QB (-) recombination could be detected. b) The flash induced transient fluorescence quantum yield changes exhibit a multiphasi relaxation kinetics shich reflect the reoxidation of Q A (-) . In control samples without exogenous acceptors this process is markedly slower than in PS II membrane fragments. The reaction becomes significantly retarded by addition of 10 μM DCMU. After dark incubation in the presence of K3[Fe(CN)6 c) Excitation of dark-adapted samples with a train of short saturating flashes gives rise to a typical pattern dominated by a high O2 yield due to the third flash and a highly damped period four oscillation. The decay of redox states S2 and S3 are dominated by short life times of 4.3 s and 1.5 s, respectively, at 20°C. The results of the present study reveal that in purified homogeneous PS II-core complexes with high oxygen evolution isolated from higher plants by β-dodecylmaltoside solubilization the thermodynamic properties and the kinetic parameters of the redox groups leading to electron transfer from water to QA are well preserved. The most obvious phenomenon is a severe modification of the QB binding site. The implications of this finding are discussed.