Three-dimensional structure of the photosystem II core dimer of higher plants determined by electron microscopy

Rapid and Highly Stable Membrane Reconstitution by LAiR Enables the Study of Physiological Integral Membrane Protein Functions

Functional reintegration into lipid environments represents a major challenge for in vitro investigation of integral membrane proteins (IMPs). Here, we report a new approach, termed LMNG Auto-insertion Reintegration (LAiR), for reintegration of IMPs into lipid bilayers within minutes. The resulting proteoliposomes displayed an unprecedented capability to maintain proton gradients and long-term stability. LAiR allowed for monitoring catalysis of a membrane-bound, physiologically relevant polyisoprenoid quinone substrate by Escherichia coli cytochromes bo ₃ (cbo ₃) and bd (cbd) under control of the proton motive force. LAiR also facilitated bulk-phase detection and physiological assessment of the "proton leak" in cbo ₃, a controversial catalytic state that previously was only approachable at the single-molecule level. LAiR maintained the multisubunit integrity and higher-order oligomeric states of the delicate mammalian F-ATP synthase. Given that LAiR can be applied to both liposomes and planar membrane bilayers and is compatible with IMPs and lipids from prokaryotic and eukaryotic sources, we anticipate LAiR to be applied broadly across basic research, pharmaceutical applications, and biotechnology.

Structural roles of lipid molecules in the assembly of plant PSII-LHCII supercomplex

In plants, photosystem II (PSII) associates with light-harvesting complexes II (LHCII) to form PSII–LHCII supercomplexes. They are multi-subunit supramolecular systems embedded in the thylakoid membrane of chloroplast, functioning as energy-converting and water-splitting machinery powered by light energy. The high-resolution structure of a PSII–LHCII supercomplex, previously solved through cryo-electron microscopy, revealed 34 well-defined lipid molecules per monomer of the homodimeric system. Here we characterize the distribution of lipid-binding sites in plant PSII–LHCII supercomplex and summarize their arrangement pattern within and across the membrane. These lipid molecules have crucial roles in stabilizing the oligomerization interfaces of plant PSII dimer and LHCII trimer. Moreover, they also mediate the interactions among PSII core subunits and contribute to the assembly between peripheral antenna complexes and PSII core. The detailed information of lipid-binding sites within PSII–LHCII supercomplex may serve as a framework for future researches on the functional roles of lipids in plant photosynthesis.

Use of Nicotiana tabacum transplastomic plants engineered to express a His-tagged CP47 for the isolation of functional photosystem II core complexes

This work focuses on the development of a molecular tool for purification of Photosystem II (PSII) from Nicotiana tabacum (L.). To this end, the chloroplast psbB gene encoding the CP47 PSII subunit was replaced with an engineered version of the same gene containing a C-terminal His-tag. Molecular analyses assessed the effective integration of the recombinant gene and its expression. Despite not exhibiting any obvious phenotype, the transplastomic plants remained heteroplasmic even after three rounds of regeneration under antibiotic selection. However, the recombinant His-tagged CP47 protein associated in vivo to the other PSII subunits allowing the isolation of a functional PSII core complex, although with low yield of extraction. These results will open up possible perspectives for further spectroscopic and structural studies.

Isolation of novel PSII-LHCII megacomplexes from pea plants characterized by a combination of proteomics and electron microscopy

In higher plants, photosystem II (PSII) is a multi-subunit pigment-protein complex embedded in the thylakoid membranes of chloroplasts, where it is present mostly in dimeric form within the grana. Its light-harvesting antenna system, LHCII, is composed of trimeric and monomeric complexes, which can associate in variable number with the dimeric PSII core complex in order to form different types of PSII-LHCII supercomplexes. Moreover, PSII-LHCII supercomplexes can laterally associate within the thylakoid membrane plane, thus forming higher molecular mass complexes, termed PSII-LHCII megacomplexes (Boekema et al. 1999a, in Biochemistry 38:2233-2239; Boekema et al. 1999b, in Eur J Biochem 266:444-452). In this study, pure PSII-LHCII megacomplexes were directly isolated from stacked pea thylakoid membranes by a rapid single-step solubilization, using the detergent n-dodecyl-α-D-maltoside, followed by sucrose gradient ultracentrifugation. The megacomplexes were subjected to biochemical and structural analyses. Transmission electron microscopy on negatively stained samples, followed by single-particle analyses, revealed a novel form of PSII-LHCII megacomplexes, as compared to previous studies (Boekema et al.1999a, in Biochemistry 38:2233-2239; Boekema et al. 1999b, in Eur J Biochem 266:444-452), consisting of two PSII-LHCII supercomplexes sitting side-by-side in the membrane plane, sandwiched together with a second copy. This second copy of the megacomplex is most likely derived from the opposite membrane of a granal stack. Two predominant forms of intact sandwiched megacomplexes were observed and termed, according to (Dekker and Boekema 2005 Biochim Biophys Acta 1706:12-39), as (C2S2)4 and (C2S2 + C2S2M2)2 megacomplexes. By applying a gel-based proteomic approach, the protein composition of the isolated megacomplexes was fully characterized. In summary, the new structural forms of isolated megacomplexes and the related modeling performed provide novel insights into how PSII-LHCII supercomplexes may bind to each other, not only in the membrane plane, but also between granal stacks within the chloroplast.

'Photosystem II: the water splitting enzyme of photosynthesis and the origin of oxygen in our atmosphere'

About 3 billion years ago an enzyme emerged which would dramatically change the chemical composition of our planet and set in motion an unprecedented explosion in biological activity. This enzyme used solar energy to power the thermodynamically and chemically demanding reaction of water splitting. In so doing it provided biology with an unlimited supply of reducing equivalents needed to convert carbon dioxide into the organic molecules of life while at the same time produced oxygen to transform our planetary atmosphere from an anaerobic to an aerobic state. The enzyme which facilitates this reaction and therefore underpins virtually all life on our planet is known as Photosystem II (PSII). It is a pigment-binding, multisubunit protein complex embedded in the lipid environment of the thylakoid membranes of plants, algae and cyanobacteria. Today we have detailed understanding of the structure and functioning of this key and unique enzyme. The journey to this level of knowledge can be traced back to the discovery of oxygen itself in the 18th-century. Since then there has been a sequence of mile stone discoveries which makes a fascinating story, stretching over 200 years. But it is the last few years that have provided the level of detail necessary to reveal the chemistry of water oxidation and O-O bond formation. In particular, the crystal structure of the isolated PSII enzyme has been reported with ever increasing improvement in resolution. Thus the organisational and structural details of its many subunits and cofactors are now well understood. The water splitting site was revealed as a cluster of four Mn ions and a Ca ion surrounded by amino-acid side chains, of which seven provide direct ligands to the metals. The metal cluster is organised as a cubane structure composed of three Mn ions and a Ca2+ linked by oxo-bonds with the fourth Mn ion attached to the cubane. This structure has now been synthesised in a non-protein environment suggesting that it is a totally inorganic precursor for the evolution of the photosynthetic oxygen-evolving complex. In summary, the overall structure of the catalytic site has given a framework on which to build a mechanistic scheme for photosynthetic dioxygen generation and at the same time provide a blue-print and incentive to develop catalysts for artificial photo-electrochemical systems to split water and generate renewable solar fuels.

Structure of spinach photosystem II-LHCII supercomplex at 3.2 Å resolution

During photosynthesis, the plant photosystem II core complex receives excitation energy from the peripheral light-harvesting complex II (LHCII). The pathways along which excitation energy is transferred between them, and their assembly mechanisms, remain to be deciphered through high-resolution structural studies. Here we report the structure of a 1.1-megadalton spinach photosystem II-LHCII supercomplex solved at 3.2 Å resolution through single-particle cryo-electron microscopy. The structure reveals a homodimeric supramolecular system in which each monomer contains 25 protein subunits, 105 chlorophylls, 28 carotenoids and other cofactors. Three extrinsic subunits (PsbO, PsbP and PsbQ), which are essential for optimal oxygen-evolving activity of photosystem II, form a triangular crown that shields the Mn4CaO5-binding domains of CP43 and D1. One major trimeric and two minor monomeric LHCIIs associate with each core-complex monomer, and the antenna-core interactions are reinforced by three small intrinsic subunits (PsbW, PsbH and PsbZ). By analysing the closely connected interfacial chlorophylls, we have obtained detailed insights into the energy-transfer pathways between the antenna and core complexes.

A comparison between plant photosystem I and photosystem II architecture and functioning

Oxygenic photosynthesis is indispensable both for the development and maintenance of life on earth by converting light energy into chemical energy and by producing molecular oxygen and consuming carbon dioxide. This latter process has been responsible for reducing the CO2 from its very high levels in the primitive atmosphere to the present low levels and thus reducing global temperatures to levels conducive to the development of life. Photosystem I and photosystem II are the two multi-protein complexes that contain the pigments necessary to harvest photons and use light energy to catalyse the primary photosynthetic endergonic reactions producing high energy compounds. Both photosystems are highly organised membrane supercomplexes composed of a core complex, containing the reaction centre where electron transport is initiated, and of a peripheral antenna system, which is important for light harvesting and photosynthetic activity regulation. If on the one hand both the chemical reactions catalysed by the two photosystems and their detailed structure are different, on the other hand they share many similarities. In this review we discuss and compare various aspects of the organisation, functioning and regulation of plant photosystems by comparing them for similarities and differences as obtained by structural, biochemical and spectroscopic investigations.

Direct energy transfer from the major antenna to the photosystem II core complexes in the absence of minor antennae in liposomes

Minor antennae of photosystem (PS) II, located between the PSII core complex and the major antenna (LHCII), are important components for the structural and functional integrity of PSII supercomplexes. In order to study the functional significance of minor antennae in the energetic coupling between LHCII and the PSII core, characteristics of PSII-LHCII proteoliposomes, with or without minor antennae, were investigated. Two types of PSII preparations containing different antenna compositions were isolated from pea: 1) the PSII preparation composed of the PSII core complex, all of the minor antennae, and a small amount of major antennae (MCC); and 2) the purified PSII dimeric core complexes without periphery antenna (CC). They were incorporated, together with LHCII, into liposomes composed of thylakoid membrane lipids. The spectroscopic and functional characteristics were measured. 77K fluorescence emission spectra revealed an increased spectral weight of fluorescence from PSII reaction center in the CC-LHCII proteoliposomes, implying energetic coupling between LHCII and CC in the proteoliposomes lacking minor antennae. This result was further confirmed by chlorophyll a fluorescence induction kinetics. The incorporation of LHCII together with CC markedly increased the antenna cross-section of the PSII core complex. The 2,6-dichlorophenolindophenol photoreduction measurement implied that the lack of minor antennae in PSII supercomplexes did not block the energy transfer from LHCII to the PSII core complex. In conclusion, it is possible, in liposomes, that LHCII transfer energy directly to the PSII core complex, in the absence of minor antennae.

Influence of thylakoid membrane lipids on the structure and function of the plant photosystem II core complex

MGDG leads to a dimerization of isolated, monomeric PSII core complexes. SQDG and PG induce a detachment of CP43 from the PSII core, thereby disturbing the intrinsic PSII electron transport. The influence of the four thylakoid membrane lipids monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG), sulfoquinovosyldiacylglycerol (SQDG) and phosphatidylglycerol (PG) on the structure and function of isolated monomeric photosystem (PS) II core complexes was investigated. Incubation with the negatively charged lipids SQDG and PG led to a loss of the long-wavelength 77 K fluorescence emission at 693 nm that is associated with the inner antenna proteins. The neutral galactolipids DGDG and MGDG had no or only minor effects on the fluorescence emission spectra of the PSII core complexes, respectively. Pigment analysis, absorption and 77 K fluorescence excitation spectroscopy showed that incubation with SQDG and PG led to an exposure of chlorophyll molecules to the surrounding medium followed by conversion to pheophytin under acidic conditions. Size-exclusion chromatography and polypeptide analysis corroborated the findings of the spectroscopic measurements and pigment analysis. They showed that the negatively charged lipid SQDG led to a dissociation of the inner antenna protein CP43 and the 27- and 25-kDa apoproteins of the light-harvesting complex II, that were also associated with a part of the PSII core complexes used in the present study. Incubation of PSII core complexes with MGDG, on the other hand, induced an almost complete dimerization of the monomeric PSII. Measurements of the fast PSII fluorescence induction demonstrated that MGDG and DGDG only had a minor influence on the reduction kinetics of plastoquinone QA and the artificial PSII electron acceptor 2,5-dimethyl-p-benzoquinone (DMBQ). SQDG and, to a lesser extent, PG perturbed the intrinsic PSII electron transport significantly.

Proteomic characterization and three-dimensional electron microscopy study of PSII-LHCII supercomplexes from higher plants

In higher plants a variable number of peripheral LHCII trimers can strongly (S), moderately (M) or loosely (L) associate with the dimeric PSII core (C2) complex via monomeric Lhcb proteins to form PSII-LHCII supercomplexes with different structural organizations. By solubilizing isolated stacked pea thylakoid membranes either with the α or β isomeric forms of the detergent n-dodecyl-D-maltoside, followed by sucrose density ultracentrifugation, we previously showed that PSII-LHCII supercomplexes of types C2S2M2 and C2S2, respectively, can be isolated [S. Barera et al., Phil. Trans. R Soc. B 67 (2012) 3389-3399]. Here we analysed their protein composition by applying extensive bottom-up and top-down mass spectrometry on the two forms of the isolated supercomplexes. In this way, we revealed the presence of the antenna proteins Lhcb3 and Lhcb6 and of the extrinsic polypeptides PsbP, PsbQ and PsbR exclusively in the C2S2M2 supercomplex. Other proteins of the PSII core complex, common to the C2S2M2 and C2S2 supercomplexes, including the low molecular mass subunits, were also detected and characterized. To complement the proteomic study with structural information, we performed negative stain transmission electron microscopy and single particle analysis on the PSII-LHCII supercomplexes isolated from pea thylakoid membranes solubilized with n-dodecyl-α-D-maltoside. We observed the C2S2M2 supercomplex in its intact form as the largest PSII complex in our preparations. Its dataset was further analysed in silico, together with that of the second largest identified sub-population, corresponding to its C2S2 subcomplex. In this way, we calculated 3D electron density maps for the C2S2M2 and C2S2 supercomplexes, approaching respectively 30 and 28Å resolution, extended by molecular modelling towards the atomic level. This article is part of a special issue entitled: photosynthesis research for sustainability: keys to produce clean energy.

Characterization of PSII-LHCII supercomplexes isolated from pea thylakoid membrane by one-step treatment with α- and β-dodecyl-D-maltoside

It was the work of Jan Anderson, together with Keith Boardman, that showed it was possible to physically separate photosystem I (PSI) from photosystem II (PSII), and it was Jan Anderson who realized the importance of this work in terms of the fluid-mosaic model as applied to the thylakoid membrane. Since then, there has been a steady progress in the development of biochemical procedures to isolate PSII and PSI both for physical and structural studies. Dodecylmaltoside (DM) has emerged as an effective mild detergent for this purpose. DM is a glucoside-based surfactant with a bulky hydrophilic head group composed of two sugar rings and a non-charged alkyl glycoside chain. Two isomers of this molecule exist, differing only in the configuration of the alkyl chain around the anomeric centre of the carbohydrate head group, axial in α-DM and equatorial in β-DM. We have compared the use of α-DM and β-DM for the isolation of supramolecular complexes of PSII by a single-step solubilization of stacked thylakoid membranes isolated from peas. As a result, we have optimized conditions to obtain homogeneous preparations of the C(2)S(2)M(2) and C(2)S(2) supercomplexes following the nomenclature of Dekker & Boekema (2005 Biochim. Biophys. Acta 1706, 12-39). These PSII-LHCII supercomplexes were subjected to biochemical and structural analyses.

Cytochrome b₅₅₉ and cyclic electron transfer within photosystem II

Cytochrome b₅₅₉ (Cyt b₅₅₉), β-carotene (Car), and chlorophyll (Chl) cofactors participate in the secondary electron-transfer pathways in photosystem II (PSII), which are believed to protect PSII from photodamage under conditions in which the primary electron-donation pathway leading to water oxidation is inhibited. Among these cofactors, Cyt b₅₅₉ is preferentially photooxidized under conditions in which the primary electron-donation pathway is blocked. When Cyt b₅₅₉ is preoxidized, the photooxidation of several of the 11 Car and 35 Chl molecules present per PSII is observed. In this review, the discovery of the secondary electron donors, their structures and electron-transfer properties, and progress in the characterization of the secondary electron-transfer pathways are discussed. This article is part of a Special Issue entitled: Photosystem II.

One-step isolation and biochemical characterization of a highly active plant PSII monomeric core

We describe a one-step detergent solubilization protocol for isolating a highly active form of Photosystem II (PSII) from Pisum sativum L. Detailed characterization of the preparation showed that the complex was a monomer having no light harvesting proteins attached. This core reaction centre complex had, however, a range of low molecular mass intrinsic proteins as well as the chlorophyll binding proteins CP43 and CP47 and the reaction centre proteins D1 and D2. Of particular note was the presence of a stoichiometric level of PsbW, a low molecular weight protein not present in PSII of cyanobacteria. Despite the high oxygen evolution rate, the core complex did not retain the PsbQ extrinsic protein although there was close to a full complement of PsbO and PsbR and partial level of PsbP. However, reconstitution of PsbP and PsbPQ was possible. The presence of PsbP in absence of LHCII and other chlorophyll a/b binding proteins confirms that LHCII proteins are not a strict requirement for the assembly of this extrinsic polypeptide to the PSII core in contrast with the conclusion of Caffarri et al. (2009).

Deg/HtrA proteases as components of a network for photosystem II quality control in chloroplasts and cyanobacteria

Organisms that perform oxygenic photosynthesis are subjected to photoinhibition of their photosynthetic function when exposed to excessive illumination. The main target of photoinhibition is the D1 protein in the reaction center of the photosystem II complex. Rapid degradation of photodamaged D1 protein and its replacement by a de novo synthesized functional copy represent an important repair mechanism crucial for cell survival under light stress conditions. This review summarizes the literature on the ATP-independent Deg/HtrA family of serine endopeptidases in cyanobacteria and chloroplasts of higher plants, and discusses their role in D1 protein degradation. We propose that Deg/HtrA proteases are part of a larger network of enzymes that ensure protein quality control, including photosystem II, in plants and cyanobacteria.

Mass spectrometric characterization of membrane integral low molecular weight proteins from photosystem II in barley etioplasts

In Photosystem II (PSII), a high number of plastid encoded and membrane integral low molecular weight proteins smaller than 10 kDa, the proteins PsbE, F, H, I, J, K, L, M, N, Tc, Z and the nuclear encoded PsbW, X, Y1, Y2 proteins have been described. Here we show that all low molecular weight proteins of PSII already accumulate in the etioplast membrane fraction in darkness, whereas PsaI and PsaJ of photosystem I (PSI) represent the only low molecular weight proteins that do not accumulate in darkness. We found by BN-PAGE separation of membrane protein complexes and selective MS that the accumulation of one-helix proteins from PSII is light independent and occurs in etioplasts. In contrast, in chloroplasts isolated from light-grown plants, low molecular weight proteins were found to specifically accumulate in PSI and II complexes. Our results demonstrate how plants grown in darkness prepare for the induction of chlorophyll dependent photosystem assembly upon light perception. We anticipate that our investigation will provide the essential means for the analysis of protein assembly in any membrane utilizing low molecular weight protein subunits.

Isolation of highly active photosystem II core complexes with a His-tagged Cyt b559 subunit from transplastomic tobacco plants

Photosystem II (PSII) is a huge multi-protein-complex consisting, in higher plants and green algae, of the PS II core and the adjacent light harvesting proteins. In the study reported here, N-terminal His-tags were added to the plastome-encoded alpha-subunit of cytochrome b559, PsbE, in tobacco plants, thus facilitating rapid, mild purification of higher plant PSII. Biolistic chloroplast transformation was used to replace the wildtype psbE gene by His-tagged counterparts. Transgenic plants did not exhibit an obvious phenotype. However, the oxygen evolution capacity of thylakoids prepared from the mutants compared to the wildtype was reduced by 10-30% depending on the length of the His-tag, although Fv/Fm values differed only slightly. Homoplasmic F1 plants were used to isolate PSII cores complexes. The cores contained no detectable traces of LHC or PsaA/B polypeptides, but the main core subunits of PSII could be identified using immunodetection and mass spectroscopy. In addition, Psb27 and PsbS were detected. The presence of the former was presumably due to the preparation method, since PSII complexes located in the stroma are also isolated. In contrast to previous reports, PsbS was solely found as a monomer on SDS-PAGE in the PSII core complexes of tobacco.

Substrate water binding and oxidation in photosystem II

This mini review presents a general introduction to photosystem II with an emphasis on the oxygen evolving complex. An attempt is made to summarise what is currently known about substrate interaction in the oxygen evolving complex of photosystem II in terms of the nature of the substrate, the timing and the location of its binding. As the nature of substrate water binding has a direct bearing on the mechanism of O-O bond formation in PSII, a discussion of O-O bond formation follows the summary of current opinion in substrate interaction.

Water splitting by Photosystem II--where do we go from here?

As this special issue shows, we know quite a lot about the workings of Photosystem II and the oxidation of water to molecular O(2). However, there are still many questions and details that remain to be answered. In this article, I very briefly outline some aspects of Photosystem II electron transport that are crucial for the efficient oxidation of water and require further studies. To fully understand Photosystem II reactions is not only a satisfying intellectual pursuit, but is also an important goal as we develop new solar technologies for the splitting of water into pure O(2) and H(2) for use as a potential fuel source. "As Students of the Past, We Send Greetings to the Students of the Future".

Photosynthetic generation of oxygen

The oxygen in the atmosphere is derived from light-driven oxidation of water at a catalytic centre contained within a multi-subunit enzyme known as photosystem II (PSII). PSII is located in the photosynthetic membranes of plants, algae and cyanobacteria and its oxygen-evolving centre (OEC) consists of four manganese ions and a calcium ion surrounded by a highly conserved protein environment. Recently, the structure of PSII was elucidated by X-ray crystallography thus revealing details of the molecular architecture of the OEC. This structural information, coupled with an extensive knowledge base derived from a wide range of biophysical, biochemical and molecular biological studies, has provided a framework for understanding the chemistry of photosynthetic oxygen generation as well as opening up debate about its evolutionary origin.

Crystal structure of the oxygen-evolving complex of photosystem II

The oxygen in our atmosphere is derived from and maintained by the water-splitting process of photosynthesis. The enzyme that facilitates this reaction and therefore underpins virtually all life on our planet is known as photosystem II (PSII). It is a multisubunit enzyme embedded in the lipid environment of the thylakoid membranes of plants, algae, and cyanobacteria. Powered by light, PSII catalyzes the chemically and thermodynamically demanding reaction of water splitting. In so doing, it releases molecular oxygen into the atmosphere and provides the reducing equivalents required for the conversion of carbon dioxide into the organic molecules of life. Recently, a fully refined structure of an isolated 700 kDa cyanobacterial dimeric PSII complex was elucidated by X-ray crystallography, which gave organizational and structural details of the 19 subunits (16 intrinsic and 3 extrinsic) that make up each monomer and provided information about the position and protein environments of the many different cofactors it binds. The water-splitting site was revealed as a cluster of four Mn ions and a Ca ion surrounded by amino acid side chains, of which six or seven form direct ligands to the metals. The metal cluster was originally modeled as a cubane-like structure composed of three Mn ions and the Ca (2+) linked by oxo bonds and the fourth Mn attached to the cubane via one of its O atoms. New data from X-ray diffraction and X-ray spectroscopy suggest some alternative arrangements. Nevertheless, all of the models are sufficiently similar to provide a basis for discussing the chemistry by which PSII splits water and makes oxygen.

Photoassembly of the Water-Oxidizing Complex in Photosystem II

The light-driven steps in the biogenesis and repair of the inorganic core comprising the O(2)-evolving center of oxygenic photosynthesis (photosystem II water-oxidation complex, PSII-WOC) are reviewed. These steps, known collectively as photoactivation, involve the photoassembly of the free inorganic cofactors to the cofactor-depleted PSII-(apo-WOC) driven by light and produce the active O(2)-evolving core comprised of Mn(4)CaO(x)Cl(y). We focus on the functional role of the inorganic components as seen through the competition with non-native cofactors ("inorganic mutants") on water oxidation activity, the rate of the photoassembly reaction, and on structural insights gained from EPR spectroscopy of trapped intermediates formed in the initial steps of the assembly reaction. A chemical mechanism for the initial steps in photoactivation is given that is based on these data. Photoactivation experiments offer the powerful insights gained from replacement of the native cofactors, which together with the recent X-ray structural data for the resting holoenzyme provide a deeper understanding of the chemistry of water oxidation. We also review some new directions in research that photoactivation studies have inspired that look at the evolutionary history of this remarkable catalyst.

The mobile thylakoid phosphoprotein TSP9 interacts with the light-harvesting complex II and the peripheries of both photosystems

The localization of the plant-specific thylakoid-soluble phosphoprotein of 9 kDa, TSP9, within the chloroplast thylakoid membrane of spinach has been established by the combined use of fractionation, immunoblotting, cross-linking, and mass spectrometry. TSP9 was found to be exclusively confined to the thylakoid membranes, where it is enriched in the stacked grana membrane domains. After mild solubilization of the membranes, TSP9 migrated together with the major light-harvesting antenna (LHCII) of photosystem II (PSII) and with PSII-LHCII supercomplexes upon separation of the protein complexes by either native gel electrophoresis or sucrose gradient centrifugation. Studies with a cleavable cross-linking agent revealed the interaction of TSP9 with both major and minor LHCII proteins as identified by mass spectrometric sequencing. Cross-linked complexes that in addition to TSP9 contain the peripheral PSII subunits CP29, CP26, and PsbS, which form the interface between LHCII and the PSII core, were found. Our observations also clearly suggest an interaction of TSP9 with photosystem I (PSI) as shown by both immunodetection and mass spectrometry. Sequencing identified the peripheral PSI subunits PsaL, PsaF, and PsaE, originating from cross-linked protein complexes of around 30 kDa that also contained TSP9. The distribution of TSP9 among the cross-linked forms was found to be sensitive to conditions such as light exposure. An association of TSP9 with LHCII as well as the peripheries of the photosystems suggests its involvement in regulation of photosynthetic light harvesting.

Deletion of PsbM in tobacco alters the QB site properties and the electron flow within photosystem II

Photosystem II, the oxygen-evolving complex of photosynthetic organisms, includes an intriguingly large number of low molecular weight polypeptides, including PsbM. Here we describe the first knock-out of psbM using a transplastomic, reverse genetics approach in a higher plant. Homoplastomic Delta psbM plants exhibit photoautotrophic growth. Biochemical, biophysical, and immunological analyses demonstrate that PsbM is not required for biogenesis of higher order photosystem II complexes. However, photosystem II is highly light-sensitive, and its activity is significantly decreased in Delta psbM, whereas kinetics of plastid protein synthesis, reassembly of photosystem II, and recovery of its activity are comparable with the wild type. Unlike wild type, phosphorylation of the reaction center proteins D1 and D2 is severely reduced, whereas the redox-controlled phosphorylation of photosystem II light-harvesting complex is reversely regulated in Delta psbM plants because of accumulation of reduced plastoquinone in the dark and a limited photosystem II-mediated electron transport in the light. Charge recombination in Delta psbM measured by thermoluminescence oscillations significantly differs from the 2/6 patterns in the wild type. A simulation program of thermoluminescence oscillations indicates a higher Q(B)/Q(-)(B) ratio in dark-adapted mutant thylakoids relative to the wild type. The interaction of the Q(A)/Q(B) sites estimated by shifts in the maximal thermoluminescence emission temperature of the Q band, induced by binding of different herbicides to the Q(B) site, is changed indicating alteration of the activation energy for back electron flow. We conclude that PsbM is primarily involved in the interaction of the redox components important for the electron flow within, outward, and backward to photosystem II.

Photosystem II: an enzyme of global significance

Photosystem II (PSII) is a multisubunit enzyme embedded in the lipid environment of the thylakoid membranes of plants, algae and cyanobacteria. Powered by light, this enzyme catalyses the chemically and thermodynamically demanding reaction of water splitting. In so doing, it releases dioxygen into the atmosphere and provides the reducing equivalents required for the conversion of CO2 into the organic molecules of life. Recently, a fully refined structure of a 700 kDa cyanobacterial dimeric PSII complex was elucidated by X-ray crystallography which gave organizational and structural details of the 19 subunits (16 intrinsic and three extrinsic) which make up each monomer and provided information about the position and protein environments of 57 different cofactors. The water-splitting site was revealed as a cluster of four Mn ions and a Ca2+ ion surrounded by amino acid side chains, of which six or seven form direct ligands to the metals. The metal cluster was modelled as a cubane-like structure composed of three Mn ions and the Ca2+ linked by oxo-bonds with the fourth Mn attached to the cubane via one of its oxygens. The overall structure of the catalytic site is providing a framework to develop a mechanistic scheme for the water-splitting process, knowledge which could have significant implications for mimicking the reaction in an artificial chemical system.

Lipids in photosystem II: interactions with protein and cofactors

Photosystem II (PSII) is a homodimeric protein-cofactor complex embedded in the thylakoid membrane that catalyses light-driven charge separation accompanied by the oxidation of water during oxygenic photosynthesis. Biochemical analysis of the lipid content of PSII indicates a number of integral lipids, their composition being similar to the average lipid composition of the thylakoid membrane. The crystal structure of PSII at 3.0 A resolution allowed for the first time the assignment of 14 integral lipids within the protein scaffold, all of them being located at the interface of different protein subunits. The reaction centre subunits D1 and D2 are encircled by a belt of 11 lipids providing a flexible environment for the exchange of D1. Three lipids are located in the dimerization interface and mediate interactions between the PSII monomers. Several lipids are located close to the binding pocket of the mobile plastoquinone Q(B), forming part of a postulated diffusion pathway for plastoquinone. Furthermore two lipids were found, each ligating one antenna chlorophyll a. A detailed analysis of lipid-protein and lipid-cofactor interactions allows to derive some general principles of lipid binding pockets in PSII and to suggest possible functional properties of the various identified lipid molecules.

Biochemical and structural analyses of a higher plant photosystem II supercomplex of a photosystem I-less mutant of barley. Consequences of a chronic over-reduction of the plastoquinone pool

Photosystem II of higher plants is a multisubunit transmembrane complex composed of a core moiety and an extensive peripheral antenna system. The number of antenna polypeptides per core complex is modulated following environmental conditions in order to optimize photosynthetic performance. In this study, we used a barley (Hordeum vulgare) mutant, viridis zb63, which lacks photosystem I, to mimic extreme and chronic overexcitation of photosystem II. The mutation was shown to reduce the photosystem II antenna to a minimal size of about 100 chlorophylls per photosystem II reaction centre, which was not further reducible. The minimal photosystem II unit was analysed by biochemical methods and by electron microscopy, and found to consist of a dimeric photosystem II reaction centre core surrounded by monomeric Lhcb4 (chlorophyll protein 29), Lhcb5 (chlorophyll protein 26) and trimeric light-harvesting complex II antenna proteins. This minimal photosystem II unit forms arrays in vivo, possibly to increase the efficiency of energy distribution and provide photoprotection. In wild-type plants, an additional antenna protein, chlorophyll protein 24 (Lhcb6), which is not expressed in viridis zb63, is proposed to associate to this minimal unit and stabilize larger antenna systems when needed. The analysis of the mutant also revealed the presence of two distinct signalling pathways activated by excess light absorbed by photosystem II: one, dependent on the redox state of the electron transport chain, is involved in the regulation of antenna size, and the second, more directly linked to the level of photoinhibitory stress perceived by the cell, participates in regulating carotenoid biosynthesis.

Environmentally modulated phosphoproteome of photosynthetic membranes in the green alga Chlamydomonas reinhardtii

Mapping of in vivo protein phosphorylation sites in photosynthetic membranes of the green alga Chlamydomonas reinhardtii revealed that the major environmentally dependent changes in phosphorylation are clustered at the interface between the photosystem II (PSII) core and its light-harvesting antennae (LHCII). The photosynthetic membranes that were isolated form the algal cells exposed to four distinct environmental conditions affecting photosynthesis: (i) dark aerobic, corresponding to photosynthetic State 1; (ii) dark under nitrogen atmosphere, corresponding to photosynthetic State 2; (iii) moderate light; and (iv) high light. The surface-exposed phosphorylated peptides were cleaved from the membrane by trypsin, methyl-esterified, enriched by immobilized metal affinity chromatography, and sequenced by nanospray-quadrupole time-of-flight mass spectrometry. A total of 19 in vivo phosphorylation sites were mapped in the proteins corresponding to 15 genes in C. reinhardtii. Amino-terminal acetylation of seven proteins was concomitantly determined. Sequenced amino termini of six mature LHCII proteins differed from the predicted ones. The State 1-to-State 2 transition induced phosphorylation of the PSII core components D2 and PsbR and quadruple phosphorylation of a minor LHCII antennae subunit, CP29, as well as phosphorylation of constituents of a major LHCII complex, Lhcbm1 and Lhcbm10. Exposure of the algal cells to either moderate or high light caused additional phosphorylation of the D1 and CP43 proteins of the PSII core. The high light treatment led to specific hyperphosphorylation of CP29 at seven distinct residues, phosphorylation of another minor LHCII constituent, CP26, at a single threonine, and double phosphorylation of additional subunits of a major LHCII complex including Lhcbm4, Lhcbm6, Lhcbm9, and Lhcbm11. Environmentally induced protein phosphorylation at the interface of PSII core and the associated antenna proteins, particularly multiple differential phosphorylations of CP29 linker protein, suggests the mechanisms for control of photosynthetic state transitions and for LHCII uncoupling from PSII under high light stress to allow thermal energy dissipation.

Refinement of the structural model for the Photosystem II supercomplex of higher plants

Recent X-ray structures determined for the Photosystem II (PSII) core complex isolated from cyanobacteria have provided important information for understanding the functionality of this photosynthetic enzyme including its water splitting activity. As yet, no high-resolution structure is available for PSII of plants or eukaryotes in general. However, crystal structures have been determined for some components of plant PSII which together with the cyanobacterial structure can be used to interpret lower resolution structures of plant PSII derived from electron cryomicroscopy (cryo-EM). Here, we utilise the published X-ray structures of a cyanobacterial PSII core, Light Harvesting Complex II (LHCII), PsbP and PsbQ proteins to construct a model of the plant LHCII-PSII supercomplex using a 17 A resolution 3D electron density map of the spinach supercomplex determined by cryo-EM and single particle analysis. In so doing, we tentatively identify the relative positioning of the chlorophylls within the supercomplex and consider energy transfer pathways between the different subunits. The modelling has also allowed density to be assigned to the three extrinsic proteins of plant PSII, PsbO, PsbP and PsbQ associated with the water splitting centre and concluded that although the position of PsbO is the same as in cyanobacteria, PsbP and PsbQ are located in different positions to the cyanobacterial extrinsic PsbU and PsbV proteins.

Evolution and unique bioenergetic mechanisms in oxygenic photosynthesis

Oxygenic photosynthesis is one example of the many bioenergetic pathways utilized by different organisms to harvest energy from the environment. These pathways revolve around a theme of coupling oxidation-reduction reactions to the formation of membrane potential and subsequent ATP synthesis. Although the basic principles underlying bioenergetics are universally conserved, the constituents of the bioenergetic pathways in different organisms have evolved unique aspects to fill an evolutionary niche. Three-dimensional structures of all of the membrane-spanning components of the electron-transfer chain of oxygenic photosynthesis have revealed those unique aspects of this fascinating process, including the unique metallocofactor for catalysis, the determinants of the uniquely high voltage cofactor, and the numerous photoprotective mechanisms that guard against radical damage.

CP43-like chlorophyll binding proteins: structural and evolutionary implications

CP43, encoded by the psbC gene, is a chlorophyll (Chl)-binding protein of Photosystem II (PSII), the water-splitting and oxygen-evolving enzyme of photosynthesis. CP47, encoded by psbB, a Chl-binding protein of PSII, is closely related to CP43. The Chl-binding six transmembrane helical unit typified by CP43, is also structurally related to the N-terminal domains of the PsaA and PsaB proteins of Photosystem I (PSI) as well as to the family of light-harvesting proteins encoded by cyanobacterial isiA genes and prochlorophyte pcb genes. Here we use recent structural information derived for PSII and PSI to review similarities and differences between the various members of the CP43-like class of light-harvesting proteins, exploring both functional and evolutionary implications.

PsbR, a missing link in the assembly of the oxygen-evolving complex of plant photosystem II

The oxygen-evolving complex of eukaryotic photosystem II (PSII) consists of three extrinsic nuclear-encoded subunits, PsbO (33 kDa), PsbP (23 kDa), and PsbQ (17 kDa). Additionally, the 10-kDa PsbR protein has been found in plant PSII and anticipated to play a role in water oxidation, yet the physiological significance of PsbR has remained obscure. Using the Arabidopsis psbR mutant, we showed that the light-saturated rate of oxygen evolution is strongly reduced in the absence of PsbR, particularly in low light-grown plants. Lack of PsbR also induced a reduction in the content of both the PsbP and the PsbQ proteins, and a near depletion of these proteins was observed under steady state low light conditions. This regulation occurred post-transcriptionally and likely involves a proteolytic degradation of the PsbP and PsbQ proteins in the absence of an assembly partner, proposed to be the PsbR protein. Stable assembly of PsbR in the PSII core complex was, in turn, shown to require a chloroplast-encoded intrinsic low molecular mass PSII subunit PsbJ. Our results provided evidence that PsbR is an important link in the PSII core complex for stable assembly of the oxygen-evolving complex protein PsbP, whereas the effects on the assembly of PsbQ are probably indirect. The physiological role of the PsbR, PsbP, and PsbQ proteins is discussed in light of their peculiar expression in response to growth light conditions.

Structure and function of photosystems I and II

Oxygenic photosynthesis, the principal converter of sunlight into chemical energy on earth, is catalyzed by four multi-subunit membrane-protein complexes: photosystem I (PSI), photosystem II (PSII), the cytochrome b(6)f complex, and F-ATPase. PSI generates the most negative redox potential in nature and largely determines the global amount of enthalpy in living systems. PSII generates an oxidant whose redox potential is high enough to enable it to oxidize H(2)O, a substrate so abundant that it assures a practically unlimited electron source for life on earth. During the last century, the sophisticated techniques of spectroscopy, molecular genetics, and biochemistry were used to reveal the structure and function of the two photosystems. The new structures of PSI and PSII from cyanobacteria, algae, and plants has shed light not only on the architecture and mechanism of action of these intricate membrane complexes, but also on the evolutionary forces that shaped oxygenic photosynthesis.

Localization of the PsbH subunit in photosystem II from the Synechocystis 6803 using the His-tagged Ni–NTA Nanogold labeling

The PsbH protein belongs to a group of small protein subunits of photosystem II (PSII) complex. This protein is predicted to have a single transmembrane helix and it is important for the assembly of the PSII complex as well as for the proper function at the acceptor side of PSII. To identify the location of the PsbH subunit, the PSII complex with His-tagged PsbH protein was isolated from the cyanobacterium Synechocystis sp. PCC 6803 and labeled by Ni(2+)-nitrilo triacetic acid Nanogold. Electron microscopy followed by single particle image analysis identified the location of the labeled His-tagged PsbH protein at the periphery of the dimeric PSII complex. These results indicate that the N terminus of the PsbH protein is located at the stromal surface of the PSII complex and close to the CP47 protein.

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.

Structural differences in the inner part of photosystem II between higher plants and cyanobacteria

A detailed comparison of key components in the Photosystem II complexes of higher plants and cyanobacteria was carried out. While the two complexes are overall very similar, significant differences exist in the relative orientation of individual components relative to one another. We compared a three-dimensional map of the inner part of plant PS II at 8 A resolution, and a 5.5 A projection map of the same complex determined by electron crystallography, to the recent 3.5-3.8 A X-ray structures of cyanobacterial complexes. The largest differences were found in the rotational alignment of the cyt b(;)559 subcomplex, and of the CP47 core antenna with respect to the D1/D2 reaction centre. Within the D1/D2 proteins, there are clear differences between plants and cyanobacteria at the stromal ends of membrane-spanning helices, even though these proteins are highly homologous. Notwithstanding these differences in the protein scaffold, the distances between the critical photosynthetic pigment cofactors seem to be precisely conserved. The different protein arrangements in the two complexes may reflect an adaptation to the two very different antenna systems, membrane-extrinsic phycobilisomes for cyanobacteria, and membrane-embedded chlorophyll a/b proteins in plants.

Acetonitrile-induced unfolding of the photosystem II manganese-stabilizing protein studied by electrospray mass spectrometry

In this paper an acetonitrile-induced unfolding of the manganese-stabilizing protein (MSP) of photosystem II was discovered. More distinct unfolding states of MSP were identified than previously by using mainly electrospray ionization mass spectrometry (ESI-MS), together with fluorescence spectra and far-UV circular dichroism (CD) at pH 2.0, 6.2 or 11.6, and with acetonitrile concentrations from 0 to 50%. At pH 6.2 with acetonitrile concentration changing from 0 to 10%, relatively broad charge-state distributions and poor intensity were observed in ESI-MS, indicating the presence of coexisting conformers. It was concluded that the structure of the MSP protein is unlikely to be a tightly folded form. When the concentration of acetonitrile was 20-40%, simulating the state in the biological membrane, changes in the state of unfolding of MSP were observed to a certain extent using ESI-MS, fluorescence and CD spectroscopy. The charge-state distribution in ESI-MS was found to move toward high states (from 13+ to 27+ to 15+ to 31+) with increasing acetonitrile concentration. At pH 2.0, the MSP structure is rearranged into an unfolded state, and at pH 11.6 the MSP structure is induced to assume another unordered state by deprotonation of appropriate residues. An interesting observation was that a second peak envelope emerged with 20-50% acetonitrile in the medium at pH 11.6.

NMR Structural Model of the Interaction of Herbicides with the Photosynthetic Reaction Center from Rhodobacter sphaeroides

The interaction of the herbicides acifluorfen and paraquat with the photosynthetic reaction center from Rhodobacter sphaeroides has been studied by NMR relaxation measurements. Interaction in aqueous solution has been demonstrated by evaluating motional features of the bound form through cross-relaxation terms of protons at fixed distances on the herbicides. Contributions to longitudinal nonselective relaxation rates different from the proton-proton dipolar relaxation were inferred, most probably due to paramagnetic effects originating from the high-spin nonheme Fe(II) ion in the reaction center. Paramagnetic contributions to proton relaxation rates were converted into distance constraints in order to build a model for the interaction. The models place paraquat in the QB site, where most herbicides interact, in agreement with docking calculations, whereas acifluorfen was placed between the metal and the QB site, as also demonstrated by the induced paramagnetic shifts. Acifluorfen could therefore act to break the electron-transfer pathway between the QA and QB sites.

The low molecular mass subunits of the photosynthetic supracomplex, photosystem II

The photosystem II (PSII) complex is located in the thylakoid membrane of higher plants, algae and cyanobacteria and drives the water oxidation process of photosynthesis, which splits water into reducing equivalents and molecular oxygen by solar energy. Electron and X-ray crystallography analyses have revealed that the PSII core complex contains between 34 and 36 transmembrane alpha-helices, depending on the organism. Of these helices at least 12-14 are attributed to low molecular mass proteins. However, to date, at least 18 low molecular mass (<10 kDa) subunits are putatively associated with the PSII complex. Most of them contain a single transmembrane span and their protein sequences are conserved among photosynthetic organisms. In addition, these proteins do not have any similarity to any known functional proteins in any type of organism, and only two of them bind a cofactor. These findings raise intriguing questions about why there are so many small protein subunits with single-transmembrane spans in the PSII complex, and their possible functions. This article reviews our current knowledge of this group of proteins. Deletion mutations of the low molecular mass subunits from both prokaryotic and eukaryotic model systems are compared in an attempt to understand the function of these proteins. From these comparisons it seems that the majority of them are involved in stabilization, assembly or dimerization of the PSII complex. The small proteins may facilitate fast dynamic conformational changes that the PSII complex needs to perform an optimal photosynthetic activity.

Structural characterization of photosystem II complex from red alga Porphyridium cruentum retaining extrinsic subunits of the oxygen-evolving complex

The structure of photosystem II (PSII) complex isolated from thylakoid membranes of the red alga Porphyridium cruentum was investigated using electron microscopy followed by single particle image analysis. The dimeric complexes observed contain all major PSII subunits (CP47, CP43, D1 and D2 proteins) as well as the extrinsic proteins (33 kDa, 12 kDa and the cytochrome c(550)) of the oxygen-evolving complex (OEC) of PSII, encoded by the psbO, psbU and psbV genes, respectively. The single particle analysis of the top-view projections revealed the PSII complex to have maximal dimensions of 22 x 15 nm. The analysis of the side-view projections shows a maximal thickness of the PSII complex of about 9 nm including the densities on the lumenal surface that has been attributed to the proteins of the OEC complex. These results clearly demonstrate that the red algal PSII complex is structurally very similar to that of cyanobacteria and to the PSII core complex of higher plants. In addition, the arrangement of the OEC proteins on the lumenal surface of the PSII complex is consistent to that obtained by X-ray crystallography of cyanobacterial PSII.

Crystal structure of the PsbP protein of photosystem II from Nicotiana tabacum

PsbP is a membrane-extrinsic subunit of the water-oxidizing complex photosystem II (PS II). The evolutionary origin of PsbP has long been a mystery because it specifically exists in higher plants and green algae but not in cyanobacteria. We report here the crystal structure of PsbP from Nicotiana tabacum at a resolution of 1.6 A. Its structure is mainly composed of beta-sheet, and is not similar to any structures in cyanobacterial PS II. However, the electrostatic surface potential of PsbP is similar to that of cyanobacterial PsbV (cyt c(550)), which has a function similar to PsbP. A structural homology search with the DALI algorithm indicated that the folding of PsbP is very similar to that of Mog1p, a regulatory protein for the nuclear transport of Ran GTPase. The structure of PsbP provides insight into its novel function in GTP-regulated metabolism in PS II.

Water, water everywhere, and its remarkable chemistry

Photosystem II (PSII), the multisubunit pigment-protein complex localised in the thylakoid membranes of oxygenic photosynthetic organisms, uses light energy to drive a series of remarkable reactions leading to the oxidation of water. The products of this oxidation are dioxygen, which is released to the atmosphere, and reducing equivalents destined to reduce carbon dioxide to organic molecules. The water oxidation occurs at catalytic sites composed of four manganese atoms (Mn(4)-cluster) and powered by the redox potential of an oxidised chlorophyll a molecule (P680(*+)). Gerald T (Jerry) Babcock and colleagues showed that electron/proton transfer processes from substrate water to P680(*+) involved a tyrosine residue (Y(Z)) and proposed an attractive reaction mechanism for the direct involvement of Y(Z) in the chemistry of water oxidation. The 'hydrogen-atom abstract/metalloradical' mechanism he formulated is an expression of his genius and a highlight of his many other outstanding contributions to photosynthesis research. A structural basis for Jerry's model is now being revealed by X-ray crystallography.

Photosystem II proteins PsbL and PsbJ regulate electron flow to the plastoquinone pool

The psbEFLJ operon of tobacco plastids encodes four bitopic low molecular mass transmembrane components of photosystem II. Here, we report the effect of inactivation of psbL on the directional forward electron flow of photosystem II as compared to that of the wild type and the psbJ deletion mutant, which is impaired in PSII electron flow to plastoquinone [Regel et al. (2001) J. Biol. Chem. 276, 41473-41478]. Exposure of Delta psbL plants to a saturating light pulse gives rise to the maximal fluorescence emission, Fm(L), which is followed within 4-6 s by a broader hitherto not observed second fluorescence peak in darkness, Fm(D). Conditions either facilitating oxidation or avoiding reduction of the plastoquinone pool do not affect the Fm(L) level of Delta psbL plants but prevent the appearance of Fm(D). The level of Fm(D) is proportional to the intensity and duration of the light pulse allowing reduction of the plastoquinone pool in dark-adapted leaves prior to the activation of PSI and oxidation of plastoquinol. Lowering the temperature decreases the Fm(D) level in the Delta psbL mutant, whereas it increases considerably the lifetime of Q(A)*- in the Delta psbJ mutant. The thermoluminescence signal generated by Q(A)*-/S(2) charge recombination is not affected; on the other hand, charge recombination of Q(B)*-/S(2,3) could not be detected in Delta psbL plants. PSII is highly sensitive to photoinhibition in Delta psbL. We conclude that PsbL prevents reduction of PSII by back electron flow from plastoquinol protecting PSII from photoinactivation, whereas PsbJ regulates forward electron flow from Q(A)*- to the plastoquinone pool. Therefore, both proteins contribute substantially to ensure unidirectional forward electron flow from PSII to the plastoquinone pool.

Identification of domains on the extrinsic 23 kDa protein possibly involved in electrostatic interaction with the extrinsic 33 kDa protein in spinach photosystem II

To elucidate the domains on the extrinsic 23 kDa protein involved in electrostatic interaction with the extrinsic 33 kDa protein in spinach photosystem II, we modified amino or carboxyl groups of the 23 kDa protein to uncharged methyl ester groups with N-succinimidyl propionate or glycine methyl ester in the presence of a water-soluble carbodiimide, respectively. The N-succinimidyl propionate-modified 23 kDa protein did not bind to the 33 kDa protein associated with PSII membranes, whereas the glycine methyl ester-modified 23 kDa protein completely bound. This indicates that positive charges on the 23 kDa protein are important for electrostatic interaction with the 33 kDa protein associated with the PSII membranes. Mapping of the N-succinimidyl propionate-modified sites of the 23 kDa protein was performed using Staphylococcus V8 protease digestion of the modified protein followed by determination of the mass of the resultant peptide fragments with MALDI-TOF MS. The results showed that six domains (Lys11-Lys14, Lys27-Lys38, Lys40, Lys90-Lys96, Lys143-Lys152, Lys166-Lys174) were modified with N-succinimidyl propionate. In these domains, Lys11, Lys13, Lys33, Lys38, Lys143, Lys166, Lys170 and Lys174 were wholly conserved in the 23 kDa protein from 12 species of higher plants. These positively charged lysyl residues on the 23 kDa protein may be involved in electrostatic interactions with the negatively charged carboxyl groups on the 33 kDa protein, the latter has been suggested to be important for the 23 kDa binding [Bricker, T.M. & Frankel, L.K. (2003) Biochemistry42, 2056-2061].

Architecture of the photosynthetic oxygen-evolving center

Photosynthesis uses light energy to drive the oxidation of water at an oxygen-evolving catalytic site within photosystem II (PSII). We report the structure of PSII of the cyanobacterium Thermosynechococcus elongatus at 3.5 angstrom resolution. We have assigned most of the amino acid residues of this 650-kilodalton dimeric multisubunit complex and refined the structure to reveal its molecular architecture. Consequently, we are able to describe details of the binding sites for cofactors and propose a structure of the oxygen-evolving center (OEC). The data strongly suggest that the OEC contains a cubane-like Mn3CaO4 cluster linked to a fourth Mn by a mono-micro-oxo bridge. The details of the surrounding coordination sphere of the metal cluster and the implications for a possible oxygen-evolving mechanism are discussed.

FTIR spectroscopy shows structural similarities between photosystems II from cyanobacteria and spinach

Photosystem II (PSII), an essential component of oxygenic photosynthesis, is a membrane-bound pigment protein complex found in green plants and cyanobacteria. Whereas the molecular structure of cyanobacterial PSII has been resolved with at least medium resolution [Zouni, A., Witt, H.-T., Kern, J., Fromme, P., Krauss, N., Saenger, W. & Orth, P. (2001) Nature (London) 409, 739-743; Kamiya, N. & Shen, J.R. (2003) Proc. Natl Acad. Sci. USA 100, 98-103], the structure of higher plant PSII is only known at low resolution. Therefore Fourier transform infrared (FTIR) difference spectroscopy was used to compare PSII from both Thermosynechococcus elongatus and Synechocystis PCC6803 core complexes with PSII-enriched membranes from spinach (BBY). FTIR difference spectra of T. elongatus core complexes are presented for several different intermediates. As the FTIR difference spectra show close similarities among the three species, the structural arrangement of cofactors in PSII and their interactions with the protein microenvironment during photosynthetic charge separation must be very similar in higher plant PSII and cyanobacterial PSII. A structural model of higher plant PSII can therefore be predicted from the structure of cyanobacterial PSII.

Crystal structure of the PsbQ protein of photosystem II from higher plants

The smallest extrinsic polypeptide of the water-oxidizing complex (PsbQ) was extracted and purified from spinach (Spinacia oleracea) photosystem II (PSII) membranes. It was then crystallized in the presence of Zn(2+) and its structure was determined by X-ray diffraction at 1.95-A resolution using the multi-wavelength anomalous diffraction method, with the zinc as the anomalous scatterer. The crystal structure shows that the core of the protein is a four-helix bundle, whereas the amino-terminal portion, which possibly interacts with the photosystem core, is not visible in the crystal. The distribution of positive and negative charges on the protein surface might explain the ability of PsbQ to increase the binding of Cl(-) and Ca(2+) and make them available to PSII.