Structural, biochemical and biophysical characterization of four oxygen-evolving Photosystem II preparations from spinach

Role of isotropic lipid phase in the fusion of photosystem II membranes

It has been thoroughly documented, by using 31P-NMR spectroscopy, that plant thylakoid membranes (TMs), in addition to the bilayer (or lamellar, L) phase, contain at least two isotropic (I) lipid phases and an inverted hexagonal (HII) phase. However, our knowledge concerning the structural and functional roles of the non-bilayer phases is still rudimentary. The objective of the present study is to elucidate the origin of I phases which have been hypothesized to arise, in part, from the fusion of TMs (Garab et al. 2022 Progr Lipid Res 101,163). We take advantage of the selectivity of wheat germ lipase (WGL) in eliminating the I phases of TMs (Dlouhý et al. 2022 Cells 11: 2681), and the tendency of the so-called BBY particles, stacked photosystem II (PSII) enriched membrane pairs of 300-500 nm in diameter, to form large laterally fused sheets (Dunahay et al. 1984 BBA 764: 179). Our 31P-NMR spectroscopy data show that BBY membranes contain L and I phases. Similar to TMs, WGL selectively eliminated the I phases, which at the same time exerted no effect on the molecular organization and functional activity of PSII membranes. As revealed by sucrose-density centrifugation, magnetic linear dichroism spectroscopy and scanning electron microscopy, WGL disassembled the large laterally fused sheets. These data provide direct experimental evidence on the involvement of I phase(s) in the fusion of stacked PSII membrane pairs, and strongly suggest the role of non-bilayer lipids in the self-assembly of the TM system.

Fluorescence quenching in thylakoid membranes induced by single-walled carbon nanotubes

The distinct photochemical and electrochemical properties of single-walled carbon nanotubes (SWCNTs) boosted the research interest in nanomaterial utilization in different in vivo and in vitro photosynthetic biohybrid setups. Aiming to unravel the yet not fully understood energetic interactions between the nanotubes and photosynthetic pigment-protein assemblies in an aqueous milieu, we studied SWCNT effects on the photochemical reactions of isolated thylakoid membranes (TMs), Photosystem II (PSII)-enriched membrane fragments and light-harvesting complexes (LHCII). The SWCNTs induced quenching of the steady-state chlorophyll fluorescence in the TM-biohybrid systems with a corresponding shortening of the average fluorescence lifetimes. The effect was not related to changes in the integrity and macroorganization of the photosynthetic membranes. Moreover, we found no evidence for direct excitation energy exchange between the SWCNTs and pigment-protein complexes, since neither the steady-state nor time-resolved fluorescence of LHCII-biohybrid systems differed from the corresponding controls. The attenuation of the fluorescence signal in the TM-biohybrid systems indicates possible leakage of photosynthetic electrons toward the nanotubes that most probably occurs at the level of the PSII acceptor site. Although it is too early to speculate on the nature of the involved electron donors and intermediate states, the observed energetic interaction could be exploited to increase the photoelectron capture efficiency of natural biohybrid systems for solar energy conversion.

Oxidative modification of LHC II associated with photosystem II and PS I-LHC I-LHC II membranes

Under aerobic conditions the production of Reactive Oxygen Species (ROS) by electron transport chains is unavoidable, and occurs in both autotrophic and heterotrophic organisms. In photosynthetic organisms both Photosystem II (PS II) and Photosystem I (PS I), in addition to the cytochrome b₆/f complex, are demonstrated sources of ROS. All of these membrane protein complexes exhibit oxidative damage when isolated from field-grown plant material. An additional possible source of ROS in PS I and PS II is the distal, chlorophyll-containing light-harvesting array LHC II, which is present in both photosystems. These serve as possible sources of ¹O₂ produced by the interaction of ³O₂ with ³chl* produced by intersystem crossing. We have hypothesized that amino acid residues close to the sites of ROS generation will be more susceptible to oxidative modification than distant residues. In this study, we have identified oxidized amino acid residues in a subset of the spinach LHC II proteins (Lhcb1 and Lhcb2) that were associated with either PS II membranes (i.e. BBYs) or PS I-LHC I-LHC II membranes, both of which were isolated from field-grown spinach. We identified oxidatively modified residues by high-resolution tandem mass spectrometry. Interestingly, two different patterns of oxidative modification were evident for the Lhcb1 and Lhcb2 proteins from these different sources. In the LHC II associated with PS II membranes, oxidized residues were identified to be located on the stromal surface of Lhcb1 and, to a much lesser extent, Lhcb2. Relatively few oxidized residues were identified as buried in the hydrophobic core of these proteins. The LHC II associated with PS I-LHC I-LHC II membranes, however, exhibited fewer surface-oxidized residues but, rather a large number of oxidative modifications buried in the hydrophobic core regions of both Lhcb1 and Lhcb2, adjacent to the chlorophyll prosthetic groups. These results appear to indicate that ROS, specifically ¹O₂, can modify the Lhcb proteins associated with both photosystems and that the LHC II associated with PS II membranes represent a different population from the LHC II associated with PS I-LHC I-LHC II membranes.

Photosystem 2 and the oxygen evolving complex: a brief overview

These special issues of photosynthesis research present papers documenting progress in revealing the many aspects of photosystem 2, a unique, one-of-a-kind complex system that can reduce a plastoquinone to a plastoquinol on every second flash of light and oxidize 2 H₂O to an O₂ on every fourth flash. This overview is a brief personal assessment of the progress observed by the author over a four-decade research career, including a discussion of some remaining unsolved issues. It will come as no surprise to readers that there are remaining questions given the complexity of PS2, and the efforts that have been needed so far to uncover its secrets. In fact, most readers will have their own lists of outstanding questions.

Effective binding of Tb3+ and La3+ cations on the donor side of Mn-depleted photosystem II

The interaction of Tb³⁺ and La³⁺ cations with different photosystem II (PSII) membranes (intact PSII, Ca-depleted PSII (PSII[-Ca]) and Mn-depleted PSII (PSII[-Mn]) membranes) was studied. Although both lanthanide cations (Ln³⁺) interact only with Ca²⁺-binding site of oxygen-evolving complex (OEC) in PSII and PSII(-Ca) membranes, we found that in PSII(-Mn) membranes both Ln³⁺ ions tightly bind to another site localized on the oxidizing side of PSII. Binding of Ln³⁺ cations to this site is not protected by Ca²⁺ and is accompanied by very effective inhibition of Mn²⁺ oxidation at the high-affinity (HA) Mn-binding site ([Mn²⁺  + H₂O₂] couple was used as a donor of electrons). The values of the constant for inhibition of electron transport K_i are equal to 2.10 ± 0.03 µM for Tb³⁺ and 8.3 ± 0.4 µM for La³⁺, whereas OEC inhibition constant in the native PSII membranes is 323 ± 7 µM for Tb³⁺. The value of K_i for Tb³⁺ corresponds to K_i for Mn²⁺ cations in the reaction of diphenylcarbazide oxidation via HA site (1.5 µM) presented in the literature. Our results suggest that Ln³⁺ cations bind to the HA Mn-binding site in PSII(-Mn) membranes like Mn²⁺ or Fe²⁺ cations. Taking into account the fact that Mn²⁺ and Fe²⁺ cations bind to the HA site as trivalent cations after light-induced oxidation and the fact that Mn cation bound to the HA site (Mn4) is also in trivalent state, we can suggest that valency may be important for the interaction of Ln³⁺ with the HA site.

Structural Dynamics of a Protein Domain Relevant to the Water-Oxidizing Complex in Photosystem II as Visualized by High-Speed Atomic Force Microscopy

Photosystem II (PSII) is a multiprotein complex that has a function of light-driven water oxidation. The catalytic site of water oxidation is the Mn₄CaO₅ cluster, which is bound to the lumenal side of PSII through amino acid residues from the D1 and CP43 proteins and is further surrounded by the extrinsic proteins. In this study, we have for the first time visualized the structural dynamics of the lumenal region of a PSII core complex using high-speed atomic force microscopy (HS-AFM). The HS-AFM images of a PSII membrane fragment showed stepwise dissociation of the PsbP and PsbO extrinsic proteins. Upon subsequent destruction of the Mn₄CaO₅ cluster, the lumenal domain of CP43 was found to undergo a conformational fluctuation. The observed structural flexibility and conformational fluctuation of the CP43 lumenal domain are suggested to play important roles in the biogenesis of PSII and the photoassembly of the Mn₄CaO₅ cluster.

The Main Structural and Functional Characteristics of Photosystem-II-Enriched Membranes Isolated from Wild Type and cia3 Mutant Chlamydomonas reinhardtii

Photosystem II (PSII)-enriched membranes retain the original PSII architecture in contrast to PSII cores or PSII supercomplexes, which are usually isolated from Chlamydomonas reinhardtii. Here, we present data that fully characterize the structural and functional properties of PSII complexes in isolated PSII-enriched membranes from C. reinhardtii. The preparations were isolated from wild-type (WT) and CAH3-deficient mutant cia3 as the influence of CAH3 on the PSII function was previously proposed. Based on the equal chlorophyll content, the PSII-enriched membranes from WT and cia3 have the same amount of reaction centers (RCs), cytochrome b559, subunits of the water-oxidizing complex, Mn ions, and carotenes. They differ in the ratio of other carotenoids, the parts of low/intermediate redox forms of cytochrome b559, and the composition of outer light-harvesting complexes. The preparations had 40% more chlorophyll molecules per RC compared to higher plants. Functionally, PSII-enriched membranes from WT and cia3 show the same photosynthetic activity at optimal pH 6.5. However, the preparations from cia3 contained more closed RCs even at pH 6.5 and showed more pronounced suppression of PSII photosynthetic activity at shift pH up to 7.0, established in the lumen of dark-adapted cells. Nevertheless, the PSII photosynthetic capacities remained the same.

Isolation and characterization of a photosystem II preparation from thylakoid membranes of the extreme halophyte Salicornia veneta Pignatti et Lausi

Salicornia veneta (Pignatti et Lausi) is an extreme halophyte living in salt marsh where NaCl concentration may be as high as 1 M. Here we report on the isolation and characterization of a PSII preparation obtained by Triton X-100 solubilisation of the thylakoid membrane. By a combination of gel electrophoresis, immunoblotting and mass spectrometry, the depletion of a number of PSII proteins such as PsbQ, PsbM and PsbT was highlighted. Moreover, the requirement of Cl^- and Ca²⁺ for optimal oxygen evolution was determined, showing that in absence of PsbQ a higher level of these ions are required. At high Cl^- concentrations, oxygen evolution was inhibited in the same way in Salicornia veneta and spinach. Reconstitution of Salicornia veneta PSII preparation with partially purified spinach PsbP and PsbQ restored oxygen evolution activity at low Cl^- and Ca²⁺ concentrations. Adaptation to high salt makes several PSII proteins dispensable.

Spectral dependence of irreversible light-induced fluorescence quenching: Chlorophyll forms with maximal emission at 700-702 and 705-710nm as spectroscopic markers of conformational changes in the core complex

The spectral dependence of the irreversible non-photochemical fluorescence quenching associated with photoinhibition in vitro has been comparatively investigated in thylakoid membranes, PSII enriched particles and PSII core complexes isolated from spinach. The analysis of the fluorescence emission spectra of dark-adapted and quenched samples as a function of the detection temperature in the 280-80K interval, indicates that Chlorophyll spectral forms having maximal emission in the 700-702nm and 705-710nm ranges gain relative intensity in concomitance with the establishment of irreversible light-induced quenching, acting thereby as spectroscopic markers. The relative enhancement of the 700-702nm and 705-710nm forms emission could be due either to an increase of their stoichiometric abundance or to their intrinsically low fluorescence quantum yields. These two factors, that can also coexist, need to be promoted by light-induced alterations in chromophore-protein as well as chromophore-chromophore interactions. The bands centred at about 701 and 706nm are also observed in the PSII core complex, suggesting their, at least partial, localisation in proximity to the reaction centre, and the occurrence of light-induced conformational changes in the core subunits.

Excitation energy transfer and charge separation are affected in Arabidopsis thaliana mutants lacking light-harvesting chlorophyll a/b binding protein Lhcb3

The composition of LHCII trimers as well as excitation energy transfer and charge separation in grana cores of Arabidopsis thaliana mutant lacking chlorophyll a/b binding protein Lhcb3 have been investigated and compared to those in wild-type plants. In grana cores of lhcb3 plants we observed increased amounts of Lhcb1 and Lhcb2 apoproteins per PSII core. The additional copies of Lhcb1 and Lhcb2 are expected to substitute for Lhcb3 in LHCII trimers M as well as in the LHCII "extra" pool, which was found to be modestly enlarged as a result of the absence of Lhcb3. Time-resolved fluorescence measurements reveal a deceleration of the fast phase of excitation dynamics in grana cores of the mutant by ~15 ps, whereas the average fluorescence lifetime is not significantly altered. Monte Carlo modeling predicts a slowing down of the mean hopping time and an increased stabilization of the primary charge separation in the mutant. Thus our data imply that absence of apoprotein Lhcb3 results in detectable differences in excitation energy transfer and charge separation.

Carotenoid triplet states in photosystem II: coupling with low-energy states of the core complex

The photo-excited triplet states of carotenoids, sensitised by triplet-triplet energy transfer from the chlorophyll triplet states, have been investigated in the isolated Photosystem II (PSII) core complex and PSII-LHCII (Light Harvesting Complex II) supercomplex by Optically Detected Magnetic Resonance techniques, using both fluorescence (FDMR) and absorption (ADMR) detection. The absence of Photosystem I allows us to reach the full assignment of the carotenoid triplet states populated in PSII under steady state illumination at low temperature. Five carotenoid triplet ((3)Car) populations were identified in PSII-LHCII, and four in the PSII core complex. Thus, four (3)Car populations are attributed to β-carotene molecules bound to the core complex. All of them show associated fluorescence emission maxima which are relatively red-shifted with respect to the bulk emission of both the PSII-LHCII and the isolated core complexes. In particular the two populations characterised by Zero Field Splitting parameters |D|=0.0370-0.0373 cm(-1)/|E|=0.00373-0.00375 cm(-1) and |D|=0.0381-0.0385 cm(-1)/|E|=0.00393-0.00389 cm(-1), are coupled by singlet energy transfer with chlorophylls which have a red-shifted emission peaking at 705 nm. This observation supports previous suggestions that pointed towards the presence of long-wavelength chlorophyll spectral forms in the PSII core complex. The fifth (3)Car component is observed only in the PSII-LHCII supercomplex and is then assigned to the peripheral light harvesting system.

Nanodomains of cytochrome b6f and photosystem II complexes in spinach grana thylakoid membranes

The cytochrome b6f (cytb6f) complex plays a central role in photosynthesis, coupling electron transport between photosystem II (PSII) and photosystem I to the generation of a transmembrane proton gradient used for the biosynthesis of ATP. Photosynthesis relies on rapid shuttling of electrons by plastoquinone (PQ) molecules between PSII and cytb6f complexes in the lipid phase of the thylakoid membrane. Thus, the relative membrane location of these complexes is crucial, yet remains unknown. Here, we exploit the selective binding of the electron transfer protein plastocyanin (Pc) to the lumenal membrane surface of the cytb6f complex using a Pc-functionalized atomic force microscope (AFM) probe to identify the position of cytb6f complexes in grana thylakoid membranes from spinach (Spinacia oleracea). This affinity-mapping AFM method directly correlates membrane surface topography with Pc-cytb6f interactions, allowing us to construct a map of the grana thylakoid membrane that reveals nanodomains of colocalized PSII and cytb6f complexes. We suggest that the close proximity between PSII and cytb6f complexes integrates solar energy conversion and electron transfer by fostering short-range diffusion of PQ in the protein-crowded thylakoid membrane, thereby optimizing photosynthetic efficiency.

Light harvesting in photosystem II

Water oxidation in photosynthesis takes place in photosystem II (PSII). This photosystem is built around a reaction center (RC) where sunlight-induced charge separation occurs. This RC consists of various polypeptides that bind only a few chromophores or pigments, next to several other cofactors. It can handle far more photons than the ones absorbed by its own pigments and therefore, additional excitations are provided by the surrounding light-harvesting complexes or antennae. The RC is located in the PSII core that also contains the inner light-harvesting complexes CP43 and CP47, harboring 13 and 16 chlorophyll pigments, respectively. The core is surrounded by outer light-harvesting complexes (Lhcs), together forming the so-called supercomplexes, at least in plants. These PSII supercomplexes are complemented by some "extra" Lhcs, but their exact location in the thylakoid membrane is unknown. The whole system consists of many subunits and appears to be modular, i.e., both its composition and organization depend on environmental conditions, especially on the quality and intensity of the light. In this review, we will provide a short overview of the relation between the structure and organization of pigment-protein complexes in PSII, ranging from individual complexes to entire membranes and experimental and theoretical results on excitation energy transfer and charge separation. It will become clear that time-resolved fluorescence data can provide invaluable information about the organization and functioning of thylakoid membranes. At the end, an overview will be given of unanswered questions that should be addressed in the near future.

Composition, architecture and dynamics of the photosynthetic apparatus in higher plants

The process of oxygenic photosynthesis enabled and still sustains aerobic life on Earth. The most elaborate form of the apparatus that carries out the primary steps of this vital process is the one present in higher plants. Here, we review the overall composition and supramolecular organization of this apparatus, as well as the complex architecture of the lamellar system within which it is harbored. Along the way, we refer to the genetic, biochemical, spectroscopic and, in particular, microscopic studies that have been employed to elucidate the structure and working of this remarkable molecular energy conversion device. As an example of the highly dynamic nature of the apparatus, we discuss the molecular and structural events that enable it to maintain high photosynthetic yields under fluctuating light conditions. We conclude the review with a summary of the hypotheses made over the years about the driving forces that underlie the partition of the lamellar system of higher plants and certain green algae into appressed and non-appressed membrane domains and the segregation of the photosynthetic protein complexes within these domains.

A functional component of the transcriptionally active chromosome complex, Arabidopsis pTAC14, interacts with pTAC12/HEMERA and regulates plastid gene expression

The SET domain-containing protein, pTAC14, was previously identified as a component of the transcriptionally active chromosome (TAC) complexes. Here, we investigated the function of pTAC14 in the regulation of plastid-encoded bacterial-type RNA polymerase (PEP) activity and chloroplast development. The knockout of pTAC14 led to the blockage of thylakoid formation in Arabidopsis (Arabidopsis thaliana), and ptac14 was seedling lethal. Sequence and transcriptional analysis showed that pTAC14 encodes a specific protein in plants that is located in the chloroplast associated with the thylakoid and that its expression depends on light. In addition, the transcript levels of all investigated PEP-dependent genes were clearly reduced in the ptac14-1 mutants, while the accumulation of nucleus-encoded phage-type RNA polymerase-dependent transcripts was increased, indicating an important role of pTAC14 in maintaining PEP activity. pTAC14 was found to interact with pTAC12/HEMERA, another component of TACs that is involved in phytochrome signaling. The data suggest that pTAC14 is essential for proper chloroplast development, most likely by affecting PEP activity and regulating PEP-dependent plastid gene transcription in Arabidopsis together with pTAC12.

Jumping mode atomic force microscopy on grana membranes from spinach

The thylakoid membrane system is a complex membrane system that organizes and reorganizes itself to provide plants optimal chemical energy from sunlight under different and varying environmental conditions. Grana membranes are part of this system and contain the light-driven water-splitting enzyme Photosystem II (PSII) and light-harvesting antenna complexes. Here, we present a direct visualization of PSII complexes within grana membranes from spinach. By means of jumping mode atomic force microscopy in liquid, minimal forces were applied between the scanning tip and membrane or protein, allowing complexes to be imaged with high detail. We observed four different packing arrangements of PSII complexes, which occur primarily as dimers: co-linear crystalline rows, nanometric domains of straight or skewed rows, and disordered domains. Upon storing surface-adhered membranes at low temperature prior to imaging, large-scale reorganizations of supercomplexes between PSII and light-harvesting complex II could be induced. The highest resolution images show the existence of membrane domains without obvious topography extending beyond supercomplexes. These observations illustrate the possibility for diffusion of proteins and smaller molecules within these densely packed membranes.

Isolation of photosystem II reaction center complexes from plants

Methods to isolate and purify 6- and 5-Chl D1/D2/Cyt b559 photosystem II (PSII) reaction center (RC) complexes from plants are presented, and the advantages and disadvantages of each procedure are discussed. One of the simpler 6-Chl procedures and a procedure for isolating 5-Chl complexes are described in detail. Furthermore, a rapid procedure that produces relatively large amounts of less pure 6-Chl material (i.e., more nonpigmented protein) is also described. Criteria to assess the purity of PSII RC preparations are presented, and problems associated with each of the isolation procedures are discussed.

Quality control of photosystem II: FtsH hexamers are localized near photosystem II at grana for the swift repair of damage

The reaction center-binding D1 protein of Photosystem II is oxidatively damaged by excessive visible light or moderate heat stress. The metalloprotease FtsH has been suggested as responsible for the degradation of the D1 protein. We have analyzed the distribution and subunit structures of FtsH in spinach thylakoids and various membrane fractions derived from the thylakoids using clear native polyacrylamide gel electrophoresis and Western blot analysis. FtsH was found not only in the stroma thylakoids but also in the Photosystem II-enriched grana membranes. Monomeric, dimeric, and hexameric FtsH proteases were present as major subunit structures in thylakoids, whereas only hexameric FtsH proteases were detected in Triton X-100-solubilized Photosystem II membranes. Importantly, among the membrane fractions examined, hexameric FtsH proteases were most abundant in the Photosystem II membranes. In accordance with this finding, D1 degradation took place in the Photosystem II membranes under light stress. Sucrose density gradient centrifugation analysis of thylakoids and the Photosystem II membranes solubilized with n-dodecyl-β-d-maltoside and a chemical cross-linking study of thylakoids showed localization of FtsH near the Photosystem II light-harvesting chlorophyll-protein supercomplexes in the grana. These results suggest that part of the FtsH hexamers are juxtapositioned to PSII complexes in the grana in darkness, carrying out immediate degradation of the photodamaged D1 protein under light stress.

Arrangement of photosystem II and ATP synthase in chloroplast membranes of spinach and pea

We used cryoelectron tomography to reveal the arrangements of photosystem II (PSII) and ATP synthase in vitreous sections of intact chloroplasts and plunge-frozen suspensions of isolated thylakoid membranes. We found that stroma and grana thylakoids are connected at the grana margins by staggered lamellar membrane protrusions. The stacking repeat of grana membranes in frozen-hydrated chloroplasts is 15.7 nm, with a 4.5-nm lumenal space and a 3.2-nm distance between the flat stromal surfaces. The chloroplast ATP synthase is confined to minimally curved regions at the grana end membranes and stroma lamellae, where it covers 20% of the surface area. In total, 85% of the ATP synthases are monomers and the remainder form random assemblies of two or more copies. Supercomplexes of PSII and light-harvesting complex II (LHCII) occasionally form ordered arrays in appressed grana thylakoids, whereas this order is lost in destacked membranes. In the ordered arrays, each membrane on either side of the stromal gap contains a two-dimensional crystal of supercomplexes, with the two lattices arranged such that PSII cores, LHCII trimers, and minor LHCs each face a complex of the same kind in the opposite membrane. Grana formation is likely to result from electrostatic interactions between these complexes across the stromal gap.

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

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

Protein diffusion and macromolecular crowding in thylakoid membranes

The photosynthetic light reactions of green plants are mediated by chlorophyll-binding protein complexes located in the thylakoid membranes within the chloroplasts. Thylakoid membranes have a complex structure, with lateral segregation of protein complexes into distinct membrane regions known as the grana and the stroma lamellae. It has long been clear that some protein complexes can diffuse between the grana and the stroma lamellae, and that this movement is important for processes including membrane biogenesis, regulation of light harvesting, and turnover and repair of the photosynthetic complexes. In the grana membranes, diffusion may be problematic because the protein complexes are very densely packed (approximately 75% area occupation) and semicrystalline protein arrays are often observed. To date, direct measurements of protein diffusion in green plant thylakoids have been lacking. We have developed a form of fluorescence recovery after photobleaching that allows direct measurement of the diffusion of chlorophyll-protein complexes in isolated grana membranes from Spinacia oleracea. We show that about 75% of fluorophores are immobile within our measuring period of a few minutes. We suggest that this immobility is due to a protein network covering a whole grana disc. However, the remaining fraction is surprisingly mobile (diffusion coefficient 4.6 +/- 0.4 x 10(-11) cm(2) s(-1)), which suggests that it is associated with mobile proteins that exchange between the grana and stroma lamellae within a few seconds. Manipulation of the protein-lipid ratio and the ionic strength of the buffer reveals the roles of macromolecular crowding and protein-protein interactions in restricting the mobility of grana proteins.

Proteomic profiles of thylakoid membranes and changes in response to iron deficiency

The proteomic profile of thylakoid membranes and the changes induced in that proteome by iron deficiency have been studied by using thylakoid preparations from Beta vulgaris plants grown in hydroponics. Two different 2-D electrophoresis approaches have been used to study these proteomes: isoelectrical focusing followed by SDS PAGE (IEF-SDS PAGE) and blue-native polyacrylamide gel electrophoresis followed by SDS PAGE (BN-SDS PAGE). These techniques resolved approximately 110-140 and 40 polypeptides, respectively. Iron deficiency induced significant changes in the thylakoid sugar beet proteome profiles: the relative amounts of electron transfer protein complexes were reduced, whereas those of proteins participating in leaf carbon fixation-linked reactions were increased. A set of polypeptides, which includes several enzymes related to metabolism, was detected in thylakoid preparations from Fe-deficient Beta vulgaris leaves by using BN-SDS PAGE, suggesting that they may be associated with these thylakoids in vivo. The BN-SDS PAGE technique has been proven to be a better method than IEF-SDS PAGE to resolve highly hydrophobic integral membrane proteins from thylakoid preparations, allowing for the identification of complexes and determination of their polypeptidic components.

Evidence for PSII donor-side damage and photoinhibition induced by cadmium treatment on rice (Oryza sativa L.)

The effects of cadmium (from 7.5 to 75 microM) on chloroplasts of rice were studied at the structural and biochemical level. Loss of pigments, reduction of thylakoids and decrease in oxygen evolution and Fv/Fm ratio occur in leaves following cadmium treatment. However, the amount of photosystem II reaction center proteins and that of its light harvesting complex is not affected, indicating that cadmium does not adversely influence the structural organization of this photosystem. In thylakoids isolated from cadmium-treated plants a loss in the capability to reduce 2,6-dichlorophenolindophenol is observed, which is partially restored if diphenylcarbazide is used as an electron donor, indicating that cadmium affects water splitting activity. In thylakoids isolated from control plants and treated with cadmium, diphenylcarbazide preserves most of the photosystem II activity lost after incubation with cadmium; most of the S(2) multiline electron paramagnetic resonance signal from the manganese cluster is lost, whereas the TyrD(+) and other signals are retained. Light-induced photosystem II damage, in vitro, is promoted by Cd-treatment as deduced from the mobility shift of the D1 protein observed by immunoblot.

The size of the population of weakly coupled chlorophyll pigments involved in thylakoid photoinhibition determined by steady-state fluorescence spectroscopy

On the basis of experiments with singlet quenchers and in agreement with previous data, it is suggested that a population of energetically weakly coupled chlorophylls may play a central role in photoinhibition in vivo and in vitro. In the present study, we have used steady state fluorescence techniques to gain direct evidence for these uncoupled chlorophylls. Due to the presence of their emission maxima, near 650 nm and more prominently in the 670--675 nm interval both chlorophylls b and a seem to be involved. A straightforward mathematical model is developed to describe the data which allows us to conclude that the uncoupled/weakly coupled population size is in the range of 1--3 molecules per photosystem.

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.

A study of molecular interactions in light-harvesting complexes LHCIIb, CP29, CP26 and CP24 by Stark effect spectroscopy

Electric field-induced absorption changes (electrochromism or Stark effect) of the light-harvesting PSII pigment-protein complexes LHCIIb, CP29, CP26 and CP24 were investigated. The results indicate the lack of strong intermolecular interactions in the chlorophyll a (Chl a) pools of all complexes. Characteristic features occur in the electronic spectrum of Chl b, which reflect the increased values of dipole moment and polarizability differences between the ground and excited states of interacting pigment systems. The strong Stark signal recorded for LHCIIb at 650-655 nm is much weaker in CP29, where it is replaced by a unique Stark band at 639 nm. Electrochromism of Chl b in CP26 and CP24 is significantly weaker but increased electrochromic parameters were also noticed for the Chl b transition at 650 nm. The spectra in the blue region are dominated by xanthophylls. The differences in Stark spectra of Chl b are linked to differences in pigment content and organization in individual complexes and point to the possibility of electron exchange interactions between energetically similar and closely spaced Chl b molecules.

Photoinhibition in vivo and in vitro involves weakly coupled chlorophyll-protein complexes

In the present study the analysis of the relation between the excited state population in the photosystem II (PSII) antenna and photoinactivation has been extended from an in vitro system, isolated thylakoids, to an in vivo system, Chlamydomonas reinhardtii cells. The results indicate that the excited state quenching by an added singlet quencher induces maximal protection against photoinhibition of about 30% of that expected on the basis of the observed light intensity-treatment time reciprocity rule. Similar results, obtained previously with thylakoids, have been interpreted in terms of damaged or incorrectly assembled complexes that play an important role in photoinhibition in the thylakoid membranes (Santabarbara, S., K. Neverov, F. M. Garlaschi, G. Zucchelli and R. C. Jennings [2001] Involvement of uncoupled antenna chlorophylls in photoinhibition in thylakoids. FEBS Lett. 491, 109-113.). In an attempt to better define this aspect, the photoinhibition action spectra were determined for mutant barley thylakoids, lacking the chlorophyll (Chl) a-b complexes of the outer antenna, and for its wild type. The results indicate that in both systems the action spectra are significantly blueshifted (2-4 nm) and are broader than the PSII absorption in the membranes. These data are interpreted in terms of a heterogeneous population of outer and inner antenna pigment-protein complexes that contain significant levels of uncoupled Chl.

Heterogeneity of the main light-harvesting chlorophyll a/b-protein complex of photosystem II (LHCII) at the level of trimeric subunits

To study organization of the main light-harvesting chlorophyll a/b-protein complex of photosystem II (LHCII) from spinach thylakoid membranes at the level of trimeric subcomplexes, we have applied non-denaturing isoelectric focusing (ndIEF) in vertical, slab polyacrylamide gels. When analyzed by two consecutive ndIEF/electroelution runs, spinach BBY membrane preparations (PSII(alpha)-enriched, stacked thylakoid membranes) were resolved into nine fractions of 100% purity, labelled 1-9 in order of decreasing pI values. Seven of these fractions (3-9) were shown by absorption spectroscopy to stand for LHCII subcomplexes. The subcomplexes were established - by monitoring their circular dichroism spectra and comparing them to the spectra of native LHCII trimers and monomers - to be structurally intact trimers. The analysis of polypeptide composition of the subcomplexes in terms of apparent molecular masses and Lhcb genes' products led us to the conclusion that each of the subcomplexes might be a mixed population of closely similar individual trimers, comprising of permutations of Lhcb1 and Lhcb2 (subcomplexes 3-7) or Lhcb1, Lhcb2 and Lhcb3 (subcomplexes 8 and 9).

Identification of Lhcb1/Lhcb2/Lhcb3 heterotrimers of the main light-harvesting chlorophyll a/b-protein complex of Photosystem II (LHC II)

Using non-denaturing isoelectric focusing in polyacrylamide vertical slab gel, we have purified to homogeneity three trimeric subcomplexes of LHC II from Arabidopsis thylakoid membranes. The polypeptide composition of the subcomplexes were studied by immunoblotting. Our results indicate the existence in vivo of LHC II heterotrimers containing Lhcb1, Lhcb2 and Lhcb3 gene products.

Involvement of uncoupled antenna chlorophylls in photoinhibition in thylakoids

Evidence is presented, by means of both fluorescence and action spectroscopy, that a small, spectroscopically heterogeneous population of both Chl a and Chl b molecules is present in isolated spinach thylakoids and is active in photoinhibition. The broadness of the action spectrum suggests that degraded or incompletely assembled pigment-protein complexes may be involved.

Arrangement of photosystem II supercomplexes in crystalline macrodomains within the thylakoid membrane of green plant chloroplasts

The chloroplast thylakoid membrane of green plants is organized in stacked grana membranes and unstacked stroma membranes. We investigated the structural organization of Photosystem II (PSII) in paired grana membrane fragments by transmission electron microscopy. The membrane fragments were obtained by a short treatment of thylakoid membranes with the mild detergent n-dodecyl-alpha, d-maltoside and are thought to reflect the grana membranes in a native state. The membranes frequently show crystalline macrodomains in which PSII is organized in rows spaced by either 26.3 nm (large-spaced crystals) or 23 nm (small-spaced crystals). The small-spaced crystals are less common but better ordered. Image analysis of the crystals by an aperiodic approach revealed the precise positions of the core parts of PSII in the lattices, as well as features of the peripheral light-harvesting antenna. Together, they indicate that the so-called C(2)S(2) and C(2)S(2)M supercomplexes form the basic motifs of the small-spaced and large-spaced crystals, respectively. An analysis of a pair of membranes with a well-ordered large-spaced crystal reveals that many PSII complexes in one layer face only light-harvesting complexes (LHCII) in the other layer. The implications of this type of organization for the efficient transfer of excitation energy from LHCII to PSII and for the stacking of grana membranes are discussed.

Contribution of LHC II complex to the electric properties of thylakoid membranes: an electric light scattering study of Chl b-less barley mutant

Electric light scattering measurements demonstrate a strong decline in the permanent electric dipole moment and electric polarizability of both thylakoid membranes and photosystem II-enriched particles of the Chlorina f2 mutant which has severely reduced levels of light-harvesting chlorophyll a/b-binding proteins compared to the wild type barley chloroplasts. The shift in the electric polarizability relaxation to higher frequencies in thylakoids and photosystem II particles from Chlorina f2 reflects higher mobility of the interfacial charges of the mutant than that of the wild type membranes. The experimental data strongly suggest that the major light-harvesting complex of photosystem II directly contribute to the electric properties of thylakoid membranes.

Multiple light-harvesting II polypeptides from maize mesophyll chloroplasts are distinct gene products

The major light-harvesting complex of photosystem II in higher plants is known as LHCII. It is composed of a number of chlorophyll-binding proteins sharing epitopes with each other. The number of apoproteins resolved by fully denaturing sodium dodecylsulfate polyacrylamide gel electrophoresis varies in different species. In order to know if this heterogeneity is caused by the expression of a number of homologous genes or if it is the product of post-translational modifications, we have resolved the six major apoproteins of Zea mays LHCII. Each protein is purified to homogeneity, subjected to direct protein sequencing and the sequences compared with those deduced from lhcb genes in maize and other organisms. All of the six proteins are distinct gene products, since they show differences in their primary structure. Three apoproteins are identified as products of type I lhcb genes and one each as type II and type III gene products. A sixth protein does not fit the requirements for any of the lhcb genes so far cloned and is therefore probably the product of an lhcb gene type not yet described. Our results clearly show that the major source of LHCII protein heterogeneity is the expression of many lhcb genes. Fractionation of maize LHCII by non-denaturing flat-bed isoelectric focusing resolves at least five major isoforms showing distinct differences in their polypeptide composition and also differing in their spectroscopic properties, thus suggesting that individual Lhcb gene products have distinct pigment-binding properties.

Multiple crystal types reveal photosystem II to be a dimer

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

Amine binding and oxidation at the catalytic site for photosynthetic water oxidation

Photosynthetic water oxidation occurs at the Mn-containing catalytic site of photosystem II (PSII). By the use of 14C-labeled amines and SDS-denaturing PAGE, covalent adducts derived from primary amines and the PSII subunits, CP47, D2/D1, and the Mn-stabilizing protein, can be observed. When PSII contains the 18- and 24-kDa extrinsic proteins, which restrict access to the active site, no 14C labeling is obtained. NaCl, but not Na2SO4, competes with 14C labeling in Mn-containing PSII preparations, and the concentration dependence of this competition parallels the activation of oxygen evolution. Formation of 14C-labeled adducts is observed in the presence or in the absence of a functional manganese cluster. However, no significant Cl- effect on 14C labeling is observed in the absence of the Mn cluster. Isolation and quantitation of the 14C-labeled aldehyde product, produced from [14C]benzylamine, gives yields of 1. 8 +/- 0.3 mol/mol PSII and 2.9 +/- 0.2 mol/mol in Mn-containing and Mn-depleted PSII, respectively. The corresponding specific activities are 0.40 +/- 0.07 micromol(micromol PSII-hr)-1 and 0.64 +/- 0.04 micromol(micromol PSII-hr)-1. Cl- suppresses the production of [14C]benzaldehyde in Mn-containing PSII, but does not suppress the production in Mn-depleted preparations. Control experiments show that these oxidation reactions do not involve the redox-active tyrosines, D and Z. Our results suggest the presence of one or more activated carbonyl groups in protein subunits that form the active site of PSII.

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.