Localization and functional characterization of the extrinsic subunits of photosystem II: an update

Evolutionary diversity of proton and water channels on the oxidizing side of photosystem II and their relevance to function

One of the reasons for the high efficiency and selectivity of biological catalysts arise from their ability to control the pathways of substrates and products using protein channels, and by modulating the transport in the channels using the interaction with the protein residues and the water/hydrogen-bonding network. This process is clearly demonstrated in Photosystem II (PS II), where its light-driven water oxidation reaction catalyzed by the Mn4CaO5 cluster occurs deep inside the protein complex and thus requires the transport of two water molecules to and four protons from the metal center to the bulk water. Based on the recent advances in structural studies of PS II from X-ray crystallography and cryo-electron microscopy, in this review we compare the channels that have been proposed to facilitate this mass transport in cyanobacteria, red and green algae, diatoms, and higher plants. The three major channels (O1, O4, and Cl1 channels) are present in all species investigated; however, some differences exist in the reported structures that arise from the different composition and arrangement of membrane extrinsic subunits between the species. Among the three channels, the Cl1 channel, including the proton gate, is the most conserved among all photosynthetic species. We also found at least one branch for the O1 channel in all organisms, extending all the way from Ca/O1 via the 'water wheel' to the lumen. However, the extending path after the water wheel varies between most species. The O4 channel is, like the Cl1 channel, highly conserved among all species while having different orientations at the end of the path near the bulk. The comparison suggests that the previously proposed functionality of the channels in T. vestitus (Ibrahim et al., Proc Natl Acad Sci USA 117:12624-12635, 2020; Hussein et al., Nat Commun 12:6531, 2021) is conserved through the species, i.e. the O1-like channel is used for substrate water intake, and the tighter Cl1 and O4 channels for proton release. The comparison does not eliminate the potential role of O4 channel as a water intake channel. However, the highly ordered hydrogen-bonded water wire connected to the Mn4CaO5 cluster via the O4 may strongly suggest that it functions in proton release, especially during the S0 → S1 transition (Saito et al., Nat Commun 6:8488, 2015; Kern et al., Nature 563:421-425, 2018; Ibrahim et al., Proc Natl Acad Sci USA 117:12624-12635, 2020; Sakashita et al., Phys Chem Chem Phys 22:15831-15841, 2020; Hussein et al., Nat Commun 12:6531, 2021).

Cell cycle of microalga Isochrysis galbana arrested by neurotoxin β-N-methylamino-l-alanine and corresponding molecular mechanisms

The phycotoxin β-N-methylamino-l-alanine (BMAA) has attracted attention due to its risks to marine organisms and human health. In this study, approximately 85 % of synchronized cells of the marine microalga Isochrysis galbana were arrested at the cell cycle G1 phase by BMAA at 6.5 μM for a 24-h exposure. The concentration of chlorophyll a (Chl a) gradually decreased, while the maximum quantum yield of PSII (F_v/F_m), the maximum relative electron transport rate (rETR_max), light utilization efficiency (α) and half-saturated light irradiance (I_k) reduced early and recovered gradually in I. galbana exposed to BMAA in 96-h batch cultures. Transcriptional expression of I. galbana analyzed at 10, 12, and 16 h disclosed multiple mechanisms of BMAA to suppress the microalgal growth. Production of ammonia and glutamate was limited by the down-regulation of nitrate transporters, glutamate synthase, glutamine synthetase, cyanate hydrolase, and formamidase. Diverse extrinsic proteins related to PSII, PSI, cytochrome b6f complex, and ATPase were influenced by BMAA at transcriptional level. Suppression of the DNA replication and mismatch repair pathways increased the accumulation of misfolded proteins, which was reflected by the up-regulated expression of proteasome to accelerate proteolysis. This study improves our understanding of the chemical ecology impacts of BMAA in marine ecosystems.

The Functions of Chloroplastic Ascorbate in Vascular Plants and Algae

Ascorbate (Asc) is a multifunctional metabolite essential for various cellular processes in plants and animals. The best-known property of Asc is to scavenge reactive oxygen species (ROS), in a highly regulated manner. Besides being an effective antioxidant, Asc also acts as a chaperone for 2-oxoglutarate-dependent dioxygenases that are involved in the hormone metabolism of plants and the synthesis of various secondary metabolites. Asc also essential for the epigenetic regulation of gene expression, signaling and iron transport. Thus, Asc affects plant growth, development, and stress resistance via various mechanisms. In this review, the intricate relationship between Asc and photosynthesis in plants and algae is summarized in the following major points: (i) regulation of Asc biosynthesis by light, (ii) interaction between photosynthetic and mitochondrial electron transport in relation to Asc biosynthesis, (iii) Asc acting as an alternative electron donor of photosystem II, (iv) Asc inactivating the oxygen-evolving complex, (v) the role of Asc in non-photochemical quenching, and (vi) the role of Asc in ROS management in the chloroplast. The review also discusses differences in the regulation of Asc biosynthesis and the effects of Asc on photosynthesis in algae and vascular plants.

Binding and functions of the two chloride ions in the oxygen-evolving center of photosystem II

Light-driven water oxidation in photosynthesis occurs at the oxygen-evolving center (OEC) of photosystem II (PSII). Chloride ions (Cl^-) are essential for oxygen evolution by PSII, and two Cl^- ions have been found to specifically bind near the Mn₄CaO₅ cluster in the OEC. The retention of these Cl^- ions within the OEC is critically supported by some of the membrane-extrinsic subunits of PSII. The functions of these two Cl^- ions and the mechanisms of their retention both remain to be fully elucidated. However, intensive studies performed recently have advanced our understanding of the functions of these Cl^- ions, and PSII structures from various species have been reported, aiding the interpretation of previous findings regarding Cl^- retention by extrinsic subunits. In this review, we summarize the findings to date on the roles of the two Cl^- ions bound within the OEC. Additionally, together with a short summary of the functions of PSII membrane-extrinsic subunits, we discuss the mechanisms of Cl^- retention by these extrinsic subunits.

D139N mutation of PsbP enhances the oxygen-evolving activity of photosystem II through stabilized binding of a chloride ion

Photosystem II (PSII) is a multisubunit membrane protein complex that catalyzes light-driven oxidation of water to molecular oxygen. The chloride ion (Cl-) has long been known as an essential cofactor for oxygen evolution by PSII, and two Cl- ions (Cl-1 and Cl-2) have been found to specifically bind near the Mn4CaO5 cluster within the oxygen-evolving center (OEC). However, despite intensive studies on these Cl- ions, little is known about the function of Cl-2, the Cl- ion that is associated with the backbone nitrogens of D1-Asn338, D1-Phe339, and CP43-Glu354. In green plant PSII, the membrane extrinsic subunits-PsbP and PsbQ-are responsible for Cl- retention within the OEC. The Loop 4 region of PsbP, consisting of highly conserved residues Thr135-Gly142, is inserted close to Cl-2, but its importance has not been examined to date. Here, we investigated the importance of PsbP-Loop 4 using spinach PSII membranes reconstituted with spinach PsbP proteins harboring mutations in this region. Mutations in PsbP-Loop 4 had remarkable effects on the rate of oxygen evolution by PSII. Moreover, we found that a specific mutation, PsbP-D139N, significantly enhances the oxygen-evolving activity in the absence of PsbQ, but not significantly in its presence. The D139N mutation increased the Cl- retention ability of PsbP and induced a unique structural change in the OEC, as indicated by light-induced Fourier transform infrared (FTIR) difference spectroscopy and theoretical calculations. Our findings provide insight into the functional significance of Cl-2 in the water-oxidizing reaction of PSII.

Assembly of D1/D2 complexes of photosystem II: Binding of pigments and a network of auxiliary proteins

Photosystem II (PSII) is the multi-subunit light-driven oxidoreductase that drives photosynthetic electron transport using electrons extracted from water. To investigate the initial steps of PSII assembly, we used strains of the cyanobacterium Synechocystis sp. PCC 6803 arrested at early stages of PSII biogenesis and expressing affinity-tagged PSII subunits to isolate PSII reaction center assembly (RCII) complexes and their precursor D1 and D2 modules (D1mod and D2mod). RCII preparations isolated using either a His-tagged D2 or a FLAG-tagged PsbI subunit contained the previously described RCIIa and RCII* complexes that differ with respect to the presence of the Ycf39 assembly factor and high light-inducible proteins (Hlips) and a larger complex consisting of RCIIa bound to monomeric PSI. All RCII complexes contained the PSII subunits D1, D2, PsbI, PsbE, and PsbF and the assembly factors rubredoxin A and Ycf48, but we also detected PsbN, Slr1470, and the Slr0575 proteins, which all have plant homologs. The RCII preparations also contained prohibitins/stomatins (Phbs) of unknown function and FtsH protease subunits. RCII complexes were active in light-induced primary charge separation and bound chlorophylls (Chls), pheophytins, beta-carotenes, and heme. The isolated D1mod consisted of D1/PsbI/Ycf48 with some Ycf39 and Phb3, while D2mod contained D2/cytochrome b559 with co-purifying PsbY, Phb1, Phb3, FtsH2/FtsH3, CyanoP, and Slr1470. As stably bound, Chl was detected in D1mod but not D2mod, formation of RCII appears to be important for stable binding of most of the Chls and both pheophytins. We suggest that Chl can be delivered to RCII from either monomeric Photosystem I or Ycf39/Hlips complexes.

Regulation of antioxidant defense and glyoxalase systems in cyanobacteria

Oxidative stress is common consequence of abiotic stress in plants as well as cyanobacteria caused by generation of reactive oxygen species (ROS), an inevitable product of respiration and photosynthetic electron transport. ROS act as signalling molecule at low concentration however, when its production exceeds the endurance capacity of antioxidative defence system, the organisms suffer oxidative stress. A highly toxic metabolite, methylglyoxal (MG) is also produced in cyanobacteria in response to various abiotic stresses which consequently augment the ensuing oxidative damage. Taking recourse to the common lineage of eukaryotic plants and cyanobacteria, it would be worthwhile to explore the regulatory role of glyoxalase system and antioxidative defense mechanism in combating abiotic stress in cyanobacteria. This review provides comprehensive information on the complete glyoxalase system (GlyI, GlyII and GlyIII) in cyanobacteria. Furthermore, it elucidates the recent understanding regarding the production of ROS and MG, noteworthy link between intracellular MG and ROS and its detoxification via synchronization of antioxidants (enzymatic and non-enzymatic) and glyoxalase systems using glutathione (GSH) as common co-factor.

Structural insights into a dimeric Psb27-photosystem II complex from a cyanobacterium Thermosynechococcus vulcanus

Photosystem II (PSII) is a multisubunit pigment-protein complex and catalyzes light-driven water oxidation, leading to the conversion of light energy into chemical energy and the release of molecular oxygen. Psb27 is a small thylakoid lumen-localized protein known to serve as an assembly factor for the biogenesis and repair of the PSII complex. The exact location and binding fashion of Psb27 in the intermediate PSII remain elusive. Here, we report the structure of a dimeric Psb27-PSII complex purified from a psbV deletion mutant (ΔPsbV) of the cyanobacterium Thermosynechococcus vulcanus, solved by cryo-electron microscopy. Our structure showed that Psb27 is associated with CP43 at the luminal side, with specific interactions formed between Helix 2 and Helix 3 of Psb27 and a loop region between Helix 3 and Helix 4 of CP43 (loop C) as well as the large, lumen-exposed and hydrophilic E-loop of CP43. The binding of Psb27 imposes some conflicts with the N-terminal region of PsbO and also induces some conformational changes in CP43, CP47, and D2. This makes PsbO unable to bind in the Psb27-PSII. Conformational changes also occurred in D1, PsbE, PsbF, and PsbZ; this, together with the conformational changes occurred in CP43, CP47, and D2, may prevent the binding of PsbU and induce dissociation of PsbJ. This structural information provides important insights into the regulation mechanism of Psb27 in the biogenesis and repair of PSII.

Trehalose matrix effects on electron transfer in Mn-depleted protein-pigment complexes of Photosystem II

The kinetics of flash-induced re-reduction of the Photosystem II (PS II) primary electron donor P₆₈₀ was studied in solution and in trehalose glassy matrices at different relative humidity. In solution, and in the re-dissolved glass, kinetics were dominated by two fast components with lifetimes in the range of 2-7 μs, which accounted for >85% of the decay. These components were ascribed to the direct electron transfer from the redox-active tyrosine Y_Z to P₆₈₀⁺. The minor slower components were due to charge recombination between the primary plastoquinone acceptor Q_A^- and P₆₈₀⁺. Incorporation of the PS II complex into the trehalose glassy matrix and its successive dehydration caused a progressive increase in the lifetime of all kinetic phases, accompanied by an increase of the amplitudes of the slower phases at the expense of the faster phases. At 63% relative humidity the fast components contribution dropped to ~50%. A further dehydration of the trehalose glass did not change the lifetimes and contribution of the kinetic components. This effect was ascribed to the decrease of conformational mobility of the protein domain between Y_Z and P₆₈₀, which resulted in the inhibition of Y_Z → P₆₈₀⁺ electron transfer in about half of the PS II population, wherein the recombination between Q_A^- and P₆₈₀⁺ occurred. The data indicate that PS II binds a larger number of water molecules as compared to PS I complexes. We conclude that our data disprove the "water replacement" hypothesis of trehalose matrix biopreservation.

Function of PsbO-Asp158 in photosystem II: effects of mutation of this residue on the binding of PsbO and function of PSII in Thermosynechococcus vulcanus

PsbO-D158 is a highly conserved residue of the PsbO protein in photosystem II (PSII), and participates in one of the hydrogen-bonding networks connecting the manganese cluster with the lumenal surface. In order to examine the role of PsbO-D158, we mutated it to E, N or K in Thermosynechococcus vulcanus and characterized photosynthetic properties of the mutants obtained. The growth rates of these three mutants were similar to that of the wild type, whereas the oxygen-evolving activity of the three mutant cells decreased to 60-64% of the wild type. Fluorescence kinetics showed that the mutations did not affect the electron transfer from Q_A to Q_B, but slightly affected the donor side of PSII. Moreover, all of the three mutant cells were more sensitive to high light and became slower to recover from photoinhibition. In the isolated thylakoid membranes from the three mutants, the PsbU subunit was lost and the oxygen-evolving activity was reduced to a lower level compared to that in the respective cells. PSII complexes isolated from these mutants showed no oxygen-evolving activity, which was found to be due to large or complete loss of PsbO, PsbV and PsbU during the process of purification. Moreover, PSII cores purified from the three mutants contained Psb27, an assembly co-factor of PSII. These results suggest that PsbO-D158 is required for the proper binding of the three extrinsic proteins to PSII and plays an important role in maintaining the optimal oxygen-evolving activity, and its mutation caused incomplete assembly of the PSII complex.

Light-driven formation of manganese oxide by today's photosystem II supports evolutionarily ancient manganese-oxidizing photosynthesis

Water oxidation and concomitant dioxygen formation by the manganese-calcium cluster of oxygenic photosynthesis has shaped the biosphere, atmosphere, and geosphere. It has been hypothesized that at an early stage of evolution, before photosynthetic water oxidation became prominent, light-driven formation of manganese oxides from dissolved Mn(2+) ions may have played a key role in bioenergetics and possibly facilitated early geological manganese deposits. Here we report the biochemical evidence for the ability of photosystems to form extended manganese oxide particles. The photochemical redox processes in spinach photosystem-II particles devoid of the manganese-calcium cluster are tracked by visible-light and X-ray spectroscopy. Oxidation of dissolved manganese ions results in high-valent Mn(III,IV)-oxide nanoparticles of the birnessite type bound to photosystem II, with 50-100 manganese ions per photosystem. Having shown that even today’s photosystem II can form birnessite-type oxide particles efficiently, we propose an evolutionary scenario, which involves manganese-oxide production by ancestral photosystems, later followed by down-sizing of protein-bound manganese-oxide nanoparticles to finally yield today’s catalyst of photosynthetic water oxidation.

Photosynthetic formation of manganese (Mn) oxides from dissolved Mn ions was proposed to occur in ancestral photosystems before oxygenic photosynthesis evolved. Here, the authors provide evidence for this hypothesis by showing that photosystem II devoid of the Mn cluster oxidises Mn ions leading to formation of Mn-oxide nanoparticles.

PsbQ-Like Protein 3 Functions as an Assembly Factor for the Chloroplast NADH Dehydrogenase-Like Complex in Arabidopsis

Angiosperms have three PsbQ-like (PQL) proteins in addition to the PsbQ subunit of the oxygen-evolving complex of photosystem II. Previous studies have shown that two PQL proteins, PnsL2 and PnsL3, are subunits of the chloroplast NADH dehydrogenase-like (NDH) complex involved in the photosystem I (PSI) cyclic electron flow. In addition, another PsbQ homolog, PQL3, is required for the NDH activity; however, the molecular function of PQL3 has not been elucidated. Here, we show that PQL3 is an assembly factor, particularly for the accumulation of subcomplex B (SubB) of the chloroplast NDH. In the pql3 mutant of Arabidopsis thaliana, the amounts of NDH subunits in SubB, PnsB1 and PsnB4, were decreased, causing a severe reduction in the NDH-PSI supercomplex. Analysis using blue native polyacrylamide gel electrophoresis suggested that the incorporation of PnsL3 into SubB was affected in the pql3 mutant. Unlike other PsbQ homologs, PQL3 was weakly associated with thylakoid membranes and was only partially protected from thermolysin digestion. Consistent with the function as an assembly factor, PQL3 accumulated independently in other NDH mutants, such as pnsl1-3. Furthermore, PQL3 accumulated in young leaves in a manner similar to the accumulation of CRR3, an assembly factor for SubB. These results suggest that PQL3 has developed a distinct function as an assembly factor for the NDH complex during evolution of the PsbQ protein family in angiosperms.

Chloride involvement in the synthesis, functioning and repair of the photosynthetic apparatus in vivo

Cl^- has long been known as a micronutrient for oxygenic photosynthetic resulting from its role an essential cofactor for photosystem II (PSII). Evidence on the in vivo Cl^- distribution in Spinacia oleracea leaves and chloroplasts shows that sufficient Cl^- is present for the involvement in PSII function, as indicated by in vitro studies on, among other organisms, S. oleracea PsII. There is also sufficient Cl^- to function, with K⁺ , in parsing the H⁺ electrochemical potential difference (proton motive force) across the illuminated thylakoid membrane into electrical potential difference and pH difference components. However, recent in vitro work on PSII from S. oleracea shows that oxygen evolving complex (OEC) synthesis, and resynthesis after photodamage, requires significantly higher Cl^- concentrations than would satisfy the function of assembled PSII O₂ evolution of the synthesised PSII with the OEC. The low Cl^- affinity of OEC (re-)assembly could be a component limiting the rate of OEC (re-)assembly.

Ultrastructural and Photosynthetic Responses of Pod Walls in Alfalfa to Drought Stress

Increasing photosynthetic ability as a whole is essential for acquiring higher crop yields. Nonleaf green organs (NLGOs) make important contributions to photosynthate formation, especially under stress conditions. However, there is little information on the pod wall in legume forage related to seed development and yield. This experiment is designed for alfalfa (Medicago sativa) under drought stress to explore the photosynthetic responses of pod walls after 5, 10, 15, and 20 days of pollination (DAP5, DAP10, DAP15, and DAP20) based on ultrastructural, physiological and proteomic analyses. Stomata were evidently observed on the outer epidermis of the pod wall. Chloroplasts had intact structures arranged alongside the cell wall, which on DAP5 were already capable of producing photosynthate. The pod wall at the late stage (DAP20) still had photosynthetic ability under well-watered (WW) treatments, while under water-stress (WS), the structure of the chloroplast membrane was damaged and the grana lamella of thylakoids were blurry. The chlorophyll a and chlorophyll b concentrations both decreased with the development of pod walls, and drought stress impeded the synthesis of photosynthetic pigments. Although the activity of ribulose-1,5-bisphosphate carboxylase (RuBisCo) decreased in the pod wall under drought stress, the activity of phosphoenolpyruvate carboxylase (PEPC) increased higher than that of RuBisCo. The proteomic analysis showed that the absorption of light is limited due to the suppression of the synthesis of chlorophyll a/b binding proteins by drought stress. Moreover, proteins involved in photosystem I and photosystem II were downregulated under WW compared with WS. Although the expression of some proteins participating in the regeneration period of RuBisCo was suppressed in the pod wall subjected to drought stress, the synthesis of PEPC was induced. In addition, some proteins, which were involved in the reduction period of RuBisCo, carbohydrate metabolism, and energy metabolism, and related to resistance, including chitinase, heat shock protein 81-2 (Hsp81-2), and lipoxygenases (LOXs), were highly expressed for the protective response to drought stress. It could be suggested that the pod wall in alfalfa is capable of operating photosynthesis and reducing the photosynthetic loss from drought stress through the promotion of the C4 pathway, ATP synthesis, and resistance ability.

Arabidopsis PsbP-Like Protein 1 Facilitates the Assembly of the Photosystem II Supercomplexes and Optimizes Plant Fitness under Fluctuating Light

In green plants, photosystem II (PSII) forms multisubunit supercomplexes (SCs) containing a dimeric core and light-harvesting complexes (LHCs). In this study, we show that Arabidopsis thaliana PsbP-like protein 1 (PPL1) is involved in the assembly of the PSII SCs and is required for adaptation to changing light intensity. PPL1 is a homolog of PsbP protein that optimizes the water-oxidizing reaction of PSII in green plants and is required for the efficient repair of photodamaged PSII; however, its exact function has been unknown. PPL1 was enriched in stroma lamellae and grana margins and associated with PSII subcomplexes including PSII monomers and PSII dimers, and several LHCII assemblies, while PPL1 was not detected in PSII-LHCII SCs. In a PPL1 null mutant (ppl1-2), assembly of CP43, PsbR and PsbW was affected, resulting in a reduced accumulation of PSII SCs even under moderate light intensity. This caused the abnormal association of LHCII in ppl1-2, as indicated by lower maximal quantum efficiency of PSII (Fv/Fm) and accelerated State 1 to State 2 transitions. These differences would lower the capability of plants to adapt to changing light environments, thereby leading to reduced growth under natural fluctuating light environments. Phylogenetic and structural analyses suggest that PPL1 is closely related to its cyanobacterial homolog CyanoP, which functions as an assembly factor in the early stage of PSII biogenesis. Our results suggest that PPL1 has a similar function, but the data also indicate that it could aid the association of LHCII with PSII.

Deletion of psbQ' gene in Cyanidioschyzon merolae reveals the function of extrinsic PsbQ' in PSII

We have successfully produced single-cell colonies of C. merolae mutants, lacking the PsbQ' subunit in its PSII complex by application of DTA-aided mutant selection. We have investigated the physiological changes in PSII function and structure and proposed a tentative explanation of the function of PsbQ' subunit in the PSII complex. We have improved the selectivity of the Cyanidioschyzon merolae nuclear transformation method by the introduction of diphtheria toxin genes into the transformation vector as an auxiliary selectable marker. The revised method allowed us to obtained single-cell colonies of C. merolae, lacking the gene of the PsbQ' extrinsic protein. The efficiency of gene replacement was extraordinarily high, allowing for a complete deletion of the gene of interest, without undesirable illegitimate integration events. We have confirmed the absence of PsbQ' protein at genetic and protein level. We have characterized the physiology of mutant cells and isolated PSII protein complex and concluded that PsbQ' is involved in nuclear regulation of PSII activity, by influencing several parameters of PSII function. Among these: oxygen evolving activity, partial dissociation of PsbV, regulation of dimerization, downsizing of phycobilisomes rods and regulation of zeaxanthin abundance. The adaptation of cellular physiology appeared to favorite upregulation of PSII and concurrent downregulation of PSI, resulting in an imbalance of energy distribution, decrease of photosynthesis and inhibition of cell proliferation.

Protein crystallization and initial neutron diffraction studies of the photosystem II subunit PsbO

The PsbO protein of photosystem II stabilizes the active-site manganese cluster and is thought to act as a proton antenna. To enable neutron diffraction studies, crystals of the β-barrel core of PsbO were grown in capillaries. The crystals were optimized by screening additives in a counter-diffusion setup in which the protein and reservoir solutions were separated by a 1% agarose plug. Crystals were cross-linked with glutaraldehyde. Initial neutron diffraction data were collected from a 0.25 mm3 crystal at room temperature using the MaNDi single-crystal diffractometer at the Spallation Neutron Source, Oak Ridge National Laboratory.

In vivo system for analyzing the function of the PsbP protein using Chlamydomonas reinhardtii

The PsbP protein is an extrinsic subunit of photosystem II (PSII) specifically developed in green-plant species including land plants and green algae. The protein-protein interactions involving PsbP and its effect on oxygen evolution have been investigated in vitro using isolated PSII membranes. However, the importance of those interactions needs to be examined at the cellular level. To this end, we developed a system expressing exogenous PsbP in the background of the Chlamydomonas BF25 mutant lacking native PsbP. Expression of His-tagged PsbP successfully restored the oxygen-evolving activity and photoautotrophic growth of the mutant, while PsbP-∆15 lacking the N-terminal 15 residues, which are crucial for the oxygen-evolving activity of spinach PSII in vitro, only partially did. This demonstrated the importance of N-terminal sequence of PsbP for the photosynthetic activity in vivo. Furthermore, the PSII-LHCII supercomplex can be specifically purified from the Chlamydomonas cells having His-tagged PsbP using a metal affinity chromatography. This study provides a platform not only for the functional analysis of PsbP in vivo but also for structural analysis of the PSII-LHCII supercomplex from green algae.

Subunit and chlorophyll organization of the plant photosystem II supercomplex

Photosystem II (PSII) is a light-driven protein, involved in the primary reactions of photosynthesis. In plant photosynthetic membranes PSII forms large multisubunit supercomplexes, containing a dimeric core and up to four light-harvesting complexes (LHCs), which act as antenna proteins. Here we solved a three-dimensional (3D) structure of the C₂S₂M₂ supercomplex from Arabidopsis thaliana using cryo-transmission electron microscopy (cryo-EM) and single-particle analysis at an overall resolution of 5.3 Å. Using a combination of homology modelling and restrained refinement against the cryo-EM map, it was possible to model atomic structures for all antenna complexes and almost all core subunits. We located all 35 chlorophylls of the core region based on the cyanobacterial PSII structure, whose positioning is highly conserved, as well as all the chlorophylls of the LHCII S and M trimers. A total of 13 and 9 chlorophylls were identified in CP26 and CP24, respectively. Energy flow from LHC complexes to the PSII reaction centre is proposed to follow preferential pathways: CP26 and CP29 directly transfer to the core using several routes for efficient transfer; the S trimer is directly connected to CP43 and the M trimer can efficiently transfer energy to the core through CP29 and the S trimer.

CyanoP is Involved in the Early Steps of Photosystem II Assembly in the Cyanobacterium Synechocystis sp. PCC 6803

Although the PSII complex is highly conserved in cyanobacteria and chloroplasts, the PsbU and PsbV subunits stabilizing the oxygen-evolving Mn4CaO5 cluster in cyanobacteria are absent in chloroplasts and have been replaced by the PsbP and PsbQ subunits. There is, however, a distant cyanobacterial homolog of PsbP, termed CyanoP, of unknown function. Here we show that CyanoP plays a role in the early stages of PSII biogenesis in Synechocystis sp. PCC 6803. CyanoP is present in the PSII reaction center assembly complex (RCII) lacking both the CP47 and CP43 modules and binds to the smaller D2 module. A small amount of larger PSII core complexes co-purifying with FLAG-tagged CyanoP indicates that CyanoP can accompany PSII on most of its assembly pathway. A role in biogenesis is supported by the accumulation of unassembled D1 precursor and impaired formation of RCII in a mutant lacking CyanoP. Interestingly, the pull-down preparations of CyanoP-FLAG from a strain lacking CP47 also contained PsbO, indicating engagement of this protein with PSII at a much earlier stage in assembly than previously assumed.

Environmental pH and the Requirement for the Extrinsic Proteins of Photosystem II in the Function of Cyanobacterial Photosynthesis

In one of the final stages of cyanobacterial Photosystem II (PS II) assembly, binding of up to four extrinsic proteins to PS II stabilizes the oxygen-evolving complex (OEC). Growth of cyanobacterial mutants deficient in certain combinations of these thylakoid-lumen-associated polypeptides is sensitive to changes in environmental pH, despite the physical separation of the membrane-embedded PS II complex from the external environment. In this perspective we discuss the effect of environmental pH on OEC function and photoautotrophic growth in cyanobacteria with reference to pH-sensitive PS II mutants lacking extrinsic proteins. We consider the possibilities that, compared to pH 10.0, pH 7.5 increases susceptibility to PS II-generated reactive oxygen species (ROS) causing photoinhibition and reducing PS II assembly in some mutants, and that perturbations to channels in the lumenal regions of PS II might alter the accessibility of water to the active site as well as egress of oxygen and protons to the thylakoid lumen. Reduced levels of PS II in these mutants, and reduced OEC activity arising from the disruption of substrate/product channels, could reduce the trans-thylakoid pH gradient (ΔpH), leading to the impairment of photosynthesis. Growth of some PS II mutants at pH 7.5 can be rescued by elevating CO2 levels, suggesting that the pH-sensitive phenotype might primarily be an indirect result of back-pressure in the electron transport chain that results in heightened production of ROS by the impaired photosystem.

The N-terminal sequence of the extrinsic PsbP protein modulates the redox potential of Cyt b559 in photosystem II

The PsbP protein, an extrinsic subunit of photosystem II (PSII) in green plants, is known to induce a conformational change around the catalytic Mn₄CaO₅ cluster securing the binding of Ca²⁺ and Cl^– in PSII. PsbP has multiple interactions with the membrane subunits of PSII, but how these affect the structure and function of PSII requires clarification. Here, we focus on the interactions between the N-terminal residues of PsbP and the α subunit of Cytochrome (Cyt) b₅₅₉ (PsbE). A key observation was that a peptide fragment formed of the first N-terminal 15 residues of PsbP, ‘pN15’, was able to convert Cyt b₅₅₉ into its HP form. Interestingly, addition of pN15 to NaCl-washed PSII membranes decreased PSII’s oxygen-evolving activity, even in the presence of saturating Ca²⁺ and Cl^– ions. In fact, pN15 reversibly inhibited the S₁ to S₂ transition of the OEC in PSII. These data suggest that pN15 can modulate the redox property of Cyt b₅₅₉ involved in the side-electron pathway in PSII. This potential change of Cyt b₅₅₉, in the absence of the C-terminal domain of PsbP, however, would interfere with any electron donation from the Mn₄CaO₅ cluster, leading to the possibility that multiple interactions of PsbP, binding to PSII, have distinct roles in regulating electron transfer within PSII.

Structural Coupling of Extrinsic Proteins with the Oxygen-Evolving Center in Photosystem II

Photosystem II (PSII), which catalyzes photosynthetic water oxidation, is composed of more than 20 subunits, including membrane-intrinsic and -extrinsic proteins. The PSII extrinsic proteins shield the catalytic Mn4CaO5 cluster from the outside bulk solution and enhance binding of inorganic cofactors, such as Ca(2+) and Cl(-), in the oxygen-evolving center (OEC) of PSII. Among PSII extrinsic proteins, PsbO is commonly found in all oxygenic organisms, while PsbP and PsbQ are specific to higher plants and green algae, and PsbU, PsbV, CyanoQ, and CyanoP exist in cyanobacteria. In addition, red algae and diatoms have unique PSII extrinsic proteins, such as PsbQ' and Psb31, suggesting functional divergence during evolution. Recent studies with reconstitution experiments combined with Fourier transform infrared spectroscopy have revealed how the individual PSII extrinsic proteins affect the structure and function of the OEC in different organisms. In this review, we summarize our recent results and discuss changes that have occurred in the structural coupling of extrinsic proteins with the OEC during evolutionary history.

Novel Features of Eukaryotic Photosystem II Revealed by Its Crystal Structure Analysis from a Red Alga

Photosystem II (PSII) catalyzes light-induced water splitting, leading to the evolution of molecular oxygen indispensible for life on the earth. The crystal structure of PSII from cyanobacteria has been solved at an atomic level, but the structure of eukaryotic PSII has not been analyzed. Because eukaryotic PSII possesses additional subunits not found in cyanobacterial PSII, it is important to solve the structure of eukaryotic PSII to elucidate their detailed functions, as well as evolutionary relationships. Here we report the structure of PSII from a red alga Cyanidium caldarium at 2.76 Å resolution, which revealed the structure and interaction sites of PsbQ', a unique, fourth extrinsic protein required for stabilizing the oxygen-evolving complex in the lumenal surface of PSII. The PsbQ' subunit was found to be located underneath CP43 in the vicinity of PsbV, and its structure is characterized by a bundle of four up-down helices arranged in a similar way to those of cyanobacterial and higher plant PsbQ, although helices I and II of PsbQ' were kinked relative to its higher plant counterpart because of its interactions with CP43. Furthermore, two novel transmembrane helices were found in the red algal PSII that are not present in cyanobacterial PSII; one of these helices may correspond to PsbW found only in eukaryotic PSII. The present results represent the first crystal structure of PSII from eukaryotic oxygenic organisms, which were discussed in comparison with the structure of cyanobacterial PSII.

Recent advances in the use of mass spectrometry to examine structure/function relationships in photosystem II

Tandem mass spectrometry often coupled with chemical modification techniques, is developing into increasingly important tool in structural biology. These methods can provide important supplementary information concerning the structural organization and subunit make-up of membrane protein complexes, identification of conformational changes occurring during enzymatic reactions, identification of the location of posttranslational modifications, and elucidation of the structure of assembly and repair complexes. In this review, we will present a brief introduction to Photosystem II, tandem mass spectrometry and protein modification techniques that have been used to examine the photosystem. We will then discuss a number of recent case studies that have used these techniques to address open questions concerning PS II. These include the nature of subunit-subunit interactions within the phycobilisome, the interaction of phycobilisomes with Photosystem I and the Orange Carotenoid Protein, the location of CyanoQ, PsbQ and PsbP within Photosystem II, and the identification of phosphorylation and oxidative modification sites within the photosystem. Finally, we will discuss some of the future prospects for the use of these methods in examining other open questions in PS II structural biochemistry.