An improved sodium dodecyl sulfate-polyacrylamide gel electrophoresis system for the analysis of membrane protein complexes

Insights into energy quenching mechanisms and carotenoid uptake by orange carotenoid protein homologs: HCP4 and CTDH

Photodamage to the photosynthetic apparatus by excessive light radiation has led to the evolution of a variety of energy dissipation mechanisms. A mechanism that exists in some cyanobacterial species, enables non-photochemical quenching of excitation energy within the phycobilisome (PBS) antenna complex by the Orange Carotenoid Protein (OCP). The OCP contains an active N-terminal domain (NTD) and a regulatory C-terminal domain (CTD). Some cyanobacteria also have genes encoding for homologs to both the CTD (CTDH) and the NTD (referred to as helical carotenoid proteins, HCP). The CTDH facilitates uptake of carotenoids from the thylakoid membranes to be transferred to the HCPs. Holo-HCPs exhibit diverse functionalities such as carotenoid carriers, singlet oxygen quenchers, and in the case of HCP4, constitutive OCP-like energy quenching. Here, we present the first crystal structure of the holo-HCP4 binding canthaxanthin molecule and an improved structure of the apo-CTDH from Anabaena sp. PCC 7120. We propose here models of the binding of the HCP4 to the PBS and the associated energy quenching mechanism. Our results show that the presence of the carotenoid is essential for fluorescence quenching. We also examined interactions within OCP-like species, including HCP4 and CTDH, providing the basis for mechanisms of carotenoid transfer from CTDH to HCPs.

Stabilization of Cereibacter sphaeroides Photosynthetic Reaction Center by the Introduction of Disulfide Bonds

The photosynthetic reaction center of the purple nonsulfur bacterium Cereibacter sphaeroides is a useful model for the study of mechanisms of photoinduced electron transfer and a promising component for photo-bio-electrocatalytic systems. The basic research and technological applications of this membrane pigment-protein complex require effective approaches to increase its structural stability. In this work, a rational design approach to genetically modify the reaction centers by introducing disulfide bonds is used. This resulted in significantly increasing the thermal stability of some of the mutant pigment-protein complexes. The formation of the S-S bonds was confirmed by X-ray crystallography as well as SDS-PAGE, and the optical properties of the reaction centers were studied. The genetically modified reaction centers presented here preserved their ability for photochemical charge separation and could be of interest for basic science and biotechnology.

Structure-function-dynamics relationships in the peculiar Planktothrix PCC7805 OCP1: Impact of his-tagging and carotenoid type

The orange carotenoid protein (OCP) is a photoactive protein involved in cyanobacterial photoprotection. Here, we report on the functional, spectral and structural characteristics of the peculiar Planktothrix PCC7805 OCP (Plankto-OCP). We show that this OCP variant is characterized by higher photoactivation and recovery rates, and a stronger energy-quenching activity, compared to other OCP studied thus far. We characterize the effect of the functionalizing carotenoid and of his-tagging on these reactions, and identify the time scales on which these modifications affect photoactivation. The presence of a his-tag at the C-terminus has a large influence on photoactivation, thermal recovery and PBS-fluorescence quenching, and likewise for the nature of the carotenoid that additionally affects the yield and characteristics of excited states and the ns-s dynamics of photoactivated OCP. By solving the structures of Plankto-OCP in the ECN- and CAN-functionalized states, each in two closely-related crystal forms, we further unveil the molecular breathing motions that animate Plankto-OCP at the monomer and dimer levels. We finally discuss the structural changes that could explain the peculiar properties of Plankto-OCP.

Assembly Apparatus of Light-Harvesting Complexes: Identification of Alb3.1-cpSRP-LHCP Complexes in the Green Alga Chlamydomonas reinhardtii

The unicellular green alga, Chlamydomonas reinhardtii, contains many light-harvesting complexes (LHCs) associating chlorophylls a/b and carotenoids; the major LHCIIs (types I, II, III and IV) and minor light-harvesting complexes, CP26 and CP29, for photosystem II, as well as nine LHCIs (LHCA1-9), for photosystem I. A pale green mutant BF4 exhibited impaired accumulation of LHCs due to deficiency in the Alb3.1 gene, which encodes the insertase involved in insertion, folding and assembly of LHC proteins in the thylakoid membranes. To elucidate the molecular mechanism by which ALB3.1 assists LHC assembly, we complemented BF4 to express ALB3.1 fused with no, single or triple Human influenza hemagglutinin (HA) tag at its C-terminus (cAlb3.1, cAlb3.1-HA or cAlb3.1-3HA). The resulting complemented strains accumulated most LHC proteins comparable to wild-type (WT) levels. The affinity purification of Alb3.1-HA and Alb3.1-3HA preparations showed that ALB3.1 interacts with cpSRP43 and cpSRP54 proteins of the chloroplast signal recognition particle (cpSRP) and several LHC proteins; two major LHCII proteins (types I and III), two minor LHCII proteins (CP26 and CP29) and eight LHCI proteins (LHCA1, 2, 3, 4, 5, 6, 8 and 9). Pulse-chase labeling experiments revealed that the newly synthesized major LHCII proteins were transiently bound to the Alb3.1 complex. We propose that Alb3.1 interacts with cpSRP43 and cpSRP54 to form an assembly apparatus for most LHCs in the thylakoid membranes. Interestingly, photosystem I (PSI) proteins were also detected in the Alb3.1 preparations, suggesting that the integration of LHCIs to a PSI core complex to form a PSI-LHCI subcomplex occurs before assembled LHCIs dissociate from the Alb3.1-cpSRP complex.

Elucidation of the essential amino acids involved in the binding of the cyanobacterial Orange Carotenoid Protein to the phycobilisome

The Orange Carotenoid Protein (OCP) is a soluble photoactive protein involved in cyanobacterial photoprotection. It is formed by the N-terminal domain (NTD) and C-terminal (CTD) domain, which establish interactions in the orange inactive form and share a ketocarotenoid molecule. Upon exposure to intense blue light, the carotenoid molecule migrates into the NTD and the domains undergo separation. The free NTD can then interact with the phycobilisome (PBS), the extramembrane cyanobacterial antenna, and induces thermal dissipation of excess absorbed excitation energy. The OCP and PBS amino acids involved in their interactions remain undetermined. To identify the OCP amino acids essential for this interaction, we constructed several OCP mutants (23) with modified amino acids located on different NTD surfaces. We demonstrated that only the NTD surface that establishes interactions with the CTD in orange OCP is involved in the binding of OCP to PBS. All amino acids surrounding the carotenoid β1 ring in the OCPR-NTD (L51, P56, G57, N104, I151, R155, N156) are important for binding OCP to PBS. Additionally, modification of the amino acids influences OCP photoactivation and/or recovery rates, indicating that they are also involved in the translocation of the carotenoid.

Structure of the far-red light utilizing photosystem I of Acaryochloris marina

Acaryochloris marina is one of the cyanobacterial species that can use far-red light to drive photochemical reactions for oxygenic photosynthesis. Here, we report the structure of A. marina photosystem I (PSI) reaction center, determined by cryo-electron microscopy at 2.58 Å resolution. The structure reveals an arrangement of electron carriers and light-harvesting pigments distinct from other type I reaction centers. The paired chlorophyll, or special pair (also referred to as P740 in this case), is a dimer of chlorophyll d and its epimer chlorophyll d'. The primary electron acceptor is pheophytin a, a metal-less chlorin. We show the architecture of this PSI reaction center is composed of 11 subunits and we identify key components that help explain how the low energy yield from far-red light is efficiently utilized for driving oxygenic photosynthesis.

Calpain-2 participates in the process of calpain-1 inactivation

Calpain-1 and calpain-2 are highly structurally similar isoforms of calpain. The calpains, a family of intracellular cysteine proteases, cleave their substrates at specific sites, thus modifying their properties such as function or activity. These isoforms have long been considered to function in a redundant or complementary manner, as they are both ubiquitously expressed and activated in a Ca²⁺- dependent manner. However, studies using isoform-specific knockout and knockdown strategies revealed that each calpain species carries out specific functions in vivo. To understand the mechanisms that differentiate calpain-1 and calpain-2, we focused on the efficiency and longevity of each calpain species after activation. Using an in vitro proteolysis assay of troponin T in combination with mass spectrometry, we revealed distinctive aspects of each isoform. Proteolysis mediated by calpain-1 was more sustained, lasting as long as several hours, whereas proteolysis mediated by calpain-2 was quickly blunted. Calpain-1 and calpain-2 also differed from each other in their patterns of autolysis. Calpain-2–specific autolysis sites in its PC1 domain are not cleaved by calpain-1, but calpain-2 cuts calpain-1 at the corresponding position. Moreover, at least in vitro, calpain-1 and calpain-2 do not perform substrate proteolysis in a synergistic manner. On the contrary, calpain-1 activity is suppressed in the presence of calpain-2, possibly because it is cleaved by the latter protein. These results suggest that calpain-2 functions as a down-regulation of calpain-1, a mechanism that may be applicable to other calpain species as well.

The circadian rhythm regulator RpaA modulates photosynthetic electron transport and alters the preferable temperature range for growth in a cyanobacterium

Cyanobacterial strains can grow within a specific temperature range that approximately corresponds to their natural habitat. However, how the preferable temperature range for growth (PTRG) is determined at the molecular level remains unclear. In this study, we detected a PTRG upshift in a mutant strain of Synechococcus elongatus PCC 7942 lacking the circadian rhythm regulator RpaA. Subsequent analyses revealed that RpaA decreases the electron transport from photosystem I to NADPH. The change in electron transport likely inhibits H₂ O₂ generation under high-temperature conditions and contributes to the observed PTRG upshift in rpaA-deficient cells. The importance of the effects of the circadian rhythm regulator on the PTRG is discussed.

Light stress in green and red Planktothrix strains: The orange carotenoid protein and its related photoprotective mechanism

Photosynthetic organisms need to sense and respond to fluctuating environmental conditions, to perform efficient photosynthesis and avoid the formation of harmful reactive oxygen species. Cyanobacteria have developed a photoprotective mechanism that decreases the energy arriving at the reaction centers by increasing thermal energy dissipation at the level of the phycobilisome, the extramembranal light-harvesting antenna. This mechanism is triggered by the photoactive orange carotenoid protein (OCP). In this study, we characterized OCP and the related photoprotective mechanism in non-stressed and light-stressed cells of three different strains of Planktothrix that can form impressive blooms. In addition to changing lake ecosystemic functions and biodiversity, Planktothrix blooms can have adverse effects on human and animal health as they produce toxins (e.g., microcystins). Three Planktothrix strains were selected: two green strains, PCC 10110 (microcystin producer) and PCC 7805 (non-microcystin producer), and one red strain, PCC 7821. The green strains colonize shallow lakes with higher light intensities while red strains proliferate in deep lakes. Our study allowed us to conclude that there is a correlation between the ecological niche in which these strains proliferate and the rates of induction and recovery of OCP-related photoprotection. However, differences in the resistance to prolonged high-light stress were correlated to a better replacement of damaged D1 protein and not to differences in OCP photoprotection. Finally, microcystins do not seem to be involved in photoprotection as was previously suggested.

Oxidative misfolding of Cu/Zn-superoxide dismutase triggered by non-canonical intramolecular disulfide formation

Misfolded Cu/Zn-superoxide dismutase (SOD1) is a pathological species in a subset of amyotrophic lateral sclerosis (ALS). Oxidative stress is known to increase in affected spinal cords of ALS and is thus considered to cause damages on SOD1 leading to the misfolding and aggregation. Despite this, it still remains elusive what triggers misfolding of SOD1 under oxidizing environment. Here, we show that a thiol group of Cys111 in SOD1 is oxidized to a sulfenic acid with hydrogen peroxide and reveal that further dissociation of the bound metal ions from the oxidized SOD1 allows another free Cys residue (Cys6) to nucleophilically attack the sulfenylated Cys111. As a result, an intra-molecular disulfide bond forms between Cys6 and Cys111. Such an abnormal SOD1 with the non-canonical disulfide bond was conformationally extended with significant cytotoxicity as well as high propensity to aggregate. Taken together, we propose a new model of SOD1 misfolding under oxidizing environment, in which formation of the non-canonical intramolecular disulfide bond plays a pivotal role.

A novel chlorophyll protein complex in the repair cycle of photosystem II

In oxygenic photosynthetic organisms, photosystem II (PSII) is a unique membrane protein complex that catalyzes light-driven oxidation of water. PSII undergoes frequent damage due to its demanding photochemistry. It must undergo a repair and reassembly process following photodamage, many facets of which remain unknown. We have discovered a PSII subcomplex that lacks 5 key PSII core reaction center polypeptides: D1, D2, PsbE, PsbF, and PsbI. This pigment-protein complex does contain the PSII core antenna proteins CP47 and CP43, as well as most of their associated low molecular mass subunits, and the assembly factor Psb27. Immunoblotting, mass spectrometry, and ultrafast spectroscopic results support the absence of a functional reaction center in this complex, which we call the "no reaction center" complex (NRC). Analytical ultracentrifugation and clear native PAGE analysis show that NRC is a stable pigment-protein complex and not a mixture of free CP47 and CP43 proteins. NRC appears in higher abundance in cells exposed to high light and impaired protein synthesis, and genetic deletion of PsbO on the PSII luminal side results in an increased NRC population, indicative that NRC forms in response to photodamage as part of the PSII repair process. Our finding challenges the current model of the PSII repair cycle and implies an alternative PSII repair strategy. Formation of this complex may maximize PSII repair economy by preserving intact PSII core antennas in a single complex available for PSII reassembly, minimizing the risk of randomly diluting multiple recycling components in the thylakoid membrane following a photodamage event.

Structural rearrangements in the C-terminal domain homolog of Orange Carotenoid Protein are crucial for carotenoid transfer

A recently reported family of soluble cyanobacterial carotenoproteins, homologs of the C-terminal domain (CTDH) of the photoprotective Orange Carotenoid Protein, is suggested to mediate carotenoid transfer from the thylakoid membrane to the Helical Carotenoid Proteins, which are paralogs of the N-terminal domain of the OCP. Here we present the three-dimensional structure of a carotenoid-free CTDH variant from Anabaena (Nostoc) PCC 7120. This CTDH contains a cysteine residue at position 103. Two dimer-forming interfaces were identified, one stabilized by a disulfide bond between monomers and the second between each monomer's β-sheets, both compatible with small-angle X-ray scattering data and likely representing intermediates of carotenoid transfer processes. The crystal structure revealed a major positional change of the C-terminal tail. Further mutational analysis revealed the importance of the C-terminal tail in both carotenoid uptake and delivery. These results have allowed us to suggest a detailed model for carotenoid transfer via these soluble proteins.

Characterization of mutants expressing thermostable D1 and D2 polypeptides of photosystem II in the cyanobacterium Synechococcus elongatus PCC 7942

Photosystem II complex embedded in thylakoid membrane performs oxygenic photosynthesis where the reaction center D1/D2 heterodimer accommodates all components of the electron transport chain. To express thermostable D1/D2 heterodimer in a cyanobacterium Synechococcus elongatus PCC 7942, we constructed a series of mutant strains whose psbA1 and psbD1 genes encoding, respectively, the most highly expressed D1 and D2 polypeptides were replaced with those of a thermophilic strain, Thermosynechococcus vulcanus. Because the C-terminal 16 amino acid sequences of D1 polypeptides should be processed prior to maturation but diverge from each other, we also constructed the psbA1ΔC-replaced strain expressing a thermostable D1 polypeptide devoid of the C-terminal extension. The psbA1/psbD1-replaced strain showed decreased growth rate and oxygen evolution rate, suggesting inefficient photosystem II. Immunoblot analyses for thermostable D1, D2 polypeptides revealed that the heterologous D1 protein was absent in thylakoid membrane from any mutant strains with psbA1, psbA1ΔC, and psbA1/psbD1-replacements, whereas the heterologous D2 protein was present in thylakoid membrane as well as purified photosystem II complex from the psbA1/psbD1-replaced strain. In the latter strain, the compensatory expression of psbA3 and psbD2 genes was elevated. These data suggest that heterologous D2 polypeptide could be combined with the host D1 polypeptide to form chimeric D1/D2 heterodimer, whereas heterologous D1 polypeptide even without the C-terminal extension was unable to make complex with the host D2 polypeptide. Since the heterologous D1 could not be detected even in the whole cells of psbA1/psbD1-replaced strain, the rapid degradation of unprocessed or unassembled heterologous D1 was implicated.

Effect of different methods of Ca2+ extraction from PSII oxygen-evolving complex on the QA- oxidation kinetics

Lumenal extrinsic proteins PsbO, PsbP, and PsbQ of photosystem II (PSII) protect the catalytic cluster Mn₄CaO₅ of oxygen-evolving complex (OEC) from the bulk solution and from soluble compounds in the surrounding medium. Extraction of PsbP and PsbQ proteins by NaCl-washing together with chelator EGTA is followed also by the depletion of Ca²⁺ cation from OEC. In this study, the effects of PsbP and PsbQ proteins, as well as Ca²⁺ extraction from OEC on the kinetics of the reduced primary electron acceptor (Q_A^-) oxidation, have been studied by fluorescence decay kinetics measurements in PSII membrane fragments. We found that in addition to the impairment of OEC, removal of PsbP and PsbQ significantly slows the rate of electron transfer from Q_A^- to the secondary quinone acceptor Q_B. Electron transfer from Q_A^- to Q_B in photosystem II membranes with an occupied Q_B site was slowed down by a factor of 8. However, addition of EGTA or CaCl₂ to NaCl-washed PSII did not change the kinetics of fluorescence decay. Moreover, the kinetics of Q_A^- oxidation by Q_B in Ca-depleted PSII membranes obtained by treatment with citrate buffer at pH 3.0 (such treatment keeps all extrinsic proteins in PSII but extracts Ca²⁺ from OEC) was not changed. The results obtained indicate that the effect of NaCl-washing on the Q_A^- to Q_B electron transport is due to PsbP and PsbQ extrinsic proteins extraction, but not due to Ca²⁺ depletion.

An enzyme kinetics study of the pH dependence of chloride activation of oxygen evolution in photosystem II

Oxygen evolution by photosystem II (PSII) involves activation by Cl^- ion, which is regulated by extrinsic subunits PsbQ and PsbP. In this study, the kinetics of chloride activation of oxygen evolution was studied in preparations of PSII depleted of the PsbQ and PsbP subunits (NaCl-washed and Na₂SO₄/pH 7.5-treated) over a pH range from 5.3 to 8.0. At low pH, activation by chloride was followed by inhibition at chloride concentrations >100 mM, whereas at high pH activation continued as the chloride concentration increased above 100 mM. Both activation and inhibition were more pronounced at lower pH, indicating that Cl^- binding depended on protonation events in each case. The simplest kinetic model that could account for the complete data set included binding of Cl^- at two sites, one for activation and one for inhibition, and four protonation steps. The intrinsic (pH-independent) dissociation constant for Cl^- activation, K _S, was found to be 0.9 ± 0.2 mM for both preparations, and three of the four pK _as were determined, with the fourth falling below the pH range studied. The intrinsic inhibition constant, K _I, was found to be 64 ± 2 and 103 ± 7 mM for the NaCl-washed and Na₂SO₄/pH7.5-treated preparations, respectively, and is considered in terms of the conditions likely to be present in the thylakoid lumen. This enzyme kinetics analysis provides a more complete characterization of chloride and pH dependence of O₂ evolution activity than has been previously presented.

Dramatic Domain Rearrangements of the Cyanobacterial Orange Carotenoid Protein upon Photoactivation

Photosynthetic cyanobacteria make important contributions to global carbon and nitrogen budgets. A protein known as the orange carotenoid protein (OCP) protects the photosynthetic apparatus from damage by dissipating excess energy absorbed by the phycobilisome, the major light-harvesting complex in many cyanobacteria. OCP binds one carotenoid pigment, but the color of this pigment depends on conditions. It is orange in the dark and red when exposed to light. We modified the orange and red forms of OCP by using isotopically coded cross-linking agents and then analyzed the structural features by using liquid chromatography and tandem mass spectrometry. Unequivocal cross-linking pairs uniquely detected in red OCP indicate that, upon photoactivation, the OCP N-terminal domain (NTD) and C-terminal domain (CTD) reorient relative to each other. Our data also indicate that the intrinsically unstructured loop connecting the NTD and CTD not only is involved in the interaction between the two domains in orange OCP but also, together with the N-terminal extension, provides a structural buffer system facilitating an intramolecular breathing motion of the OCP, thus helping conversion back and forth from the orange to red form during the OCP photocycle. These results have important implications for understanding the molecular mechanism of action of cyanobacterial photoprotection.

Utilization of light by fucoxanthin-chlorophyll-binding protein in a marine centric diatom, Chaetoceros gracilis

The major light-harvesting pigment protein complex (fucoxanthin-chlorophyll-binding protein complex; FCP) was purified from a marine centric diatom, Chaetoceros gracilis, by mild solubilization followed by sucrose density gradient centrifugation, and then characterized. The dynamic light scattering measurement showed unimodality, indicating that the complex was highly purified. The amount of chlorophyll a (Chl a) bound to the purified FCP accounted for more than 60 % of total cellular Chl a. The complex was composed of three abundant polypeptides, although there are nearly 30 FCP-related genes. The two major components were identified as Fcp3 (Lhcf3)- and Fcp4 (Lhcf4)-equivalent proteins based on their internal amino acid sequences and a two-dimensional isoelectric focusing electrophoresis analysis developed in this work. Compared with the thylakoids, the FCP complex showed higher contents of fucoxanthin and chlorophyll c but lower contents of the xanthophyll cycle pigments diadinoxanthin and diatoxanthin. Fluorescence excitation spectra analyses indicated that light harvesting, rather than photosystem protection, is the major function of the purified FCP complex, which is associated with more than 60 % of total cellular Chl a. These findings suggest that the huge amount of Chl bound to the FCP complex composed of Lhcf3, Lhcf4, and an unidentified minor protein has a light-harvesting function to allow efficient photosynthesis under the dim-light conditions in the ocean.

Multiple copies of the PsbQ protein in a cyanobacterial photosystem II assembly intermediate complex

Photosystem II (PSII) undergoes frequent damage owing to the demanding electron transfer chemistry it performs. To sustain photosynthetic activity, damaged PSII undergoes a complex repair cycle consisting of many transient intermediate complexes. By purifying PSII from the cyanobacterium Synechocystis sp. PCC 6803 using a histidine-tag on the PsbQ protein, a lumenal extrinsic subunit, a novel PSII assembly intermediate was isolated in addition to the mature PSII complex. This new complex, which we refer to as PSII-Q4, contained four copies of the PsbQ protein per PSII monomer, instead of the expected one copy. In addition, PSII-Q4 lacked two other lumenal extrinsic proteins, PsbU and PsbV, which are present in the mature PSII complex. We suggest that PSII-Q4 is a late PSII assembly intermediate that is formed just before the binding of PsbU and PsbV, and we incorporate these results into an updated model of PSII assembly.

Whole-blood immunoassay for γH2AX as a radiation biodosimetry assay with minimal sample preparation

The current state of the art in high-throughput minimally invasive radiation biodosimetry involves the collection of samples in the field and analysis at a centralized facility. We have developed a simple biological immunoassay for radiation exposure that could extend this analysis out of the laboratory into the field. Such a forward placed assay would facilitate triage of a potentially exposed population. The phosphorylation and localization of the histone H2AX at double-stranded DNA breaks has already been proven to be an adequate surrogate assay for reporting DNA damage proportional to radiation dose. Here, we develop an assay for phosphorylated H2AX directed against minimally processed sample lysates. We conduct preliminary verification of H2AX phosphorylation using irradiated mouse embryo fibroblast cultures. Additional dosimetry is performed using human blood samples irradiated ex vivo. The assay reports H2AX phosphorylation in human blood samples in response to ionizing radiation over a range of 0-5 Gy in a linear fashion, without requiring filtering, enrichment, or purification of the blood sample.

Sortase-mediated ligation of PsaE-modified photosystem I from Synechocystis sp. PCC 6803 to a conductive surface for enhanced photocurrent production on a gold electrode

Sortase-mediated ligation was used to attach the photosystem I (PSI) complex from Synechocystis sp. PCC 6803 in a preferential orientation to enhance photoinduced electron flow to a conductive gold surface. Ideally, this method can result in a uniform monolayer of protein, covalently bound unidirectionally to the electrode surface. The exposed C-termini of the psaE subunits of the PSI trimer were targeted to contain an LPETG-sortase recognition sequence to increase noncompeting electron transfer by uniformly orienting the PSI stromal side proximal to the surface. Surface characterization with atomic force microscopy suggested that monolayer formation and optimal surface coverage occurred when the gold surfaces were incubated with peptide at 100 to 500 μM concentrations. When photochronoamperometry with potassium ferrocyanide and ferricyanide as redox mediators was used, photocurrents in the range of 100 to 200 nA/cm(2) were produced, which is an improvement over other attachment techniques for photosystem monolayers that produce approximately 100 nA/cm(2) or less. This work demonstrated that sortase-mediated ligation aided in the control of PSI orientation on modified gold surfaces with a distribution of 94% stromal side proximal and 6% lumenal side proximal to the surface for current-producing PSI.

Multistrip Western blotting: a tool for comparative quantitative analysis of multiple proteins

The qualitative and quantitative measurements of protein abundance and modification states are essential in understanding their functions in diverse cellular processes. Typical Western blotting, though sensitive, is prone to produce substantial errors and is not readily adapted to high-throughput technologies. Multistrip Western blotting is a modified immunoblotting procedure based on simultaneous electrophoretic transfer of proteins from multiple strips of polyacrylamide gels to a single membrane sheet. In comparison with the conventional technique, Multistrip Western blotting increases data output per single blotting cycle up to tenfold; allows concurrent measurement of up to nine different total and/or posttranslationally modified protein expression obtained from the same loading of the sample; and substantially improves the data accuracy by reducing immunoblotting-derived signal errors. This approach enables statistically reliable comparison of different or repeated sets of data and therefore is advantageous to apply in biomedical diagnostics, systems biology, and cell signaling research.

MS-based cross-linking analysis reveals the location of the PsbQ protein in cyanobacterial photosystem II

PsbQ is a luminal extrinsic protein component that regulates the water splitting activity of photosystem II (PSII) in plants, algae, and cyanobacteria. However, PsbQ is not observed in the currently available crystal structures of PSII from thermophilic cyanobacteria. The structural location of PsbQ within the PSII complex has therefore remained unknown. Here, we report chemical cross-linking followed by immunodetection and liquid chromatography/tandem MS analysis of a dimeric PSII complex isolated from the model cyanobacterium, Synechocystis sp. PCC 6803, to determine the binding site of PsbQ within PSII. Our results demonstrate that PsbQ is closely associated with the PsbO and CP47 proteins, as revealed by cross-links detected between (120)K of PsbQ and (180)K and (59)K of PsbO, and between (102)K of PsbQ and (440)D of CP47. We further show that genetic deletion of the psbO gene results in the complete absence of PsbQ in PSII complexes as well as the loss of the dimeric form of PSII. Overall, our data provide a molecular-level description of the enigmatic binding site of PsbQ in PSII in a cyanobacterium. These results also help us understand the sequential incorporation of the PsbQ protein during the PSII assembly process, as well as its stabilizing effect on the oxygen evolution activity of PSII.

Specificity of the cyanobacterial orange carotenoid protein: influences of orange carotenoid protein and phycobilisome structures

Cyanobacteria have developed a photoprotective mechanism that decreases the energy arriving at the reaction centers by increasing thermal energy dissipation at the level of the phycobilisome (PB), the extramembranous light-harvesting antenna. This mechanism is triggered by the photoactive Orange Carotenoid Protein (OCP), which acts both as the photosensor and the energy quencher. The OCP binds the core of the PB. The structure of this core differs in diverse cyanobacterial strains. Here, using two isolated OCPs and four classes of PBs, we demonstrated that differences exist between OCPs related to PB binding, photoactivity, and carotenoid binding. Synechocystis PCC 6803 (hereafter Synechocystis) OCP, but not Arthrospira platensis PCC 7345 (hereafter Arthrospira) OCP, can attach echinenone in addition to hydroxyechinenone. Arthrospira OCP binds more strongly than Synechocystis OCP to all types of PBs. Synechocystis OCP can strongly bind only its own PB in 0.8 m potassium phosphate. However, if the Synechocystis OCP binds to the PB at very high phosphate concentrations (approximately 1.4 m), it is able to quench the fluorescence of any type of PB, even those isolated from strains that lack the OCP-mediated photoprotective mechanism. Thus, the determining step for the induction of photoprotection is the binding of the OCP to PBs. Our results also indicated that the structure of PBs, at least in vitro, significantly influences OCP binding and the stabilization of OCP-PB complexes. Finally, the fact that the OCP induced large fluorescence quenching even in the two-cylinder core of Synechococcus elongatus PBs strongly suggested that OCP binds to one of the basal allophycocyanin cylinders.

Overexpression of flv3 improves photosynthesis in the cyanobacterium Synechocystis sp. PCC6803 by enhancement of alternative electron flow

BACKGROUND

To ensure reliable sources of energy and raw materials, the utilization of sustainable biomass has considerable advantages over petroleum-based energy sources. Photosynthetic algae have attracted attention as a third-generation feedstock for biofuel production, because algae cultivation does not directly compete with agricultural resources, including the requirement for productive land and fresh water. In particular, cyanobacteria are a promising biomass feedstock because of their high photosynthetic capability.

RESULTS

In the present study, the expression of the flv3 gene, which encodes a flavodiiron protein involved in alternative electron flow (AEF) associated with NADPH-coupled O2 photoreduction in photosystem I, was enhanced in Synechocystis sp. PCC6803. Overexpression of flv3 improved cell growth with corresponding increases in O2 evolution, intracellular ATP level, and turnover of the Calvin cycle. The combination of in vivo (13)C-labeling of metabolites and metabolomic analysis confirmed that the photosynthetic carbon flow was enhanced in the flv3-overexpressing strain.

CONCLUSIONS

Overexpression of flv3 improved cell growth and glycogen production in the recombinant Synechocystis sp. PCC6803. Direct measurement of metabolic turnover provided conclusive evidence that CO2 incorporation is enhanced by the flv3 overexpression. Increase in O2 evolution and ATP accumulation indicates enhancement of the AEF. Overexpression of flv3 improves photosynthesis in the Synechocystis sp. PCC6803 by enhancement of the AEF.

Phycobilisomes supply excitations to both photosystems in a megacomplex in cyanobacteria

In photosynthetic organisms, photons are captured by light-harvesting antenna complexes, and energy is transferred to reaction centers where photochemical reactions take place. We describe here the isolation and characterization of a fully functional megacomplex composed of a phycobilisome antenna complex and photosystems I and II from the cyanobacterium Synechocystis PCC 6803. A combination of in vivo protein cross-linking, mass spectrometry, and time-resolved spectroscopy indicates that the megacomplex is organized to facilitate energy transfer but not intercomplex electron transfer, which requires diffusible intermediates and the cytochrome b6f complex. The organization provides a basis for understanding how phycobilisomes transfer excitation energy to reaction centers and how the energy balance of two photosystems is achieved, allowing the organism to adapt to varying ecophysiological conditions.

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.

Mass spectrometry-based footprinting reveals structural dynamics of loop E of the chlorophyll-binding protein CP43 during photosystem II assembly in the cyanobacterium Synechocystis 6803

The PSII repair cycle is required for sustainable photosynthesis in oxygenic photosynthetic organisms. In cyanobacteria and higher plants, proteolysis of the precursor D1 protein (pD1) to expose a C-terminal carboxylate group is an essential step leading to coordination of the Mn4CaO5 cluster, the site of water oxidation. Psb27 appears to associate with both pD1- and D1-containing PSII assembly intermediates by closely interacting with CP43. Here, we report that reduced binding affinity between CP43 and Psb27 is triggered by the removal of the C-terminal extension of the pD1 protein. A mass spectrometry-based footprinting strategy was adopted to probe solvent-exposed aspartic and glutamic acid residues on the CP43 protein. By comparing the extent of footprinting between HT3ΔctpAΔ27PSII and HT3ΔctpAPSII, two genetically modified PSII assembly complexes, we found that Psb27 binds to CP43 on the side of Loop E distal to the pseudo-symmetrical D1-D2 axis. By comparing a second pair of PSII assembly complexes, we discovered that Loop E of CP43 undergoes a significant conformational rearrangement due to the removal of the pD1 C-terminal extension, altering the Psb27-CP43 binding interface. The significance of this conformational rearrangement is discussed in the context of recruitment of the PSII lumenal extrinsic proteins and Mn4CaO5 cluster assembly. In addition to CP43's previously known function as one of the core PSII antenna proteins, this work demonstrates that Loop E of CP43 plays an important role in the functional assembly of the Water Oxidizing Center (WOC) during PSII biogenesis.

Two types of fucoxanthin-chlorophyll-binding proteins I tightly bound to the photosystem I core complex in marine centric diatoms

Intact fucoxanthin (Fucox)-chlorophyll (Chl)-binding protein I-photosystem I supercomplexes (FCPI-PSIs) were prepared by a newly developed simple fast procedure from centric diatoms Chaetoceros gracilis and Thalassiosira pseudonana to study the mechanism of their efficient solar energy accumulation. FCPI-PSI purified from C. gracilis contained 252 Chl a, 23 Chl c, 56 Fucox, 34 diadinoxanthin+diatoxanthin, 1 violaxanthin, 21 ß-carotene, and 2 menaquinone-4 per P700. The complex showed a high electron transfer activity at 185,000μmolmg Chl a(-1)·h(-1) to reduce methyl viologen from added cytochrome c6. We identified 14 and 21 FCP proteins in FCPI-PSI of C. gracilis and T. pseudonana, respectively, determined by N-terminal and internal amino acid sequences and liquid chromatography-tandem mass spectrometry (LC-MS/MS) analyses. PsaO and a red lineage Chla/b-binding-like protein (RedCAP), Thaps3:270215, were also identified. Severe detergent treatment of FCPI-PSI released FCPI-1 first, leaving the FCPI-2-PSI-core complex. FCPI-1 contained more Chl c and showed Chl a fluorescence at a shorter wavelength than FCPI-2, suggesting an excitation-energy transfer from FCPI-1 to FCPI-2 and then to the PSI core. Fluorescence emission spectra at 17K in FCPI-2 varied depending on the excitation wavelength, suggesting two independent energy transfer routes. We formulated a model of FCPI-PSI based on the biochemical assay results.

Psb28 is involved in recovery of photosystem II at high temperature in Synechocystis sp. PCC 6803

Psb28 is an extrinsic protein of photosystem II (PSII), which is conserved among photosynthetic organisms from cyanobacteria to higher plants. A unicellular cyanobacterium, Synechocystis sp. PCC 6803, has two homologs of Psb28, Psb28-1 and Psb28-2. However, the role of these proteins remains poorly understood. In this study, we disrupted the psb28-1 (sll1398) and psb28-2 (slr1739) genes in wild-type Synechocystis sp. PCC 6803 and examined their photosynthetic properties to elucidate the physiological role of Psb28 in photosynthesis. We also disrupted the psb28-1 gene in a dgdA mutant defective in the biosynthesis of digalactosyldiacylglycerol, in which Psb28-1 significantly accumulates in PSII. The disruption of the psb28-1 gene in the wild-type resulted in growth retardation under high-light conditions at high temperatures with a low rate of restoration of photodamaged photosynthetic machinery. Similar phenomena were observed at normal growth temperatures in the psb28-1/dgdA double mutant. In contrast, disruption of psb28-2 in the wild-type and dgdA mutant did not affect host strain phenotype, suggesting that Psb28-2 does not contribute to the recovery of PSII. In addition, protein analysis using strains expressing His-tagged Psb28-1 revealed that Psb28-1 is mainly associated with the CP43-less PSII monomer. In the dgdA mutant, the CP43-less PSII monomer accumulated to a greater extent than in the wild-type, and its accumulation caused greater accumulation of Psb28-1 in PSII. These results demonstrate that Psb28-1 plays an important role in PSII repair through association with the CP43-less monomer, particularly at high temperatures.

Gene sequencing and characterization of the light-harvesting complex 2 from thermophilic purple sulfur bacterium Thermochromatium tepidum

In this study, gene sequences coding for the light-harvesting (LH) 2 polypeptides from a thermophilic purple sulfur bacterium Thermochromatium tepidum are reported and characterization of the LH2 complex is described. Three sets of pucBA genes have been identified, and the gene products have been analyzed by electrophoresis and reversed-phase chromatography. The result shows that all of the genes are expressed but the distribution of the expression is not uniform. The gene products undergo post-translational modification, where two of the β-polypeptides appear to be N-terminally methylated. Absorption spectrum of the purified LH2 complex exhibits Q (y) transitions at 800 and 854 nm in dodecyl β-maltopyranoside solution, and the circular dichroism spectrum shows a "molischianum"-like characteristic. No spectral change was observed for the LH2 when the bacterium was cultured under different conditions of light intensity. In lauryl dimethylamine N-oxide (LDAO) solution, significant changes in the absorption spectrum were observed. The B850 peak decreased and blue-shifted with increasing the LDAO concentration, whereas the B800 intensity increased without change in the peak position. The spectral changes can be partially or almost completely reversed by addition of metal ions, and the divalent cations seem to be more effective. The results indicate that ionic interactions may exist between LH2, detergent molecules and metal ions. Possible mechanisms involved in the detergent- and cation-induced spectral changes are discussed.

Psb27, a transiently associated protein, binds to the chlorophyll binding protein CP43 in photosystem II assembly intermediates

Photosystem II (PSII), a large multisubunit pigment-protein complex localized in the thylakoid membrane of cyanobacteria and chloroplasts, mediates light-driven evolution of oxygen from water. Recently, a high-resolution X-ray structure of the mature PSII complex has become available. Two PSII polypeptides, D1 and CP43, provide many of the ligands to an inorganic Mn(4)Ca center that is essential for water oxidation. Because of its unusual redox chemistry, PSII often undergoes degradation followed by stepwise assembly. Psb27, a small luminal polypeptide, functions as an important accessory factor in this elaborate assembly pathway. However, the structural location of Psb27 within PSII assembly intermediates has remained elusive. Here we report that Psb27 binds to CP43 in such assembly intermediates. We treated purified genetically tagged PSII assembly intermediate complexes from the cyanobacterium Synechocystis 6803 with chemical cross-linkers to examine intermolecular interactions between Psb27 and various PSII proteins. First, the water-soluble 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) was used to cross-link proteins with complementary charged groups in close association to one another. In the His27△ctpAPSII preparation, a 58-kDa cross-linked species containing Psb27 and CP43 was identified. This species was not formed in the HT3△ctpA△psb27PSII complex in which Psb27 was absent. Second, the homobifunctional thiol-cleavable cross-linker 3,3'-dithiobis(sulfosuccinimidylpropionate) (DTSSP) was used to reversibly cross-link Psb27 to CP43 in His27△ctpAPSII preparations, which allowed the use of liquid chromatography/tandem MS to map the cross-linking sites as Psb27K(63)↔CP43D(321) (trypsin) and CP43K(215)↔Psb27D(58)AGGLK(63)↔CP43D(321) (chymotrypsin), respectively. Our data suggest that Psb27 acts as an important regulatory protein during PSII assembly through specific interactions with the luminal domain of CP43.

A genetically tagged Psb27 protein allows purification of two consecutive photosystem II (PSII) assembly intermediates in Synechocystis 6803, a cyanobacterium

Photosystem II (PSII) is a large membrane bound molecular machine that catalyzes light-driven oxygen evolution from water. PSII constantly undergoes assembly and disassembly because of the unavoidable damage that results from its normal photochemistry. Thus, under physiological conditions, in addition to the active PSII complexes, there are always PSII subpopulations incompetent of oxygen evolution, but are in the process of undergoing elaborate biogenesis and repair. These transient complexes are difficult to characterize because of their low abundance, structural heterogeneity, and thermodynamic instability. In this study, we show that a genetically tagged Psb27 protein allows for the biochemical purification of two monomeric PSII assembly intermediates, one with an unprocessed form of D1 (His27ΔctpAPSII) and a second one with a mature form of D1 (His27PSII). Both forms were capable of light-induced charge separation, but unable to photooxidize water, largely because of the absence of a functional tetramanganese cluster. Unexpectedly, there was a significant amount of the extrinsic lumenal PsbO protein in the His27PSII, but not in the His27ΔctpAPSII complex. In contrast, two other lumenal proteins, PsbU and PsbV, were absent in both of these PSII intermediate complexes. Additionally, the only cytoplasmic extrinsic protein, Psb28 was detected in His27PSII complex. Based on these data, we have presented a refined model of PSII biogenesis, illustrating an important role of Psb27 as a gate-keeper during the complex assembly process of the oxygen-evolving centers in PSII.

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).

S4 protein Sll1252 is necessary for energy balancing in photosynthetic electron transport in Synechocystis sp. PCC 6803

Sll1252 was identified as a novel protein in photosystem II complexes from Synechocystis sp. PCC 6803. To investigate the function of Sll1252, the corresponding gene, sll1252, was deleted in Synechocystis 6803. Despite the homology of Sll1252 to YlmH, which functions in the cell division machinery in Streptococcus, the growth rate and cell morphology of the mutant were not affected in normal growth medium. Instead, it seems that cells lacking this polypeptide have increased sensitivity to Cl(-) depletion. The growth and oxygen evolving activity of the mutant cells was highly suppressed compared with those of wild-type cells when Cl(-) and/or Ca(2+) was depleted from the medium. Recovery of photosystem II from photoinhibition was suppressed in the mutant. Despite the defects in photosystem II, in the light, the acceptor side of photosystem II was more reduced and the donor side of photosystem I was more oxidized compared with wild-type cells, suggesting that functional impairments were also present in cytochrome b(6)/f complexes. The amounts of cytochrome c(550) and cytochrome f were smaller in the mutant in the Ca(2+)- and Cl(-)-depleted medium. Furthermore, the amount of IsiA protein was increased in the mutant, especially in the Cl(-)-depleted medium, indicating that the mutant cells perceive environmental stress to be greater than it is. The amount of accompanying cytochrome c(550) in purified photosystem II complexes was also smaller in the mutant. Overall, the Sll1252 protein appears to be closely related to redox sensing of the plastoquinone pool to balance the photosynthetic electron flow and the ability to cope with global environmental stresses.

Germ-line mitochondria exhibit suppressed respiratory activity to support their accurate transmission to the next generation

Mitochondria are accurately transmitted to the next generation through a female germ cell in most animals. Mitochondria produce most ATP, accompanied by the generation of reactive oxygen species (ROS). A specialized mechanism should be necessary for inherited mitochondria to escape from impairments of mtDNA by ROS. Inherited mitochondria are named germ-line mitochondria, in contrast with somatic ones. We hypothesized that germ-line mitochondria are distinct from somatic ones. The protein profiles of germ-line and somatic mitochondria were compared, using oocytes at two different stages in Xenopus laevis. Some subunits of ATP synthase were at a low level in germ-line mitochondria, which was confirmed immunologically. Ultrastructural histochemistry using 3,3'-diaminobenzidine (DAB) showed that cytochrome c oxidase (COX) activity of germ-line mitochondria was also at a low level. Mitochondria in one oocyte were segregated into germ-line mitochondria and somatic mitochondria, during growth from stage I to VI oocytes. Respiratory activity represented by ATP synthase expression and COX activity was shown to be low during most of the long gametogenetic period. We propose that germ-line mitochondria that exhibit suppressed respiration alleviate production of ROS and enable transmission of accurate mtDNA from generation to generation.

Protection by α-tocopherol of the repair of photosystem II during photoinhibition in Synechocystis sp. PCC 6803

α-Tocopherol is a lipophilic antioxidant that is an efficient scavenger of singlet oxygen. We investigated the role of α-tocopherol in the protection of photosystem II (PSII) from photoinhibition using a mutant of the cyanobacterium Synechocystis sp. PCC 6803 that is deficient in the biosynthesis of α-tocopherol. The activity of PSII in mutant cells was more sensitive to inactivation by strong light than that in wild-type cells, indicating that lack of α-tocopherol enhances the extent of photoinhibition. However, the rate of photodamage to PSII, as measured in the presence of chloramphenicol, which blocks the repair of PSII, did not differ between the two lines of cells. By contrast, the repair of PSII from photodamage was suppressed in mutant cells. Addition of α-tocopherol to cultures of mutant cells returned the extent of photoinhibition to that in wild-type cells, without any effect on photodamage. The synthesis de novo of various proteins, including the D1 protein that plays a central role in the repair of PSII, was suppressed in mutant cells under strong light. These observations suggest that α-tocopherol promotes the repair of photodamaged PSII by protecting the synthesis de novo of the proteins that are required for recovery from inhibition by singlet oxygen.

Quantitative analysis of cell surface membrane proteins using membrane-impermeable chemical probe coupled with 18O labeling

We report a mass spectrometry-based strategy for quantitative analysis of cell surface membrane proteome changes. The strategy includes enrichment of surface membrane proteins using a membrane-impermeable chemical probe followed by stable isotope (18)O labeling and LC-MS analysis. We applied this strategy for enriching membrane proteins expressed by Shewanella oneidensis MR-1, a Gram-negative bacterium with known metal-reduction capability via extracellular electron transfer between outer membrane proteins and extracellular electron receptors. LC/MS/MS analysis resulted in the identification of about 400 proteins with 79% of them being predicted to be membrane localized. Quantitative aspects of the membrane enrichment were shown by peptide level (16)O and (18)O labeling of proteins from wild-type and mutant cells (generated from deletion of a type II secretion protein, GspD) prior to LC-MS analysis. Using a chemical probe labeled pure protein as an internal standard for normalization, the quantitative data revealed reduced abundances in Delta gspD mutant cells of many outer membrane proteins including the outer membrane c-type cytochromes OmcA and MtrC, in agreement with a previous report that these proteins are substrates of the type II secretion system.

Structural determinants underlying photoprotection in the photoactive orange carotenoid protein of cyanobacteria

The photoprotective processes of photosynthetic organisms involve the dissipation of excess absorbed light energy as heat. Photoprotection in cyanobacteria is mechanistically distinct from that in plants; it involves the orange carotenoid protein (OCP), a water-soluble protein containing a single carotenoid. The OCP is a new member of the family of blue light-photoactive proteins; blue-green light triggers the OCP-mediated photoprotective response. Here we report structural and functional characterization of the wild type and two mutant forms of the OCP, from the model organism Synechocystis PCC6803. The structural analysis provides high resolution detail of the carotenoid-protein interactions that underlie the optical properties of the OCP, unique among carotenoid-proteins in binding a single pigment per polypeptide chain. Collectively, these data implicate several key amino acids in the function of the OCP and reveal that the photoconversion and photoprotective responses of the OCP to blue-green light can be decoupled.

Identification of a protein required for recovery of full antenna capacity in OCP-related photoprotective mechanism in cyanobacteria

High light can be lethal for photosynthetic organisms. Similar to plants, most cyanobacteria protect themselves from high irradiance by increasing thermal dissipation of excess absorbed energy. The photoactive soluble orange carotenoid protein (OCP) is essential for the triggering of this photoprotective mechanism. Light induces structural changes in the carotenoid and the protein, leading to the formation of a red active form. Through targeted gene interruption we have now identified a protein that mediates the recovery of the full antenna capacity when irradiance decreases. In Synechocystis PCC 6803, this protein, which we called the fluorescence recovery protein (FRP), is encoded by the slr1964 gene. Homologues of this gene are present in all of the OCP-containing strains. The FRP is a 14-kDa protein, strongly attached to the membrane, which interacts with the active red form of the OCP. In vitro this interaction greatly accelerates the conversion of the red OCP form to the orange form. We propose that in vivo, FRP plays a key role in removing the red OCP from the phycobilisome and in the conversion of the free red OCP to the orange inactive form. The discovery of FRP and its characterization are essential elements in the understanding of the OCP-related photoprotective mechanism in cyanobacteria.

An alternative method for calcium depletion of the oxygen evolving complex of photosystem II as revealed by the dark-stable multiline EPR signal

The dark-stable multiline EPR signal of photosystem II (PSII) is associated with a slow-decaying S(2) state that is due to Ca(2+) loss from the oxygen evolving complex. Formation of the signal was observed in intact PSII in the presence of 100-250 mM NaCl at pH 5.5. Both moderately high NaCl concentration and decreased pH were required for its appearance in intact PSII. It was estimated that only a portion of oxygen evolving complexes was responsible for the signal (about 20% in 250 mM NaCl), based on the loss of the normal S(2)-state multiline signal. The formation of the dark-stable multiline signal in intact PSII at pH 5.5 could be reversed by addition of 15 mM Ca(2+) in the presence of moderately high NaCl, confirming that it was the absence of Ca(2+) that led to its appearance. Formation of the dark-stable multiline signal in NaCl-washed PSII, which lacks the PsbP (23 kDa) and PsbQ (17 kDa) subunits, was observed in about 80% of the sample in the presence of 150 mM NaCl at pH 5.5, but some signal was also observed under normal buffer conditions. In both intact and NaCl-washed PSII, the S(2)Y(Z). signal, which is also characteristic of Ca(2+) depletion, appeared upon subsequent illumination. Formation of the dark-stable multiline signal took place in the absence of Ca(2+) chelator or polycarboxylic acids, indicating that the signal did not require their direct binding as has been proposed previously. The conditions used here were milder than those used to produce the signal in previous studies and included a preillumination protocol to maximize the dark-stable S(2) state. Based on these conditions, it is suggested that Ca(2+) release occurred through protonation of key residues that coordinate Ca(2+) at low pH, followed by displacement of Ca(2+) with Na(+) by mass action at the moderately high NaCl concentration.