Intermolecular and intramolecular interactions of the 33-kDa protein in photosystem II

Poly(styrene-co-maleic acid)-mediated isolation of supramolecular membrane protein complexes from plant thylakoids

Derivatives of poly(styrene-co-maleic acid) (pSMA), have recently emerged as effective reagents for extracting membrane protein complexes from biological membranes. Despite recent progress in using SMAs to study artificial and bacterial membranes, very few reports have addressed their use in studying the highly abundant and well characterized thylakoid membranes. Recently, we tested the ability of twelve commercially available SMA copolymers with different physicochemical properties to extract membrane protein complexes (MPCs) from spinach thylakoid membrane. Based on the efficacy of both protein and chlorophyll extraction, we have found five highly efficient SMA copolymers: SMA® 1440, XIRAN® 25010, XIRAN® 30010, SMA® 17352, and SMA® PRO 10235, that show promise in extracting MPCs from chloroplast thylakoids. To further advance the application of these polymers for studying biomembrane organization, we have examined the composition of thylakoid supramolecular protein complexes extracted by the five SMA polymers mentioned above. Two commonly studied plants, spinach (Spinacia oleracea) and pea (Pisum sativum), were used for extraction as model biomembranes. We found that the pSMAs differentially extract protein complexes from spinach and pea thylakoids. Based on their differential activity, which correlates with the polymer chemical structure, pSMAs can be divided into two groups: unfunctionalized polymers and ester derivatives.

Physiological and proteomic analysis of maize seedling response to water deficiency stress

Low water availability is a major abiotic factor limiting photosynthesis and the growth and yield of crops. Maize (Zea mays) is among the most drought-sensitive cereal crops. Herein, the physiological and proteomic changes of maize seedlings caused by polyethylene-glycol-induced water deficit were analyzed. The results showed that malondialdehyde and proline contents increased continuously in the treated seedlings. Soluble sugar content and superoxide dismutase activity were upregulated initially but became downregulated under prolonged water deficit. A total of 104 proteins were found to be differentially accumulated under water stress. The identified proteins were mainly involved in photosynthesis, carbohydrate metabolism, stress defense, energy production, and protein metabolism. Interestingly, substantial incongruence between protein and transcript levels was observed, indicating that gene expression in water-stressed maize seedlings is controlled by complex mechanisms. Finally, we propose a hypothetical model that includes the different molecular, physiological, and biochemical changes that occurred during the response and tolerance of maize seedlings to water deficiency. Our study provides valuable insight for further research into the overall mechanisms underlying drought response and tolerance in maize and other plants.

The Use of Advanced Mass Spectrometry to Dissect the Life-Cycle of Photosystem II

Photosystem II (PSII) is a photosynthetic membrane-protein complex that undergoes an intricate, tightly regulated cycle of assembly, damage, and repair. The available crystal structures of cyanobacterial PSII are an essential foundation for understanding PSII function, but nonetheless provide a snapshot only of the active complex. To study aspects of the entire PSII life-cycle, mass spectrometry (MS) has emerged as a powerful tool that can be used in conjunction with biochemical techniques. In this article, we present the MS-based approaches that are used to study PSII composition, dynamics, and structure, and review the information about the PSII life-cycle that has been gained by these methods. This information includes the composition of PSII subcomplexes, discovery of accessory PSII proteins, identification of post-translational modifications and quantification of their changes under various conditions, determination of the binding site of proteins not observed in PSII crystal structures, conformational changes that underlie PSII functions, and identification of water and oxygen channels within PSII. We conclude with an outlook for the opportunity of future MS contributions to PSII research.

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.

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.

Probing the N-terminal sequence of spinach PsbO: evidence that essential threonine residues bind to different functional sites in eukaryotic photosystem II

The N-terminal ¹E-⁶L domain of the manganese-stabilizing protein (PsbO) from spinach prevents non-specific binding of the subunit to photosystem II (PSII) and deletions of the ¹E-⁷T or ¹E-¹⁵T sequences from the PsbO N-terminus reduce or impair, respectively, functional binding of PsbO to PSII (Popelkova et al., Biochemistry 42:6193-6200, 2003). The work presented here provides deeper insights into the interaction of PsbO with PSII. The data show that a single mutation, ¹⁵T → A in mature PsbO from spinach reduces the stoichiometry of its functional binding from two to one subunit per PSII and decreases reconstitution of activity to about 45 % of the wild-type control. Replacement of the ¹E-⁶L domain with ⁶M in the T15A PsbO mutant has no additional negative effect on recovery of O₂ evolution activity, but it significantly weakens both functional and nonspecific binding of the truncated mutant to PSII. These results suggest that the ¹⁵T side-chain by itself is essential for binding of one of two PsbO subunits to eukaryotic PSII and that specific PSII-binding sites for PsbO are distinguishable; one PSII-binding site does not require PsbO-¹⁵T and probably interacts with the other N-terminal domain of PsbO. Identity of the latter domain is revealed by a requirement for the presence of the ¹E-⁶L sequence that is shown here to be necessary for high-affinity binding of PsbO to PSII. When combined with previous results, the data presented here lead to a more detailed model for PsbO binding in eukaryotic PSII.

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.

CE detection of N-(3-dimethylaminopropyl)-N-carbodiimide/N-hydroxysuccinimide-coupled proteins after homo- and hetero-crosslinking reactions

Protein-protein conjugates formed by carbodiimide crosslinking reactions have been analyzed for the first time using CE. Lysozyme and BSA were chosen as model proteins to study the efficacy of N-(3-dimethylaminopropyl)-N-ethylcarbodiimide and N-hydroxysuccinimide as crosslinkers. Detection of the molecular mass increase was checked by SDS-PAGE. Commercially available, PVA-coated capillaries showed appropriate selection, while phospho-deactivated and dynamic PVA-coated capillaries did not give suitable resolution. CE was found to be an efficient tool to characterize homo- (lysozyme-lysozyme) and hetero- (lysozyme-BSA) protein coupling by suitable variations of electrophoretic mobilities.

Structural and functional aspects of the MSP (PsbO) and study of its differences in thermophilic versus mesophilic organisms

The Manganese Stabilizing Protein (MSP) of Photosystem II (PSII) is a so-called extrinsic subunit, which reversibly associates with the other membrane-bound PSII subunits. The MSP is essential for maximum rates of O(2) production under physiological conditions as stabilizes the catalytic [Mn(4)Ca] cluster, which is the site of water oxidation. The function of the MSP subunit in the PSII complex has been extensively studied in higher plants, and the structure of non-PSII associated MSP has been studied by low-resolution biophysical techniques. Recently, crystal structures of PSII from the thermophilic cyanobacterium Thermosynechococcus elongatus have resolved the MSP subunit in its PSII-associated state. However, neither any crystal structure is available yet for MSP from mesophilic organisms, higher plants or algae nor has the non-PSII associated form of MSP been crystallized. This article reviews the current understanding of the structure, dynamics, and function of MSP, with a particular focus on properties of the MSP from T. elongatus that may be attributable to the thermophilic ecology of this organism rather than being general features of MSP.

Binding of manganese stabilizing protein to photosystem II: identification of essential N-terminal threonine residues and domains that prevent nonspecific binding

The N-terminus of spinach photosystem II manganese stabilizing protein (MSP) contains two amino acid sequences, (4)KRLTYD(10)E and (15)TYL(18)E, that are necessary for binding of two copies of this subunit to the enzyme [Popelkova et al. (2002) Biochemistry 41, 10038-10045]. To better understand the basis of MSP-photosystem II interactions, the role of threonine residues in the highly conserved motifs T(Y/F)DE and TY has been characterized. Deletion mutants lacking the first 5, 6, 7, and 15 amino acid residues at the N-terminus of the protein were examined for their ability to reconstitute activity in MSP-depleted photosystem II. The results reported here show that truncations of five and six amino acid residues (mutants DeltaR5M and DeltaL6M mutants) have no negative effect on recovery of oxygen evolution activity or on binding of MSP to photosystem II. Deletion of seven residues (mutant DeltaT7M) decreases reconstitution activity to 40% of the control value and reduces functional binding of the mutant protein to photosystem II from two to one copy. Deletion of 15 amino acid residues (mutant DeltaT15M) severely impairs functional binding of MSP, and lowers O(2) evolution activity to less than 20% of the control. DeltaT7M is the only mutant that exhibited neither nonspecific binding to photosystem II nor changes in tertiary structure. These, and previous results, show that the highly conserved Thr7 and Thr15 residues of MSP are required for functional binding of two copies of the eukaryotic protein to photosystem II. Although the N-terminal domains, (1)EGGKR(6)L, (8)YDEIQS(14)K, and (16)YL(18)E of spinach MSP are unnecessary for specific, functional binding interactions, these sequences are necessary to prevent nonspecific binding of the protein to photosystem II.

Carboxylate groups on the manganese-stabilizing protein are required for efficient binding of the 24 kDa extrinsic protein to photosystem II

The effects of the modification of carboxylate groups on the manganese-stabilizing protein on the binding of the 24 kDa extrinsic protein to Photosystem II were investigated. Carboxylate groups on the manganese-stabilizing protein were modified with glycine methyl ester in a reaction facilitated by 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide. The manganese-stabilizing protein which was modified while associated with NaCl-washed membranes could bind to calcium chloride-washed PS II membranes and reconstitute oxygen evolution in a manner similar to that observed for unmodified manganese-stabilizing protein (Frankel, L.K, Cruz, J. C. and Bricker, T. M. (1999) Biochemistry 38, 14271-14278). However, PS II membranes reconstituted with this modified protein were defective in their ability to bind the extrinsic 24 kDa protein of Photosystem II. Mapping of the sites of modification was carried out by trypsin and Staphylococcus V8 protease digestion of the modified protein and analysis by MALDI mass spectrometry. These studies indicated that the domains (1)E-(71)D, (97)D-(144)D, and (180)D-(187)E are labeled when the manganese-stabilizing protein is bound to NaCl-washed Photosystem II membranes. We hypothesize that modified carboxylates, possibly residues (1)E, (32)E, (139)E, and/or (187)E, in these domains are responsible for the altered binding affinity of the 24 kDa protein observed.

N-terminal truncations of manganese stabilizing protein identify two amino acid sequences required for binding of the eukaryotic protein to photosystem II and reveal the absence of one binding-related sequence in cyanobacteria

Manganese stabilizing protein (MSP) is an intrinsically disordered extrinsic subunit of photosystem II that regulates the stability and kinetic performance of the tetranuclear manganese cluster that oxidizes water to oxygen. An earlier study showed that deletion of the (1)E-(3)G domain of MSP caused no loss of activity reconstitution, whereas deletion of the (4)K-(10)E domain reduced binding of the protein from 2 to 1 mol of MSP/mol of photosystem II and lowered activity reconstitution to about 50% of the control value [Popelkova et al. (2002) Biochemistry 41, 2702-2711]. In this work we present evidence that deletion of 13 or 14 amino acid residues from the MSP N-terminus (mutants DeltaS13M and DeltaK14M) does not interfere either with functional binding of one copy of MSP to photosystem II or with reconstitution of oxygen evolution activity to 50% of the control level. Both of these mutants exhibit nonspecific binding to photosystem II at higher protein concentrations. Truncation of the MSP sequence by 18 amino acids (mutant DeltaE18M), however, causes a loss of protein binding and activity reconstitution. This result demonstrates that the N-terminal domain (15)T-(18)E is required for binding of at least one copy of MSP to photosystem II. Analyses of CD spectra reveal changes in the structure of DeltaE18M (loss of beta-sheet, gain of unordered structure). Use of the information gained from these experiments in analyses of N-terminal sequences of MSP from a number of species indicates that higher plants and algae possess two recognition domains that are required for MSP binding to PSII, whereas cyanobacteria lack the first N-terminal domain found in eukaryotes. This may explain the absence of a second copy of MSP in the crystal structure of PSII from Synechococcus elongatus [Zouni et al. (2001) Nature 409, 739-743].

A domain of the manganese-stabilizing protein from Synechococcus elongatus involved in functional binding to photosystem II

Site-directed mutagenesis was performed to investigate whether the two protease-sensitive sequences Phe(156)-Gly(163) and Arg(184)-Ser(191), of the manganese-stabilizing protein (MSP) from a thermophilic cyanobacterium, Synechococcus elongatus (Motoki, A., Shimazu, T., Hirano, M., and Katoh, S. (1998) Biochim. Biophys. Acta 1365, 492-502), are involved in functional interaction with photosystem II (PSII). The ability of MSP to bind to its functional site on the PSII complex and to reactivate oxygen evolution was dramatically reduced by the substitution of Arg(152), Asp(158), Lys(160), or Arg(162) with uncharged residues, by insertion of a single residue between Phe(156) and Leu(157), or by deletion of Leu(157). Substitution of each of the four charged residues with an identically charged residue showed that the charges at Asp(158), and possibly Lys(160), are important for the electrostatic interaction with PSII. The reactivating ability was also strongly affected by the alteration of Phe(156) to Leu. Replacement of Lys(188), the only strictly conserved charged residue in the Arg(184)-Ser(191) sequence, by Gln had only a marginal effect on the function of MSP. High affinity binding of MSP to PSII was also affected significantly by mutation at Arg(152), which is located in a region (Val(148)-Arg(152)) strictly conserved among the 14 sequences so far reported. These results imply that the Val(148)-Gly(163) sequence, which is well conserved among MSPs from cyanobacteria to higher plants, is a domain of MSP for functional interaction with PSII.

Increased tolerance to thermal inactivation of oxygen evolution in spinach Photosystem II membranes by substitution of the extrinsic 33-kDa protein by its homologue from a thermophilic cyanobacterium

Photosynthetic oxygen evolution is an extremely heat-sensitive process and incubation of spinach Photosystem II (PSII) membranes at 40 degrees C for only several minutes leads to its complete inactivation. Substitution experiments of the spinach 33-kDa manganese stabilizing protein by a homologue protein, isolated either from the thermophilic cyanobacterium Phormidium laminosum, or from Escherichia coli as a recombinant thermophilic cyanobacterial protein, showed a significant increase in tolerance to heat inactivation of the oxygen-evolving activity. The results allow us to suggest that thermal inactivation of oxygen evolution in higher plant PSII membranes is due to dissociation of the 33-kDa protein as a consequence of temperature-induced conformational changes, and stabilization can be provided by substitution by a thermostable homologue whose secondary structure and binding to PSII remain unaltered at moderately high temperatures.

N-terminus of the photosystem II manganese stabilizing protein: effects of sequence elongation and truncation

The importance of the N-terminal domain of manganese stabilizing protein in binding to photosystem II has been previously demonstrated [Eaton-Rye and Murata (1989) Biochim. Biophys. Acta 977, 219-226; Odom and Bricker (1992) Biochemistry 31, 5616-5620]. In this paper, we report results from a systematic study of functional and structural consequences of N-terminal elongation and truncation of manganese stabilizing protein. Precursor manganese stabilizing protein is the unprocessed wild-type protein, which carries an N-terminal extension of 84 amino acids in the form of its chloroplastic signal peptide. Despite its increased size, this protein is able to reconstitute O(2) evolution activity to levels observed with the mature, processed protein, but it also binds nonspecifically to PSII. Truncation of wild-type manganese stabilizing protein by site-directed mutagenesis to remove three N-terminal amino acids, resulting in a mutant called DeltaG3M, causes no loss of activity reconstitution, but this protein also exhibits nonspecific binding. Further truncation of the wild-type protein by ten N-terminal amino acids, producing DeltaE10M, limits binding of manganese stabilizing protein to 1 mol/mol of photosystem II and decreases activity reconstitution to about 65% of that obtained with the wild-type protein. Because two copies of wild type normally bind to photosystem II, amino acids in the domain (4)K-(10)E must be involved in the binding of one copy of manganese stabilizing protein to photosystem II. Spectroscopic analysis (CD and UV spectra) reveals that N-terminal elongation and deletion of manganese stabilizing protein influence its overall conformation, even though secondary structure content is not perturbed. Our data suggest that the solution structure of manganese stabilizing protein attains a more compact solution structure upon removal of N-terminal amino acids.

Mutations of arginine 64 within the putative Ca(2+)-binding lumenal interhelical a-b loop of the photosystem II D1 protein disrupt binding of the manganese stabilizing protein and cytochrome c(550) in Synechocystis sp. PCC6803

Mutations D1-R64E, D1-R64Q, and D1-R64V in the putative calcium-binding lumenal interhelical a-b loop of the photosystem II (PSII) D1 protein were characterized in terms of impact on growth, extrinsic protein binding, photoactivation, and properties of the H(2)O-oxidation complex. The D1-R64E charge reversal mutation greatly weakened the binding of the extrinsic manganese-stabilizing protein (MSP) and, to a considerably lesser extent, weakened the binding of cytochrome c(550) (c550). Both D1-R64Q and D1-R64E exhibited an increased requirement for Ca(2+) in the cell growth medium. Bare platinum electrode measurements of O(2)-evolving membranes showed a retarded appearance of O(2) following single turn-over flashes, especially in the case of the D1-R64E mutant. The D1-R64E mutant also had a pronounced tendency to lose O(2) evolution activity in the dark and exhibited an increased relative quantum yield of photoactivation, which are characteristics shared by mutants that lack extrinsic proteins. S(2) and S(3) decay measurements in the isolated membranes indicate that D1-R64E and D1-R64Q have faster decays of these higher S-states as compared to the wild-type. However, fluorescence decay in the presence of DCMU, which monitors primarily Q(A)(-) charge recombination with PSII donors, showed somewhat slower decays. Taken together, the fluorescence and S-state decay indicate that the midpoint of either Q(B)(-) has been modified to be more negative in the mutants or that a recombination path presumably involving either Q(B)(-) or Y(D) has become kinetically more accessible.

Molecular characterization of cDNA encoding oxygen evolving enhancer protein 1 increased by salt treatment in the mangrove Bruguiera gymnorrhiza

Young plants of the common Okinawa mangrove species Bruguiera gymnorrhiza were transferred from freshwater to a medium with seawater salt level (500 mM NaCl). Two-dimensional gel electrophoresis revealed in the leaf extract of the plant a 33 kDa protein with pI 5.2, whose quantity increased as a result of NaCl treatment. The N-terminal amino acids sequence of this protein had a significant homology with mature region of oxygen evolving enhancer protein 1 (OEE1) precursor. The cloning of OEE1 precursor cDNA fragment was carried out by means of reverse transcription-PCR (RT-PCR) using degenerated primers. Both 3'- and 5'-regions were isolated by rapid amplification of cDNA ends (RACE) method. The deduced amino acid sequence consisted of 322 amino acids and was 87% identical to that of Nicotiana tabacum. In B. gymnorrhiza, the predicted amino acid sequence of the mature protein starts at the residue number 85 of the open reading frame. The first 84-amino acid residues correspond to a typical transit sequence for the signal directing OEE1 to its appropriate compartment of chloroplast. The expression of OEE1 was analyzed together with other OEE subunits and D1 protein of photosystem II. The transcript levels of all the three OEEs were enhanced by NaCl treatment, but the significant increase of D1 protein was not observed.

Deprotonation of the 33-kDa, extrinsic, manganese-stabilizing subunit accompanies photooxidation of manganese in photosystem II

Photosystem II catalyzes photosynthetic water oxidation. The oxidation of water to molecular oxygen requires four sequential oxidations; the sequentially oxidized forms of the catalytic site are called the S states. An extrinsic subunit, the manganese-stabilizing protein (MSP), promotes the efficient turnover of the S states. MSP can be removed and rebound to the reaction center; removal and reconstitution is associated with a decrease in and then a restoration of enzymatic activity. We have isotopically edited MSP by uniform (13)C labeling of the Escherichia coli-expressed protein and have obtained the Fourier transform infrared spectrum associated with the S(1) to S(2) transition in the presence either of reconstituted (12)C or (13)C MSP. (13)C labeling of MSP is shown to cause 30-60 cm(-1) shifts in a subset of vibrational lines. The derived, isotope-edited vibrational spectrum is consistent with a deprotonation of glutamic/aspartic acid residues on MSP during the S(1) to S(2) transition; the base, which accepts this proton(s), is not located on MSP. This finding suggests that this subunit plays a role as a stabilizer of a charged transition state and, perhaps, as a general acid/base catalyst of oxygen evolution. These results provide a molecular explanation for known MSP effects on oxygen evolution.

Carboxylate groups on the manganese-stabilizing protein are required for its efficient binding to photosystem II

The effects of the modification of carboxylate groups on the manganese-stabilizing protein of photosystem II were investigated. Carboxylate groups (including possibly the C-terminus) on the manganese-stabilizing protein were modified with glycine methyl ester in a reaction facilitated by 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide. The manganese-stabilizing protein that was modified while associated with NaCl-washed photosystem II membranes contained 1-2 modified carboxylates, whereas the protein that was modified while free in solution contained 4 modified carboxylates. Both types of modified protein could reconstitute oxygen evolution at high manganese-stabilizing protein to photosystem II reaction center ratios. However, the protein that had been modified in solution exhibited a dramatically altered binding affinity for photosystem II. No such alteration in binding affinity was observed for the protein that had been modified while associated with the photosystem. Mapping of the sites of modification was carried out by trypsin and Staphylococcus V8 protease digestion of the modified proteins and analysis by matrix-assisted laser desorption/ionization mass spectrometry. These studies indicated that the domains (157)D-(168)D and (212)E-(247)Q (C-terminus) are labeled only when the manganese-stabilizing protein is modified in solution. Modified carboxylates in these domains are responsible for the altered binding affinity of this protein for the photosystem.

The carboxyl-terminal tripeptide of the manganese-stabilizing protein is required for quantitative assembly into photosystem II and for high rates of oxygen evolution activity

The extrinsic manganese stabilizing protein of photosystem II is required for Mn retention by the O2-evolving complex, accelerates the rate of O2 evolution, and protects photosytem II against photoinhibition. We report results from studies of the in vitro reconstitution of spinach photosytem II with recombinant manganese stabilizing protein with C-terminal deletions of two, three, and four amino acids. The deletions were the result of amber mutations introduced by site-directed mutagenesis. Removal of the C-terminal dipeptide (Glu-Gln) did not diminish the ability of the manganese stabilizing protein either to rebind to or to restore high rates of O2 evolution to photosystem II preparations depleted of the native protein. Deletion of the C-terminal tripeptide (Leu-Glu-Gln) resulted in weakened but specific binding of manganese stabilizing protein to photosystem II and minimal recovery of O2 evolution activity. Removal of the C-terminal tetrapeptide (Gln-Leu-Glu-Gln) eliminated the ability of the subunit to interact stably with all of its available binding sites on photosystem II, as evidenced by the fact that this mutant was totally inactive in restoring O2 evolution activity. Evidence is presented to indicate that these mutational effects on the binding and function of the manganese stabilizing protein may be due to major changes in tertiary structure. The truncation mutations lacking either the C-terminal tri- or tetrapeptide exhibit apparent size increases of 25 and 40%, respectively, when compared either to a mutant lacking the C-terminal dipeptide or to the wild-type protein.

Photoassembly of the photosystem II (Mn)4 cluster in site-directed mutants impaired in the binding of the manganese-stabilizing protein

Photoactivation is the light-dependent ligation of Mn2+ into the H2O oxidation complex of photosystem II (PSII) and culminates in the formation of an enzymatically active complex containing Ca2+ and four Mn>/=3+. Previous kinetic analysis demonstrated that the genetic removal of the extrinsic manganese-stabilizing protein (MSP) increases the quantum yield of photoactivation 4-fold relative to that of the wild type, consistent with the hypothesis that MSP hinders access of Mn2+ to a site of photoligation [Burnap, R. L., et al. (1996) Biochemistry35, 874-882]. In this report, several Synechocystis sp. PCC6803 mutants with defined amino acid substitutions in the N-terminal region of MSP or the e-loop of intrinsic PSII protein CP47 [Putnam-Evans, C., et al. (1996) Biochemistry 35, 4046-4053] were characterized in terms of the binding of MSP to the intrinsic portion of the PSII complex and in terms of photoactivation kinetics. The charge-pair switch mutation, Arg384Arg385 --> Glu384Glu385 in the lumenal e-loop of CP47 (CP47 RR384385EE), exhibited the most severe impairment of MSP binding, whereas the Arg384Arg385 --> Gly384Gly385 (CP47 RR384385GG) mutation caused a more moderate impairment in binding. Single-substitution mutations at the highly conserved Asp9 or Asp10 positions in the amino-terminal region of MSP also resulted in a reduced binding affinity, but not as severe as that in CP47 RR384385EE. The relative quantum yield of photoactivation of hydroxylamine-extracted mutant PSII was generally found to correlate with the degree of MSP binding impairment, with the CP47 RR384385 mutants exhibiting the highest quantum yields. A two-locus, double-mutant construct involving deletion of MSP in the CP47 RR384385EE background was found to be only slightly more impaired in H2O oxidation activity than either of the corresponding single-locus mutant derivatives, indicating that mutations at these genetically separate loci encode physically interacting products affecting the same reaction parameter during H2O oxidation. Taken together, the results reinforce the concept that MSP interacts with the e-loop of CP47 at Arg384Arg385 and that disruption of this interaction causes significant alterations of the site of H2O oxidation in terms of assembly and enzymatic activity of the Mn cluster.