The topology of a membrane protein: the orientation of the 32 kd Qb-binding chloroplast thylakoid membrane protein

Charge separation in photosystem II: a comparative and evolutionary overview

Our current understanding of the PSII reaction centre owes a great deal to comparisons to the simpler and better understood, purple bacterial reaction centre. Here we provide an overview of the similarities with a focus on charge separation and the electron acceptors. We go on to discuss some of the main differences between the two kinds of reaction centres that have been highlighted by the improving knowledge of PSII. We attempt to relate these differences to functional requirements of water splitting. Some are directly associated with that function, e.g. high oxidation potentials, while others are associated with regulation and protection against photodamage. The protective and regulatory functions are associated with the harsh chemistry performed during its normal function but also with requirements of the enzyme while it is undergoing assembly and repair. Key aspects of PSII reaction centre evolution are also addressed. This article is part of a Special Issue entitled: Photosystem II.

Relocating a gene for herbicide tolerance: A chloroplast gene is converted into a nuclear gene

The chloroplast gene psbA codes for the photosynthetic quinone-binding membrane protein Q(B), which is the target of the herbicide atrazine. This gene has been converted into a nuclear gene. The psbA gene from an atrazine-resistant biotype of Amaranthus hybridus has been modified by fusing its coding region to transcription-regulation and transit-peptide-encoding sequences of a bona fide nuclear gene. The constructs were introduced into the nuclear genome of tobacco by using the Agrobacterium tumor-inducing (Ti) plasmid system, and the protein product of nuclear psbA has been identified in the photosynthetic membranes of chloroplasts. Recovery of atrazine-tolerant transgenic plants shows that the product of the transplanted gene functions in photosynthesis. These experiments show that it is possible to modify chloroplast-gene-specified functions via nuclear-genome transformation and also raise evolutionary questions.

Functional asymmetry of photosystem II D1 and D2 peripheral chlorophyll mutants of Chlamydomonas reinhardtii

The peripheral accessory chlorophylls (Chls) of the photosystem II (PSII) reaction center (RC) are coordinated by a pair of symmetry-related histidine residues (D1-H118 and D2-H117). These Chls participate in energy transfer from the proximal antennae complexes (CP43 and CP47) to the RC core chromophores. In addition, one or both of the peripheral Chls are redox-active and participate in a low-quantum-yield electron transfer cycle around PSII. We demonstrate that conservative mutations of the D2-H117 residue result in decreased Chl fluorescence quenching efficiency attributed to reduced accumulation of the peripheral accessory Chl cation, Chl(Z)(+). In contrast, identical symmetry-related mutations at residue D1-H118 had no effect on Chl fluorescence yield or quenching kinetics. Mutagenesis of the D2-H117 residue also altered the line width of the Chl(Z)(+) EPR signal, but the line shape of the D1-H118Q mutant remained unchanged. The D1-H118 and D2-H117 mutations also altered energy transfer properties in PSII RCs. Unlike wild type or the D1-H118Q mutant, D2-H117N RCs exhibited a reduced CD doublet in the red region of Chl absorbance band, indicative of reduced energetic coupling between P680 and the peripheral accessory Chl. In addition, transient absorption measurements of D2-H117N RCs, excited on the blue side of the Chl absorbance band, exhibited a ( approximately 400 fs) pheophytin Q(X) band bleach lifetime component not seen in wild-type or D1-H118Q RCs. The origin of this component may be related to delayed fast-energy equilibration of the excited state between the core pigments of this mutant.

Photosystem II: the solid structural era

Understanding the precise role of photosystem II as an element of oxygenic photosynthesis requires knowledge of the molecular structure of this membrane protein complex. The past few years have been particularly exciting because the structural era of the plant photosystem II has begun. Although the atomic structure has yet to be determined, the map obtained at 6 A resolution by electron crystallography allows assignment of the key reaction center subunits with their associated pigment molecules. In the following, we first review the structural details that have recently emerged and then discuss the primary and secondary photochemical reaction pathways. Finally, in an attempt to establish the evolutionary link between the oxygenic and the anoxygenic photosynthesis, a framework structure common to all photosynthetic reaction centers has been defined, and the implications have been described.

In vitro random mutagenesis of the D1 protein of the photosystem II reaction center confers phototolerance on the cyanobacterium Synechocystis sp. PCC 6803

The D1 protein of the photosystem II reaction center is thought to be the most light-sensitive component of the photosynthetic machinery. To understand the mechanisms underlying the light sensitivity of D1, we performed in vitro random mutagenesis of the psbA gene that codes for D1, transformed the unicellular cyanobacterium Synechocystis sp. PCC 6803 with mutated psbA, and selected phototolerant transformants that did not bleach in high intensity light. A region of psbA2 coding for 178 amino acids of the carboxyl-terminal portion of the peptide was subjected to random mutagenesis by low fidelity polymerase chain reaction amplification or by hydroxylamine treatment. This region contains the binding sites for Q(B), D2 (through Fe), and P680. Eighteen phototolerant mutants with single and multiple amino acid substitutions were selected from a half million transformants exposed to white light at 320 micromol m(-2) s(-1). A strain transformed with non-mutagenized psbA2 became bleached under the same conditions. Site-directed mutagenesis has confirmed that one or more substitutions of amino acids at residues 234, 254, 260, 267, 322, 326, and 328 confers phototolerance. The rate of degradation of D1 protein was not appreciably affected by the mutations. Reduced bleaching of mutant cyanobacterial cells may result from continued buildup of photosynthetic pigment systems caused by changes in redox signals originating from D1.

The D1 C-terminal processing protease of photosystem II from Scenedesmus obliquus. Protein purification and gene characterization in wild type and processing mutants

Polypeptide D1 of the photosystem II reaction center of oxygenic photosynthesis is expressed in precursor form (pre-D1), and it must be proteolytically processed at its C terminus to enable assembly of the manganese cluster responsible for photosynthetic water oxidation. A rapid and highly sensitive enzyme-linked immunosorbent assay-based microtiter plate method is described for assaying this D1 C-terminal processing protease. A protocol is described for the isolation and purification to homogeneity of the enzyme from the green alga, Scenedesmus obliquus. Amino acid sequence information on the purified protease was used to clone the corresponding gene, the translated sequence of which is presented. A comparison of the gene product with homologous proteases points to a region of conserved residues that likely corresponds to the active site of a new class of serine protease. The LF-1 mutant strain of Scenedesmus (isolated by Dr. Norman Bishop) is incapable of processing pre-D1. We show here that the C-terminal processing protease gene in this strain contains a single base deletion that causes a frame shift and a premature stop of translation within the likely active site of the enzyme. A suppressor strain, LF-1-RVT-1, which is photoautotrophic and capable of processing pre-D1 has a nearby single base insertion that restores the expression of active enzyme. These observations provide the first definitive proof that the enzyme isolated is responsible for in vivo proteolytic processing of pre-D1 and that no other protease can compensate for its loss.

Site-directed mutations at residue 251 of the photosystem II D1 protein of Chlamydomonas that result in a nonphotosynthetic phenotype and impair D1 synthesis and accumulation

In Cyanobacteria and Chlamydomonas reinhardtii, substitution of valine for alanine at position 251 of the photosystem II D1 protein in the loop between transmembrane helices IV and V confers resistance to herbicides that reduce photosystem II function and increases sensitivity to photoinhibition. Using site-directed mutagenesis and chloroplast transformation in Chlamydomonas we have examined further the role of residue 251 in relation to D1 structure, function, and photosynthetic performance. Of the 12 different amino acid substitutions for Ala251 introduced at this position, five (Arg, Asp, Gln, Glu, and His) resulted in a nonphotosynthetic phenotype. Transformants with the Arg251 substitution synthesize a normal sized 32-kDa D1 protein with greatly reduced stability. The Gln, Glu, His, and Asp transformants make a 33-34-kDa form of the D1 protein of varying stability as well as an immunologically related polypeptide of 24-25 kDa corresponding to the N-terminal portion of D1 that is unstable and appears to be an aborted D1 translation product. All mutant forms of the D1 protein are intrinsic to the thylakoids. In contrast to previous studies in Cyanobacteria showing that residues in the IV-V loop can be mutated or deleted without loss of photosynthetic competence, our results suggest that Ala251 has a key role in the structure and function of the IV-V loop region.

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

Intermolecular and intramolecular interactions of the extrinsic 33-kDa protein in photosystem II were investigated by cross-linking with a water-soluble carbodiimide as cross-linking agent. This zero-length cross-linker is known to cross-link the 33-kDa protein to the chlorophyll-alpha-binding protein CP47 [Bricker, T. M., Odom, W. R. & Queirolo, C. B. (1988) FEBS Lett. 231, 111-117; Enami, I., Kaneko, M., Kitamura, N., Koike, H., Sonoike, K., Inoue, Y & Katoh, S. (1991) Biochim. Biophys. Acta 1960, 224-232]. In this work, cross-linking was observed not only to CP47 but also to a small intrinsic subunit. In addition, through the use of a high-resolution SDS-gel system, three intramolecular cross-linked products of the 33-kDa protein were detected. To search for additional cross-linking sites that might not be accessible to the cross-linker in intact photosystem II, the isolated 33-kDa protein was activated for cross-linking and subsequently bound to CaCl2-washed photosystem II. In the complementary experiment, CaCl2-washed photosystem II was activated, then reconstituted with the 33-kDa protein. The results of the cross-linking reactions demonstrated that all carboxylic acid groups involved in cross-linking were located on the 33-kDa protein and all primary amines were located on intrinsic membrane proteins. No cross-linking other than those observed in cross-linking experiments with intact photosystem II were observed. This indicated that the 33-kDa protein is bound to CP47 and a small subunit but not to the photosystem II reaction centre. This observation is consistent with the finding that cross-linking was independent of the presence or absence of the manganese cluster. Possible residues on the 33-kDa protein and CP47 involved in cross-linking are suggested.

The lumenal loop connecting transmembrane helices I and II of the D1 polypeptide is important for assembly of the photosystem two complex

Current structural models indicate that the D1 and D2 polypeptides of the Photosystem two reaction center complex (PS II RC) each span the thylakoid membrane five times. In order to assess the importance of the lumenal extrinsic loop that connects transmembrane helices I and II of D1 we have constructed five deletion mutants and two double mutants in the cyanobaterium Synechocystic sp. PCC 6803. Four of the deletion mutants (Δ59-65, Δ69-74, Δ79-86 and Δ109-110) are obligate photoheterotrophs unable to accumulate D1 in the membrane as assayed by immunoblotting experiments or pulse-labelling experiments using [(35)S]-methionine. In contrast deletion mutant Δ100 which lacks A100 behaved very similarly to the WT control strain in terms of photoautotrophic growth rate, saturated rates of oxygen evolution, flash-induced oxygen evolution, fluorescence induction and decay, and thermoluminescence. Δ100 is the first example of an internal deletion on the lumenal side of the D1 polypeptide that is benign to photosystem two function. Double mutant D103G/E104A also behaves similarly to the WT control strain leading to the conclusion that residues D103 and E104 are unlikely to be involved in ligating the metal ions Mn or Ca(2+), which are needed for photosynthetic oxygen evolution. Double mutant, G109A/G110A, was constructed to assess the significance of this GlyGly motif which is also conserved in the L subunit of purple bacterial reaction centres. The G109A/G110A mutant is able to evolve oxygen at approximately 50-70% of WT rates but is unable to grow phatoautotrophically apparently because of an enhanced sensitivity to photoinactivation than the WT control strain. A photoautotropic revertant was isolated from this strain and shown to result from a mutation that restored the WT codon at position 109. Pulse-chase experiments in cells using [(35)S]-methionine showed that resistance to photoinhibition in the revertant correlated with an enhanced rate of incorporation of D1 into the membrane compared to mutant G109A/G110A. The sensitivity to photoinhibition shown by the G109A/G110A mutant is therefore consistent with a perturbation to the D1 repair cycle possibly at the level of D1 synthesis or incorporation of D1 into the PS II complex.

Modeling of the D1/D2 proteins and cofactors of the photosystem II reaction center: implications for herbicide and bicarbonate binding

A three-dimensional model of the photosystem II (PSII) reaction center from the cyanobacterium Synechocystis sp. PCC 6803 was generated based on homology with the anoxygenic purple bacterial photosynthetic reaction centers of Rhodobacter sphaeroides and Rhodopseudomonas viridis, for which the X-ray crystallographic structures are available. The model was constructed with an alignment of D1 and D2 sequences with the L and M subunits of the bacterial reaction center, respectively, and by using as a scaffold the structurally conserved regions (SCRs) from bacterial templates. The structurally variant regions were built using a novel sequence-specific approach of searching for the best-matched protein segments in the Protein Data Bank with the "basic local alignment search tool" (Altschul SF, Gish W, Miller W, Myers EW, Lipman DJ, 1990, J Mol Biol 215:403-410), and imposing the matching conformational preference on the corresponding D1 and D2 regions. The structure thus obtained was refined by energy minimization. The modeled D1 and D2 proteins contain five transmembrane alpha-helices each, with cofactors (4 chlorophylls, 2 pheophytins, 2 plastoquinones, and a non-heme iron) essential for PSII primary photochemistry embedded in them. A beta-carotene, considered important for PSII photoprotection, was also included in the model. Four different possible conformations of the primary electron donor P680 chlorophylls were proposed, one based on the homology with the bacterial template and the other three on existing experimental suggestions in literature. The P680 conformation based on homology was preferred because it has the lowest energy. Redox active tyrosine residues important for P680+ reduction as well as residues important for PSII cofactor binding were analyzed. Residues involved in interprotein interactions in the model were also identified. Herbicide 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) was also modeled in the plastoquinone QB binding niche using the structural information available from a DCMU-binding bacterial reaction center. A bicarbonate anion, known to play a role in PSII, but not in anoxygenic photosynthetic bacteria, was modeled in the non-heme iron site, providing a bidentate ligand to the iron. By modifying the previous hypothesis of Blubaugh and Govindjee (1988, Photosyn Res 19:85-128), we modeled a second bicarbonate and a water molecule in the QB site and we proposed a hypothesis to explain the mechanism of QB protonation mediated by bicarbonate and water. The bicarbonate, stabilized by D1-R257, donates a proton to QB2- through the intermediate of D1-H252; and a water molecule donates another proton to QB2-. Based on the discovery of a "water transport channel" in the bacterial reaction center, an analogous channel for transporting water and bicarbonate is proposed in our PSII model. The putative channel appears to be primarily positively charged near QB and the non-heme iron, in contrast to the polarity distribution in the bacterial water transport channel. The constructed model has been found to be consistent with most existing data.

Two-dimensional crystals of photosystem II: biochemical characterization, cryoelectron microscopy and localization of the D1 and cytochrome b559 polypeptides

Photosystem II (PS II) is a photosynthetic reaction center found in higher plants which has the unique ability to evolve oxygen from water. Several groups have formed two-dimensional PS II crystals or have isolated PS II complexes and studied them by electron microscopy and image analysis. The majority of these specimens have not been well characterized biochemically and have yielded relatively low resolution two-dimensional projection maps with a variety of unit cell sizes. We report the characterization of the polypeptide and lipid content of tubular crystals of PS II. The crystals contain the reaction center core polypeptides D1, D2, cytochrome b559, as well as the chlorophyll-binding polypeptides (CP) CP47, CP43, CP29, CP26, CP24, and CP22. The lipid composition was similar to the lipids found in the stacked portion of thylakoids. We also report a 2.0-nm resolution projection map determined by electron microscopy and image analysis of frozen, hydrated PS II crystals. This projection map includes information on the portion of the complex buried in the lipid bilayer. The unit cell is a dimer with unit vectors of 17.0 and 11.4 nm separated by an angle of 106.6 degrees. In addition, Fab fragments against D1 and cytochrome b559 were used to localize those two polypeptides, and thus the reaction center, within the PS II complex. The results indicate that D1 and cytochrome b559 are found within one of the heaviest densities of the monomeric unit.

In vitro synthesis and assembly of photosystem II core proteins. The D1 protein can be incorporated into photosystem II in isolated chloroplasts and thylakoids

The D1 reaction center protein of the membrane-bound photosystem II complex (PSII) has a much higher turnover rate than the other PSII proteins. Thus, the D1 protein has to be replaced while the other PSII components are not newly synthesized. In this study, this D1 protein replacement into PSII complexes was followed in two in vitro translation systems: isolated chloroplasts and a homologous run-off translation system consisting primarily of isolated thylakoids with attached ribosomes. The incorporation of newly synthesized radiolabeled products into different (sub)complexes was analyzed by sucrose density gradient centrifugation of n-dodecyl beta -D-maltoside-solubilized thylakoid membranes. This analysis allowed us to follow the release of the nascent polypeptide chains from the ribosomes and identification of at least four assembly steps of the PSII complex, as shown below. (i) Both in isolated chloroplasts and in thylakoids, newly synthesized D1 protein is predominantly incorporated into existing PSII subcomplexes, indicating that synthesis and import of nuclear-encoded factors is not needed for D1 protein replacement. (ii) In chloroplasts, D1 protein incorporation into PSII core complexes is more efficient than during translation in isolated thylakoids. In the thylakoid translation system, a large percentage of radiolabeled D1 protein is found in smaller PSII subcomplexes, like PSII reaction center particles, and as unassembled protein in the membrane. This indicates that stromal factors are required in the replacement process of the D1 protein. (iii) Both in isolated chloroplasts and in thylakoids, the other PSII core proteins D2, CP43, and CP47 are also synthesized and released from the membrane-bound ribosomes, but incorporation into PSII complexes occurs to a much smaller extent than the D1 protein. Instead they accumulate predominantly as unassembled proteins in the thylakoid membrane. (iv) In chloroplasts, synthesis of the D1 protein seems to be adjusted according to the possibilities of incorporation into PSII complexes, while synthesis of the D2 protein, CP43, and CP47 is less regulated and their accumulation as unassembled protein in the membrane is abundant.

A nuclear-encoded subunit of the photosystem II reaction center

A nuclear-encoded polypeptide of 6.1 kDa was identified in isolated photosystem II (PSII) reaction center from Spinacia oleracea. The hydrophobic membrane protein easily escapes staining procedures such as Coomassie R-250 or silver staining, but it is clearly detected by immunodecoration with peptide-directed IgG. This additional subunit was found to be present in PSII reaction centers previously known to contain only the D1/D2/cytb559 proteins and the psbI gene product. Furthermore, cross-linking experiments using 1-(3-dimethylaminopropyl-) 3-ethylcarbodiimide showed that the nearest neighbors were the D1 and D2 proteins and the cytb559. The 6.1-kDa protein was purified by immune affinity chromatography. N-terminal sequence analysis of the isolated protein confirmed the identity of the 6.1-kDa protein and enabled finding of strong similarities with a randomly obtained cDNA from Arabidopsis thaliana. Using enzyme-linked immunosorbent assay in combination with thylakoid membrane preparations of different orientation, the N terminus of the protein, predicted to span the membrane once, is suggested to be exposed at the lumen side of the membrane. Consequently the 6.1-kDa protein seems to be the only subunit in the PSII reaction center that is nuclear encoded and has its N terminus on the lumen side of the membrane. These findings open for new interesting suggestions concerning the properties of photosystem II reaction center with respect to the photosynthetic activity, regulation and assembly in higher plants.

Modeling the quinone-B binding site of the photosystem-II reaction center using notions of complementarity and contact-surface between atoms

Functional identity and significant similarities in cofactors and sequence exist between the L and M reaction center proteins of the photosynthetic bacteria and the D1 and D2 photosystem-II reaction center proteins of cyanobacteria, algae, and plants. A model of the quinone (QB) binding site of the D1 protein is presented based upon the resolved structure of the QB binding pocket of the L subunit, and introducing novel quantitative notions of complementarity and contact surface between atoms. This model, built without using traditional methods of molecular mechanics and restricted to residues in direct contact with QB, accounts for the experimentally derived functional state of mutants of the D1 protein in the region of QB. It predicts the binding of both the classical and phenol-type PSII herbicides and rationalizes the relative levels of tolerance of mutant phenotypes.

Vir-115 gene product is required to stabilize D1 translation intermediates in chloroplasts

The nuclear gene mutant of barley, vir-115, shows a developmentally induced loss of D1 synthesis that results in inactivation of Photosystem II. Translation in plastids isolated from 1 h illuminated vir-115 seedlings is similar to wild type. In wild-type barley, illumination of plants for 16 to 72 h results in increased radiolabel incorporation into the D1 translation intermediates of 15-24 kDa. In contrast, these D1 translation intermediates were not observed in vir-115 plastids isolated from plants illuminated for 16-72 h. In addition, after 72 h of illumination, radiolabel incorporation into D1 was undetectable in vir-115 plastids. The level and distribution of psbA mRNA in membrane-associated polysomes was similar in wild-type and vir-115 mutant plastids isolated from plants illuminated for 16-72 h. Toeprint analysis showed similar levels of translation initiation complexes on psbA mRNA in vir-115 and wild-type plastids. These results indicate that translation initiation and elongation of D1 is not significantly altered in the mutant plastids. Ribosome pausing on psbA mRNA was observed in wild-type and vir-115 mutant plastids. Therefore, the absence of D1 translation intermediates in mutant plastids is not due to a lack of ribosome pausing on psbA mRNA. Based on these results, it is proposed that vir-115 lacks or contains a modified nuclear-encoded gene product which normally stabilizes the D1 translation intermediates. In wild-type plastids, ribosome pausing and stabilization of D1 translation intermediates is proposed to facilitate assembly of cofactors such as chlorophyll with D1 allowing continued D1 synthesis and accumulation in mature chloroplasts.

Lumenal side histidine mutations in the D1 protein of Photosystem II affect donor side electron transfer in Chlamydomonas reinhardtii

Site-directed mutants of the D1 protein generated in Chlamydomonas reinhardtii have been characterized to determine whether specific lumenal side histidine residues participate in or directly influence electron transfer. Histidine 195 (H195), a conserved residue located near the amino-terminal end of the D1 transmembrane alpha-helix containing the putative P680 chlorophyll ligand H198, was changed to asparagine (H195N), aspartic acid (H195D), and tyrosine (H195Y). These H195 mutants displayed essentially wild-type rates of electron transfer from the water-oxidizing complex to 2,6-dichlorophenolindophenol. Flash-induced chlorophyll a (Chl a) fluorescence yield rise and decay measurements for Mn-depleted membranes of the H195Y and H195D mutants, however, revealed modified YZ to P680+ electron transfer kinetics. The rate of the variable Chl a fluorescence rise was reduced approximately 10-fold in H195Y and H195D relative to the wild type. In addition, the rate of Chl a fluorescence decay in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea was approximately 50-fold more rapid in H195D than in the wild type. These results can be accommodated by a change in the midpoint potential of YZ+/YZ which is apparent only upon the removal of the Mn cluster. In addition, we have generated a histidine to phenylalanine substitution at histidine 190 (H190), a conserved residue located near the lumenal thylakoid surface of D1 in close proximity to the secondary donor YZ. The H190F mutant is characterized by an inability to oxidize water associated with the loss of the Mn cluster and severely altered donor side kinetics. These and other results suggest that H190 may participate in redox reactions leading to the assembly of the Mn cluster.

Molecular topology of the Photosystem II chlorophyll a binding protein, CP 43: Topology of a thylakoid membrane protein

We have used antibodies generated against synthetic peptides to determine the topology of the 43 kD chlorophyll a binding protein (CP 43) of Photosystem II. Based on the pattern of proteolytic fragments detected (on western blots) by peptide specific antibodies, a six transmembrane span topological model, with the amino and carboxyl termini located on the stromal membrane surface, is predicted. This structure is similar to that predicted for CP 47, a PS II chlorophyll a binding protein (Bricker T (1990) Photosynth Res 24: 1-13). The model is discussed in reference to the possible location of chlorophyll binding sites.

Spectroscopic evidence from site-directed mutants of Synechocystis PCC6803 in favor of a close interaction between histidine 189 and redox-active tyrosine 160, both of polypeptide D2 of the photosystem II reaction center

The reaction center of photosystem II of oxygenic photosynthesis contains two redox-active tyrosines called Z and D, each of which can act as an electron donor to the oxidized primary electron donor, P680+. These tyrosines are located in homologous positions on the third transmembrane alpha-helix of each of the two homologous polypeptides, D1 and D2, that comprise the reaction center. Tyrosine D of polypeptide D2 has been proposed, upon oxidation, to give up its phenolic proton to a nearby basic amino acid residue, forming a neutral radical. Modeling studies have pointed to His190 (spinach numbering) as a likely candidate for this basic residue. As a test of this hypothesis, we have constructed three site-directed mutations in the D2 polypeptide of the cyanobacterium Synechocystis sp. PCC6803. His189 (the Synechocystis homologue of His190 of spinach) has been replaced by glutamine, aspartate, or leucine. Instead of the normal D. EPR signal (g = 2.0046; line width 16-19 G), PSII core complexes isolated from these three mutants show an altered dark-stable EPR signal with a narrowed line width (11-13 G), and g values of 2.0046, 2.0043, and 2.0042 for the His189Gln, His189Asp, and His189Leu mutants, respectively. Despite the reduced line width, these EPR signals show g values and microwave-power saturation properties similar to the normal D. signal. Furthermore, specific deuteration in one of those mutants at the 3 and 5 positions of the phenol ring of the photosystem II reaction center tyrosines results in a loss of hyperfine structure of the EPR signal, proving that the signal indeed arises from tyrosine.2+ This observation provides support for a model in which an imidazole nitrogen of His189 accepts the phenolic proton of Tyr160 upon oxidation of D, forming a back hydrogen bond to the phenolic oxygen of the neutral tyrosyl radical.

Effect of trypsin on D1/D 2-cytochrom b 559 Photosystem 2 reaction center complex and reaction center from Rhodopseudomonas viridis

Proteolytic enzyme (trypsin) was used to structurally alter the RCs isolated from plant and bacterium as a way of probing the relation between structure (chromophore-apoprotein interactions) and function (photochemical activity). It was found that neither spectral characteristics (absorption spectrum, the 4th derivative of absorption spectrum) nor photochemical activity (pheophytine photoreduction, P680 photooxidation, etc.) were changed dramatically in D1/D2/cytochrom b 559 PS 2 reaction center complex digested with trypsin. The PS 2 RC treated with trypsin migrates by one green band during electrophoresis with dodecylmaltoside. The peptides with a molecular mass higher than 3-4 kDa were not separated from PS 2 RC. These data indicate that digestion of D1 and D2 proteins does not disturb yet the conformation of peptides or their interactions in so-called 'core' of RC and the native state of pigments. In contrast to that, the RC from Rhodopseudomonas viridis treated with enzyme has changed absorption spectrum and lost photochemical activity. The stability of the bacterial RC increased after exchange of LDAO by dodecylmaltoside.

Photosynthetic water oxidation: The protein framework

Approximately 20 protein subunits are associated with the PS II complex, not counting subunits of peripheral light-harvesting antenna complexes. However, it is not yet established which proteins specifically are involved in the water-oxidation process. Much evidence supports the concept that the D1/D2 reaction center heterodimer not only plays a central role in the primary photochemistry of Photosystem II, but also is involved in electron donation to P680 and in ligation of the manganese cluster. This evidence includes (a) the primary donor to P680 has been shown to be a redox-active tyrosyl residue (Tyr161) in the D1 protein, and (b) site-directed mutagenesis and computer-assisted modeling of the reaction center heterodimer have suggested several sites with a possible function in manganese ligation. These include Asp170, Gln165 and Gln189 of the D1 protein and Glu69 of the D2 protein as well as the C-terminal portion of the mature D1 protein. Also, hydrophilic loops of the chlorophyll-binding protein CP43 that are exposed at the inner thylakoid surface could be essential for the water-splitting process.In photosynthetic eukaryotes, three lumenal extrinsic proteins, PS II-O (33 kDa), PS II-P (23 kDa) and PS II-Q (16 kDa), influence the properties of the manganese cluster without being involved in the actual catalysis of water oxidation. The extrinsic proteins together may have multiple binding sites to the integral portion of PS II, which could be provided by the D1/D2 heterodimer and CP47. A major role for the PS II-O protein is to stabilize the manganese cluster. Most experimental evidence favors a connection of the PS II-P protein with binding of the Cl(-) and Ca(2+) ions required for the water oxidation, while the PS II-Q protein seems to be associated only with the Cl(-) requirement. The two latter proteins are not present in PS II of prokaryotic organisms, where their functions may be replaced by a 10-12 kDa subunit and a newly discovered low-potential cytochrome c-550.

Role of the carboxy terminus of polypeptide D1 in the assembly of a functional water-oxidizing manganese cluster in photosystem II of the cyanobacterium Synechocystis sp. PCC 6803: assembly requires a free carboxyl group at C-terminal position 344

The D1 polypeptide of the photosystem II (PSII) reaction center is synthesized as a precursor polypeptide which is posttranslationally processed at the carboxy terminus. It has been shown in spinach that such processing removes nine amino acids, leaving Ala344 as the C-terminal residue [Takahashi, M., Shiraishi, T., & Asada, K. (1988) FEBS Lett. 240, 6-8; Takahashi, Y., Nakane, H., Kojima, H., & Satoh, K. (1990) Plant Cell Physiol. 31, 273-280]. We show here that processing on the carboxy side of Ala344 also occurs in the cyanobacterium Synechocystis 6803, resulting in the removal of 16 amino acids. By constructing a deletion strain of Synechocystis 6803 that lacks the three copies of the psbA gene encoding D1, we have developed a system for generating psbA mutants. Using this system, we have constructed mutants of Synechocystis 6803 that are modified in the region of the C-terminus of the D1 polypeptide. Characterization of these mutants has revealed that (1) processing of the D1 polypeptide is blocked when the residue after the cleavage site is changed from serine to proline (mutant Ser345Pro) with the result that the manganese cluster is unable to assemble correctly; (2) the C-terminal extension of 16 amino acid residues can be deleted with little consequence either for insertion of D1 into the thylakoid membrane or for assembly of D1 into a fully active PSII complex; (3) removal of only one more residue (mutant Ala344stop) results in a loss of assembly of the manganese cluster; and (4) the ability of detergent-solubilized PSII core complexes (lacking the manganese cluster) to bind and oxidize exogenous Mn2+ by the secondary donor, Z+, is largely unaffected in the processing mutants (the Ser345Pro mutant of Synechocystis 6803 and the LF-1 mutant of Scenedesmus obliquus) and the truncation mutant Ala344stop. Our results are consistent with a role for processing in regulating the assembly of the photosynthetic manganese cluster and a role for the free carboxy terminus of the mature D1 polypeptide in the ligation of one or more manganese ions of the cluster.

A three-dimensional model of the Photosystem II reaction centre of Pisum sativum

A three-dimensional model of the core proteins D1 and D2, including the cofactors, that form the Photosystem II reaction centre of pea (Pisum sativum), has been generated. This model was built with a rule-based computer modelling system using the information from the crystal structures of the photosynthetic reaction centres of Rhodopseudomonas viridis and Rhodobacter sphaeroides. An alignment of the primary sequences of twenty three D1, nine D2, eight bacterial L and eight bacterial M subunits predicts strong similarity between bacterial and higher plant reaction centres, especially in the transmembrane region where the cofactors responsible for electron transport are located. The sequence to be modelled was aligned to the bacterial structures using environment-dependent substitution tables to construct matrices, improving the alignment procedure. The ancestral relationship between the bacteria and higher plant sequences allowed both the L and M subunits to be used as structural templates as they were equally related to the higher plant polypeptides. The regions with the highest predicted structural homology were used as a framework for the construction of the structurally conserved regions. The structurally conserved region of the model shows strong similarity to the bacterial reaction centre in the transmembrane helices. The stromal and lumenal loops show greater sequence variation and are therefore predicted to be the structurally variable regions in the model. The key sidechain assignments and residues that may interact with cofactors are discussed.

Sequence analysis of photoaffinity-labelled peptides derived by proteolysis of photosystem-2 reaction centres from thylakoid membranes treated with [14C]azidoatrazine

Photosystem-2 reaction centres were prepared from pea thylakoid membranes that had been photoaffinity labelled with [14C]-azidoatrazine (2-azido-4-ethylamino-6-isopropylamino-s-triazine), a derivative of the herbicide atrazine which binds to the secondary plastoquinone electron-acceptor site of photosystem 2. SDS/PAGE of the 14C-labelled reaction centres followed by fluorography revealed photoaffinity-labelled proteins of apparent molecular masses 30 kDa and 55 kDa, which corresponded to the D1 polypeptide and to an SDS-stable heterodimer of the D1 and D2 polypeptides, respectively. To obtain sequence information on the site of photoaffinity labelling, an 8-kDa photoaffinity-labelled peptide, generated by proteolysis of the reaction-centre material with trypsin, was isolated and purified to apparent homogeneity using reverse-phase and size-exclusion HPLC techniques. The amino terminus of the photoaffinity-labelled peptide was determined to be Leu-Gly-Met-Arg-Pro-Xaa-Ile-Ala-Val-Ala-Tyr by Edman sequencing. This corresponds to the amino terminus of a predicted tryptic peptide of D1 and confirms that azidoatrazine photolabels the D1 polypeptide of photosystem 2 in the region Leu137-Arg225. Chymotrypsin/trypsin digestion of photoaffinity-labelled reaction centres followed by reverse-phase HPLC was used to isolate a smaller photoaffinity-labelled peptide. On Edman sequencing, Ser-Ala were identified as the first two residues and 14C was released on the third cycle, after which further degradation was blocked. The two potential peptide fragments with Ser-Ala at the amino terminus in the region Leu137-Arg225 are Ser148-Ala-Pro and Ser212-Ala-Met. Proline is an unlikely target for reaction with the nitrene of the photoactivated azidoatrazine, and the data are thus consistent with Met214 as the site of photoaffinity labelling on D1 when thylakoid membranes are illuminated with ultraviolet irradiation in the presence of [14C]azidoatrazine.

Breakdown of the photosystem II reaction center D1 protein under photoinhibitory conditions: identification and localization of the C-terminal degradation products

Illumination of a suspension of thylakoids with light at high intensity causes inhibition of the photosystem II electron transport activity and loss from the membrane of the D1 protein of the photosystem II reaction center. Impairment of the electron transport activity and depletion of D1 protein from the thylakoid membrane of pea were investigated with reference to the presence or absence of oxygen in the suspension. The breakdown products of the D1 protein were identified by immunoblotting with anti-D1 polyclonal antibodies which were proven to recognize mainly the C-terminal region of the protein. The results obtained show that (i) the light-induced inactivation of the photosystem II electron transport activity under anaerobic conditions is faster than in the presence of oxygen; (ii) depletion of D1 protein is observed on a longer time scale with respect to loss of electron transport activity and is faster when photoinhibition is performed in the presence of oxygen; (iii) C-terminal fragments of D1 are only observed when photoinhibition is carried out anaerobically and are mainly localized in the stroma-exposed regions; and (iv) the fragments observed after anaerobic photoinhibition are quickly degraded on further illumination of the thylakoid suspension in the presence of oxygen.

Photosynthetic electron transport in genetically altered photosystem II reaction centers of chloroplasts

Using a cotransformation system to identify chloroplast transformants in Chlamydomonas reinhardtii, we converted histidine-195 of the photosystem II reaction center D1 protein to a tyrosine residue. The mutants were characterized by a reduced quantum efficiency for photosynthetic oxygen evolution, which varied in a pH-dependent manner, a reduced capacity to oxidize artificial donors to photosystem II, and P680+ reduction kinetics (microsecond) that were essentially similar to wild type. In addition, a dark-stable radical was detected by ESR in mutant photosystem II particles but not in wild-type particles. This radical was similar in g value and lineshape to chlorophyll or carotenoid cations but could have arisen from a tyrosine-195 cation. The ability of the photosystem II trap (P680+) to oxidize tyrosine residues suggests that the mutant tyrosine residue could be used as a redox-sensitive probe to investigate the environment around the photosystem II trap.