Changes in electrophoretic mobility of a chloroplast membrane polypeptide associated with the loss of the oxidizing side of photosystem II in low fluorescent mutants of Scenedesmus

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

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

Processing of D1 Protein: A Mysterious Process Carried Out in Thylakoid Lumen

In oxygenic photosynthetic organisms, D1 protein, a core subunit of photosystem II (PSII), displays a rapid turnover in the light, in which D1 proteins are distinctively damaged and immediately removed from the PSII. In parallel, as a repair process, D1 proteins are synthesized and simultaneously assembled into the PSII. On this flow, the D1 protein is synthesized as a precursor with a carboxyl-terminal extension, and the D1 processing is defined as a step for proteolytic removal of the extension by a specific protease, CtpA. The D1 processing plays a crucial role in appearance of water-oxidizing capacity of PSII, because the main chain carboxyl group at carboxyl-terminus of the D1 protein, exposed by the D1 processing, ligates a manganese and a calcium atom in the Mn₄CaO₅-cluster, a special equipment for water-oxidizing chemistry of PSII. This review focuses on the D1 processing and discusses it from four angles: (i) Discovery of the D1 processing and recognition of its importance: (ii) Enzyme involved in the D1 processing: (iii) Efforts for understanding significance of the D1 processing: (iv) Remaining mysteries in the D1 processing. Through the review, I summarize the current status of our knowledge on and around the D1 processing.

The carboxyl-terminal processing of precursor D1 protein of the photosystem II reaction center

The D1 protein, a key subunit of photosystem II reaction center, is synthesized as a precursor form with a carboxyl-terminal extension, in oxygenic photosynthetic organisms with some exceptions. This part of the protein is removed by the action of an endopeptidase, and the proteolytic processing is indispensable for the manifestation of oxygen-evolving activity in photosynthesis. The carboxyl-terminus of mature D1 protein, which appears upon the cleavage, has recently been demonstrated to be a ligand for a manganese atom in the Mn(4)Ca-cluster, which is responsible for the water oxidation chemistry in photosystem II, based on the isotope-edited Fourier transform infrared spectroscopy and the X-ray crystallography. On the other hand, the structure of a peptidase involved in the cleavage of precursor D1 protein has been resolved at a higher resolution, and the enzyme-substrate interactions have extensively been analyzed both in vivo and in vitro. The present article briefly summarizes the history of research and the present state of our knowledge on the carboxyl-terminal processing of precursor D1 protein in the photosystem II reaction center.

Function of the 23 kDa extrinsic protein of Photosystem II as a manganese binding protein and its role in photoactivation

The function of the extrinsic 23 kDa protein of Photosystem II (PSII) was studied with respect to Mn binding and its ability to supply Mn to PSII during photoactivation, i.e. the light-dependent assembly of the tetramanganese cluster. The extrinsic proteins and the Mn cluster were removed by TRIS treatment from PSII-enriched membrane fragments and purified by anion exchange chromatography. Room temperature EPR spectra of the purified 23 kDa protein demonstrated the presence of Mn. Photoactivation was successful with low Mn concentrations when the 23 kDa protein was present, while in its absence a higher Mn concentration was needed to reach the same level of oxygen evolution activity. In addition, the rate of photoactivation was significantly accelerated in the presence of the 23 kDa protein. It is proposed that the 23 kDa protein plays an important role in providing Mn during the process of PSII assembly and that it acquires Mn during the light-induced turnover of D1 in the PSII damage-repair cycle and delivers Mn to repaired PSII.

Evidence that D1 processing is required for manganese binding and extrinsic protein assembly into photosystem II

Photosystem II (PSII) is a large membrane protein complex that catalyzes oxidation of water to molecular oxygen. During its normal function, PSII is damaged and frequently turned over. The maturation of the D1 protein, a key component in PSII, is a critical step in PSII biogenesis. The precursor form of D1 (pD1) contains a C-terminal extension, which is removed by the protease CtpA to yield PSII complexes with oxygen evolution activity. To determine the temporal position of D1 processing in the PSII assembly pathway, PSII complexes containing only pD1 were isolated from a CtpA-deficient strain of the cyanobacterium Synechocystis 6803. Although membranes from the mutant cell had nearly 50% manganese, no manganese was detected in isolated DeltactpAHT3 PSII, indicating a severely decreased manganese affinity. However, chlorophyll fluorescence decay kinetics after a single saturating flash suggested that the donor Y(Z) was accessible to exogenous Mn(2+) ions. Furthermore, the extrinsic proteins PsbO, PsbU, and PsbV were not present in PSII isolated from this mutant. However, PsbO and PsbV were present in mutant membranes, but the amount of PsbV protein was consistently less in the mutant membranes compared with the control membranes. We conclude that D1 processing precedes manganese binding and assembly of the extrinsic proteins into PSII. Interestingly, the Psb27 protein was found to be more abundant in DeltactpAHT3 PSII than in HT3 PSII, suggesting a possible role of Psb27 as an assembly factor during PSII biogenesis.

Metallo-radical hypothesis for photoassembly of (Mn)4-cluster of photosynthetic oxygen evolving complex

A new hypothetical mechanism is proposed for photoassembly of the (Mn)4-cluster of the photosynthetic oxygen evolving complex (OEC). In this process, a neutral radical of Y(Z) tyrosine plays a role in oxidizing Mn2+ associated with an apo-OEC, and also in abstracting a proton from a water molecule bound to the Mn2+ ion, together with D1-His190. This is in a similar fashion to the metallo-radical mechanism proposed for photosynthetic water oxidation by the (Mn)4-cluster. The model insists that a common mechanism participates in the photoassembly of the (Mn)4-cluster and the photosynthetic water oxidation.

Genetic engineering of thylakoid protein complexes by chloroplast transformation in Chlamydomonas reinhardtii

Chloroplast transformation of Chlamydomonas reinhardtii has developed into a powerful tool for studying the structure, function and assembly of thylakoid protein complexes in a eukaryotic organism. In this article we review the progress that is being made in the development of procedures for efficient chloroplast transformation. This focuses on the development of selectable markers and the use of Chlamydomonas mutants, individually lacking thylakoid protein complexes, as recipients. Chloroplast transformation has now been used to engineer all four major thylakoid protein complexes, photosystem II, photosystem I, cytochrome b 6/f and ATP synthase. These results are discussed with an emphasis on new insights into assembly and function of these complexes in chloroplasts as compared with their prokaryotic counterparts.

Assembly of the Photosystem II oxygen-evolving complex is inhibited in psbA site-directed mutants of Chlamydomonas reinhardtii. Aspartate 170 of the D1 polypeptide

Photosystem II catalyzes the photooxidation of water to molecular oxygen, providing electrons to the photosynthetic electron transfer chain. The D1 and D2 chloroplast-encoded reaction center polypeptides bind cofactors essential for Photosystem II function. Transformation of the chloroplast genome of the eukaryotic green alga Chlamydomonas reinhardtii has allowed us to engineer site-directed mutants in which aspartate residue 170 of D1 is replaced by histidine (D170H), asparagine (D170N), threonine (D170T), or proline (D170P). Mutants D170T and D170P are completely deficient in oxygen evolution, but retain normal (D170T) or 50% (D170P) levels of Photosystem II reaction centers. D170H and D170N accumulate wild-type levels of PSII centers, yet evolve oxygen at rates approximately 45% and 15% those of control cells, respectively. Kinetic analysis of chlorophyll fluorescence in the mutants reveals a specific defect in electron donation to the reaction center. Measurements of oxygen flash yields in D170H show, however, that those reaction centers capable of evolving oxygen function normally. We conclude that aspartate residue 170 of the D1 polypeptide plays a critical role in the initial binding of manganese as the functional chloroplast oxygen-evolving complex is assembled.

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.

Evidence from directed mutagenesis that aspartate 170 of the D1 polypeptide influences the assembly and/or stability of the manganese cluster in the photosynthetic water-splitting complex

To identify amino acid residues that influence the assembly or stability of the manganese cluster in photosystem II, we have generated site-directed mutations in the D1 polypeptide of the cyanobacterium, Synechocystis sp. PCC 6803. Indirect evidence has suggested that the D1 polypeptide provides some of the ligands that are required for metal binding. Mutations at position 170 of D1 were selected for characterization, since an aspartate to asparagine mutation (DN170D1) at this position completely abolishes photoautotrophic growth, while retention of a carboxylic acid at this position (aspartate to glutamate, DE170D1) supports photoautotrophic growth. Photosystem II particles were purified from control, DE170D1, and DN170D1 cells by a procedure that retains high rates of oxygen evolution activity in control particles [Noren, G.H., Boerner, R.J., & Barry, B.A. (1991) Biochemistry 30, 3943-3950]. Spectroscopic analysis shows that the tyrosine radical, Z+, which normally oxidizes the manganese cluster, is rapidly reduced in the DE170D1 mutant, but not in the DN170D1 mutant. A possible explanation of this block or dramatic decrease in the rate of electron transfer between the manganese cluster and tyrosine Z is an alteration in the properties of the metal center. Quantitation of manganese in these particles is consistent with aspartate 170 influencing the stability or assembly of the manganese cluster, since the aspartate to asparagine mutation results in a decrease in the manganese content per reaction center. Photosystem II particles from DN170D1 show a 60% decrease in the amount of specifically bound manganese per reaction center, when compared to control particles. Also, we observe a 70% decrease in the amount of specifically bound manganese per reaction center in partially purified DN170D1 particles and at least an 80% decrease in the amount of hydroxylamine-reducible manganese in DN170D1 thylakoid membranes. Single-turnover fluorescence assays and steady-state EPR measurements demonstrate that the remaining, endogenous manganese does not rapidly reduce tyrosine Z+ in the DN170D1 mutant. Additional evidence that aspartate 170 influences the assembly or stability of the metal site comes from analysis of the DE170D1 mutant. Although this mutant assembles a functional manganese cluster, as assessed by oxygen evolution and spectroscopic assays, the properties of the manganese site are perturbed.

Nuclear mutations affecting plastoquinone accumulation in maize

We have isolated and characterized two nuclear mutations which affect plastoquinone accumulation in maize. The mutations, hcf103 and hcf114, modify the same genetic locus. Plants homozygous for either mutant allele exhibit reduced PS II electron transport activity, reduced variable chlorophyll fluorescence and reduced delayed fluorescence yield. In these ways, hcf103 and hcf114 resemble previously described PS II mutants which lack stably assembled PS II reaction center complexes. However, unlike most previously described PS II mutants, hcf103 and hcf114 possess stable membrane-associated PS II complexes. Plastoquinone (PQ-9), which performs a variety of redox functions essential to normal non-cyclic electron transport, is severely depleted in the mutants. The lack of PS II electron transport activity is attributed to the absence of PQ-9. This is the first report of mutants deficient in PQ which do not also suffer serious pleiotropic defects.

Regeneration of the high-affinity manganese-binding site in the reaction center of an oxygen-evolution deficient mutant of Scenedesmus by protease action

The O2-evolution deficient mutant (LF-1) of Scenedesmus obliquus inserts an unprocessed D1 protein into the thylakoid membrane and binds less than half the wild type (WT) level of Mn. LF-1 photosystem II (PS II) membrane fragments lack that part of the high-affinity Mn(2+)-binding site found in WT membranes which may be associated with histidine residues on the D1 protein (Seibert et al. 1989 Biochim Biophys Acta 974: 185-191). Hsu et al. (1987 Biochim Biophys Acta 890: 89-96) purport that the high-affinity site (characterized by competitive inhibition of DPC-supported DCIP photoreduction by μM concentrations of Mn(2+)) in Mn-extracted PS II membranes is also the binding site for Mn functional in O2 evolution. Proteases (papain, subtilisin, and carboxypeptidase A) can be used to regenerate the high-affinity Mn(2+)-binding site in LF-1 PS II membranes but not in thylakoids. Experiments with the histidine modifier, DEPC, suggest that the regenerated high-affinity Mn(2+)-binding sites produced by either subtilisin or carboxypeptidase A treatments were the same sites observed in WT membranes. However, none of the protease treatments produced LF-1 PS II membranes that could be photoactivated. Reassessment of the processing studies of Taylor et al. (1988 FEBS Lett 237: 229-233) lead us to believe that their procedure also does not result in substantial photoactivation of LF-1 PS II membranes. We conclude that (1) the unprocessed carboxyl end of the D1 protein in LF-1 is located on the lumenal side of the PS II membrane, (2) the unprocessed fragment physically obstructs or perturbs that part of the high-affinity Mn(2+)-binding site undetectable in LF-1, and (3) the D1 protein must be processed at the time of insertion into the membrane for normal O2-evolution function to result.

The effect of mono- and divalent salts on the rise and decay kinetics of EPR signal II in Photosystem II preparations from spinach

The rise and decay kinetics of EPR signal II have been used to probe the organization of the donor side of Photosystem II (PS II) before and after extraction of PS II preparations with high concentrations of salt. 800 mM NaCl or 500-800 mM NaBr substantially depletes the preparations of the 16 and 24 kDa proteins and decreases the steady-state rate of O2-evolution by 70-80% from control rates. These treatments do not largely alter the decay kinetics of Signal II; the rise kinetics remain in the instrument limited time range (2 microseconds or less) during the first 8-12 flashes. Treating PS II preparations with 800 mM CaCl2 removes the 16, 24 and 33 kDa proteins with at least 95% inhibition of the steady-state rates of O2 evolution. The additional removal of the 33 kDa polypeptide decreases the rates of oxidation and rereduction of Z, the species responsible for Signal II. Preparations treated with either mono- or divalent salts show a steady-state light-induced increase in Signal II similar to that seen in Tris-washed samples. Such a steady-state increase indicates that the rate of electron transport from water to Z is greatly decreased or blocked. The data are interpreted within a model in which there is an intermediate electron carrier between the O2 evolving complex and Z.