The azido[14 C]atrazine photoaffinity technique labels a 34-kDa protein in Scenedesmus which functions on the oxidizing side of photosystem II

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.

High Yield Non-detergent Isolation of Photosystem I-Light-harvesting Chlorophyll II Membranes from Spinach Thylakoids: IMPLICATIONS FOR THE ORGANIZATION OF THE PS I ANTENNAE IN HIGHER PLANTS

Styrene-maleic acid copolymer was used to effect a non-detergent partial solubilization of thylakoids from spinach. A high density membrane fraction, which was not solubilized by the copolymer, was isolated and was highly enriched in the Photosystem (PS) I-light-harvesting chlorophyll (LHC) II supercomplex and depleted of PS II, the cytochrome b6/f complex, and ATP synthase. The LHC II associated with the supercomplex appeared to be energetically coupled to PS I based on 77 K fluorescence, P700 photooxidation, and PS I electron transport light saturation experiments. The chlorophyll (Chl) a/b ratio of the PS I-LHC II membranes was 3.2 ± 0.9, indicating that on average, three LHC II trimers may associate with each PS I. The implication of these findings within the context of higher plant PS I antenna organization is discussed.

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.

Introduction to competition between continuous cultures of Phaeodactylum tricornutum and Dunaliella tertiolecta

When the diatom, Phaeodactylum tricornutum, and the microalga, Dunaliella tertiolecta, are cultured together in a chemostat at dilution factors of approximately 0.5 day-1, the diatom develops the higher population density. At dilution factors above 1.2 day-1 the inability of the diatom to assimilate nutrient as fast as it flows into the chemostat results in the microalga generating the larger population. This change in population densities is accompanied by an increase in the chlorophyll content of the diatom and a decrease in the chlorophyll content of the microalga. Two species of phytoplankton can coexist when they compete for nutrient in a chemostat providing they do not otherwise interact. When the species do interact coexistence in a stable steady state is possible providing intraspecies interactions exceed the interactions between the species. Both species adjust their consumption to minimise the concentration of nutrient in the chemostat and their growth is modified to match the dilution factor of the flow.

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.

Site-directed photosystem II mutants with perturbed oxygen-evolving properties. 2. Increased binding or photooxidation of manganese in the absence of the extrinsic 33-kDa polypeptide in vivo

Several site-directed photosystem II mutants with substitutions at Asp-170 or in the carboxyterminal region of the D1 polypeptide were characterized in vivo in the absence of the extrinsic 33-kDa polypeptide. Site-directed mutations were constructed in the cyanobacterium Synechocystis sp. PCC 6803. The 33-kDa polypeptide was removed by insertional inactivation of the Synechocystis psbO gene. Mutants were characterized by measuring changes in the yield of variable chlorophyll a fluorescence following a saturating flash or brief illumination in the presence of an electron-transfer inhibitor or following each of a series of saturating flashes in the absence of inhibitor [Chu, H.-A., Nguyen, A. P., & Debus, R. J. (1994) Biochemistry (preceding paper in this issue)]. In the presence of the extrinsic 33-kDa polypeptide, many site-directed mutants contained a significant fraction of photosystem II reaction centers that lacked photooxidizable Mn ions. This fraction decreased dramatically in the absence of the extrinsic 33-kDa polypeptide, even in mutants having a significantly perturbed high-affinity Mn binding site (e.g., in the mutants D170A and D170T). These results show that, in vivo, the extrinsic 33-kDa polypeptide directly or indirectly governs the occupancy of the high-affinity Mn binding site by Mn ions or the ability of bound Mn ions to reduce YZ+.

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.

Formation of the S2 state and structure of the Mn complex in photosystem II lacking the extrinsic 33 kilodalton polypeptide

Electron paramagnetic resonance (EPR) spectroscopy and O2 evolution assays were performed on photosystem II (PSII) membranes which had been treated with 1 M CaCl2 to release the 17, 23 and 33 kilodalton (kDa) extrinsic polypeptides. Manganese was not released from PSII membranes by this treatment as long as a high concentration of chloride was maintained. We have quantitated the EPR signals of the several electron donors and acceptors of PSII that are photooxidized or reduced in a single stable charge separation over the temperature range of 77 to 240 K. The behavior of the samples was qualitatively similar to that observed in samples depleted of only the 17 and 23 kDa polypeptides (de Paula et al. (1986) Biochemistry25, 6487-6494). In both cases, the S2 state multiline EPR signal was observed in high yield and its formation required bound Ca(2+). The lineshape of the S2 state multiline EPR signal and the magnetic properties of the manganese site were virtually identical to those of untreated PSII membranes. These results suggest that the structure of the manganese site is unaffected by removal of the 33 kDa polypeptide. Nevertheless, in samples lacking the 33 kDa polypeptide a stable charge separation could only be produced in about one half of the reaction centers below 160 K, in contrast to the result obtained in untreated or 17 and 23 kDa polypeptide-depleted PSII membranes. This suggests that one function of the 33 kDa polypeptide is to stabilize conformations of PSII that are active in secondary electron transfer events.

Effects of trypsin and calcium chloride on signal IIs in oxygen-evolving PS II preparations

Photosystem II oxygen-evolving preparations exhibited a reversible loss of signal IIs hyperfine structure when treated with 1.0 M CaCl2. A progressive irreversible loss of hyperfine structure was observed upon trypsin treatment of these preparations. These treatments appear to alter the environment of the radical responsible for signal IIs. Gel electrophoresis of trypsin-treated photosystem II preparations indicates that three polypeptides (45, 32-34, and 26 kDa) are altered with the same kinetics as observed for the trypsin-induced loss of signal IIs. Two of these polypeptides (45 and 32-34 kDa) are core components of photosystem II.

Inhibitors of photosystem II and the topology of the herbicide and QB binding polypeptide in the thylakoid membrane

The folding through the thylakoid membrane of the D-1 herbicide binding polypeptide and of the homologous D-2 subunit of photosystem II is predicted from comparison of amino acid sequences and hydropathy index plots with the folding of the subunits L and M of a bacterial photosystem. As the functional amino acids involved in Q and Fe binding in the bacterial photosystem of R. viridis, as indicated by the X-ray structure, are conserved in the homologous D-1 and D-2 subunits of photosystem II, a detailed topology of the binding niche of QB and of herbicides on photosystem II is proposed. The model is supported by the observed amino acid changes in herbicide tolerant plants and algae. These changes are all in the binding domain on the matrix side of the D-1 polypeptide, and turn out to be of functional significance in the QB binding.New inhibitors of QB function are described. Their chemical structure, i.e. pyridones, quinolones, chromones and benzodiones, contains the features of the phenolic type herbicides. Their essential elements, π-charges at particular atoms, QSAR and steric requirements for optimal inhibitory potency are discussed and compared with the "classical" herbicides of the urea/triazine type.