Effect of the 33-kDa protein on the S-state transitions in photosynthetic oxygen evolution

Jean Lavorel (1928-2021): a great scientist, a model of integrity

This paper is a tribute to a great scientist and an authentic "honest man" that was Jean Lavorel (1928-2021). He was a pioneer in research on the primary events of photosynthesis in algae, plants, and photosynthetic bacteria. He focused his attention on chlorophyll fluorescence and luminescence, and also on oxygen evolution, both experimentally (with laboratory-built refined apparatus) and theoretically. Many of his results are classical now. Besides a survey of his main achievements, most of them obtained by him alone, different reminiscences on the researcher and the person he was illustrate the rich personality of Jean Lavorel.

Lysine Acetylome Analysis Reveals Photosystem II Manganese-stabilizing Protein Acetylation is Involved in Negative Regulation of Oxygen Evolution in Model Cyanobacterium Synechococcus sp. PCC 7002

Nε-Acetylation of lysine residues represents a frequently occurring post-translational modification widespread in bacteria that plays vital roles in regulating bacterial physiology and metabolism. However, the role of lysine acetylation in cyanobacteria remains unclear, presenting a hurdle to in-depth functional study of this post-translational modification. Here, we report the lysine acetylome of Synechococcus sp. PCC 7002 (hereafter Synechococcus) using peptide prefractionation, immunoaffinity enrichment, and coupling with high-precision liquid chromatography-tandem mass spectrometry analysis. Proteomic analysis of Synechococcus identified 1653 acetylation sites on 802 acetylproteins involved in a broad range of biological processes. Interestingly, the lysine acetylated proteins were enriched for proteins involved in photosynthesis, for example. Functional studies of the photosystem II manganese-stabilizing protein were performed by site-directed mutagenesis and mutants mimicking either constitutively acetylated (K99Q, K190Q, and K219Q) or nonacetylated states (K99R, K190R, and K219R) were constructed. Mutation of the K190 acetylation site resulted in a distinguishable phenotype. Compared with the K190R mutant, the K190Q mutant exhibited a decreased oxygen evolution rate and an enhanced cyclic electron transport rate in vivo Our findings provide new insight into the molecular mechanisms of lysine acetylation that involved in the negative regulation of oxygen evolution in Synechococcus and creates opportunities for in-depth elucidation of the physiological role of protein acetylation in photosynthesis in cyanobacteria.

Mechanism of interaction of Al3+ with the proteins composition of photosystem II

The inhibitory effect of Al3+on photosystem II (PSII) electron transport was investigated using several biophysical and biochemical techniques such as oxygen evolution, chlorophyll fluorescence induction and emission, SDS-polyacrylamide and native green gel electrophoresis, and FTIR spectroscopy. In order to understand the mechanism of its inhibitory action, we have analyzed the interaction of this toxic cation with proteins subunits of PSII submembrane fractions isolated from spinach. Our results show that Al 3+, especially above 3 mM, strongly inhibits oxygen evolution and affects the advancement of the S states of the Mn4O5Ca cluster. This inhibition was due to the release of the extrinsic polypeptides and the disorganization of the Mn4O5Ca cluster associated with the oxygen evolving complex (OEC) of PSII. This fact was accompanied by a significant decline of maximum quantum yield of PSII (Fv/Fm) together with a strong damping of the chlorophyll a fluorescence induction. The energy transfer from light harvesting antenna to reaction centers of PSII was impaired following the alteration of the light harvesting complex of photosystem II (LHCII). The latter result was revealed by the drop of chlorophyll fluorescence emission spectra at low temperature (77 K), increase of F0 and confirmed by the native green gel electrophoresis. FTIR measurements indicated that the interaction of Al 3+ with the intrinsic and extrinsic polypeptides of PSII induces major alterations of the protein secondary structure leading to conformational changes. This was reflected by a major reduction of α-helix with an increase of β-sheet and random coil structures in Al 3+-PSII complexes. These structural changes are closely related with the functional alteration of PSII activity revealed by the inhibition of the electron transport chain of PSII.

An intrinsically disordered photosystem II subunit, PsbO, provides a structural template and a sensor of the hydrogen-bonding network in photosynthetic water oxidation

Photosystem II (PSII) is a membrane-bound enzyme that utilizes solar energy to catalyze the photooxidation of water. Molecular oxygen is evolved after four sequential light-driven oxidation reactions at the Mn4CaO5 oxygen-evolving complex, producing five sequentially oxidized states, Sn. PSII is composed of 17 membrane-spanning subunits and three extrinsic subunits, PsbP, PsbQ, and PsbO. PsbO is intrinsically disordered and plays a role in facilitation of the water oxidizing cycle. Native PsbO can be removed and substituted with recombinant PsbO, thereby restoring steady-state activity. In this report, we used reaction-induced Fourier transform infrared spectroscopy to obtain information concerning the role of PsbP, PsbQ, and PsbO during the S state cycle. Light-minus-dark difference spectra were acquired, monitoring structural changes associated with each accessible flash-induced S state transition in a highly purified plant PSII preparation (Triton X-100, octylthioglucoside). A comparison of S2 minus S1 spectra revealed that removal of PsbP and PsbQ had no significant effect on the data, whereas amide frequency and intensity changes were associated with PsbO removal. These data suggest that PsbO acts as an organizational template for the PSII reaction center. To identify any coupled conformational changes arising directly from PsbO, global (13)C-PsbO isotope editing was employed. The reaction-induced Fourier transform infrared spectra of accessible S states provide evidence that PsbO spectral contributions are temperature (263 and 277 K) and S state dependent. These experiments show that PsbO undergoes catalytically relevant structural dynamics, which are coupled over long distance to hydrogen-bonding changes at the Mn4CaO5 cluster.

pH optimum of the photosystem II H₂O oxidation reaction: effects of PsbO, the manganese-stabilizing protein, Cl- retention, and deprotonation of a component required for O₂ evolution activity

Hydroxide ion inhibits Photosystem II (PSII) activity by extracting Cl(-) from its binding site in the O(2)-evolving complex (OEC) under continuous illumination [Critchley, C., et al. (1982) Biochim. Biophys. Acta 682, 436]. The experiments reported here examine whether two subunits of PsbO, the manganese-stabilizing protein, bound to eukaryotic PSII play a role in protecting the OEC against OH(-) inhibition. The data show that the PSII binding properties of PsbO affect the pH optimum for O(2) evolution activity as well as the Cl(-) affinity of the OEC that decreases with an increasing pH. These results suggest that PsbO functions as a barrier against inhibition of the OEC by OH(-). Through facilitation of efficient retention of Cl(-) in PSII [Popelkova, H., et al. (2008) Biochemistry 47, 12593], PsbO influences the ability of Cl(-) to resist OH(-)-induced release from its site in the OEC. Preventing inhibition by OH(-) allows for normal (short) lifetimes of the S(2) and S(3) states in darkness [Roose, J. L., et al. (2011) Biochemistry 50, 5988] and for maximal steady-state activity by PSII. The data presented here indicate that activation of H(2)O oxidation occurs with a pK(a) of ∼6.5, which could be a function of deprotonation of one or more amino acid residues that reside near the OEC active site on the D1 and CP43 intrinsic subunits of the PSII reaction center.

The extrinsic proteins of Photosystem II

In this review we examine the structure and function of the extrinsic proteins of Photosystem II. These proteins include PsbO, present in all oxygenic organisms, the PsbP and PsbQ proteins, which are found in higher plants and eukaryotic algae, and the PsbU, PsbV, CyanoQ, and CyanoP proteins, which are found in the cyanobacteria. These proteins serve to optimize oxygen evolution at physiological calcium and chloride concentrations. They also shield the Mn(4)CaO(5) cluster from exogenous reductants. Numerous biochemical, genetic and structural studies have been used to probe the structure and function of these proteins within the photosystem. We will discuss the most recent proposed functional roles for these components, their structures (as deduced from biochemical and X-ray crystallographic studies) and the locations of their proposed binding domains within the Photosystem II complex. This article is part of a Special Issue entitled: Photosystem II.

Characterization of photosystem II photochemistry in transgenic tobacco plants with lowered Rubisco activase content

Rubisco activase plays an important role in the regulation of CO(2) assimilation. However, it is unknown how activase regulates photosystem II (PSII) photochemistry. To investigate the effects of Rubisco activase on PSII photochemistry, we obtained transgenic tobacco (Nicotiana tabacum) plants with 50% (i7), 25% (i28), and 5% (i46) activase levels as compared to wild type plants by using a gene encoding tobacco activase for the RNAi construct. Both CO(2) assimilation and PSII activity were significantly reduced only in transgenic i28 and i46 plants, suggesting that activase deficiency led to decreased PSII activity. Flash-induced fluorescence kinetics indicated that activase deficiency resulted in a slow electron transfer between Q(A) (primary quinine electron acceptor of PSII) and Q(B) (secondary quinone electron acceptor of PSII). Thermoluminescence measurements revealed that activase deficiency induced a shift of S(2)Q(A)(-) and S(2)Q(B)(-) recombinations to higher temperatures in parallel, and a decrease in the intensities of the thermoluminescence emissions. Activase deficiency also dampened the period-four oscillation of the thermoluminescence B-band. Protein gel blot analysis showed that activase deficiency resulted in a significant decrease in the content of D1, D2, CP43, CP47, and PsbO proteins. Transmission electron microscopy analysis demonstrated that activase deficiency induced a significant decrease in the number of grana stacks per chloroplast and discs per grana stack. Our results suggest that activase plays an important role in maintaining PSII function and chloroplast development.

Interaction of bicarbonate with the manganese-stabilizing protein of photosystem II

The effect of reversible removal of HCO(3)(-) on structural re-arrangements in the Mn-stabilizing protein (MSP) of photosystem II, isolated from pea leaves, was studied using measurements of characteristic alterations in fluorescence of hydrophobic probe 8-anilino-1-naphthalene-sulfonic acid (ANS). It was shown that the treatments capable of removal of HCO(3)(-) (or CO(2)) from possible binding sites in MSP (pH lowering from 6.5 to 3.5, addition of a structurally similar anion HCO(3)(-) in concentration 1-20mM or air evacuation at pH 3.5) result in a significant (up to 370%) increase of ANS fluorescence (indicative of structural changes in MSP), whereas HCO(3)(-) lowers the ANS fluorescence to the initial level observed in untreated protein at pH 6.5. Since the effects are revealed at (sub)micromolar concentrations of HCO(3)(-), the specific high-affinity binding of HCO(3)(-) (or CO(2)) to MSP (required for its native structure preservation) is proposed. Possible bicarbonate binding sites and its physiological role within the water-oxidizing complex of photosystem II are discussed.

FTIR evidence that the PsbP extrinsic protein induces protein conformational changes around the oxygen-evolving Mn cluster in photosystem II

Extrinsic proteins of photosystem II (PSII) regulate the oxygen-evolving reaction performed at the Mn cluster by controlling the binding properties of the indispensable cofactors Ca(2+) and Cl(-). However, the molecular mechanism underlying this regulation is not yet understood. We have investigated the structural couplings of the extrinsic proteins PsbO, PsbP, and PsbQ of higher plants with the Mn cluster using Fourier transform infrared (FTIR) spectroscopy. Light-induced FTIR difference spectra upon the S(1) --> S(2) transition were measured using spinach PSII membranes, and the effects of the selective depletion of extrinsic proteins were examined. Depletion of the PsbP and PsbQ proteins by NaCl washing revealed clear changes in the amide I bands with no appreciable changes in the bands of carboxylate and imidazole groups, whereas the depletion of all three proteins by CaCl(2) washing did not cause further changes. The original amide I features were recovered by reconstitution of the NaCl-washed PSII with PsbP, and the same recovery was observed with (13)C-labeled PsbP. These results indicate that the PsbP protein, but not PsbQ and PsbO, affects the protein conformation around the Mn cluster in the intrinsic proteins without changing the ligand structure. Reconstitution with Delta15-PabP, in which the 15 N-terminal residues were truncated, did not restore the amide I bands, indicating that the interaction of the N-terminal region induces the conformational changes. This observation correlates well with a previous finding that Delta15-PabP did not restore the Ca(2+) and Cl(-) retention ability upon rebinding to PSII [Ifuku, K., et al. (2005) Photosynth. Res. 84, 251-255]. Therefore, the evidence strongly suggests that protein conformational changes around the Mn cluster induced by PsbP through its N-terminal region affect the binding properties of Ca(2+) and Cl(-) and enhance their retention.

The psbo1 mutant of Arabidopsis cannot efficiently use calcium in support of oxygen evolution by photosystem II

The Arabidopsis thaliana mutant psbo1 contains a point mutation in the psbO-1 gene (At5g66570) leading to the loss of expression of the PsbO-1 protein and overexpression of the PsbO-2 protein (Murakami, R., Ifuku, K., Takabayashi, A., Shikanai, T., Endo, T., and Sato, F. (2002) FEBS Lett. 523, 138-142). Previous characterization of fluorescence induction and decay kinetics by our laboratory documented defects on both the oxidizing and reducing sides of Photosystem II. Additionally, anomalous flash oxygen yield patterns indicated that the mutant contains a defective oxygen-evolving complex that appears to exhibit anomalously long-lived S(2) and S(3) oxidation states (Liu, H., Frankel, L. K., and Bricker, T. M. (2007) Biochemistry 46, 7607-7613). In this study, we have documented that the S(2) and S(3) states in psbo1 thylakoids decay very slowly. The total flash oxygen yield of the psbo1 mutant was also significantly reduced, as was its stability. Incubation of psbo1 thylakoids at high NaCl concentrations did not increase the rate of S(2) and S(3) state decay. The oxygen-evolving complexes of the mutant did, however, exhibit somewhat enhanced stability following this treatment. Incubation with CaCl(2) had a significantly more dramatic effect. Under this condition, both the S(2) and S(3) states of the mutant decayed at nearly the same rate as the wild type, and the total oxygen yield and its stability following CaCl(2) treatment were indistinguishable from that of the wild type. These results strongly suggest that the principal defect in the psbo1 mutant is an inability to effectively utilize the calcium associated with Photosystem II. We hypothesize that the PsbO-2 protein cannot effectively sequester calcium at the oxygen-evolving site.

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.

Functional analysis of photosystem II in a PsbO-1-deficient mutant in Arabidopsis thaliana

The Arabidopsis thaliana mutant psbo1 (formerly the mutant LE18-30), which contains a point mutation in the psbO-1 gene leading to defective expression of the PsbO-1 protein, has recently been described [Murakami, R. et al. (2002) FEBS Lett. 523, 138-142]. This mutant completely lacks the PsbO-1 protein and overexpresses the PsbO-2 protein. To further study the effect of PsbO-1 deficiency on the function of photosystem II, the polyphasic chlorophyll a fluorescence rise and flash fluorescence induction and decay of the relative fluorescence quantum yield were measured in whole leaves from wild type and the psbo1 mutant. Additionally, flash oxygen yield experiments were performed on thylakoid membranes isolated from wild type and the psbo1 mutant. The results obtained indicate that during fluorescence induction the psbo1 gene exhibited an enhanced O to P transition. Additionally, while the J to I transition in wild type accounted for more than 30% of the total fluorescence yield, in the mutant it accounted for less than 2% rise in the total. Analysis of the flash-induced fluorescence rise in the presence of DCMU indicated that in wild type the ratio of PS IIalpha to PS IIbeta reaction centers was approximately 1.2 while in the mutant the ratio was approximately 0.3. Fluorescence decay kinetics in the absence of DCMU indicated that electron transfer to QB was significantly altered in the mutant. Fluorescence decay kinetics in the presence of DCMU indicated that the charge recombination between QA- and the S2 state of the oxygen-evolving complex was retarded. Furthermore, flash oxygen yield analysis indicated that both the S2 and S3 states exhibited significantly longer lifetimes in the psbo1 mutant than in wild type. Our data indicate that while PsbO-1-deficient plants can grow photoautotrophically (although at a reduced growth rate) the photochemistry of PS II is significantly altered.

Spare quinones in the QB cavity of crystallized photosystem II from Thermosynechococcus elongatus

The recent crystallographic structure at 3.0 A resolution of PSII from Thermosynechococcus elongatus has revealed a cavity in the protein which connects the membrane phase to the binding pocket of the secondary plastoquinone Q(B). The cavity may serve as a quinone diffusion pathway. By fluorescence methods, electron transfer at the donor and acceptor sides was investigated in the same membrane-free PSII core particle preparation from T. elongatus prior to and after crystallization; PSII membrane fragments from spinach were studied as a reference. The data suggest selective enrichment of those PSII centers in the crystal that are intact with respect to O(2) evolution at the manganese-calcium complex of water oxidation and with respect to the integrity of the quinone binding site. One and more functional quinone molecules (per PSII monomer) besides of Q(A) and Q(B) were found in the crystallized PSII. We propose that the extra quinones are located in the Q(B) cavity and serve as a PSII intrinsic pool of electron acceptors.

Structure and activity of the photosystem II manganese-stabilizing protein: role of the conserved disulfide bond

The 33-kDa manganese-stabilizing protein (MSP) of Photosystem II (PS II) maintains the functional stability of the Mn cluster in the enzyme's active site. This protein has been shown to possess characteristics similar to those of the intrinsically disordered, or natively unfolded proteins. Alternately it was proposed that MSP should be classified as a molten globule, based in part on the hypothesis that its lone disulfide bridge is necessary for structural stability and function in solution. A site-directed mutant MSP (C28A,C51A) that eliminates the disulfide bond reconstitutes O(2) evolution activity and binds to MSP-free PS II preparations at wild-type levels. This mutant was further characterized by incubation at 90 degrees C to determine the effect of loss of the disulfide bridge on MSP thermostability and solution structure. After heating at 90 degrees C for 20 min, C28A,C51A MSP was still able to bind to PS II preparations at molar stoichiometries similar to those of WT MSP and reconstitute O(2) evolution activity. A fraction of the protein aggregates upon heating, but after resolubilization, it regains the ability to bind to PS II and reconstitute O(2) evolution activity. Characterization of the solution structure of C28A,C51A MSP, using CD spectroscopy, UV absorption spectroscopy, and gel filtration chromatography, revealed that the mutant has a more disordered solution structure than WT MSP. The disulfide bond is therefore unnecessary for MSP function and the intrinsically disordered characteristics of MSP are not dependent on its presence. However, the disulfide bond does play a role in the solution structure of MSP in vivo, as evidenced by the lability of a C20S MSP mutation in Synechocystis 6803.

The Manganese-stabilizing Protein Is Required for Photosystem II Assembly/Stability and Photoautotrophy in Higher Plants

Interfering RNA was used to suppress the expression of two genes that encode the manganese-stabilizing protein of photosystem II in Arabidopsis thaliana, MSP-1 (encoded by psbO-1, At5g66570), and MSP-2 (encoded by psbO-2, At3g50820). A phenotypic series of transgenic plants was recovered that expressed high, intermediate, and low amounts of these two manganese-stabilizing proteins. Chlorophyll fluorescence induction and decay analyses were performed. Decreasing amounts of expressed protein led to the progressive loss of variable fluorescence and a marked decrease in the fluorescence quantum yield (F(v)/F(m)) in both the absence and the presence of dichloromethylurea. This result indicated that the amount of functional photosystem II reaction centers was compromised in the plants that exhibited intermediate and low amounts of the manganese-stabilizing proteins. An analysis of the decay of the variable fluorescence in the presence of dichlorophenyldimethylurea indicated that charge recombination between Q ((A-)) and the S(2) state of the oxygen-evolving complex was seriously retarded in the plants that expressed low amounts of the manganese stabilizing proteins. This may have indicated a stabilization of the S(2) state in the absence of the extrinsic component. Immunological analysis of the photosystem II protein complement indicated that significant losses of the CP47, CP43, and D1 proteins occurred upon the loss of the manganese-stabilizing proteins. This indicated that these extrinsic proteins were required for photosystem II core assembly/stability. Additionally, although the quantity of the 24-kDa extrinsic protein was only modestly affected by the loss of the manganese-stabilizing proteins, the 17-kDa extrinsic protein dramatically decreased. The control proteins ribulose bisphosphate carboxylase and cytochrome f were not affected by the loss of the manganese-stabilizing proteins; the photosystem I PsaB protein, however, was significantly reduced in the low expressing transgenic plants. Finally, it was determined that the transgenic plants that expressed low amounts of the manganese-stabilizing proteins could not grow photoautotrophically.

Cytochrome c550 in the Cyanobacterium Thermosynechococcus elongatus

Cytochrome c(550) is one of the extrinsic Photosystem II subunits in cyanobacteria and red algae. To study the possible role of the heme of the cytochrome c(550) we constructed two mutants of Thermosynechococcus elongatus in which the residue His-92, the sixth ligand of the heme, was replaced by a Met or a Cys in order to modify the redox properties of the heme. The H92M and H92C mutations changed the midpoint redox potential of the heme in the isolated cytochrome by +125 mV and -30 mV, respectively, compared with the wild type. The binding-induced increase of the redox potential observed in the wild type and the H92C mutant was absent in the H92M mutant. Both modified cytochromes were more easily detachable from the Photosystem II compared with the wild type. The Photosystem II activity in cells was not modified by the mutations suggesting that the redox potential of the cytochrome c(550) is not important for Photosystem II activity under normal growth conditions. A mutant lacking the cytochrome c(550) was also constructed. It showed a lowered affinity for Cl(-) and Ca(2+) as reported earlier for the cytochrome c(550)-less Synechocystis 6803 mutant, but it showed a shorter lived S(2)Q(B)(-) state, rather than a stabilized S(2) state and rapid deactivation of the enzyme in the dark, which were characteristic of the Synechocystis mutant. It is suggested that the latter effects may be caused by loss (or weaker binding) of the other extrinsic proteins rather than a direct effect of the absence of the cytochrome c(550).

Biosynthetic Ca2+/Sr2+ Exchange in the Photosystem II Oxygen-evolving Enzyme of Thermosynechococcus elongatus

The thermophilic cyanobacterium, Thermosynechococcus elongatus, has been grown in the presence of Sr2+ instead of Ca2+ with the aim of biosynthetically replacing the Ca2+ of the oxygen-evolving enzyme with Sr2+. Not only were the cells able to grow normally with Sr2+, they actively accumulated the ion to levels higher than those of Ca2+ in the normal cultures. A protocol was developed to purify a fully active Sr(2+)-containing photosystem II (PSII). The modified enzyme contained a normal polypeptide profile and 1 strontium/4 manganese, indicating that the normal enzyme contains 1 calcium/4 manganese. The Sr(2+)- and Ca(2+)-containing enzymes were compared using EPR spectroscopy, UV-visible absorption spectroscopy, and O2 polarography. The Ca2+/Sr2+ exchange resulted in the modification of the EPR spectrum of the manganese cluster and a slower turnover of the redox cycle (the so-called S-state cycle), resulting in diminished O2 evolution activity under continuous saturating light: all features reported previously by biochemical Ca2+/Sr2+ exchange in plant PSII. This allays doubts that these changes could be because of secondary effects induced by the biochemical treatments themselves. In addition, the Sr(2+)-containing PSII has other kinetics modifications: 1) it has an increased stability of the S3 redox state; 2) it shows an increase in the rate of electron donation from TyrD, the redox-active tyrosine of the D2 protein, to the oxygen-evolving complex in the S3-state forming S2; 3) the rate of oxidation of the S0-state to the S1-state by TyrD* is increased; and 4) the release of O2 is slowed down to an extent similar to that seen for the slowdown of the S3TyrZ* to S0TyrZ transition, consistent with the latter constituting the limiting step of the water oxidation mechanism in Sr(2+)-substituted enzyme as well as in the normal enzyme. The replacement of Ca2+ by Sr2+ appears to have multiple effects on kinetics properties of the enzyme that may be explained by S-state-dependent shifts in the redox properties of both the manganese complex and TyrZ as well as structural effects.

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.

Substrate water exchange in photosystem II depends on the peripheral proteins

The (18)O exchange rates for the substrate water bound in the S(3) state were determined in different photosystem II sample types using time-resolved mass spectrometry. The samples included thylakoid membranes, salt-washed Triton X-100-prepared membrane fragments, and purified core complexes from spinach and cyanobacteria. For each sample type, two kinetically distinct isotopic exchange rates could be resolved, indicating that the biphasic exchange behavior for the substrate water is inherent to the O(2)-evolving catalytic site in the S(3) state. However, the fast phase of exchange became somewhat slower (by a factor of approximately 2) in NaCl-washed membrane fragments and core complexes from spinach in which the 16- and 23-kDa extrinsic proteins have been removed, compared with the corresponding rate for the intact samples. For CaCl(2)-washed membrane fragments in which the 33-kDa manganese stabilizing protein (MSP) has also been removed, the fast phase of exchange slowed down even further (by a factor of approximately 3). Interestingly, the slow phase of exchange was little affected in the samples from spinach. For core complexes prepared from Synechocystis PCC 6803 and Synechococcus elongatus, the fast and slow exchange rates were variously affected. Nevertheless, within the experimental error, nearly the same exchange rates were measured for thylakoid samples made from wild type and an MSP-lacking mutant of Synechocystis PCC 6803. This result could indicate that the MSP has a slightly different function in eukaryotic organisms compared with prokaryotic organisms. In all samples, however, the differences in the exchange rates are relatively small. Such small differences are unlikely to arise from major changes in the metal-ligand structure at the catalytic site. Rather, the observed differences may reflect subtle long range effects in which the exchange reaction coordinates become slightly altered. We discuss the results in terms of solvent penetration into photosystem II and the regional dielectric around the catalytic site.

The oxidation state of the photosystem II manganese cluster influences the structure of manganese stabilizing protein

Exposure of photosystem II membranes to trypsin that has been treated to inhibit chymotrypsin activity produces limited hydrolysis of manganese stabilizing protein. Exposure to chymotrypsin under the same conditions yields substantial digestion of the protein. Further probing of the unusual insensitivity of manganese stabilizing protein to trypsin hydrolysis reveals that increasing the temperature from 4 to 25 degrees C will cause some acceleration in the rate of proteolysis. However, addition of low (100 microM) concentrations of NH2OH, that are sufficient to reduce, but not destroy, the photosystem II Mn cluster, causes a change in PS II-bound manganese stabilizing protein that causes it to be rapidly digested by trypsin. Immunoblot analyses with polyclonal antibodies directed against the N-terminus of the protein, or against the entire sequence show that trypsin cleavage produces two distinct peptide fragments estimated to be in the 17-20 kDa range, consistent with proposals that there are 2 mol of the protein/mol photosystem II. The correlation of trypsin sensitivity with Mn redox state(s) in photosystem II suggest that manganese stabilizing protein may interact either directly with Mn, or alternatively, that the polypeptide is bound to another protein of the photosystem II reaction center that is intimately involved in binding and redox activity of Mn.

Two functionally distinct manganese clusters formed by introducing a mutation in the carboxyl terminus of a photosystem II reaction center polypeptide, D1, of the green alga Chlamydomonas reinhardtii

To study the function of the carboxyl-terminal domain of a photosystem II (PSII) reaction center polypeptide, D1, chloroplast mutants of the green alga Chlamydomonas reinhardtii have been generated in which Leu-343 and Ala-344 have been simultaneously or individually replaced by Phe and Ser, respectively. The mutants carrying these replacements individually, L343F and A344S, showed a wild-type phenotype. In contrast, the double mutant, L343FA344S, evolved O2 at only 20-30% of the wild-type rate and was unable to grow photosynthetically. In this mutant, PSII accumulated to 60% of the wild-type level, indicating that the O2-evolving activity per PSII was reduced to approximately half that of the wild-type. However, the amount of Mn atom detected in the thylakoids suggested that a normal amount of Mn cluster was assembled. An investigation of the kinetics of flash-induced fluorescence yield decay revealed that the electron transfer from Q(-)(A) to Q(B) was not affected. When a back electron transfer from Q(-)(A) to a donor component was measured in the presence of 3-(3,4-dichlorophenol)-1,1-dimethylurea, a significantly slower component of the Q(-)(A) oxidation was detected in addition to the normal component that corresponds to the back electron transfer from the Q(-)(A) to the S(2)-state of the Mn cluster. Thermoluminescence measurements revealed that L343FA344S cells contained two functionally distinct Mn clusters. One was equivalent to that of the wild-type, while the other was incapable of water oxidation and was able to advance the transition from the S(1)-state to the S(2)-state. These results suggested that a fraction of the Mn cluster had been impaired by the L343FA344S mutation, leading to decreased O2 evolution. We concluded that the structure of the C-terminus of D1 is critical for the formation of the Mn cluster that is capable of water oxidation, in particular, transition to higher S-states.

Origin and properties of fluorescence emission from the extrinsic 33 kDa manganese stabilizing protein of higher plant water oxidizing complex

The fluorescence properties of the isolated extrinsic 33 kDa subunit acting as 'manganese stabilizing protein' (MSP) of the water oxidizing complex in photosynthesis was analyzed in buffer solution. Measurements of the emission spectra as a function of excitation wavelength, pH and temperature led to the following results: (a) under all experimental conditions the spectra monitored were found to be the composite of two contributions referred to as '306 nm band' and 'long-wavelength band', (b) the excitation spectra of these two bands closely resemble those of tyrosine and tryptophan in solution, respectively, (c) the spectral shape of the '306 nm band' is virtually independent on pH but its amplitude drastically decreases in the alkaline with a pK of 11.7, (d) the amplitude of the 'long-wavelength' emission band at alkaline pH slightly increases when the pH rises from 7.2 to about 11.3 followed by a sharp decline at higher pH, and (e) the shape of the overall spectrum at pH 7.2 is only slightly changed upon heating to 90 degrees C whereas the amplitude significantly declines. Based on these findings the two distinct fluorescence bands are ascribed to tyrosine(s) ('306 nm band') and the only tryptophan residue W241 of MSP from higher plants ('long-wavelength band') as emitters which are both embedded into a rather hydrophobic environment.

Amino acid residues that modulate the properties of tyrosine Y(Z) and the manganese cluster in the water oxidizing complex of photosystem II

The catalytic site for photosynthetic water oxidation is embedded in a protein matrix consisting of nearly 30 different polypeptides. Residues from several of these polypeptides modulate the properties of the tetrameric Mn cluster and the redox-active tyrosine residue, Y(Z), that are located at the catalytic site. However, most or all of the residues that interact directly with Y(Z) and the Mn cluster appear to be contributed by the D1 polypeptide. This review summarizes our knowledge of the environments of Y(Z) and the Mn cluster as obtained from the introduction of site-directed, deletion, and other mutations into the photosystem II polypeptides of the cyanobacterium Synechocystis sp. PCC 6803 and the green alga Chlamydomonas reinhardtii.

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.

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.

Functional reconstitution of photosystem II with recombinant manganese-stabilizing proteins containing mutations that remove the disulfide bridge

The 33-kDa extrinsic subunit of PSII stabilizes the O2-evolving tetranuclear Mn cluster and accelerates O2 evolution. We have used site-directed mutagenesis to replace one or both Cys residues in spinach MSP with Ala. Previous experiments using native and reduced MSP led to the conclusion that a disulfide bridge between these two cysteines is essential both for its binding and its functional properties. We report here that the disulfide bridge, though essential for MSP stability, is otherwise dispensible. The mutation C51A by itself had a delayed effect on MSP function: [C51A]MSP restored normal rates of O2 evolution to PSII but was defective in stabilizing this activity during extended illumination. In contrast, the Cys-free double mutant, [C28A,C51A]MSP, was functionally identical to the wild-type protein. Based on results presented here, we propose a light-dependent interaction between MSP and PSII that occurs only during the redox cycling of the Mn cluster and which is destabilized by the single mutation, C51A.

A current assessment of photosystem II structure

This review covers the recent progress in the elucidation of the structure of photosystem II (PSII). Because much of the structural information for this membrane protein complex has been revealed by electron microscopy (EM), the review will also consider the specific technical and interpretation problems that arise with EM where they are of particular relevance to the structural data. Most recent reviews of photosystem II structure have concentrated on molecular studies of the PSII genes and on the likely roles of the subunits that they encode or they were mainly concerned with the biophysical data and fast absorption spectroscopy largely relating to electron transfer in various purified PSII preparations. In this review, we will focus on the approaches to the three-dimensional architecture of the complex and the lipid bilayer in which it is located (the thylakoid membrane) with special emphasis placed upon electron microscopical studies of PSII-containing thylakoid membranes. There are a few reports of 3D crystals of PSII and of associated X-ray diffraction measurements and although little structural information has so far been obtained from such studies (because of the lack of 3D crystals of sufficient quality), the prospects for such studies are also assessed.

Improved 5-step modeling of the Photosystem II S-state mechanism in cyanobacteria

We present a model of the S-state mechanism, as well as an improved eigenvalue analysis, that integrate into a coherent ensemble several features found since the S-state model was initially developed. These features include the presence of S-1, deactivations in the dark interval between flashes, and the change in the number of active PS II centers by photoinhibition or photoactivation. A new feature is the capacity to predict the steady-state distribution of S-states under conditions of steady photoinhibition or photoactivation. The improved eigenvalue analysis allowed the calculation of the initial S-state distribution. In addition, the model resolved 'true' photochemical misses from apparent misses due to deactivations in the dark interval between flashes. The model suggested that most of the misses that are commonly reported are due to deactivations, and not to an intrinsic inefficiency of the photochemical mechanism of PS II. Because models that allow double-hits encompassing the S2 to S3 transition often predict negative initial quantities of S2 in cyanobacteria, our proposed model specifically prohibited them. The model accounts for inhomogeneous misses and a steady-state distribution of the type (S2)≈(S1)>(S3)≈(S0). This 5-step model uses only 4 probabilities, and is therefore easy to handle. The use of this model is critical for the analysis of several cyanobacterial strains, as well as for any species that show non-negligible deactivations in the dark interval between flashes.

Secondary structure of the 33 kDa, extrinsic protein of Photosystem II: a far-UV circular dichroism study

The 33 kDa extrinsic protein of Photosystem II is an important component of the oxygen-evolving apparatus which functions to stabilize the manganese cluster at physiological chloride concentrations and to lower the calcium requirement for oxygen evolution. Chou-Fasman analysis of the amino-acid sequence of this protein suggests that this component contains a high proportion of alpha-helical structure and only relatively small amounts of beta-sheet structure. A computational study using more sophisticated techniques (Beauregard, M. (1992) Environ. Exp. Bot. 32, 411-429) concluded that the protein contained little periodically ordered secondary structure. In this study, we have directly measured the relative proportions of secondary structure present in the 33 kDa protein using far-ultraviolet circular dichroism spectroscopy. Our results indicate that, in solution, this protein contains a large proportion of beta-sheet structure (38%) and relatively small amounts of alpha-helical structure (9%). A structural model of the 33 kDa protein based on a constrained Chou-Fasman analysis (Teeter, M.M. and Whitlow, M (1988) Proteins 4, 262-273) is presented.

Reconstitution of the spinach oxygen-evolving complex with recombinant Arabidopsis manganese-stabilizing protein

The psbO gene of cyanobacteria, green algae and higher plants encodes the precursor of the 33 kDa manganese-stabilizing protein (MSP), a water-soluble subunit of photosystem II (PSII). Using a pET-T7 cloning/expression system, we have expressed in Escherichia coli a full-length cDNA clone of psbO from Arabidopsis thaliana. Upon induction, high levels of the precursor protein accumulated in cells grown with vigorous aeration. In cells grown under weak aeration, the mature protein accumulated upon induction. In cells grown with moderate aeration, the ratio of precursor to mature MSP decreased as the optical density at induction increased. Both forms of the protein accumulated as inclusion bodies from which the mature protein could be released under mildly denaturing conditions that did not release the precursor. Renatured Arabidopsis MSP was 87% as effective as isolated spinach MSP in restoring O2 evolution activity to MSP-depleted PSII membranes from spinach; however, the heterologous protein binds to spinach PSIIs with about half the affinity of the native protein. We also report a correction to the previously published DNA sequence of Arabidopsis psbO (Ko et al., Plant Mol Biol 14 (1990) 217-227).

The role of CP 47 in the evolution of oxygen and the binding of the extrinsic 33-kDa protein to the core complex of Photosystem II as determined by limited proteolysis

In order to identify the domain within Photosystem II complexes that functions in the evolution of oxygen, we performed limited proteolysis with lysylendopeptidase of the core complex of Photosystem II which had been depleted of the extrinsic 33-kDa protein (Mn-stabilizing protein). The cleavage sites were estimated from the amino-terminal sequences of the degradation fragments, their apparent molecular masses and amino-acid compositions. Under certain conditions, the D2 protein was cleaved at Lys13; and a chlorophyll a-binding protein, CP 47, was cleaved at Lys227 and Lys389. Another chlorophyll a-binding protein, CP 43, was degraded more rapidly than CP 47. The oxygen-evolving activity and the capacity for rebinding of the 33-kDa protein to the core complex of Photosystem II decreased in parallel, with kinetics very similar to those of the cleavage of CP 47 at Lys389. These observations strongly suggest that the hydrophilic domain around Lys389 of CP 47, which are located on the lumenal side, is important in the binding of the 33-kDa protein and in maintaining the oxygen-evolving activity of the Photosystem II complex.

Slow oxygen release on the first two flashes in chemically stressed Photosystem II membrane fragments results from hydrogen peroxide oxidation

Flash-induced amperometric signals were measured with a Joliot-type O2 rate electrode in spinach Photosystem II (PS II) membrane fragments exposed to very low concentrations of added hydroxylamine or hydrogen peroxide. In both cases 'anomalous O2 signals' were observed on the first two flashes, and oscillating four-flash patterns were observed on subsequent flashes. The anomalous signals were eliminated in the presence of catalase but not EDTA. The rise times of the O2-release kinetics associated with the anomalous signals were slow (ca. 20 ms with NH2OH and ca. 120 ms with H2O2) compared to the kinetics of O2 release on subsequent flashes and in control membranes (3-6 ms). It is proposed that when the intact PS II O2-evolving complex is perturbed with small concentrations of added reductant, H2O2 can gain access and bind to the complex. Bound H2O2 can then reduce lower S states in some centers leading to anomalous O2 signals on the first two flashes. A model is presented to explain both types of anomalous O2 production. Oxygen observed on the third and subsequent flashes is due to the normal photosynthetic O2-evolution process arising from the S3-state. Anomalous O2 production could be a protective mechanism in PS II centers subjected to stress conditions.

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.

Interaction of CPa-1 with the manganese-stabilizing protein of photosystem II: identification of domains cross-linked by 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide

The structural organization of photosystem II proteins has been investigated by use of the zero-length protein cross-linking reagent 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide and monoclonal and polyclonal antibody reagents. Photosystem II membranes were treated with 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide which cross-links amino groups to carboxyl groups which are in van der Waals contact. This treatment did not affect the oxygen evolution rates of these membranes and increased the retention of oxygen evolution after CaCl2 washing. Analysis of the proteins cross-linked by this treatment indicated that two cross-linked species with apparent molecular masses of 95 and 110 kDa were formed which cross-reacted with antibodies against both the 33-kDa manganese-stabilizing protein and the chlorophyll protein CPa-1. Cleavage of the 110-kDa cross-linked species with cyanogen bromide followed by N-terminal sequence analysis was used to identify the peptide fragments of CPa-1 and the manganese-stabilizing protein which were cross-linked. Two cyanogen bromide fragments were identified with apparent molecular masses of 50 and 25 kDa. N-Terminal sequence analysis of the 50-kDa cyanogen bromide fragment indicates that this consists of the C-terminal 16.7-kDa fragment of CPa-1 and the intact manganese-stabilizing protein. This strongly suggests that the manganese-stabilizing protein is cross-linked to the large extrinsic loop domain of CPa-1. N-Terminal analysis of the 25-kDa cyanogen bromide fragment indicates that this consists of the C-terminal 16.7-kDa peptide of CPa-1 and the N-terminal 8-kDa peptide of the manganese-stabilizing protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Oxygen-evolving photosystem II preparation from wild type and photosystem II mutants of Synechocystis sp. PCC 6803

We present here a simple and rapid method which allows relatively large quantities of oxygen-evolving photosystem II- (PS-II-) enriched particles to be obtained from wild-type and mutants of the cyanobacterium Synechocystis 6803. This method is based on that of Burnap et al. [Burnap, R., Koike, H., Sotiropoulou, G., Sherman, L. A., & Inoue, Y. (1989) Photosynth. Res. 22, 123-130] but is modified so that the whole preparation, from cells to PS-II particles, is achieved in 10 h and involves only one purification step. The purified preparation exhibits a 5-6-fold increase of O2-evolution activity on a chlorophyll basis over the thylakoids. The ratio of PS-I to PS-II is about 0.14:1 in the preparation. The secondary quinone electron acceptor, QB, is present in this preparation as demonstrated by thermoluminescence studies. These PS-II particles are well-suited to spectroscopic studies as demonstrated by the range of EPR signals arising from components of PS-II that are easily detectable. Among the EPR signals presented are those from a formal S3-state, attributed to an oxidized amino acid interacting magnetically with the Mn complex in Ca(2+)-deficient PS-II particles, and from S2 modified by the replacement of Ca2+ by Sr2+. Neither of these signals has been previously reported in cyanobacteria. Their detection under these conditions indicates a similar lesion caused by Ca2+ depletion in both plants and cyanobacteria. The protocol has also been applied to mutants which have site-specific changes in PS-II. Data are presented on mutants having changes on the electron donor (Y160F) and electron acceptor (G215W) side of the D2 polypeptide.